the history of the core dynamos of mars and the moon inferred … · 2019. 8. 23. · draft 2 24...
TRANSCRIPT
Draft
The History of the Core Dynamos of Mars and the Moon Inferred From Their Crustal Magnetization: A Brief Review
Journal: Canadian Journal of Earth Sciences
Manuscript ID cjes-2018-0068.R1
Manuscript Type: Article
Date Submitted by the Author: 06-Jun-2018
Complete List of Authors: Arkani-Hamed, Jafar; University of Toronto, Physics
Keyword:true polar wander of Mars and Moon, dynamo reversals of Mars and Moon, isolated magnetic anomalies of Mars and Moon, core dynamo of Mars and Moon
Is the invited manuscript for consideration in a Special
Issue? :
Understanding magnetism and electromagnetism and their implications: A tribute to David W. Strangway
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
1
1
2
The History of the Core Dynamos of Mars and the Moon Inferred From Their Crustal 3
Magnetization: A Brief Review 4
5
Jafar Arkani-Hamed1,2 6
7
1Department of Physics, University of Toronto, Toronto, Canada, M5S-1A7, Tel: 905-822-0232, 8
Fax: 416-978-7606, E-Mail: [email protected] 9
2Department of Earth and Planetary Sciences, McGill University, Montreal, Canada, H3A-0E8. 10
11
12
13
14
15
16
17
18
19
20
21
22
23
Page 1 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
2
Abstract. 24
The core dynamos of Mars and the Moon have distinctly different histories. Mars had no core 25
dynamo at the end of accretion. It took ~100 Myr for the core to create a strong dynamo that 26
magnetized the martian crust. Giant impacts during 4.2-4.0 Ga crippled the core dynamo 27
intermittently, until a thick stagnant lithosphere developed on the surface and reduced the heat 28
flux at the core-mantle boundary, killing the dynamo at ~3.8 Ga. On the other hand, the Moon 29
had a strong core dynamo at the end of accretion that lasted ~100 Myr and magnetized its 30
primordial crust. Either precession of the core, or thermo-chemical convection in the mantle, or 31
chemical convection in the core created a strong core dynamo that magnetized the sources of the 32
isolated magnetic anomalies in later times. Mars and the Moon indicate dynamo reversals and 33
true polar wander. The polar wander of the Moon is easier to explain compared to that of Mars. 34
It was initiated by the mass deficiency at South Pole Aitken basin which moved the basin 35
southward by ~68o relative to the dipole axis of the core field. The formation of mascon maria at 36
later times introduced positive mass anomalies at the surface, forcing the Moon to make an 37
additional ~52o degree polar wander. Interaction of multiple impact shock waves with the 38
dynamo, the abrupt angular momentum transfer to the mantle by the impactors, and the global 39
overturn of the core after each impact were probably the factors causing the dynamo reversal. 40
41
42
Key Words: 43
True polar wander of Mars and Moon, Dynamo reversals of Mars and Moon 44
Isolated magnetic anomalies of Mars and Moon, Core dynamo of Mars and Moon 45
46
Page 2 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
3
1. Introduction: 47
Mars and the Moon are two terrestrial bodies with similar magnetic field characteristics, both 48
have highly magnetized crusts but no core dynamos at present. They had strong core dynamos in 49
the past. The histories of their core dynamos are distinctly different, mainly because of their 50
different accretion processes and different core sizes. Terrestrial planets accrete from the 51
planetary nebula of dust and gas through four distinct stages (Wetherill and Stewart, 1989; 52
Weidenschilling, 1997; Chambers and Wetherill, 1998; Agnor et al., 1999; Kokubo and Ida, 53
1996; Chambers, 2004; Kokubo and Genda, 2010). First, dust particles collide and stick 54
together, forming some centimeter size grains that settle in the mid-plane of the nebula, orbiting 55
the Sun. Second, localized gravitational instabilities collapse the cloud of the centimeter size 56
grains and create planetesimals with a few kilometer radii. Third, through their mutual 57
gravitational attraction, planetesimals collide and merge to produce planetary embryos that can 58
grow to the Moon- to Mars-sized bodies. Dynamic friction caused by the planetesimals and 59
remaining nebula dust and gas maintains the growing embryos in almost circular orbits about the 60
Sun. By the end of this stage majority of planetesimals and small grains are used up in making 61
the embryos, and almost the entire nebula gas is dissipated. Fourth, the mutual gravitational 62
attraction of the well-grown embryos in the absence of appreciable dynamic friction increases 63
their orbital eccentricities, leading to orbit crossings and high-velocity collisions in the process of 64
forming a large planet like Earth. Based on its small mass and rapid formation timescale 65
obtained from 182Hf–182W chronometry, Kobayashi and Dauphas (2013) proposed that Mars 66
likely formed in a massive disk from planetesimals smaller than 10 km in radius. The 67
Thorium/Tungsten and Thorium/Hafnium ratios in the martian mantle, confirm that the growth 68
time of Mars was on the order of 2 Myr (Dauphas and Pourmand, 2011), well within the upper 69
Page 3 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
4
limit of 10 Myr suggested by Hansen (2009). It is not clear whether Mars is a runaway grown 70
embryo without experiencing the embryo–embryo scale high velocity collisions, or it has 71
actually experienced a few such collisions in its last stage of accretion. On the other hand, the 72
Moon is formed through the impact of a high-velocity Mars size body with the Earth (e.g., 73
Canup, 2012; Canup, and Asphaug, 2001; Canup, et al., 2013), or by collision of two half-Earth 74
size bodies (Nakajima and Stevenson, 2014, 2015). 75
The most important factor that probably resulted in two different histories of the martian and 76
lunar core dynamos is the size of the two bodies, with radii of ~1700 km and 390 km, 77
respectively. The core volume of the Moon is only 1% of the Moon’s volume and that of Mars is 78
about 12% of martian volume. The lunar core is small enough to lose the initially excited 79
thermally driven core dynamo within ~100 Myr after its accretion (Arkani-Hamed and Boutin, 80
2017). Because no thermally driven core dynamo could last longer, the magnetic source bodies 81
in the lunar crust that formed at later times must have been magnetized by a mechanically driven 82
core dynamo (Dwyer et al., 2011, Le Bars et al., 2011), or a vigorous thermo-chemical mantle 83
convection (Stegman et al., 2003), or chemical convection due to possible core solidification 84
(Laneuville, et al. 2014). On the other hand, the superheated large martian core at the end of 85
accretion could generate an appreciable core dynamo once it recovered from the crippling effect 86
of the formation of northern Lowland. 87
When a core dynamo initiated and when it ceased, was there only a single initiation and 88
cessation episode or there were multiple episodes, what was the mechanism powering the core 89
dynamo, and what was the cause of the dynamo cessation? These are the main outstanding 90
issues we investigate on the basis of the major characteristics of the magnetic anomalies of Mars 91
and the Moon. Section 2 is concerned with the core dynamo of Mars, while that of the Moon is 92
Page 4 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
5
addressed in Section 3. Section 4 presents major conclusions of this investigation. Appendix A 93
is devoted to the analysis of a small and isolated magnetic anomaly, which is the common 94
feature of Mars and the Moon used in this study. Because of the small core sizes of both Mars 95
and the Moon compared to their surface radii, it is assumed in this study that the crustal 96
magnetization is due to the dipolar component of the core field. The higher degree components 97
of the core field decrease much faster than the dipole component as they propagate from the 98
core-mantle boundary to the surface. 99
100
2. Magnetic Anomalies of Mars: 101
Acuna et al. (1999) derived, for the first time, a global magnetic anomaly map of Mars using 102
Mars Global Surveyor (MGS) magnetic data acquired at 85–200 km altitudes. Several magnetic 103
anomaly maps of Mars were published within 5 years after the first publication (e.g., Acuna et 104
al., 2001; Purucker et al., 2000; Connerney et al., 2001; Arkani-Hamed, 2002a; Cain et al., 2003; 105
Langlais et al., 2004). A highly coherent model of martian magnetic field was derived using the 106
night time mapping-phase MGS magnetic data measured at 360-420 km altitudes (Arkani-107
Hamed, 2004a). The entire data were divided into two almost equal sets, and each set was 108
expressed in terms of the spherical harmonics of degree up to 90. The power spectra of these two 109
models were almost identical over harmonics of degree up to 62. Figure 1 shows the radial 110
component of the martian magnetic field at 370 km altitude, derived using the co-varying 111
harmonics of degrees up to 62 of the two models, which have degree correlation coefficients 112
higher than 0.85 over the entire harmonics retained, and higher than 0.95 over the harmonics of 113
degrees lower than 50. By removing the higher degree harmonics and presenting the magnetic 114
field at a constant altitude, the spherical harmonic model significantly reduces the low-115
Page 5 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
6
correlating non-crustal noise and removes the effects of the spacecraft altitude variations. In the 116
absence of a core field at present, the anomalies are of crustal origin. The magnetic anomalies of 117
Mars’ crust are about an order of magnitude stronger than those of Earth’s crust at comparable 118
altitudes. This emphasizes that the magnetic source bodies of Mars’ crust are highly magnetic 119
compared to the terrestrial ones. This is because the estimates of the paleointensity of Mars’ core 120
field based on the ~4.1 Ga meteorite ALH 84001 (e.g., Weiss et al., 2008) and magnetostrophic 121
balance calculations (Arkani-Hamed, et al., 2008) are comparable to, if not weaker than, Earth’s 122
magnetic field. 123
Figure 1 is dominated by strong and closely spaced complex magnetic anomalies over terrae 124
Cimmeria and Sirenum, likely arising from large and closely located magnetic bodies. The 125
proximity of these strong anomalies precludes the determination of the paleomagnetic pole 126
position of Mars on the basis of these anomalies. However, there are 10 small and well-isolated 127
anomalies that can be successfully modeled by simple prisms of uniform magnetization (Arkani-128
Hamed, 2001). Using isolated magnetic anomalies has an advantage that a given magnetic 129
anomaly is simple enough to provide a reliable direction of the magnetizing field, hence the 130
paleomagnetic pole position. The resulting paleomagnetic poles are clustered, 7 of the poles are 131
clustered within a circle of 30o radius centered at about 50oE and 25oS (Arkani-Hamed, 2002b). 132
The cluster partly overlaps the paleomagnetic pole locations suggested by Sprenke and Baker 133
(2000). Also, the paleomagnetic poles obtained by modeling two isolated anomalies (Hood and 134
Zakharian, 2001; Hood and Richmond, 2002) fall inside the 30o radius circle. The lack of core 135
dynamo at present and the clustering of the paleomagnetic poles, while their magnetic source 136
bodies are widely distributed over the globe, indicate that the martian crust has been magnetized 137
by a long lasting global magnetic field in the past, likely produced by a core dynamo. 138
Page 6 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
7
139
140
141
2.1 History of Martian Core Dynamo: 142
The thermal evolution models of a growing proto-Mars (e.g., Senshu et al., 2002) show that 143
partial melting occurred in the upper parts of the mantle when the radius of the proto-Mars 144
exceeded ~80% of the present radius of Mars. The geochemical analyses of martian meteorites 145
imply a magma ocean of 700–800 km deep in the upper parts of the mantle by the end of 146
accretion (Righter et al., 1998). Once the temperature exceeds the solidus temperatures of 147
silicates, iron melts completely due to its lower melting temperature compared to those of the 148
silicates. The high-density iron blobs descend through the underlying mantle, initiating the core 149
formation. The gravitational energy released by the descending iron blobs results in a 150
superheated molten core with temperatures over 700K above the temperature at the base of the 151
lower mantle (e.g., Spohn et al., 2001). The descent time of an iron blob strongly depends on the 152
size of the blob and the viscosity of the mantle (Samuel and Tackley, 2008), it is usually shorter 153
than 1 Myr (Monteux and Arkani-Hamed, 2013). Concurrent with the core formation, the 154
differentiation of magma ocean in the upper mantle produces a primordial crust which floats on 155
the surface and cools rapidly. The cooling of the superheated core to the mantle in the early 156
stages after the accretion is expected to power a thermally driven core dynamo that is capable of 157
magnetizing the newly forming primordial crust as the crust cools below the magnetic blocking 158
temperatures of its minerals (e.g., Spohn et al., 2001; Arkani-Hamed, 2005). However, no core 159
dynamo probably existed in the early history of Mars. 160
Page 7 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
8
Figure 1 shows some major geologic structures such as the northern Lowland, the Tharsis bulge, 161
and the Tharsis volcanic mountains that have almost no magnetic signatures, which provide 162
constraints on the active period of the core dynamo. The lack of strong magnetic anomalies 163
inside the Lowland, and the similarity of the magnetic dichotomy to the topographic dichotomy 164
surrounding the Lowland imply that the Lowland formation process has likely demagnetized the 165
underlying crust, and no appreciable magnetic field existed to re-magnetize the newly formed 166
volcanic crust inside the Lowland. The Lowland is probably created in the later stages of 167
accretion of Mars by either a giant impact (Wilhelms and Squyres, 1984; Cameron, A.G.W., 168
1997), or several large but not giant impacts (Frey et al., 2002), or a giant hemispheric mantle 169
plume beneath the Lowland (Roberts and Zhong, 2006; Citron et al. 2018), or a giant impact 170
probably occurred at the antipodal location of the Lowland (Reese et. al., 2010). The single 171
giant impact at the Lowland site has been supported by many investigators (Andrews-Hanna et 172
al., 2008; Marinova et al., 2008; Nimmo et al., 2008; Arkani-Hamed, 2010) and it is adopted in 173
this study. Accordingly, the shock wave generated by the giant impact propagates in the mantle 174
and the core, resulting in differential heating with strong heating directly beneath the impact site. 175
The fast spinning and differentially heated low viscosity core stably stratifies, resulting in 176
spherically symmetric temperature that increases with radius and prevents the thermal 177
convection in the core and the initiation of a thermally driven core dynamo (Arkani-Hamed and 178
Olson, 2010). The stagnant lid thermal evolution models of Mars (Arkani-Hamed, 2010, 2012) 179
suggest that it takes about 50–120 Myr for the core to overcome the crippling effects of the giant 180
impact and establish a vigorous convection, powering a dynamo. During this long period, the 181
newly created volcanic crust inside the Lowland cools below the magnetic blocking temperature 182
range of its minerals, and cannot acquire thermoremanent magnetization in the absence of a core 183
Page 8 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
9
field (Arkani-Hamed and Boutin, 2012a). The lack of a magnetic anomaly associated with 184
Utopia basin, a ~3400 km diameter basin centered at 45oN and 115oE inside the Lowland, 185
supports this scenario. This is because a large impact that created the basin at about 350 Myr 186
after the formation of the Lowland could demagnetize the entire volcanic crust beneath the 187
impact site if the crust had been magnetized prior to the impact. The sharp change of the crustal 188
magnetization across the basin boundary is expected to create an appreciable magnetic anomaly, 189
called the magnetic edge effect, at satellite altitudes of about 400 km. Such a magnetic anomaly 190
is not observed (see Figure 1). 191
There are other evidence indicating the non-magnetic primordial crust. Figure 2 shows no 192
appreciable magnetic anomalies over a vast area south of 30oS and in the longitude range from 193
west of Hellas basin to Argyre basin, referred to as the South Province (Arkani-Hamed and 194
Boutin, 2012a), which is comparable in area to Tharsis bulge. The figure also shows no magnetic 195
anomalies over Tempe Terra in the northernmost part of highlands on Mars, extending from 196
30oN to 55oN and from 270oE to 300oE. Both areas have high densities of impact craters and 197
impact-related Quasi Circular Depressions (QCD, Frey et al., 2002) indicating that the 198
underlying crust is very ancient, likely primordial. Using MOLA topography data together with 199
the JPL gravity model (jgmro-110B2), Arkani-Hamed and Boutin (2012a and 2012b) determined 200
the crustal structure underlying impact craters and impact-related QCDs larger than 200 km in 201
diameter in both premordial areas and concluded that the impacts that have created these features 202
were capable of significantly disturbing and demagnetizing the entire underlying crust. The 203
craters and QCDs are expected to have well-defined magnetic edge effects if the surrounding 204
crust is magnetic. The lack of distinct magnetic anomalies associated with these features 205
indicates that the surrounding primordial crust is not magnetic. 206
Page 9 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
10
Investigations of the heating of Mars by giant impactors capable of producing craters larger than 207
500 km in diameter (e.g., Roberts et al., 2009; Arkani-Hamed, 2010, 2012, Roberts and Arkani-208
Hamed, 2012, 2014, 2017; Kuang et al., 2014) have concluded that the impactors could have 209
crippled the core dynamo, but only for a short period. The stagnant lid thermal evolution model 210
of martian mantle after the Borealis impact that created the martian Lowland (Arkani-Hamed, 211
2010) suggests that the impact-induced stratified core cools to the mantle while creating a thin 212
convecting shell at the top, which increases in thickness in due time. The shell becomes thick 213
enough capable of generating a core dynamo after about 50-120 Myr. The strong magnetic 214
anomalies over Cimmeria and Sinerum terrae indicate that the core dynamo remained active to 215
magnetize the underlying crust once it overcame the crippling effects of Borealis impactor. 216
When the core dynamo actually ceased is a matter of debate. Rock magnetic measurements of 217
the oldest martian meteorite ALH84001 (e.g., Collinson, 1997; Kirschvink et al., 1997; Weiss et 218
al., 2002; Antretter et al., 2003) indicate that it was magnetized on the martian surface during 219
Noachian period, between 4.1 and 3.8 Ga (Carr and Head, 2010). The lack of magnetic 220
anomalies inside giant impact basins Hellas, Argyre and Isidis suggests that the impacts have 221
demagnetized the crust, and there has been no active core dynamo to re-magnetize it (Acuna et 222
al., 1999; Mohit and Arkani-Hamed, 2004; Hood et al., 2003). The shock wave generated by a 223
giant impact propagates in the crust and demagnetizes the crust within less than 1 hour, but it 224
may take several million years for the demagnetized crust to acquire magnetization in the 225
presence of a dynamo field. This is largely because the remagnetization is controlled by the 226
cooling of the crust below the magnetic blocking temperatures of its minerals by thermal 227
conduction, which is a very slow process. Lillis et al. (2008, 2013) investigated the magnetic 228
field over 20 impact basins formed between 4.22 and 3.81 Ga (Frey, 2008) and concluded that 229
Page 10 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
11
the core field decayed rapidly within less than 100 Myr at around 4.1 Ga. The impact heating of 230
the martian core by the 8 largest of the 20 impacts, chronologically Daedalia, Ares, Amazonis, 231
Chryse, Scopolus, Acidalia, Utopia, and Hellas shows that each of the impacts is capable of 232
crippling the core dynamo for a limited time (Arkani-Hamed, 2012). The thermal evolution 233
models of Mars, based on coupling a spherically symmetric 1D core model with a 3D mantle 234
convection model while using a temperature- and pressure-dependent mantle viscosity, show that 235
the collective battering the core dynamo by the impacts and the gradual thickening of the 236
stagnant lid at the surface eventually killed the dynamo (Roberts and Arkani-Hamed, 2017). 237
Figure 3 shows the crippling effects of each impact on the core, the larger the impact the stronger 238
is the crippling. The core dynamo was strong until Ares impact (we name the impacts after the 239
giant basins). Being the second largest impact, Ares introduced substantial perturbations to the 240
core temperature and together with the following Amazonis impact constrained the core 241
convection to the upper about 200 km for about 30 Myr. During this long period, the downward 242
heat conduction from the high temperature stratified region near the core-mantle boundary and 243
the upward heat conduction from the deeper parts of the core along the adiabatic gradient 244
reduced the super-adiabatic deeper parts of the core to sub-adiabatic. The strong dynamo that 245
existed deep in the core prior to Ares impact might have retained its strength while it was 246
gradually decreasing from super-critical to sub-critical until Acidalia impact, the third largest 247
impact, which likely killed the subcritical dynamo. The core convection has never become 248
vigorous enough to regenerate a strong dynamo capable of magnetizing the crust since ~3.8 Ga. 249
Figure 1 shows no appreciable magnetic anomalies associated with Syria planum, implying no 250
core dynamo was active during the long-lived volcanism that lasted from late Noachian to early 251
Amazonian, 3.8 to 3.0 Ga (Carr and Head, 2010). The entire volcanic layer could have acquired 252
Page 11 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
12
appreciable magnetization if the core dynamo were active. Likewise, no significant magnetic 253
anomalies are associated with Tharsis bulge, formed through major volcanic activities in 254
Noachian to early Hesperian, 4.1 to 3.7 Ga (Carr and Head, 2010), although minor volcanism 255
likely continued to the recent past (Hartmann and Neukum, 2001). Many places of Tharsis 256
buldge have been affected by tectonic activities, such as Valles Marineris and shield volcanoes, 257
which could have created detectable magnetic anomalies if the Tharsis plains were appreciably 258
magnetized. The Valles has a mean depth of ~5 km, a width of 100–400 km, and a length of 259
~3500 km. If the Tharsis plains were magnetized prior to the formation of the canyon, an 260
appreciable magnetic edge effect is expected at satellite altitudes. Moreover, the large shield 261
volcanoes, Olympus, Arsia, Pavonis, and Ascreaus have poured out a huge amount of volcanic 262
lava during their formation from early Hesperian to late Amazonian 3.7 to 1.5 Ga (Carr and 263
Head, 2010). The magnetization of the thick lava by an existing core field can easily be detected 264
at the satellite altitude of ~400 km even if the magnetizing core field was an order of magnitude 265
weaker than the present Earth’s core field (Hood and Hartdegen, 1997). The lack of magnetic 266
signatures associated with the shield volcanos indicate that no core dynamo was active during the 267
formation of the volcanoes. The pre-existing Tharsis plains that are overlain by the shield 268
volcanoes are not appreciably magnetized either. If they were, the formation of the volcanoes 269
could have thermally demagnetized the underlying plains and created low-magnetic patches 270
giving rise to detectable magnetic anomalies. The lack of appreciable magnetic anomalies over 271
Tharsis bulge is related to the absence of a strong core dynamo when the major parts of the bulge 272
were forming (Arkani-Hamed, 2004b; Johnson and Phillips, 2004). 273
274
2.2 Driving Mechanism of Martian Core Dynamo: 275
Page 12 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
13
Planetary scientists have not yet reached consensus about the driving mechanism of the martian 276
core dynamo that operated probably between 4.4 and 3.8 Ga. The same is also true about the 277
processes that killed the dynamo. One possible scenario is that the core dynamo was maintained 278
by a vigorous thermal convection in a superheated liquid core (e.g., Stevenson et al., 1983; 279
Nimmo and Stevenson, 2000; Stevenson, 2001; Breuer and Spohn, 2003). Accordingly, once a 280
stagnant lithosphere developed on the convecting mantle, and Mars became a one-plate planet, it 281
hampered heat loss from the mantle and subsequently from the core. The core dynamo ceased 282
because of the reduction of heat flux from the core that decreased the vigor of core convection. 283
The formation of Mars through accreting small planetesimals and the core formation in the later 284
stages of accretion result in a superheated core that supports this driving mechanism (Spohn et 285
al., 2001). This scenario is viable ~50-120 Myr after the accretion of Mars, bearing in mind the 286
crippling effect of the Borealis impact that occurred almost at the end of accretion. Another 287
scenario relates the generation of the core dynamo to chemical convection due to solidification of 288
an inner core that could release light elements causing core convection (Schubert et al., 2000), 289
and argues that the core dynamo actually started later than 4 Ga when the martian core cooled 290
enough to start solidifying the inner core. However, the oldest martian meteorite (ALH84001) 291
was probably magnetized on the martian surface before 4 Ga, as mentioned above. Moreover, 292
the upper part of the core is probably liquid at present (Yoder et al., 2003). It is likely that the 293
lower mantle of Mars was heated up by the radioactive elements in the early history causing 294
decrease in the core cooling rate, hence hampering the core solidification (Spohn, et al., 2001; 295
Arkani-Hamed, 2005). 296
Mechanical excitation of the planetary core dynamos has been proposed in the last two decades 297
(e.g., Lacaze et al., 2006; Tilgner, 2005, 2007; Wu and Roberts, 2009, 2012; Lin et al., 2015; 298
Page 13 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
14
Reddy, et al., 2018). The proximity in time of the impacts that created the giant basins on Mars 299
and the cessation of the martian core dynamo (Lillis et al., 2008, 2013) suggests a causative 300
relationship between them (Kuang et al., 2014; Roberts and Arkani-Hamed, 2017). On the basis 301
of experimental, numerical and theoretical studies (e.g., Moffatt, 1970; Olson, 1981; Singer and 302
Olson, 1984; Kerswell, 1994; Tilgner, 2005; Lacaze et al., 2004, 2005, 2006), Arkani-Hamed et 303
al. (2008) proposed that tidally induced elliptical instability in the martian core by a large 304
retrograde satellite could have excited the core dynamo of Mars for hundreds of millions of 305
years. 306
Figure 4 shows an example for the orbital evolution of an asteroid assumed to be captured by 307
Mars as a retrograde satellite. It is determined using the two-body orbital dynamic equation 308
(Bertotti and Farinella, 1990), 309
dR/dt = -3κ sin(2δ)(m/M)(a5/R
11/2)[G(M +m)]1/2, 310
where κ (= 1.53) is the Love number of Mars (Yoder et al., 2003), a (= 3390 km) is the radius of 311
Mars, R is the orbital radius of the satellite, G is the gravitational constant, and M and m are the 312
masses of Mars (6.39x1023 kg) and asteroid (1.95x1020 kg), respectively. δ is the phase angle of 313
the maximum tidal deformation of Mars relative to the Mars–satellite line, produced because of 314
the time delay between the maximum tidal force exerted on Mars and the maximum deformation 315
of Mars due to its an-elastic response. Based on the scaling relationship between the mass and 316
velocity of an impactor and the resulting size of a crater [Holsapple and Schmidt, 1982; Schmidt 317
and Housen, 1987] the satellite could be capable of creating a Utopia size basin upon impacting 318
on Mars. Included in Figure 4 is the rate of tidal energy dissipation in Mars determined by 319
dE/dt = d/dt [(Im Ω2 – GMm/r)/2] (5) 320
Page 14 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
15
where Im and Ω are the moment of inertia and the angular velocity of Mars, respectively. The 321
rate of tidal energy dissipation is over two orders of magnitude greater than the rate of Ohmic 322
dissipation, ~108 W, expected in the martian core [Arkani-Hamed et al., 2008]. 323
Arkani-Hamed (2009) identified 4 subsets of basins from the 20 giant impact basins reported by 324
Frey (2008) that trace great circles on Mars. The probability that out of 20 randomly distributed 325
points on a sphere a given subset traces a great circle within +/- 3 degrees latitude was calculated 326
to be less than 6%. The author suggested that a given great circle was not due to a random 327
chance, rather it was the prevailing equator of Mars when the parent asteroid of a corresponding 328
subset of impactors was orbiting Mars as a satellite. The tidal energy dissipated in Mars due to 329
any of the 4 parent asteroids, ranging in mass from 1021 to 3x1021 kg, would be well over 2 330
orders of magnitude larger than the energy needed to maintain a strong core dynamo. Even if 331
only one of the satellites were retrograde, it could have lasted for over 800 million years (see 332
Figure 4). The tidal forces exerted on Mars could have created an ellipsoidal core-mantle 333
boundary that could have enhanced the mechanical energy transfer from the mantle to the core 334
(e.g., Wu and Roberts, 2009). If only 1% of the tidal energy was partitioned to the core, it was 335
sufficient to power a strong core dynamo. The spin-orbit coupling of Mars and the parent 336
retrograde satellite gradually reduced the orbital radius of the parent body. Shortly after the 337
parent body entered the Roche limit of Mars and disintegrated, the fragments likely followed the 338
same great circle and impacted on Mars producing the corresponding impact basins. Based on 339
the ages and age limits of the basins provided by Frey (2008), the basins of a given subset were 340
probably formed in a short period. Upon fragmentation the tidal force of the parent body, that 341
was deforming the streamlines in the martian core and exciting the elliptical instability, 342
diminished and the dynamo ceased. 343
Page 15 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
16
The tidally driven scenario of the martian core dynamo seems viable, but requires a detailed 344
dynamic verification. The capture of a large asteroid as a retrograde satellite can be complicated 345
largely due to the small mass of Mars. Agnor and Hamilton (2006) investigated the capture of 346
Triton by Neptune. The authors suggested that Triton was one of a two-body system, like Pluto 347
and Sharon, that approached Neptune. The larger of the two bodies was captured by Neptune but 348
the smaller one escaped. The capture process is not considered in the present study. Our starting 349
point in the above example is after the capture when the asteroid becomes a retrograde satellite. 350
A large asteroid can also be captured by Mars as a prograde satellite. If it is captured at a 351
distance farther that the co-orbiting radius ~20600 km, where the orbital period of the satellite 352
equals to the rotational period of Mars, the satellite recedes from Mars. On the other hand, if it is 353
captured at a distance shorter than the co-orbiting radius, it spirals down and impacts Mars. An 354
above-mentioned Utopia type asteroid captured as a prograde satellite at a distance slightly 355
shorter than the co-orbiting radius impacts Mars within about 70,000 years (See Figure 4 of 356
Arkani-Hamed, 2009). Although the tidal deformation of Mars would be appreciable and the 357
elliptical instability of the core would be strong, the entire period is too short to relate the long 358
lasting core dynamo of Mars to the tidal excitation by a prograde satellite. However, a 359
retrograde satellite captured at distances much longer than the co-orbiting radius remains orbiting 360
Mars for a long period as seen in Figure 4. 361
362
3. Magnetic Anomalies of the Moon: 363
The rock magnetic measurements of lunar rock samples during the Apollo era (e.g., Nagata et al., 364
1970; Runcorn et al., 1970; Strangway et al., 1970) and the surface magnetometer surveys by 365
Apollo 12 and Apollo 14 (Dyal et al., 1970, 1971) revealed for the first time that the lunar rocks 366
Page 16 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
17
are appreciably magnetized. The Apollo 15 and 16 sub satellites measurements in the equatorial 367
region and at about 100 km altitudes showed that the lunar crust is coherently magnetized over 368
tens to hundreds of km (Coleman et al., 1972; Dyal et al., 1974; Hood et al., 1981). The global 369
measurements of the magnetic field of the Moon by the Lunar Prospector’s magnetometer and 370
electron reflectometer (Lin et al., 1998; Purucker, 2008), and by Selene (Kaguya) mission 371
(Tsunakawa et al., 2010, 2014, 2015) established that the lunar crust is appreciably magnetized 372
on a global scale, emphasizing that the magnetizing field was likely generated by a core dynamo. 373
Figure 5 shows three versions of the radial component of the lunar magnetic field: the 150 degree 374
spherical harmonic model at 30 km altitude of the Lunar Prospector data (Purucker, 2008), the 375
Kaguya model at 100 km altitude (Tsunakawa, H. et al., 2010), and the Lunar Prospector Level-2 376
(LP-2) data at variable spacecraft altitudes ranging from 10 km to 100 km. The strong 377
correlation among the models emphasizes the validity of the anomalies. 378
379
3.1 History of the Lunar Core Dynamo: 380
The nature of the magnetizing field was debated during the Apollo era. Nine different 381
mechanisms were proposed: 1. A magnetic field associated with the solar wind, 2. The terrestrial 382
magnetic field when the Moon was very close to Earth, 3. An ancient magnetized solid core, 4. 383
Lunar core dynamo generated in a liquid core, 5. Small pockets of Fe-FeS eutectic close to the 384
surface that generated magnetic dynamo, 6. A local unipolar dynamo where current is induced in 385
a highly conducting lava basin by the electric field near the surface, 7. A thermoelectric dynamo 386
generated in the cooling lunar crust, 8. Electric current systems in ionized volcanic ash flows, 387
and 9. Crustal piezo-remanent magnetization induced by impact related shock waves propagating 388
in the iron grains of the crust. Daily and Dyal (1979) examined the viability of these 9 389
Page 17 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
18
mechanisms in a review article and only three were selected plausible: the impact-related shock 390
magnetization, the magnetization by an early solar wind field, and the magnetization by a liquid 391
core dynamo. On the basis of limited rock magnetic measurements during the Apollo missions, 392
Strangway et al. (1971) and Runcorn et al. (1971) suggested that the magnetizing field was 393
global in nature and was created by the core dynamo that was powered by thermal convection in 394
a liquid iron core of the Moon. The lunar rock samples were from loose rocks on the surface, 395
hence the directions of the core field at the Apollo landing sites were not constrained. 396
The paleomagnetic investigations have revealed a long lasting core dynamo with magnetic field 397
intensity of about 10-100 µT on the lunar surface at around 4.2-3.5 Ga (e.g., Cisowski et al., 398
1983; Fuller and Cisowski, 1987; Garrick-Bethell et al., 2009; Cournède et al., 2012; Shea et al., 399
2012; Suaveta et al., 2013; Weiss and Tikoo, 2014). Analysis of the crustal magnetic anomalies 400
of the Moon has also led to the conclusion that the lunar dynamo existed for a long period. The 401
electron reflectometer data over some Nectarian age basins (Crisium, Mendel–Rydberg, Bailly, 402
Humboldtianum and Moscoviense) show magnetic anomalies near their centers (e.g., Halekas et 403
al., 2003). The anomalies are related to the thermoremanent magnetization of the impact melt 404
rocks acquired during the Nectarian period, between the formation of Nectaris basin and 405
Imbrium basin, 3.92–3.85 Ga (Stoffler and Ryder, 2001). Moreover, majority of early Nectarian 406
basins have central magnetic anomalies with high intensities, implying a strong core field in the 407
early Nectarian period. Hood (2011) derived magnetic anomalies over Moscoviense, Mendel–408
Rydberg, Humboldtianum, and Crisium basins, using the low-altitude Lunar Prospector 409
magnetometer data and suggested thermoremanent magnetization of impact melt rocks in a long 410
lasting magnetizing field, hence supporting the existence of a core dynamo at the formation 411
times of the basins. The two magnetic anomalies located inside the South Pole-Aitken basin 412
Page 18 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
19
(SPAB) at the antipodal regions of young impact basins Imbrium and Serenitatis, are related to 413
the shock remanent magnetization created by converging impact induced ionized plasma and 414
shock waves (e.g., Hood and Artemieva, 2008). The antipodal magnetic anomalies can be used 415
to argue for the existence of a core dynamo with appreciable intensity at the impact time, 416
because the magnetic field of the solar wind alone may not be strong enough to magnetize a 417
thick crustal layer capable of creating a magnetic anomaly detectable at the satellite altitudes. 418
It is worth mentioning that there is no appreciable magnetic anomaly over the antipodal zone of 419
South Pole-Aitken basin. With a diameter of about 2500 km, it is the largest impact basin on the 420
Moon formed at about 200 Myr prior to the formation of Imbrium and Serenitatis basins (Merle 421
et al., 2014). Although the shock heating of the lunar core was strong enough to cripple a likely 422
pre-existing core dynamo, the magnetic field of the core could not vanish abruptly. In the 423
absence of the dynamo, the core field freely decays within about (Rc/π)2 / µ = 500 years, where 424
Rc is the lunar core radius (~390 km) and µ is the magnetic diffusivity of the core, (~ 1 m2/s, 425
Arkani-Hamed and Olson, 2010). The core field actually did not decay appreciably in a very 426
short time of a few hours during the convergence of the impact-induced ionized plasma at the 427
antipodal zone (e.g., Hood and Artemieva, 2008), hence it could have been strong to magnetize 428
the crust by the shock magnetization. The lack of magnetic anomalies in the antipodal region of 429
the SPAB may imply the lack of core dynamo at the impact time that created the SPAB, or a less 430
important contribution of the shock magnetization to the total magnetization of the crust. More 431
probably, however, the antipodal zone of SPAB coincides with Procellarum Terrane, which is 432
underlain by a thick KREEP (Potassium, Rare-Earth Elements, and Phosphorus) layer that kept 433
the temperature of the upper mantle and the lower parts of the crust well above the magnetic 434
blocking temperatures for a long time (Laneuville et al., 2013). Moreover, the upper parts of the 435
Page 19 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
20
crust possibly bearing shock remanent magnetization was probably thermally demagnetized by 436
the extensive volcanism that covered almost all of the Terrane. 437
Arkani-Hamed and Boutin (2014) studied the demagnetization of the ancient lunar highland crust 438
by impacts that have created craters with diameters larger than 100 km, to investigate the 439
probability that a strong core dynamo existed when the newly forming lunar crust was cooling 440
below the magnetic blocking temperatures of its minerals in the early history. The impacts that 441
are capable of creating the craters are also capable of significantly disturbing the crust directly 442
beneath and demagnetizing the entire crust of ~60 km thickness (e.g., Reindler and Arkani-443
Hamed, 2001; Arkani-Hamed and Boutin, 2012a). Such a pervasive demagnetization can 444
produce a distinct magnetic anomaly, the edge effect, at satellite altitudes if the surrounding 445
primordial crust is magnetic. Using the vertical component of the LP-2 data, the least 446
contaminated data set, the authors modeled the magnetic anomalies associated with 20 craters 447
with distinct edge effects, all located on the ancient crust of the lunar far side (see Appendix A). 448
The resulting paleomagnetic pole positions are clustered while craters are widely distributed, 449
implying a stable magnetizing field of a core dynamo in the very early history of the Moon. 450
Also investigated by Arkani-Hamed and Boutin (2014) were the isolated magnetic anomalies on 451
the highlands with no obvious topographic signatures, indicating that the magnetic source bodies 452
are deep seated. The authors modeled 10 isolated magnetic anomalies having sufficient data to 453
obtain reliable magnetic maps. Note that the formation times of the source bodies of the isolated 454
anomalies are not well constrained, hence the models can only show the basic characteristics of 455
the source bodies, but cannot provide reliable information about the time they were magnetized. 456
There is evidence for possible dynamo reversals and true polar wander in the early history of the 457
Moon. Allowing for dynamo reversals, the paleomagnetic poles obtained from the magnetic 458
Page 20 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
21
signatures associated with the craters trace a consistent path suggestive of a true polar wander of 459
the Moon and a stable core dynamo. However, the polar wander is less evident from the 460
magnetic poles determined using isolated magnetic anomalies. The poles cluster in three 461
locations, implying that the core dynamo was less stable during the magnetization of the source 462
bodies. 463
The complex group of strong magnetic anomalies concentrated on the northern rim of SPAB are 464
interpreted by the magnetization of highly magnetic ejecta from an iron-rich impactor that 465
created the basin (Wieczorek et al., 2012). According to this hypothesis, the magnetic source 466
bodies were created within a very short time after the impact. However, detailed modeling of the 467
magnetic anomalies (Arkani-Hamed and Boutin, 2017) concluded that the magnetic source 468
bodies are large intrusive, created during a very long period. It takes a long time for an intrusive 469
source body to cool below its magnetic blocking temperatures and acquire magnetization (e.g., 470
Arkani-Hamed and Celetti, 1989; Purucker et al., 2012). The associated magnetic anomaly 471
arises from the lateral variations of the vertically integrated magnetization of the source body. 472
Figure 6a shows the vertical component of the magnetic field over SPAB derived using the radial 473
component of the LP-2 data. The magnetic anomalies, identified by circles that contain major 474
part of the anomalies, show two distinctly different polarities. In the absence of radioactive age 475
data the anomalies are numbered chronologically based on the polar wander path of the Moon 476
traced by the paleomagnetic poles (see below). Anomalies 1 to 5 have positive lobes in the north 477
of their negative lobes, whereas anomalies 7 to 14 have negative lobes in the north of their 478
positive lobes. The well-isolated strong but small anomaly near the center of Leibnitz crater 479
(anomaly 3) belongs to the first set of anomalies and anomaly 9 that is located close but outside 480
of the basin has the same polarity of the second category. Judging from the impact 481
Page 21 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
22
demagnetization of martian crust by a large impacting body that created Hellas basin on Mars 482
(Mohit and Arkani-Hamed, 2004; Lillis et al., 2010), which is comparable in size to SPAB, the 483
impact that has created SPAB has most likely demagnetized the entire crust within a radius of 484
~1.4 times the basin radius. Halekas et al. (2003) investigated the magnetic field over 34 multi-485
ring impact basins of the Moon using the LP electron reflectometer data, and concluded that all 486
of the impacts have significantly reduced the magnetization of the crust to distances of about 487
1.5–2 basins’ radii. Hence, anomaly 9 is likely created after the formation of the SPAB. We 488
note the overlap of two anomalies with distinctly different polarities. The small anomaly 14 is 489
embedded in the large anomaly 5, implying that the source body of the small anomaly is formed 490
well after the magnetization of the source body of the large anomaly. Otherwise, the formation 491
of the large body would have obliterated the pre-existing source of the small anomaly. The very 492
existence of the magnetic anomalies inside SPAB, and anomaly 9 outside but within ~1.4 basin 493
radius, indicate that the source bodies were magnetized after the formation of the basin that 494
occurred at around 4.1 Ga. It is, therefore, feasible that the source bodies of the first category 495
anomalies are magnetized after the formation of SPAB, but earlier than the magnetization time 496
of the source bodies of the second category, and that there was a dynamo reversal between these 497
two periods. 498
The two categories of the magnetic anomalies inside SPAB indicate two distinctly different polar 499
wander paths (Figure 6b). In the absence of the large mascon maria such as Smythii, Crisium, 500
Serenitatis, Imbrium, and Orientale, the huge mass deficiency associated with the formation of 501
SPAB was likely the main driving force which moved the basin southward. The north pole of 502
the dipole component of the core field, i.e., the paleomagnetic pole, was moving northward 503
relative to the basin as the source bodies were getting younger. Figure 6b emphasizes that the 504
Page 22 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
23
magnetic source bodies of the first category magnetic anomalies were forming and being 505
magnetized by the core dynamo as the true polar wander driven by the SPAB was taking place. 506
Therefore, anomaly 1 is the oldest and anomaly 6 is the youngest among the first category 507
anomalies. The source body of the paleomagnetic pole #6 is actually the north Crisium anomaly 508
(Hood, 2011) which is outside the SPAB (Arkani-Hamed and Boutin, 2017), hence is not 509
included in Figure 6a. Based on the well-defined linear polar wander path expected from the 510
true polar wander theory (e.g., Matsuyama, et al, 2006), the paleomagnetic poles trace the true 511
polar wander path of the Moon for about 68o. Moreover, the almost linear trace of 512
paleomagnetic poles and their locations relative to the SPAB indicate that the true polar wander 513
was mainly driven by a single source, the mass deficiency associated with SPAB. 514
Figure 6b shows core dynamo reversal after the formation of the north Crisium anomaly. 515
Athough the reversal may have occurred spontaneously, the interaction of the shock waves 516
produced by the Serenitatis and Imbrium impacts with the core dynamo, the sudden transfer of 517
angular momentum to the mantle by the impactors, and the dynamic overturn of the core through 518
stratification following the impacts probably triggered the dynamo reversal. The shock wave 519
produced by either Imbrium or Serenitatis impacts propagates in the lunar mantle as an almost 520
spherical shock front. The shape of the shock front, however, changes to a drastically complex 521
pattern upon impinging the spherical core-mantle boundary and entering the core (e.g., Ivanov et 522
al., 2010; Arkani-Hamed and Ivanov, 2014). Moreover, the shock front propagating in the core 523
partly reflects and partly refracts at the antipodal core-mantle boundary. The core is shocked 524
twice, first by the direct shockwave propagating away from the impact site and next by the 525
reflected shock wave. It seems quite feasible that the core dynamo gets perturb drastically. The 526
interaction of the double shock waves with the core dynamo remains to be investigated. 527
Page 23 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
24
The formation of major surface mass concentrations (mascons) with different masses and 528
locations on the front side of the Moon, associated with large impact basins Smythii, Crisium, 529
Serenitatis, Imbrium, and Orientale, initiated a new polar wander at about 200 Myr after the 530
formation of SPAB, and moved the major mascons basins toward the equator. Moreover, a 531
pervasive volcanism due to concentration of KREEP in a thick layer beneath the present 532
Procellarum Terrane (Wieczorek and Phillips, 2000; Laneuville, et al., 2018), which likely 533
moved a huge amount of mass upward in the lunar interior (Laneuville et al., 2013), contributed 534
to the excess mass near the surface and resulted in further polar wander. The magnetic source 535
bodies that were forming inside SPAB after the dynamo reversal were being magnetized when 536
the north pole of the core field had already flipped. Therefore, their north paleomagnetic poles 537
were moving southward as the source bodies were getting younger. Accordingly, the source 538
body of anomaly 7 is the oldest and that of anomaly 14 is the youngest among the source bodies 539
formed after the core dynamo reversal. Due to the different formation times and locations of the 540
mascons, the polar wander path described by the second category magnetic anomalies is 541
somewhat scattered compared to that of the first category anomalies, as seen in Figure 6b. 542
The lunar interior was hot during the first 1 Gyr of its history. The majority of mare flooding 543
occurred between 3.6 and 3.8 Ga (Head, 1976; Geiss et al, 1977; Schultz and Spudis, 1983). 544
Bearing in mind that the time it took for the polar wander strongly depended on the poorly 545
estimated viscosity of the lunar mantle at about 4 Ga, it is not feasible to estimate the total time 546
required for the entire polar wander of about 120o. 547
548
3.2 Driving Mechanism of the Lunar Core Dynamo: 549
Page 24 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
25
The paleomagnetic poles obtained from the isolated features on the primordial crust of the Moon, 550
whether the isolated intrusive bodies or the impact craters, are clustered while the source bodies 551
are widely distributed, indicating that the source bodies were magnetized by a dipole field with 552
consistent direction which could have been a core dynamo field. 553
The Moon-forming models, either by collision of a Mars size body with the Earth (e.g., Benz et 554
al., 1986; Canup and Asphaug, 2001; Canup, 2004, 2012; Canup et al., 2013) or by the collision 555
of two half-Earth size bodies (Nakajima and Stevenson, 2014, 2015), suggest that a partially 556
molten silicate disk is formed in Earth orbit, and the Moon is formed as a hot body by accretion 557
of the high-temperature disk during 100 to 1000 years (Thompson and Stevenson, 1988). The 558
core formation in the partially molten Moon took place during its accretion and resulted in a 559
superheated core. The Moon cooled from the surface down and a cold stagnant lid formed on the 560
surface with increasing thickness and viscosity as time passed. The thermal evolution models of 561
the Moon calculated on the basis of the stagnant lid thermal convection simulations in the mantle 562
(Arkani-Hamed and Boutin, 2017) show high heat flux from the superheated core to the mantle. 563
The fast cooling of the core was capable of generating a thermally driven core dynamo in the 564
first about 100 Myr, i.e., during the differentiation of the silicate mantle and formation of the 565
lunar primordial crust. There is no evidence that the Moon was impacted by a large planetary 566
embryo in the later stages of its accretion to hamper the initiation of a thermally driven dynamo, 567
which was likely the case for Mars. The primordial lunar crust acquired thermoremanent 568
magnetization as it cooled through its magnetic blocking temperatures in the presence of the core 569
field. The magnetized primordial crust is consistent with the magnetic anomalies associated 570
with the large craters of more than 100 km in diameters resulted from the impact 571
demagnetization of the primordial crust (Arkani-Hamed and Boutin, 2014). 572
Page 25 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
26
Stegman et al. (2003) suggested that a high-density ilmenite layer probably produced in the 573
upper mantle at the later stages of solidification of the magma ocean sank and covered the core. 574
It subsequently heated up by its highly concentrated radioactive elements and became buoyant, 575
resulting in vigorous mantle convection which enhanced the heat loss from the core and 576
generated a strong thermally driven core dynamo for a few hundred million years. This 577
mechanism took about 400 Myr to generate a strong core dynamo. During this long period the 578
entire primordial crust could cool well below the magnetic blocking temperatures of its minerals 579
in the absence of a strong core field, hence could not acquire thermoremanent magnetization. 580
The cold crust could acquire weak induced magnetization in the presence of a core dynamo in a 581
later time, but the induced magnetization would decay rapidly once the core dynamo diminishes. 582
Dwyer et al. (2011) proposed that the rotation axes of the liquid core and the solid mantle of the 583
Moon did not initially coincide, and the Earth-driven precession resulted in differential motion of 584
the core and mantle of the Moon, which stirred the core and powered a core dynamo. The 585
precession driven dynamo was capable of creating a magnetic field of 1-10µT at the lunar 586
surface at around 4 Ga. The field decreased gradually below 0.01µT at around 2.7 Ga, when the 587
Earth–Moon distance exceeded ~48 times Earth’s radius and the dissipated power in the core 588
was insufficient to drive a dynamo. However, the precession driven dynamo was not probably 589
active in the first ~70 Myr of the lunar history when the Earth-Moon distance was less than ~26 590
times Earth radius, because the core closely followed the mantle (Meyer and Wisdom, 2011). 591
Hence, it cannot account for the magnetization of the lunar primordial crust. 592
Large oblique impacts may transfer a considerable amount of angular moment to the lunar 593
mantle, causing reorientation of its rotation axis relative to that of the liquid core and resulting in 594
differential rotation of the core and mantle that may power a core dynamo (e.g., Melosh, 1975; 595
Page 26 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
27
Wieczorek, M. A. and Le Feuvre, M., 2009). A precession driven dynamo initiated by a large 596
impactor that is capable of creating a crater ~700 km in diameter, may not last longer than 597
10,000 years (Le Bars et al., 2011). Because of the random locations and random impact angles 598
of the impactors during the very early history of the Moon, the pole positions of the possible 599
impact-driven core fields must have been random. This is in direct contradiction with the well-600
clustered paleomagnetic poles determined from the magnetic anomalies associated with the 601
impact craters, implying that the primordial crust of the Moon was not magnetized by a core 602
dynamo driven by random impacts. 603
The seismic detection of the lunar core revealed a solid inner core of about 240 km radius 604
overlain by a liquid outer core of about 390 km radius (Weber et al., 2011). The existence of an 605
inner core led Laneuville et al. (2014) to propose a chemically driven lunar core dynamo. 606
However, the core was very hot and core solidification may not have started when the primordial 607
crust was forming in the very early history. Hence, a chemically driven core dynamo might not 608
have existed to magnetize the primordial crust of the Moon. 609
The above mention mechanically and chemically driven core dynamo models are not consistent 610
with the magnetization of the primordial lunar crust, emphasizing that the thermally driven core 611
dynamo in the very early history of the Moon, due to the fast cooling of the initially superheated 612
core, was probably the responsible mechanism. However, such a thermally driven core dynamo 613
could not have lasted long enough to magnetize the lunar rock during 3.5-4.2 Ga. On the other 614
hand, the buoyant ilmenite layer model of Stegman et al. (2003), the precession driven dynamo 615
model of Dwyer et al. (2011), and a late activation of the chemically driven dynamo model of 616
Laneuville et al. (2014) could have created a strong core field on the lunar surface at around 3.5–617
4.2 Ga. 618
Page 27 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
28
619
4. Conclusions: 620
Mars and the Moon had superheated cores at the end of their accretion, due to the release of the 621
gravitational energy by the descent of iron inside the silicate mantle in the making of the cores. 622
Overlain by relatively colder mantles, both cores were capable of cooling fast and generating 623
core dynamos. However, the giant Borealis impact at the end of accretion of Mars prevented 624
core convention and initiation of a core dynamo. It took about 50-120 Myrs for the martian core 625
to generate a core dynamo, hence the primordial crust that was formed during the first ~100 626
Myrs was not magnetized. Each of the 8 large impacts that occurred during 4.2 to 4.0 Ga 627
crippled the existing core dynamo of Mars. However, having a large volume the martian core 628
was zealous, it regenerated a dynamo a new after each impact. In the mean time, the stagnant 629
lithosphere at the surface thickened and reduced the heat loss from the mantle. The temperature 630
difference between the core and the lower mantle decreased in time and the core convection 631
gradually weakened to a point that it could no longer support a core dynamo. After 9 episodes of 632
crippling and reactivation, by Borealis and the large 8 impactors, the core dynamo of Mars 633
ceased for good at ~3.8 Ga. 634
With no giant impact occurring at the end of accretion, the superheated lunar core generated a 635
strong thermally driven core dynamo that lasted about 100 Myr and magnetized the newly 636
forming primordial lunar crust. The magnetic source bodies of the Moon created at later times 637
were magnetized by either a precession driven dynamo, or a thermally driven dynamo due to 638
possibly vigorous thermo-chemical mantle convection, or a chemically driven dynamo resulting 639
from the core solidification. The lunar dynamo had two episodes of initiation and cessation. The 640
first initiation was at the later stages of accretion, which lasted ~100 Myr. The second initiation 641
Page 28 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
29
was some times before the heavy bombardment, ~4.1 Ga. There is no viable estimate as when 642
the second core dynamo of the Moon ceased to exist. Suffice to say that, similar to the case of 643
Mars, the gradual thickening of the stagnant lid at the surface and the relatively small size of the 644
lunar core were probably the major factors that gradually diminished the core dynamo. 645
646
Analysis of the isolated magnetic anomalies of both intrusive and impact craters provided a 647
means to determine the direction of the magnetizing field and reveal polar wander and core 648
dynamo reversals of both Mars and the Moon. The polar wander path of the Moon is simple to 649
interpret. The formation of SPAB at around 4.1 Ga created a huge surface mass deficiency that 650
derived ~68o true polar wander. The formation of large mascon basins at around 3.9 Ga initiated 651
a new true polar wander of about ~52o in later times. There was also a core dynamo reversal 652
between these two polar wander periods. 653
654
Acknowledgement. This research was supported by Natural Sciences and Engineering Research 655
Council (NSERC) of Canada, special Grant at McGill University, NSERC-GRF 241305, I would 656
like to thank two anonymous reviewers for their constructive comments. 657
Page 29 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
30
References:
Acuna, M.H. et al., 1999. Global distribution of crustal magnetization discovered by the Mars
Global Surveyor MAG/ER experiment. Science 284, 790–793.
Acuna, M.H. et al., 2001. Magnetic field of Mars: Summary of results from the aerobraking and
mapping orbits. J. Geophys. Res. 106 (El0), 23403–23417.
Agnor, C.B., and D. P. Hamilton, (2006). Neptune’s capture of its moon Triton in a
binary–planet gravitational encounter, Nature, 441, 192-194.
Agnor, C. B., Canup, R.M., and Leviston, H. F. 1999. On the character and consequences of
large impacts in the late stage of terrestrial planet formation, Icarus, 142, 219–237.
Andrews-Hanna, J.C., Zuber M.T., and Banerdt, W.B. 2008. The Borealis basin and the origin of
the martian crustal dichotomy, Nature, 453, 1212-1215.
Antretter, M., et al. 2003. Paleomagnetic record of Martian meteorite ALH84001, J. Geophys.
Res. 108(E6), 5049.
Arkani-Hamed, J. 2001. Paleomagnetic pole positions and pole reversals of Mars, Geophys. Res.
Lett., 28(17), 3409–3412.
Arkani-Hamed, J. 2002a. An improved 50-degree spherical harmonic model of the magnetic
field of Mars derived from both high-altitude and low-altitude data, J. Geophys. Res.,107,
10,1029/2001JE001835, 13-1, 13-8.
Arkani-Hamed, J. 2002b. Magnetization of the Martian crust, J. Geophys. Res., 107, E5, 5032,
10.1029/2001JE001496.
Arkani-Hamed, J. 2004a. A coherent model of the crustal magnetic field of Mars, J. Geophys.
Res., 109, E09005, doi:10.1029/2004JE002265.
Page 30 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
31
Arkani-Hamed, J. 2004b. Timing of the Martian core dynamo, J. Geophys. Res., 109, E03006,
doi:10.1029/2003JE002195.
Arkani-Hamed, J. 2005. Magnetic crust of Mars, J. Geophys. Res., 110, E08005,
doi:10.1029/2004JE002397.
Arkani-Hamed, J., 2009. Did tidal deformation power the core dynamo of Mars?, Icarus 201, 31–
43.
Arkani-Hamed, J. 2010. Possible crippling of the core dynamo of Mars by Borealis impact, J.
Geophys. Res., 115, E12021, doi:10.1029/2010JE003602.
Arkani-Hamed, J. 2012. Life of the Martian dynamo, Physics of the Earth and Planetary Interiors
196-197 (2012) 83–96.
Arkani-Hamed, J., and Boutin, D. 2012a. Low-magnetic crust underlying South Province of
Mars. Icarus 217, 209–230.
Arkani-Hamed, J., and Boutin, D. 2012b. Is the primordial crust of Mars magnetized?, Icarus,
221, 192–207.
Arkani-Hamed, J., and Boutin, D. 2014. Analysis of isolated magnetic anomalies and magnetic
signatures of impact craters: Evidence for a core dynamo in the early history of the Moon, Icarus,
237, 262–277.
Arkani-Hamed, J., Boutin, D. 2017. South Pole Aitken Basin magnetic anomalies: Evidence for
the true polar wander of Moon and a lunar dynamo reversal, J. Geophys. Res. Planets, 122,
doi:10.1002/2016JE005234.
Arkani-Hamed, J., and Celetti, G., Effects of thermal remanent magnetization on the magnetic
anomalies of intrusives, J. Geophys. Res., 94, 7364-7378, 1989.
Page 31 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
32
Arkani-Hamed, J. and B. A. Ivanov, (2014). Shock wave propagation in layered planetary
embryos, Phys. Earth and Planet. Inter. 230, 45-59.
Arkani-Hamed, J., and Olson, P. 2010. Giant impact stratification of the martian core, Geophys.
Res. Lett., 37, L02201, doi:10.1029/2009GL041417.
Arkani-Hamed, J., Seyed-Mahmoud, B., and Aldridge, K. 2008. Tidal excitation of Elliptical
Instability in the Martian core: Possible Mechanism for Generating the Core Dynamo, J.
Geophys. Res., 113, E06003.
Benz,W., Slattery, W.L., and Cameron, A.G.W. 1986. The origin of the Moon and the single-
impact hypothesis I, Icarus, 66, 515–535.
Bertotti,B., and P. Farinella, Physics of the Earth and Solar System, KluwerAcademic
Publishers, Boston, 1990.
Breuer, D., and Spohn, T. 2003. Early plate tectonics versus single-plate tectonics on Mars:
Evidence from magnetic field history and crust evolution, J. Geophys. Res., 108(E7), 5072,
doi:10.1029/2002JE001999.
Cain, J.C., Ferguson, B.B., and Mozzoni, D. 2003., An n = 90 internal potential function of the
Martian crustal magnetic field, J. Geophys. Res., 108(E2), 5008, doi:10.1029/2000JE001487.
Cameron, A.G.W. 1997. The origin of the Moon and the single impact hypothesis. V. Icarus 126
(1), 126–137
Canup, R.M. 2004. Simulations of a late lunar-forming impact, Icarus, 168, 433–456.
Canup, R.M. 2012. Forming a Moon with an Earth-like composition via a giant impact, Science,
338, 1052–1055.
Canup, R.M., and Asphaug, E. 2001. Origin of the Moon in a giant impact near the end of the
Earth’s formation, Nature, 412, 708–712.
Page 32 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
33
Canup, R. M., Barr, A.C., and Crawford, D.A. 2013. Lunar-forming impacts: High-resolution
SPH and AMR-CTH simulations, Icarus, 222, 200–219.
Carr, H.M. and Head III, J. W. ,2010. Geologic history of Mars, Earth and Planetary Science
Letters, 294, 185-203.
Chambers, J.E. 2004. Planetary accretion in the inner Solar System. Earth Planet. Sci. Lett. 223,
241–252.
Chambers, J. E., and Wetherill, G.W. 1998. Making the terrestrial planets: N-body integrations
of planetary embryos in three dimensions, Icarus, 136, 304–327.
Cisowski, S.M., Collinson, D.W., Runcorn, S.K., Stephenson, A., and Fuller, M. 1983. A review
of lunar paleointensity data and implications for the origin of lunar paleomagnetism, Proc. Lunar
Planet. Sci. Conf. l3th, Part 2, J. Geophys. Res., 88, A691–A704.
Citron, R. I., Manga, M., Tan, E. (2018) A hybrid origin of the Martian crustal dichotomy:
Degree-
1 convection antipodal to a giant impact, Earth Planet. Sci. Lett. 491, 58–66.
Coleman, P.J., Lichtenstein, B.R., Russell, C.T., Sharp, L.R., and Schubert, G. 1972. Magnetic
fields near the Moon, Geochim. Cosmochim. Acta, 36 (suppl. 3), 2271–2286.
Collinson, D.W. 1997. Magnetic properties of Martian meteorites: implications for an ancient
Martian magnetic field. Planet. Sci. 32, 803–811.
Connerney, J.E.P., et al. 2001. The global magnetic field of Mars and implications for crustal
evolution, Geophys. Res. Lett., 28, 4015– 4018.
Cournède, C., Gattacceca, J., and Rochette, P. 2012. Magnetic study of large Apollo samples:
Possible evidence for an ancient centered dipolar field on the Moon, Earth Planet. Sci. Lett.,
331–332, 31–42.
Daily, W.D., Dyal, P., 1979. Theories for the origin of lunar magnetism. Phys. Earth
Planet. Inter. 20, 255–270.
Page 33 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
34
N. Dauphas, N., and A. Pourmand, 2011. Hf-W-Th evidence for rapid growth of Mars and its
status as a planetary embryo. Nature, 473, 489–492.
Dwyer, C.A., Stevenson, D.J., and Nimmo, F. 2011. A long-lived lunar dynamo driven by
continuous mechanical stirring, Nature, 479, 212–214.
Dyal, P., Parkin, C.W., Sonerr, C.P. 1970. Lunar Surface Magnetometer Experiment. Apollo 12
Preliminary Science Report. NASA Spec. Publ. 235, pp. 55–73.
Dyal, P., Parkin, C.W., Sonett, C.P., Dubois, R.L., and Simmons, G. 1971. Lunar Portable
Magnetometer Experiment. Apollo 14 Preliminary Science Report. NASA Spec. Publ. 272, p.
227.
Dyal, P., Parkin, C.W., and Daily, W.D. 1974. Magnetism and the interior of the Moon.
Frey, H.V., Roark, J.H., Shockey, K.M., Frey, E.L., Sakimoto, S.E.H. 2002. Ancient lowlands on
Mars. Geophys. Res. Lett. 29, 1384, doi: http://dx.doi.org/10.1029/2005JE002,480.
Frey, H. V. 2008. Ages of very large impact basins on Mars: Implications for the late heavy
bombardment in the inner solar system, Geophys. Res. Lett., 35, L13203,
doi:10.1029/2008GL033515.
Fuller, M., and Cisowski, S.M. 1987. Lunar paleomagnetism, in Geomagnetism, vol. 2, edited by
J. Jacobs, pp. 307–455, Academic Press, Orlando, Fla.
Garrick-Bethell, I., B. P. Weiss, D. L. Shuster, and J. Buz (2009), Early lunar magnetism,
Science, 323, 356–359.
Geiss, J., et al. 1977. Absolute time scale of lunar mare formation and filling, Phil. Trans. R. Soc.
London, Ser. A., 285, 151-158.
Halekas, J.S., Lin, R.P., Mitchell, D.L. 2003. Magnetic fields of lunar multi-ring impact basins.
Meteorit. Planet. Sci. 38, 565–578.
Page 34 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
35
Hansen, R.M.S., 2009. Formation of the terrestrial planets from a narrow annulus, Astrophys. J.
703, 1131–1140.
Hartmann, W.K., Neukum, G. 2001. Cratering chronology and the evolution of Mars. Space Sci.
Rev. 96, 165–194.
Head, J. 1976. Lunar Volcanism in space and time, Rev. Geophys., 14, 265-300.
Holsapple, K.A., and R.M. Schmidt On the scaling of crater dimensions: 2. Impact processes. J.
Geophys. Res. 87, 1849-1870, 1982.
Hood, L.L. 2011. Central magnetic anomalies of Nectarian-aged lunar impact basins: Probable
evidence for an early core dynamo, Icarus,211, 1109–1128.
Hood, L.L., and Artemieva, N.A. 2008. Antipodal effects of lunar basin-forming impacts: Initial
3D simulations and comparisons with observations, Icarus, 193, 485–502.
Hood, L.L., and Hartdegen, K. 1997. A crustal magnetization model for the magnetic field of
Mars: A preliminary study of the Tharsis region, Geophys. Res. Lett., 24, 727–730.
Hood, L.L., and Richmond, N.C. 2002. Mapping and modeling of major Martian magnetic
anomalies, Lunar Planet. Sci., XXXIII, abstract 1128.
Hood, L.L., Richmond, N.C., Pierazzo, E., and Rochette, P. 2003. Distribution of crustal
magnetic fields on Mars: Shock effects of basinforming impacts, Geophys. Res. Lett., 30(6),
1281, doi:10.1029/2002GL016657.
Hood, L.L., and Zakharian, A. 2001. Mapping and modeling of magnetic anomalies in the
northern polar region of Mars, J. Geophys. Res., 106, 14,601–14,619.
Hood, L.L., Russell, C.T., and Coleman Jr., P.J. 1981. Contour maps of lunar remanent magnetic
fields. J. Geophys. Res. 86, 1055–1069.
Ivanov, B.A., Melosh, H.J., Pierazzo, E., 2010. Basin-forming impacts: reconnaissance
Page 35 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
36
modeling. Geological Society of America Special Paper 465, pp. 29–49.
Johnson, C.L., Phillips, R.J. 2004. Evolution of the Tharsis region of Mars: Insights from
magnetic field observations. Earth Planet. Sci. Lett. 230, 241–254.
Kerswell, R.R. 1994. Tidal excitation of hydrodynamic waves and their damping in the Earth. J.
Fluid Mech. 274, 219–241.
Kirschvink, J.L., Maine, A.T., Vali, H. 1997. Paleomagnetic evidence of a low temperature
origin of carbonate in the Martian meteorite ALH84001. Science, 275, 1629–1633
Kobayashi, H., Dauphas, N. 2013. Small planetesimals in a massive disk formed Mars, Icarus
225 (2013) 122–130.
Kokubo, E., and Ida, S. 1996. On runaway growth of planetesimals, Icarus, 123, 180– 191.
Kokubo, E., and Genda, H. 2010. Formation of the terrestrial planets form proto-planets under a
realistic accretion condition, Astrophys. J. Lett., 714, L21–L25.
Kuang, W., Jiang, W., Roberts J., and Frey H.V. 2014. Could giant basin-forming impacts have
killed Martian dynamo?, Geophy. Res. Lette., 28, 8006–8012.
Lacaze, L., Le Gal, P., and Le Dize`s, S. 2004. Elliptical instability in a rotating spheroid, Phys.
Earth. Planet. Inter., 505, 1 – 22.
Lacaze, L., Le Gal, P., and Le Dize`s, S. 2005. Elliptical instability of the flow in a rotating shell,
Phys. Earth. Planet. Inter., 151, 194–205.
Lacaze, L., Herreman, W., Le Bars, M., Le Dize`s, S., and Le Gal, P. 2006. Magnetic field
induced by elliptical instability in a rotating spheroid, Geophys. Astrophys. Fluid Dyn., 100,
299– 317.
Page 36 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
37
Laneuville, M., Taylor, J., and Wieczorek, M. 2018. Lunar radioactive heat source distribution
and magnetic field generation, 49th Lunar and Planetary Science Conference 2018 (LPI
Contrib. No. 2083), Abstract 1722.pdf.
Laneuville, M., Wieczorek, M., Breuer, D., and Tosi, N. 2013. Asymmetric thermal evolution of
the Moon, J. Geophys. Res. Planets, 118, 1435–1452, doi:10.1002/jgre.20103.
Laneuville, M., et al. 2014. A long-lived lunar dynamo powered by core crystallization. Lunar
Planet. Sci. 45. Abstract 1819.pdf.
Langlais, B., Purucker, M.E., and Mandea, M. 2004. Crustal magnetic field of Mars, J. Geophys.
Res., 109, E02008, doi:10.1029/2003JE002048.
Le Bars, M., Wieczorek, M.A., Karatekin, Ö., Cébron, D., and Laneuville, M. 2011. An impact-
driven dynamo for the early Moon, Nature, 479, 215–218.
Lillis, R. J., et al, .2010. Study of impact demagnetization at Mars using Monte Carlo modeling
and multiple altitude data, J. Geophys. Res., 115, E07007, doi:10.1029/2009JE003556
Lillis, R.J., et al., 2008. Rapid decrease in Martian crustal magnetization in the Noachian era:
implications for the dynamo and climate of early Mars. Geophys. Res. Lett.35, L14203.
Lillis, R.I., Robbins, S., Manga, M., Halekas, J.S., and H.V. Frey .2013. Time history of the
Martian dynamo from crater magnetic field analysis, J. Geophys. Res. Planets, 118, 1488–1511,
doi:10.1002/jgre.20105.
Lin, R.P., et al. 1998. Lunar surface magnetic fields and their interaction with the solar wind:
Results from Lunar Prospector, Science, 281, 1480–1484.
Lin, Y., P. Marti, and J. Noir, 2015. Shear-driven parametric instability in a precessing sphere;
Physics of Fluids 27, 046601.
Page 37 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
38
Marinova, M.M., Aharonson, O., and Asphaug, E. 2008. Mega-impact formation of the Mars
hemispheric dichotomy, Nature, 453, 1216-1219.
Matsuyama, I., et al. .2006. Rotational stability of dynamic planets with elastic lithospheres, J.
Geophys. Res., 111, E02003, doi:10.1029/2005JE002447.
Melosh, H.J. 1975. Large impact craters and the Moon orientation, Earth Planet. Sci. Lett., 26,
353–360.
Meyer, J., and Wisdom, J. 2011. Precession of the lunar core, Icarus, 211, 921–92.
Merle, R.E., et al. 2014. High resolution U-Pb ages of Ca-phosphates in Apollo 14 breccias:
Implications for the age of the Imbrium impact, Meteorit. Planet. Sci., 49(12), 2241–2251.
Moffatt, H. K. 1970. Dynamo action associated with random inertial waves in a rotating
conducting fluid, J. Fluid Mech., 44, 705– 719.
Mohit, P. S., and Arkani-Hamed, J. 2004. Impact demagnetization of the Martian crust, Icarus,
168, 305–317.
Monteux, J., and J. Arkani-Hamed, (2014). Consequences of giant impacts in early Mars: Core
merging and Martian dynamo evolution, J. Geophys. Res.Planets, 119,
doi:10.1002/2013JE004587.
Nagata, T., et al. 1970. Magnetic properties and natural remanent magnetization of lunar
materials, Geochim. Cosmochim. Acta, 34(suppl. 1), 2325–2340.
Nakajima, M., and Stevenson, D.J. 2014. Investigation of the initial state of the Moon-forming
disk: Bridging SPH simulations and hydrostatic models, Icarus 233, 259–267.
Nakajima, M., and Stevenson, D.J. 2015. Melting and mixing states of the Earth’s mantle after
the Moon-forming impact, Earth and Planetary Science Letters, 427, 286–295.
Page 38 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
39
Nimmo, F., Stevenson, D.J. 2000. Influence of early plate tectonics on the thermal evolution and
magnetic field of Mars. J. Geophys. Res. 105, 1969–1979.
Nimmo, F., Hart, S.D., Korycansky, D.G., and Agnor, C.B. 2008. Implications of an impact
origin for the martian hemispheric dichotomy, Nature, 453, 1220-1224.
Olson, P. 1981. A simple physical model for the terrestrial dynamo, J. Geophys. Res., 86,
10,875–10,880.
Pierazzo, E., A. M. Vickery, and H. J. Melosh (1997), A re-evaluation of impact melt
production, Icarus, 127, 408– 423.
Purucker, M., et al. 2000. An altitude-normalized magnetic map of Mars and its interpretation,
Geophys. Res. Lett., 27, 2449– 2452.
Purucker, M.E. 2008. A global model of the internal magnetic field of the Moon based on Lunar
Prospector magnetometer observations. Icarus 197, 19–23.
Purucker, M. E., Head III, J.W., and Wilson, L. 2012. Magnetic signature of the lunar South
Pole-Aitken basin: Character, origin, and age, J. Geophys. Res., 117 , E05001,
doi:10.1029/2011JE003922.
Reddy, K. S., B. Favier, and M. Lebars, 2018. Turbulent Kinematic Dynamos in Ellipsoids
Driven by Mechanical Forcing, Geophys. Res. Lett. https://doi.org/10.1002/2017GL076542.
Reese., C. C., C. P. Orth, and V. S. Solomatov, 2010. Impact origin for the Martian crustal
dichotomy: Half emptied or half filled? J. Geophys. Res., 115, 10.1029/2009JE003506.
Reindler, L., Arkani-Hamed, J. 2001. The compensation state of intermediate size lunar craters.
Icarus 153, 71–88.
Page 39 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
40
Righter, K., Hervig, R.J., Kring, D.A. 1998. Accretion and core formation on Mars Molybdenum
contents of melt inclusion glasses in three SNC meteorites. Geochim. Cosmochim. Acta 62,
2167–2177.
Roberts, J.H., Zhong, S. 2006. Degree-1 convection in the martian mantle and the origin of the
hemispheric dichotomy. J. Geophys. Res. 111, E06013, doi: http://
dx.doi.org/10.1029/2005JE002668.
Roberts, J. H., Lillis, R.J., and Manga, M. 2009. Giant impacts on early Mars and the cessation
of the Martian dynamo, J. Geophys. Res., 114, E04009, doi:10.1029/2008JE003287.
Roberts, J. H., and Arkani-Hamed, J. 2012. shock-induced mantle dynamics on Mars, Icarus,
218, 278–289.
Roberts, J. H., and Arkani-Hamed, J. 2014. Impact heating and coupled core cooling and mantle
dynamics on Mars, J. Geophys. Res. Planets, 119, 729–744, doi:10.1002/2013JE004603.
Roberts, J. H., and Arkani-Hamed, J. 2017. Effects of Basin-Forming Impacts on the Thermal
Evolution and Magnetic Field Of Mars, Earth Planet. Sci. Lett., 478, 192–202.
Runcorn, S.K. et al. 1970. Magnetic properties of Apollo 11 lunar samples. Geochim.
Cosmochim. Acta 34 (Suppl. 1), 2369–2387.
Runcorn, S. K., et al. 1971. Magnetic properties of Apollo ’11 lunar samples, in Proceedings of
the Apollo 11 Lunar Science Conference, vol. 3, pp. 2369–2387, Pergamon, Oxford.
Samuel, H., and P. J. Tackley (2008), Dynamics of core formation and equilibration by negative
diapirism, Geochem. Geophys. Geosyst.,9, Q06011, doi:10.1029/2007GC001896.
Schmidt, R.M., and K.R. Housen, Some recent advances in the scaling of impact and explosion
cratering. Int. J. Impact Eng. 5, 543-560, 1987.
Page 40 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
41
Schubert, G., Russell, C.T., and Moore, W.B. 2000. Timing of the Martian dynamo. Nature 408,
666–667.
Schultz, P.H., and Spudis, P.D. 1983. Beginning and end of lunar Mare Volcanism, Nature 302,
133-236.
Senshu, H., Kuramoto, K., Matsui, T. 2002. Thermal evolution of a growing Mars. J. Geophys.
Res. 107 (E12), 5118. http://dx.doi.org/10.1029/2001JE001819.
Shea, E. K., et al. 2012. A long-lived lunar core dynamo, Science, 335, 453–456.
Singer, H. A., and P.L. Olson .1984. Dynamo action in a stably stratified core, Geophys. J., 78,
371– 387.
Spohn, T., M. H. Acuna, D. Breuer, M. Golombek, R. Greeley, A. Halliday,E. Hauber, R.
Jaumann, and F. Sohl (2001), Geophysical constraints on the evolution of Mars, Space Sci. Rev.,
96, 231– 262
Sprenke, K. F., and Baker, L.L. 2000. Magnetization, paleomagnetic poles, and polar wander on
Mars, Icarus, 147, 26–34.
Stegman, D. R., et al. 2003. An early lunar core dynamo driven by thermochemical mantle
convection, Nature, 421, 143–146.
Stevenson, D. J., Spohn, T., and Schubert, G. 1983. Magnetism and thermal evolution of the
terrestrial planets, Icarus, 54, 466– 489.
Stevenson, D.J. 2001. Mars core and magnetism. Nature 412, 214–219.
Stoffler, D. and G. Ryder, 2001., stratigraphy and isotope ages of lunar geologic Units:
Chronological standard for the inner solar system, Space Science Reviews, 96, 9-54.
Strangway, D.W., Larsona, E.E., and Pearce, G.W. 1970. Magnetic studies of lunar samples
Breccia and fines, Geochim, Cosmochim Acta, 34(suppl. 1), 2435–2451.
Page 41 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
42
Strangway, D.W., Pearce, G.W., Gose, G.W., and Timme, R.W. 1971. Remanent magnetization
of lunar samples, Earth Planet. Sci. Lett., 13, 43–52.
Suavet, C., et al. 2013. Persistence and origin of the lunar core dynamo, Proc. Natl. Acad. Sci.
U.S.A., 110, 8453–8458.
Tilgner, A. 2005. Precession driven dynamos. Phys. Fluids 17, 0341042005.
Tilgner, A. 2007. Kinematic dynamos with precession driven flow in a sphere, Geophysical and
Astrophysical Fluid Dynamics 100, Issue 1, p.1-9.
Thompson, C., and Stevenson, D.J. 1988. Gravitational instability of two-phase disks and the
origin of the Moon, Astrophys. J., 333, 452–481.
Tsunakawa, H., et al. 2010. Lunar magnetic field observations and initial global mapping of
lunar magnetic anomalies by MAP-LMAG onboard SELENE (Kaguya), Space Sci. Rev., 152,
doi:10.1007/s11214-010-9652-0.
Tsunakawa, H., et al. 2014. Regional mapping of the lunar magnetic anomalies at the surface:
Method and its application to strong and weak magnetic anomaly regions, Icarus, 228, 35–53.
Tsunakawa, H., et al. 2015. Surface vector mapping of magnetic anomalies over the Moon using
Kaguya and Lunar Prospector observations, J. Geophys. Res. Planets, 120, 1160–1185,
doi:10.1002/2014JE004785.
Weber, R. C., et al. 2011. Seismic detection of the lunar core, Science, 331, 309–312.
Weidenschilling, S.J. 1997. Aerodynamics of solid bodies in the solar nebula. Mon. Not. R.
Astron. Soc. 180, 57–70.
Weiss, B. P., and Tikoo, S.M. 2014. The lunar dynamo, Science, 346(6214), 1246753–1–
1246753-10.
Page 42 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
43
Weiss, B.P., et al. 2002. Records of an ancient Martian field in ALH84001, Earth Planet. Sci.
Lett.
201, 449–463.
Weiss, B.P., et al. 2008. Paleointensity of the 835 ancient Martian magnetic field. GRL 35,
L23207. doi:10.1029/2008GL035585
Wetherill, G.W., Stewart, G.R. 1989. Accumulation of a swarm of small planetesimals. Icarus
77, 350–357.
Wieczorek, M. A., and Le Feuvre, M. 2009. Did a large impact reorient the Moon?, Icarus, 200,
358–366.
Wieczorek, M. A., and Phillips, R.J. 2000. The “Procellarum KREEP Terrane”: Implications for
mare volcanism and lunar evolution, J. Geophys. Res., 105, 20,417–20,430.
Wieczorek, M. A., Weiss, B.P., and Stewart S.T. 2012. An impactor origin for lunar magnetic
anomalies, Science, 335, 1212–1215.
Wilhelms, D.E., Squyres, S.W. 1984. The martian hemispheric dichotomy may be due to a giant
impact. Nature 309, 138–140.
Wu, C.C., and P.H. Roberts, 2009. On a dynamo driven by spheroidal precession , Geophys. &
Astrophys. Fluid Dynam., 103, 467-501.
Wu, C.C., and P.H. Roberts, 2012. On a dynamo driven topographically by longitudinal libra-
Tion , Geophys. & Astrophys. Fluid Dynam.,107, doi.org/10.1080/03091929.2012.682990
Yoder, C.F., Konopliv, A.S., Yuan, D.N., Standish, E.M., Folkner, W.M. 2003. Fluid core size of
Mars from detection of the solar tide. Science 300, 299–303.
Page 43 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
44
Figure Captions:
Figure 1. Radial component of the 62-degree spherical harmonic model of Mars magnetic field
at 370 km altitude (Arkani-Hamed, 2004a).
Figure 2. The vertical component of the magnetic field of Mars derived using MGS high altitude
nighttime magnetic data acquired during (A) 1999–2002 and (B) 2003–2006. (C) Shows the
locations of the craters (in green), the impact-related QCDs (in red), and the none-impact-related
QCDs (in blue), only in the non-magnetic South Province, which has a pink background in
panels A and B (Arkani-Hamed and Boutin, 2012a).
Figure 3. Evolution of the martian core properties for two viscosity models of the martian
mantle (Roberts and Arkani-Hamed, 2017). Black curves are for the Newtonian viscosity and
red curves are for the stress-dependent viscosity. The heat flux panel distinctly shows that shortly
after an impact the heat flux jumps due to the juxtaposition of the stratified high temperature
uppermost core to the colder mantle. The spikes from right to left specify the heat flux
Page 44 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
45
immediately after Daedalia, Ares, Amazonis, Chryse, Scopolus, Acidalia, Utopia, and Hellas
impacts, respectively. Note that the two viscosity models have overall similar behaviors.
Figure 4. The orbital radius of a retrograde asteroids (top) and the rate of tidal energy dissipated
inside Mars (bottom). The numbers on the curves denote the tidal delay time in minutes (see
text for details).
Figure 5. The global distributions of the radial component of the lunar magnetic field (Arkani-
Hamed and Boutin, 2014). (A) the 150-degree spherical harmonic model at 30 km altitude, (B)
the Kaguya data at 100 km altitude, and (C) the LP-2 data at the variable spacecraft altitudes.
The black stripes are the regions with no LP-2 data. The map projection is centered at 180E.
Figure 6. Magnetic anomalies of SPAB (A) and their paleomagnetic north pole positions (B)
(Arkani-Hamed and Boutin, 2017). The horizontal axis of panel A shows the east longitude and
the vertical axis is the latitude, both in degrees. Note that the north Crisium magnetic anomaly is
not included in panel A, because it is outside the frame of the panel, but its paleomagnetic pole
(#6) is included in panel B. The background gray color shows the regions with LP-2 magnetic
data, while the white strips denote places with no LP-2 magnetic data.
Figure A1. The vertical, upward component of the magnetic field (first two rows) and the
magnetic intensity (second two rows) at 50 and 100 km altitudes of a uniformly magnetized
vertical circular disk of 100 km diameter (Arkani-Hamed and Boutin, 2014). The numbers on
the top of the panels show the dip angle of the magnetization vector, 0 for horizontal and 90 for
vertical, upward. The vertically integrated bulk magnetization of the disk is 3x104 A. The
numbers on the horizontal and vertical axes of the lowest right panel show the rectangular
coordinate distances in km relative to the center of the prism.
Page 45 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
46
647
Page 46 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
47
648
649
650
651
Figure 1 652
653
Page 47 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
48
654
Figure 2 655
Page 48 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
49
656
657
658
Figure 3 659
660
661
Page 49 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
50
662
Figure 4 663
664
665
Page 50 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
51
666
Figure 5 667
Page 51 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
52
668
Figure 6 669
Page 52 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
53
670
671
672
Figure A1 673
674
675
676
Page 53 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
54
677
Appendix A: Modeling an Isolated Magnetic Anomaly: 678
A spherical crust of constant thickness with uniformly distributed magnetic minerals is a 679
magnetic annihilator that creates no magnetic field outside when it is magnetized by an internal 680
magnetic field regardless of the complexity of the field and magnetization intensity of the crust 681
(Runcorn, 1975). This is actually the simplest magnetic annihilator among very many complex 682
annihilators (e.g., Arkani-Hamed and Dyment, 1996). Such a crust cannot be distinguished 683
from a non-magnetic crust on the basis of magnetic anomaly analysis. On a local scale, a thin 684
flat and uniformly magnetized extended layer of constant thickness does not create magnetic 685
anomalies except near its edges. The layer is regarded a magnetic annihilator far from its edges. 686
We consider well-isolated magnetic anomalies because they are usually associated with simple 687
source bodies and allow modeling by simple elliptical prisms, hence determining the 688
paleomagnetic pole positions when the bodies acquired magnetization. Modeling a magnetic 689
anomaly is based on an implicit assumption that the surrounding crust creates no magnetic field. 690
We model an intrusive magnetic source body with no topographic signature by a uniformly 691
magnetized cylindrical prism of 10 km thickness with an elliptical horizontal cross section. The 692
prism is located on the surface vertically and extends to a 10 km depth. An elliptical prism 693
allows more degrees of freedom than a circular prism, thus provides a better fit between the data 694
and the model. The three components of the magnetization vector of the prism are determined 695
by fitting the vertical component of the model field to the vertical component of the Lunar 696
Prospector Level-2 (LP-2) data at the observation points, hence taking into account the actual 697
spacecraft altitudes (Arkani-Hamed and Boutin, 2014). The magnetic intensity anomaly of a 698
circular prism overlies the prism, with its major part inside a circle with a radius about twice the 699
Page 54 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
55
radius of the prism (Figure A1). It is worth mentioning that such a simplistic model is highly 700
idealized, it provides a first order estimate of the magnetic anomaly of an almost 701
equidimensional magnetic body. 702
The lack of a topographic signature obscures the exact location of a deep-seated source body. As 703
a first step, we estimate the horizontal projection of the center of the body using the magnetic 704
intensity anomaly. We then calculate the minimum misfit elliptical prism model by moving its 705
center at 20 km intervals inside a square that extends the major areal coverage of a magnetic 706
anomaly. We change its semi-major axis from 60 to 200 km with 20 km increments and its 707
semi-minor axis from 60 km to the semi-major axis again with 20 km increments. We also 708
change the orientation of the major axis of the prism with respect to north from 0 to 180o at 20o 709
intervals. We adopt the same procedure for modeling a magnetic anomaly associated with the 710
impact demagnetization of the crust beneath an impact crater, except that the cross-section of the 711
prism is circular, its center is at the center of the crater, and the magnetic data are over a circular 712
area with a radius twice the radius of the crater. The magnetization of the surrounding crust is in 713
the opposite direction of the magnetization of the model prism. 714
The paleomagnetic pole position associated with the magnetization vector of a source body is 715
determined adopting the procedure by Arkani-Hamed (2001) and assuming that the magnetizing 716
field near the surface is the dipole component of the core field. Because of the very small core 717
radii of the Moon and Mars compared to their actual radii the quadrupole component of the core 718
field decreases by about 4-5 times faster than the dipole component as it propagates from the 719
core-mantle boundary to the surface, and higher degree components decrease even further. 720
721
References: 722
Page 55 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences
Draft
56
Arkani-Hamed, J. 2001. Paleomagnetic pole positions and pole reversals of Mars, Geophys. Res. 723
Lett., 28(17), 3409–3412. 724
Arkani-Hamed, J., and Boutin, D. 2014. Analysis of isolated magnetic anomalies and magnetic 725
signatures of impact craters: Evidence for a core dynamo in the early history of the Moon, Icarus, 726
237, 262–277. 727
Arkani-Hamed, J., and Dyment, J. 1996. The magnetic potential and magnetization contrasts of 728
the Earth's lithosphere, J. Geophys. Res., 101, 11,401-11,425. 729
Runcorn, S.K. 1975. On the interpretations of lunar magnetism. Phys. Earth Planet. Inter. 10, 730
327. 731
732
Page 56 of 56
https://mc06.manuscriptcentral.com/cjes-pubs
Canadian Journal of Earth Sciences