sediments of the moon and earth as end-members for comparative planetology

SEDIMENTS OF THE MOON AND EARTH AS END-MEMBERS FORCOMPARATIVE PLANETOLOGY

ABHIJIT BASU1 and EMANUELA MOLINAROLI2

1Department of Geological Sciences, Indiana University, Bloomington, IN 47405, U.S.A. (E-mail:[email protected]); 2Dipartimento di Scienze Ambientali, Universitá di Venezia, Dorsoduro 2137,

30123 Venezia, Italy (E-mail: [email protected])

Abstract. Processes of production, transport, deposition, lithification, and preservation of sedimentsof the Moon and Earth are extremely different. The differences arise primarily from the dissimilarityin the origins and sizes of the Moon and Earth. The consequence is that the Moon does not havean atmosphere, a hydrosphere (the Moon is totally dry), a biosphere (the Moon is totally life-less),a magnetosphere, and any tectonic force. Pristine rocks on the exposed surface of the Moon areprincipally anorthositic and basaltic, but those on the Earth are granitic (discounting suboceanicrocks). Sediments on these two bodies probably represent two end-members on rocky planetarybodies. Sediments on other rocky planetary bodies (atmosphere-free Mercury and asteroids, Venuswith a thick atmosphere but possibly no water on its surface, and Mars with a currently dry surfacesculptured by running water in the past) are intermediate in character. New evidence suggests thatcharacteristics of Martian sediments may be in-between those of the Moon and Earth. For example,impacts generate most Martian sediments as on the Moon, and, Martian sediments are wind-blownto form dunes as on Earth. A comparative understanding of sediments of the Moon and Earth helpsus anticipate and interpret the sedimentary record of other planetary bodies.

Impact processes, large and small, have produced the sediments of the Moon. Unlike Earth,the surface of the Moon is continuously bombarded by micrometeorites and solar wind. Processesof chemical and mechanical weathering aided by biological activity produce sediments on Earth,fixing a significant amount of carbon in the solid state. Whereas solar wind produces minor chemicalchanges in lunar sediments, chemical weathering significantly alters and affects the character of Earthsediments. Primarily ballistic and electrostatic forces transport lunar sediments but Earth sedimentsare transported by air, water, and ice. Whereas Earth sediments accumulate mostly in basins createdslowly by tectonic forces, lunar sediments are deposited in craters (excavated instantaneously byimpacts) or even on high grounds. Rubble, sand, mud, and carbonate material on Earth are lithifiedthrough burial, expulsion of water, and precipitation of cement from H2O-solutions. In contrast, lunarsediments are lithified through presumably low-energy shock waves that sinter and bind clastic grainsinto regolith breccias. Surface processes and morphological features on the Moon are dominated byimpact cratering and ejecta deposition, while those on Earth are sculptured by water, ice, and air.

However, comparisons in two areas assist in planning planetary exploration. (1) Dust, i.e., smallparticles elevated above the solid surface of a planetary body, is ubiquitous on the Moon and Earth.The composition of dust is related to but is different from the source rocks, especially where dustis transported over long distances as on Earth. Dust obscures observation of a planetary body andinterferes with remote sensing; dust may also affect climate on planetary bodies with an atmosphere.(2) Because Earth’s lithosphere has been recycled many times, sediments shed from rocks and regionsthat do not exist any more are the principal guides to the ancient Earth and its crustal evolution. Be-cause the lunar surface is completely covered by regolith, and no bedrock has been directly observedor sampled, sediment is the principal guide to the lunar crust, past and present. Provenance analysisof lunar and terrestrial sediments is accomplished using the same methods and principles.

Earth, Moon and Planets 85–86: 25–43, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

26 ABHIJIT BASU AND EMANUELA MOLINAROLI

1. Introduction

Sediments and sedimentary processes on the Moon and Earth are very different. Inthe absence of water, an atmosphere, the magnetosphere, and much less oxygenin its rocks, the Moon has neither clay minerals nor carbonates, and no Fe3+.Mechanical weathering by impacts is the principal process of sediment generationon the Moon; on Earth, chemical weathering predominates. Whereas processes ofsediment transport are principally ballistic on the Moon, movement by air, waterand ice prevail on the Earth. The radical differences between Earth and Moonsediments make them useful end-members between which all sediments of allterrestrial planetary bodies are expected to lie.

The purpose of this paper is (1) to compare and contrast major characteristicsof the origin, transportation, deposition, and preservation of sediments, especiallydust, in the Earth and the Moon, and (2) to suggest how sediments of other rockyplanetary bodies, especially Mars, may fit in-between the sediments of the Earthand the Moon.

The differences between the sediments of the Earth and the Moon mentionedabove arise primarily from the dissimilarity in the origins and sizes of the Moon(diameter: 3476 km; mass: 0.7 × 1023 kg) and Earth (diameter: 12756 km; mass:59.7 × 1023 kg). In consequence, the Moon is devoid of an atmosphere, a hy-drosphere (the Moon is totally dry), a biosphere (the Moon is totally life-less), amagnetosphere, and any tectonic force. Pristine rocks on the exposed surface ofthe Moon are principally anorthositic and basaltic, but those on Earth are granitic(discounting suboceanic rocks). Sediments on these two bodies probably representtwo end-members of those on rocky planetary bodies. Sediments on other rockybodies (atmosphere-free Mercury and asteroids, Venus with a thick atmospherebut possibly no water on its surface, and Mars with a currently dry surface thathad been sculptured by running water in the past) should be intermediate in char-acter. New evidence suggests that the characteristics of Martian sediments maybe intermediate in character between those of the Moon and Earth. For example,impacts generate most Martian sediments as on the Moon, and, Martian sedimentsare wind-blown to form dunes as in deserts on Earth. A comparative understandingof sediments of the Moon and Earth helps us anticipate and interpret the sedi-mentary record of other planetary bodies, especially that of Mars. In our opinion,taking stock of the sedimentary characteristics of the Earth and the Moon, vis a viscomparative planetology and new data on Mars, is timely.

2. Importance of Planetary Sediments

Several reasons for the justification of our purpose as stated above follow. First,there has been no systematic assessment of the similarities and dissimilaritiesbetween the sediments of these two planetary bodies. Such knowledge is necessary

SEDIMENTS AS END-MEMBERS FOR COMPARATIVE PLANETOLOGY 27

for a comprehensive understanding of comparative planetology. Books on com-parative planetology (e.g., Morrison and Owen, 1996; Hartmann, 1983) generallyconsider sediments on the Earth and the Moon separately instead of consideringprocesses and products that are common to both. Second, and it follows from theabove, such a comparison aids in understanding the processes and products onplanetary bodies that may be intermediate between the Earth and the Moon, Marsin particular. Currently, Mars has no significant magnetosphere, no surface water,and its atmosphere is very thin (0.07 bar). However, it used to have a significantmagnetic field, copious running water, and an atmosphere dense enough to allowliquid water on the surface. Mars has changed from being somewhat earth-like toperhaps becoming somewhat moon-like. A comparison of the Earth–Moon sedi-mentary systems is important in understanding the evolution of the sedimentarysystem on Mars. Third, dust affects surficial properties of both the Earth and theMoon, especially affecting remote sensing signals (Pieters and Englert, 1993). Acomparative understanding should aid interpreting remote sensing signals fromother planetary bodies with (e.g., Venus, Mars) or without (e.g., asteroids, Mer-cury) an atmosphere. Fourth, siliciclastic sediments, common to both the Earthand the Moon, are the principal guides in provenance determination. Much of thepaleogeologic reconstruction of the ancient Earth and most of our understandingof the distribution of rock-types on the Moon are dependent on provenance inter-pretation of siliciclastic sediments (Dickinson, 1988; Heiken et al., 1991; Spudis,1996; Basu and Riegsecker, 2000). Feedback between understanding siliciclasticsediments on the Earth and the Moon has led to refinements of concepts and ap-proaches in provenance interpretation (Basu et al., 1981). Finally, in the absenceof biological activity, lunar sediments comprise one lifeless prototype as opposedto Earth sediments that are affected by abundant life. Any investigation into find-ing evidence of life in other planetary bodies must be guided by the nature ofsediments, which should be as earth-like as possible and as different from lunarsediments as possible.

3. Sediments

Sediment may be defined as the material that is deposited at the surface of a plan-etary body by physical, chemical, or biological agents (cf. Press and Siever, 1994,p. 570). Characteristics of sediments are functions of the processes of their produc-tion, transportation, and deposition. These processes on Earth and the Moon arefundamentally different, although and obviously the same physical and chemicalprinciples control the processes. Below we compare and contrast some of the char-acteristics of significant sediments on the Earth and the Moon generally under therubric of processes. We deviate from this strategy and separately consider dust – thefinest grains in sediments – which can be levitated above the planetary surface and


moved in planetary atmospheres. Dust is ubiquitous in sediments of all planetarybodies and deserves special treatment.

3.1. PRODUCTION (ORIGIN)

Impact processes, large and small, have produced the sediments of the Moon.Large impacts that produced craters with diameters ranging from 1 km to morethan 1000 km were much more common in ancient times (>3 b.y.) than during thesubsequent time up to the present (Figure 4.15 in Heiken et al., 1991; Wetherill,1989; Hartmann, 1983). The product of the process, impact ejecta consisting oflarge blocks (>100 m) through fine dust (<1 µm), constitutes the bulk of lunarsediments. Ancient ejecta, i.e. sediments, are mixed in with newer ejecta andsurvive as a regolith component even today (Kerridge, 1975, 1980). Some of theancient ejecta have been preserved in regolith breccias (McKay et al., 1986, 1989),and some have been covered with subsequent basalt flows or other ejecta blankets.Whereas the Earth has undergone the same impact history, sediments produced byimpacts have been subjected to immediate reprocessing by such terrestrial agentsas wind, water, ice, and the biota. Thus, only at very few places of recent impacton Earth do we find sediments produced by impacts (Grant and Schultz, 1993).Some ejecta on the Earth have been caught up in impact-melts and are preserved asclasts a few of which have been digested to various degrees (Bogard et al., 1988;Engelhardt and Graup, 1984; Newsom et al., 1986). Impact melt breccias are thusfound both on the Earth and the Moon.

Earth’s atmosphere and its magnetosphere shield it from micrometeorites andsolar wind (a stream of energetic particles, e.g., elements in an ionized state, thatescape from the Sun and travel at about 400 km/sec through the solar system).The Moon, however, has no such shield and its sediments are continuously bom-barded and processed by micrometeoritic and solar wind. The process, essentiallyanhydrous, is called “space weathering” (Pieters, 1998) and leads to maturation(Heiken et al., 1991). Micrometeoritic bombardment on one hand pulverizes thesediments to finer and finer sizes and on the other hand melts a little dust at thepoint of impact, which scavenges other nearby grains and congeals to form a newconstructional grain called agglutinates. Eventually a steady state of grain size isreached as the rates of pulverization and agglutination match each other (McKayet al., 1974). However, larger impacts may excavate larger grains, many of whichare fresh mineral and rock fragments, from below the top layer of sediments on theMoon and replenish the surficial material, commonly called lunar soil. Such eventsdisturb the steady state and a new cycle of maturation begins (McKay et al., 1974;Mendell and McKay, 1975; Basu, 1977; McKay and Basu, 1983). Earth sedimentsdo not undergo any comparable set of processes to achieve a steady state.

Solar wind bombardment of the surface of the Moon implants such elements asH, He, C, N, Ne, Ar, Kr, Xe in the outer rinds of all exposed grains. Of these H ismost abundant and is also chemically reactive. Micrometeoritic bombardment not


Figure 1. Back-scattered electron image of the interior of an agglutinate grain from Apollo 15 soil15221. Globules of Fe0-metal are entrained in flows of silicate glass swirling around clasts of mineralgrains (irregular pods of various gray shades) and gas-vesicles (black circles).

only melts some dust (see above) but also vaporizes a substantial quantity of thetarget (Cintala, 1992). The reactive H, both in the melt and in the vapor, reducesthe Fe in Fe-bearing minerals to metallic Fe0 that form as single domain superpara-magnetic nanophase Fe0. If in the vapor, grains of nanophase Fe0 are deposited ingrain coatings as the vapor condenses at the lunar surface (Hapke, 1975; Keller andMcKay, 1993; Keller et al., 2000). If in the melt, grains of nanophase Fe0 becomepart and parcel of the quench-product, i.e., glass that binds other soil grains andproduces agglutinates (McKay et al., 1972; Housely et al., 1974; Morris, 1980).Because micrometeoritic and solar wind bombardments are continuous processes,older agglutinates become incorporated in newer agglutinates (Basu and Meinsch-ein, 1976; Basu, 1977; McKay and Basu, 1983). In the process, grains of nanophaseFe0 become re-mobilized and coagulate to form larger globules of Fe0 that becomeentrained in agglutinitic melt defining flow lines (Figure 1). Overall, space weath-ering on the Moon (1) brings about a change in mineralogic composition of lunarsoils by converting many minerals into glass, and (2) despite losing a minor amountof volatile elements (e.g., Si, Fe, Na, K) that escape the Moon, retains the chemicalcomposition of lunar soils.

Processes of chemical and mechanical weathering of pre-existing rocks, in thepresence of water, aided by biological activity produce sediments on Earth. Tec-tonic forces on the Earth uplift mountains and downwarp basins creating conditionsof extensive transport and differentiation of sediments by running water. Very little


of the sediments produced on Earth stays at the site of production; transport mixessediments from different sources; and, energetics in the environment of depositionsort and segregate sediments into fractions of different properties. Both chemicaland mineralogic compositions of Earth sediments are usually vastly different fromthose of their parent rocks (Dickinson, 1988; Zuffa, 1985). In contrast, althoughlarge basin forming impacts in the distant past (>3 b.y.) have produced and movedlarge amounts of sediments (ejecta) through long distances, lunar sediments arecurrently produced and stirred in situ by micrometeoritic bombardment (Heiken etal., 1991).

Biological intervention in the production, alteration, and preservation of sedi-ments is overwhelmingly extensive if not unique on Earth. Chemical, biochemical,and physical changes are wrought upon rocks and sediments by the smallestbacteria and the largest animal. Perhaps a unique phenomenon is the fixing ofa large amount of carbon in the Earth’s crust as limestone (CaCO3), dolostone(CaMg(CO3)2), and coal and petroleum (C–H–O compounds). The phenomenonhas apparently changed the composition of the primitive atmosphere of the Earthfrom being rich in CO2 to one dominated by nitrogen and oxygen. As an aside, notethat CO2 constitutes 95% of the atmosphere of Mars, which suggests that carbon-fixing in Mars may not have been significant. No comparison with the Moon ispermissible because the Moon has neither any life nor enough mass to support anatmosphere.

3.1.1. DustContemporary dust grains in the Earth’s atmosphere originate from six principalsources (Pye, 1987). (1) Interplanetary dust grains rain on Earth and some (<10µm) float on top of the atmosphere; but larger grains penetrate the atmosphere,slow down, and eventually concentrate on sea bottom as micrometeorites. Theannual flux on to the Earth is in the order of 105 to 106 ton yr−1 (Grun et al., 1985;Love and Brownlee, 1993; Taylor et al., 1998) and is probably proportionately lesson the smaller sized Moon. (2) Wind blown sediments and soils, deflated mostlyfrom arid and semi-arid areas, contribute the largest amount of dust, commonly>2 µm, into the atmosphere (Pye, 1993, 1987; Coudé-Gaussen, 1984; Prospero,1981; Prospero et al., 1983). Whereas this process does not have any counterpartin the Moon, wind processes currently produce dust storms on Mars. (3) Volcanicdust and aerosols are intermittently injected into the atmosphere, which take a fewyears to settle (Braitseva et al., 1996; Kohno et al., 1999). (4) Degraded biolo-gical products contribute either directly or through pedogenic processing in soils.Natural forest fires and biomass burning for deforestation and agriculture (Suman,1996) contribute mostly <5 µm grains. This biological source of dust is uniqueto Earth. (5) Salts, an important source of atmospheric aerosols, can enter theterrestrial dust regime not only from sabkha soils but also from drying spray of seawaves (Fairall et al., 1983). (6) Finally, industrial emissions introduced by civiliz-


ation are increasing exponentially, the total flux of which is probably comparableto the influx of cosmic dust, if not higher.

In contrast, only a single process, meteoritic impact, produces dust-size grainson the Moon. In the virtual absence of an atmosphere, incoming cosmic dust andmicrometeorites impinge on the lunar surface at high speed (∼15 km sec−1) andgenerate additional dust.

3.2. GRAIN SIZE AND MINERALOGY

Any photograph taken at the lunar surface would show large boulders and imprintson fine dust (Figure 2). Estimates of the total distribution of grain size of lunarsediments, from boulders to fine dust, have not been made. For regolith samplesobtained by astronauts, NASA curatorial facility determined the distribution of“grains” > 1 cm; however, it is not certain if the sampling has been comprehensiveup to and beyond 10 cm. Lunar “soils”, defined for the ease of handling to be >1mm, have been sieved. However, allocations have been generally 0.5 g or so (Graf,1993; Heiken et al. 1991; Morris et al., 1983), which might have rendered someallocations non-representative. The longer a lunar soil has been exposed to micro-meteoritic bombardment, the finer it becomes despite concomitant agglutination(see above). In general, most of the lunar soils have a mean grain size of about50 µm with a large standard deviation; about 25% of the submillimeter fraction is<20 µm in size. The grain size distribution of an Apollo 11 soil is shown in Figure3 as an example.

The literature on the mineralogy and grain size distribution of terrestrial sedi-ments is vast. Useful summaries are found in common textbooks (e.g., Blatt, 1982;Tucker, 1991) and are not repeated here. Instead, we include a short discussion ondust.

3.2.1. DustBecause there are so many sources of terrestrial dust as mentioned above, no gen-eralization can be made for the grain size distribution of any single type (Pye,1987). Grain size distributions of dust deposits depend on the strength and velocit-ies of wind speed and distance from source. For illustrative purposes we comparethe grain size distributions of deposits of Saharan dust in Sardegna (Italy) and inEngland (Figure 4).

Mineral compositions of dust deposits on all planetary bodies depend on (1) theweathering products and soil ingredients in source regions, and (2) size sortingaccomplished in flight. Larger grain size fractions are understandably richer incomposite grains such as polymineralic rock fragments and aggregated productssuch as clay-pellets and calcrete/silcrete/ferricrete on the Earth, and agglutinatesand regolith breccia on the Moon. On the Earth, clay minerals dominate the <2µm size fraction with increasing amounts of quartz and Na-K-feldspar in lar-ger grain sizes (Table Ia) indicating a generally granitic source. On the Moon,


Figure 2. (a) Strewn field of boulders near the Van Serg Crater at the Apollo 17 site; Rover withan astronaut in foreground for scale. (NASA photograph from the Lunar and Planetary Institute,Houston). (b) An astronaut boot depresses lunar soil and dust (NASA photograph from the Lunar andPlanetary Institute, Houston; http://www.lpi.usra.edu/expmoon/Apollo11/A11_MP.MissionLP.gif).(c) Dimensions of imprints such as this allow calculations to estimate physical and technicalproperties of lunar soils (NASA photograph from the Lunar and Planetary Institute, Houston;http://www.lpi.usra.edu/expmoon/Apollo11/A11_MP.SurfaceAct4FS.gif).

Figure 3. Grain size distribution of the submillimeter fraction of Apollo 11 soil 10084, the first lunarsoil collected by astronauts and returned to Earth.


Figure 4. Grain size distribution of Saharan dust collected in England in 1968 (adapted from Pye,1987), and in Sardegna (Italy) in 1990 (dry precipitation) and in 1993 (wet precipitation). Strengthsof storms determine the mean grain size that could be transported to different distances; wet precip-itation is controlled by raindrop sizes in addition to the sizes of airborne grains and shows a differentgrain size distribution pattern.

the dominant constituents are agglutinates, breccias, rock fragments (basaltic andanorthositic), pyroxene, Ca-rich plagioclase feldspar, glass, ilmenite, and olivine(Table Ib) indicating basaltic and anorthositic sources.

3.3. TRANSPORTATION

Primarily ballistic and electrostatic forces transport lunar sediments but Earth sedi-ments are transported by wind, water, and ice. The contrast in transport mechanismproduces two kinds of sediments.

In ballistic transport, all fragmentary materials are entrained in the ejecta flow,their trajectories determined primarily by the angle and the force of ejection. Largerimpacts also produce sufficient vapor that propels the movement of base surgescontaining fragments from the cratering process. Base surges and ballistic transportinitiated by large impacts (e.g., producing >100 km diameter craters) have traveledfor 100 s of kilometers to more than 1000 km at the lunar surface. Smaller craters,some are submicron in size, obviously transport ejecta to very limited distances.Fragments transported as ejecta hardly undergo any change in size or shape orcomposition. Such conservation of sediment characteristics assists in provenancestudies of lunar sediments.


TABLE Ia

Modal distribution of minerals in airborne dust collected inSardegna in the size range between 0.5 µm and 62 µm. Thecontrast between the Saharan and local dust shows that car-bonate minerals are almost exclusively exotic. In addition, thesignificant presence of quartz, K-feldspar, and Na-rich plagio-clase suggests the dominance of granite as the principal pristinesource rock. (SEM and EDS determination)

Saharan Local

Constituent modal % modal %

Quartz 26.1 11.0

K-feldspar 4.0 5.0

Plagioclase (Na-rich) 2.4 5.5

Carbonates 10.0 0.6

Illite 18.1 43.7

Smectite 14.3 9.6

Chlorite 7.6 13.0

Kaolinite 12.2 0.5

Palygorskite 4.3 0.0

Talc 0.0 5.2

Amphibole 0.0 1.0

Fe-Ti oxides 1.0 5.0

Total 100 100

n (grains) 575 500

Electrostatic transport (Rennilson and Criswell, 1974; Criswell, 1972) of prin-cipally submicron grains at the surface of the Moon occurs at the terminator region(boundary between daylight and night darkness) across which there occurs a largetemperature differential. In the virtual absence of an atmosphere, the immediateenvironment of the lunar surface is charged with free-flowing ions. Electrostaticforces at the terminator facilitate preferential levitation of the finest grains againstgravitational forces. The net result is that the very surface of the Moon is a veneerof submicron dust covering the grains below. This has significant implication forremote sensing of the Moon and other atmosphere-free bodies because the compos-ition of the finest fraction of lunar soils is different from that of the bulk (Papike,1981; Walker and Papike, 1981; Basu and McKay, 1985; Taylor et al., 2000).

Although hardly any compositional change occurs during the transport ofterrestrial sediments in ice sheets and glaciers (Nesbitt and Young, 1996), gla-cial grinding changes the size and shapes of sedimentary fragments drastically.Such grinding also produces considerable amounts of dust, as do other processes


TABLE Ib

Modal distribution of constituents in Apollo 11 soil10084 in the 20 µm–1 mm size range. The signific-ant presence of basaltic and anorthositic rock fragments,and of pyroxene and Ca-rich plagioclase suggests thedominance of basalt and anorthosite as the principalpristine source rocks on the Moon. (Optical microscopicdetermination)

Constituent Modal %

Plagioclase (Ca-rich) 2.9

Pyroxene 16.8

Olivine 0.3

Ilmenite/chromite 3.3

Anorthositic fragments 0.7

Basaltic fragments 18.8

Regolith Breccia 16.0

Agglutinate 35.4

Grey glass 3.0

Green glass 0.2

Other glass 2.6

Miscellaneous 0.1

Total 100

n (grains) 1477

(Boulton, 1978; Nahon and Trompette, 1982). Depending on climatic conditions,sediments could be transported over distances ranging from 10s of km to a few1000 km. Transport by wind and water (river, ocean currents) takes place princip-ally in three modes: traction of sediments along the fluid-solid interface, saltation(jumping grains), and suspension. The efficiency of such transport and the sizesof grains that may be transported in any of the modes depend on a combinationof the velocity and the turbulence in the fluid medium. For example in the Earth’satmosphere, at a mean wind speed of 15 m s−1 and at a turbulence coefficient (ameasure of the degree of vertical mixing in air) of 104 cm2 s−1, a 50 µm quartzsphere may travel only 1 km but a 10 µm quartz sphere may travel 106 km (Tsoarand Pye, 1987).

3.4. DEPOSITION

Deposition takes place when gravitational forces overcome the inertial forces act-ing upon a grain entrained in a flow. Deceleration of velocity of the medium is theprincipal cause for deposition of sediments in transport. Whereas this principle is


universally true, two special conditions affect the process of deposition of very finesediments on the Moon and Earth. On the Moon, electrostatic forces levitate smallgrains the deposition of which occurs as the terminator region rotates (see above;also Criswell, 1972). On the Earth, fine dust may be forced up to the tropospherethrough dust storms and in dust plumes where they are commonly encapsulated incloud-moisture and precipitate with raindrops, i.e., wet deposition (e.g., Guerzoniet al., 1997; Molinaroli and Ibba, 1995; Molinaroli, 1996). Eventually, terrestrialsediments accumulate mostly in basins created slowly by tectonic forces, suchas the oceans, lakes, alluvial plains, and temporary storage in high grounds untilaggradational processes remove such sediment. On the Moon, sediments are de-posited in craters (excavated instantaneously by impacts) or even on high groundsthat may be ascended by base surge type of mass movement and where they cometo rest.

3.5. PRESERVATION (LITHIFICATION)

Nearly all terrestrial sediments are lithified upon burial, the notable exception be-ing the lithification of some soil-crusts such as silcrete, ferricrete, and calcrete,and, direct precipitation of some salts and oozes. Expulsion of water is the firstprocess that sediments undergo upon burial, which is followed by precipitation ofcementing material between detrital grains from pore fluids. Because burial is alsoaccompanied by an increase in temperature up to about 200 ◦C, reactions betweensome minerals and pore water also take place. This process, diagenesis, changes themineralogy of the detrital sediments; some minerals may dissolve fully or in partand may be replaced by others (Morton, 1984; McBride, 1987; Rooney and Basu,1994). In addition, new minerals are commonly precipitated. Common mineralsthat cement terrestrial sediments are calcite (CaCO3), dolomite (CaMg(CO3)2),quartz (SiO2), and clays (hydrated alumino-silicates with a sheet structure); somevolcaniclastic sediments are cemented by zeolite (H2O-bearing alumino-silicateswith a framework structure). These diagenetic minerals owe their origin to thedissolution of detrital grains, and at places may be the only material remainingin a rock. The conversion of limestone to dolostone is an example; in rare cases,some volcaniclastic sandstones have been totally zeolitized.

In contrast, lunar sediments are lithified through presumably low-energy shockwaves that sinter and bind clastic grains into regolith breccias (Kieffer, 1975).In the absence of water, no reactions akin to those in terrestrial sediments takeplace. Thus, lunar sediments do not undergo the drastic changes in compositionof terrestrial sediments. However, shock processes do disturb some of the internalatomic arrangements of the minerals in the target, may produce an incipient meltfilm, and sinter all minerals in dust sizes beyond recognition.


3.6. DISCUSSION

Processes of production, transportation, deposition, and lithification of sedimentssignificantly alter the mineralogic and chemical composition of parent rocks. Thedegree of deviation is much more pronounced in terrestrial sediments than in lunarsediments. Thus the adage that provenance is “one of the most difficult problemsthe sedimentary petrographer is called on to solve” (Pettijohn et al., 1987, p. 255)applies with less rigor for lunar sediments. In the absence of any sampled bedrock,and because of the presence of a mega-regolith that covers all rock exposureson the Moon (Heiken et al., 1991), lunar sediments are the only material thatprovide sample data for provenance interpretation and for deciphering the geologyof the Moon. In fact, much more quantitative knowledge has been gained aboutthe provenance of lunar sediments (e.g., Papike et al., 1982; Korotev, 1997; Basuand Bower, 1977) than that on the Earth (Valloni and Basu, in press; Basu, 1998;Molinaroli et al., 1991; Ibbeken and Schleyer, 1991; Zuffa, 1986; Mack, 1984).

4. Planetary Exploration

We surmise that exploration of solar system planets in the foreseeable future ofabout a century will be confined to their surface. Remote sensing of reflectancespectra (uv-vis-ir), γ -rays, x-ray fluorescence, neutron interactions, etc. will besupplemented by landings and limited traverses to collect surficial material for insitu analyses and possibly return to the Earth. The material from which remotesignals are received and that collected at planetary surfaces will be entirely re-stricted to sediments. Recent and current examples of such exploration are thoseof Clementine and Prospector missions to the Moon, the Pathfinder and the MarsGlobal Surveyor to Mars, NEAR-Shoemaker to Eros, and Galileo fly-bys of Gaspraand Ida. It is against this forecast that future exploration results will be interpreted,most of which will depend on the knowledge of terrestrial and lunar sediments forground- truth and calibration. All dry atmosphere-free rocky planetary bodies arelikely to be moon-like and all wet rocky planetary bodies (of the past if not thepresent as well) are likely to be earth-like sans extensive life.

4.1. DUSTS ON ROCKY PLANETARY BODIES

It is probably clear from the above that dust is ubiquitous. Dust is recognizedeven on small planetary bodies such as Gaspra (12 km average diameter) andIda (56 × 15 km croissant shaped) not to speak of larger bodies with largergravitational fields (Chapman, 1996). We have also shown that the compositionof dust is related to but different from the rocks from which they originate. Ad-ditionally, bodies with any atmosphere suffer dust storms that may carry dustall around the body before deposition. On Earth, dusts that travel in plumes(see http://www.nrlmry.navy.mil/aerosol/Case_studies/20000226_sah ara/) avoid


extensive mixing; their provenance can be recognized or inferred regardless of dis-tance traveled (Prospero, 1996, 1999; Molinaroli, 1996). On Mars, currently dry atthe surface, global dust storms are common which mix them thoroughly to producea homogeneous dust all around the planet (McSween and Keil, 2000; McLennan,2000). Properties of dust, therefore, are likely to reflect the local material on bodieswithout an atmosphere, and, both local and distant source-material in bodies withan atmosphere. Further, in bodies with thin atmosphere, dust storms are likely tohomogenize the dust on a global scale.

Remote observation of the surface of a planetary body – be it from a fly-by, fromthe Hubble telescope and other orbiting spacecrafts, or from the Earth itself – is theprincipal method of planetary exploration. The veneer of dust on a planetary bodyaffects the remote determination of chemical and mineralogical composition ofcrustal rocks of the body. Much of this is due to the fact that remotely sensed signalsintegrate over a limited depth from the surface. For example, on the Moon, γ -raysmay integrate to a depth of about 50 cm, the reflectance of sunlight (uv-vis-ir spec-tra) may integrate only up to 2 cm, and x-rays fluoresced by primary solar x-raysmay be sourced only in a few uppermost nanometers. Thus, dust or any sediment-cover of a planetary body will affect measurements if not also obscure observation.In addition, floating dust interferes with transmission and measurement of anyelectromagnetic radiation, including those used for photography.

However, several methods are in place to measure the mass distribution and thecomposition of airborne particles (e.g., Schutz et al., 1981; Huffman, 1992). Theresults assist in filtering and isolating signals from the surface obscured by dust. Forexample, the composition of Martian rocks estimated from APXS (alpha protonx-ray spectrometer) signals is obtained by subtracting the estimated compositionof the dust veneer on rocks (Rieder et al., 1997). Weathering may produce a thinfilm of deposits on some rocks, and the films and not the rocks per se are sensedremotely. However, if the weathering products are sensitive to parent material, re-mote sensing may distinguish between the parent materials. For example, remotelysensed thermal emission (infrared) of the weathering films on an 1843 and a 1935pahoehoe flow easily separate the two although they are “almost indistinguishablein the field” (Plate 5.13 and Figure 5.15 in Pieters and Englert, 1993).

4.2. PROVENANCE OF SEDIMENTS OF ROCKY PLANETARY BODIES

On Earth, crustal rocks are exposed at the surface. A limestone caps Mt. Everest(Odell, 1967) and deep-seated mantle material is obducted at the surface in Oman(Searle and Cox, 1999). Many have been eroded and possibly lost forever. Sedi-ments shed from exposed rocks carry some signature of the parent subject to themodifications discussed above. Provenance analysis of sediments and sedimentaryrocks lead us to reconstruct the geological evolution of the Earth (e.g., Johnssonand Basu, 1993) in conjunction with the study of other rocks. Because no bedrock


has been studied so far either on the Moon or on Mars, we are totally dependent onprovenance analysis of the surficial material of these two bodies.

If on the Earth, we had access to only unconsolidated sediments and strewnpebbles and boulders, we would conclude that granitic and carbonate materialconstitute the continental crust of the Earth, with minor input from basaltic rocks.For the material of the Moon, we have so far concluded that basaltic, anorthositic,and a “potassium-rare earth element-phosphorus”-rich material constitute its crust(Taylor, 1982). As described above, processes that modify the sediments of theEarth and the Moon are vastly different. Earth processes destroy provenance signa-tures much more than those on the Moon. Early Mars (Clifford and Parker, 1999;Carr, 1999, 1996) likely was warm and wet sustaining liquid water at the surface,much like the Earth. Processes controlling the composition of the then sedimentsshould have been akin to those of the Earth (sans life). At least for the last 3 billionyears Mars has been dry at the surface, has suffered meteorite impacts ejectingmaterial from deeper parts of the crust somewhat like the Moon, and have hadvolcanism until recently (Hartmann et al., 1999, 2000). Because the atmosphere ofMars at 0.07 bar is apparently sufficient to retard the velocities of micrometeorites,agglutination does not occur. On the other hand, ultraviolet radiation oxidizes thematerial at the surface where hematite (γ -Fe2O3) is preserved and boulders andpebbles are still comminuted (Morris, 1999). Identification of these Martian pro-cesses renders surficial material of Mars as products of processes that are commonto either Earth or the Moon.

We conclude that properties of sediments on Mars and other rocky planetarybodies are intermediate between those on Earth and on the Moon.

Acknowledgements

We are very grateful to Dr. Cesare Barbieri and Dr. Francesca Rampazzi for givingus the opportunity to present this paper. This work was partially supported by CEE-STEP Programme (Contract STEP-CTN090-0080) and ex MURST 60% fundsto EM, and, by NASA grant NAG5-9369 and Indiana University (Department ofGeological Sciences and International Programs) to AB.

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