clark r. chapman (swri) , r.g. strom (univ. ariz.),
Post on 18-Mar-2016
26 Views
Preview:
DESCRIPTION
TRANSCRIPT
Clark R. Chapman Clark R. Chapman (SwRI)(SwRI), R.G. Strom , R.G. Strom (Univ. Ariz.),(Univ. Ariz.),S.C. Solomon S.C. Solomon (DTM, Carnegie Institution)(DTM, Carnegie Institution),,
J.W. Head III J.W. Head III (Brown Univ.)(Brown Univ.), , and and W.J. Merline W.J. Merline (SwRI)(SwRI)
AAS Division for Planetary SciencesAAS Division for Planetary SciencesCornell University, Ithaca NYCornell University, Ithaca NY
14 October 200814 October 2008
MESSENGERMESSENGER Perspectives on Perspectives on Mercury’s CrateringMercury’s Cratering
Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering
with thanks to the whole MESSENGER Teamwith thanks to the whole MESSENGER Team
“Mariner 10 Imaged 45% of Surface.” Vivaldi Crater: Then and Now
Mariner 10 Image & Shaded Relief MESSENGER image
Vivaldi Crater at Sunset & SunriseM1 M2
Long, Linear Secondary Crater Chains
Prominence of secondary craters recognized from M10 images
Long, linear chains of craters radiate from large peak-ring crater Eminescu they are obviously
not SL9-like could they be pit
craters from crustal fractures?
how are ejecta launched in such a co-linear fashion?
Note: unusual orientation of some chains: one curves!
Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering
Long, Linear Rays (M2 Flyby)
Rays seen by Mariner 10 now understood to be a global system of long, narrow rays emanating from a never-before-seen 110 km diameter fresh crater
Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering
Basins on Mercury
Caloris Basin, MSGR M1 + Mariner 10Caloris Basin, MSGR M1 + Mariner 10 New Basin, MSGR M2 + M1New Basin, MSGR M2 + M1
Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering
Double-Ring Basin Raditladi
Questions about Cratering on Mercury:
Relationship of Mercury’s basins to lunar basins and the Late Heavy Bombardment
Raditladi basin: could it have formed quite recently? Early “Population 1” highlands cratering: is the relative
lack of craters <40 km diameter related to formation of “intercrater plains”? If so, how?
Stratigraphy of Caloris basin (by crater densities, SFDs) Are interior plains impact melt or subsequent volcanics? Are exterior plains due to Caloris ejecta (e.g. Cayley
plains) or the result of volcanism? If plains are volcanic, did interior and exterior volcanism
end at the same time or different times? Secondary cratering on Mercury: how does it compare
with secondary cratering on the Moon? Absolute chronology: for basin formation, cratering,
darkening of fresh-crater rays by space-weathering, etc.
Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering
Study regions for statistics of small craters: Smooth plains
west of Caloris Fresh double-
ring basin Raditladi
Ejecta blanket Flooded floor
Secondary crater field from several fresh, large primary craters
Heavily cratered terrain
Coverage key Yellow = area
studied Orange = focus
on small craters
Caloris
Raditladi
Double-Ring Basin: Flooded Floor and Ejecta Blanket
Segment above excluded region is on ejecta deposits
Segment below is floor of basin
Craters on rare non-flooded regions ex-cluded from analysis of floor
Note the very fresh, crater-free terrains
Summary: Small Craters in Diverse Terrains
Slopes of SFDs for craters <10 km in different regions vary; they may reflect varying contributions by the generally very steep SFD for secondaries shown in pink
Craters reach empirical saturation densities at large diameters in heavily cratered terrain and at diameters < a few km in the heavily cratered terrain and in the secondary crater field
If smooth plains are post-LHB, then the straightforward conclusion is that Raditladi (both ejecta and interior plains) is < 1 Gy in age, but depends on stochastics of secondaries
Caloris Interior and Exterior Plains
Counts of craters >8 km diameter within plains units, both inside and exterior to Caloris
New counts from best images from Mariner 10 and first MESSENGER flyby
Interior Plains
Exterior Plains
Mariner 10
MESSENGER M1
Exterior Plains
Caloris Interior Plains ~25% Older than Exterior Plains
Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering
Caloris Basin Cratering Stratigraphy
Caloris mountains on rim (measured by Caleb Fassett) show old, Pop. 1 signature Crater density much
higher than on plains SFD shape resembles
that of highlands on Moon and Mercury
Hence interior plains must have volcanic origin, cannot be contemporaneous impact melt
Interior and exterior plains have low density, and flat Pop. 2 signa-ture…so they formed mainly after the LHB had ended
Important issue raised by these resultsImportant issue raised by these results: If exterior plains are volcanic, then interpretation : If exterior plains are volcanic, then interpretation of knobby texture of Odin Formation as Cayley-Plains-like Caloris ejecta may be wrongof knobby texture of Odin Formation as Cayley-Plains-like Caloris ejecta may be wrong
Cratering Components
New data consistent with M10 view: Pop. 1 (LHB), Pop. 2 (recent NEAs) Secondary branch upturn near 8 km (vs 2 km on Moon) Variations in R near 2 km due to proportions of Pop. 1, chains, clusters Smooth plains are ~25% younger than plains on floor of Caloris; both post-date rim
Popula
tion 1
Population 2
Secondaries
Caloris plains ~25% older thanthe smooth exterior plains
Sample of MSGR cratered terrains more densely cratered than Mar. 10 avg.
Deficit w.r.t. Pop. 1 due to intercrater plains?
top related