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?