Clark R. Chapman (SwRI) , R.G. Strom (Univ. Ariz.),

1. Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering MESSENGER Perspectives on Mercury’s Cratering Clark R. Chapman (SwRI), R.G. Strom (Univ. Ariz.), S.C. Solomon (DTM, Carnegie Institution),J.W. Head III (Brown Univ.), and W.J. Merline (SwRI) with thanks to the whole MESSENGER Team AAS Division for Planetary Sciences Cornell University, Ithaca NY 14 October 2008

2. “Mariner 10 Imaged 45% of Surface.” Vivaldi Crater: Then and Now Mariner 10 Image & Shaded Relief MESSENGER image

3. Vivaldi Crater at Sunset & Sunrise M1 M2

4. Long, Linear Secondary Crater Chains Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering • 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!

5. Long, Linear Rays (M2 Flyby) Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering • 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

6. Basins on Mercury Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering Caloris Basin, MSGR M1 + Mariner 10 New Basin, MSGR M2 + M1

7. Double-Ring Basin Raditladi

8. Questions about Cratering on Mercury: Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering • 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.

9. 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

10. 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

11. 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

12. 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 MESSENGER M1 Exterior Plains Interior Plains Exterior Plains Mariner 10

13. Caloris Interior Plains ~25% Older than Exterior Plains Clark R. Chapman, et al. MESSENGER Perspectives on Mercury’s Cratering

14. Caloris Basin Cratering Stratigraphy Important issue raised by these results: If exterior plains are volcanic, then interpretation of knobby texture of Odin Formation as Cayley-Plains-like Caloris ejecta may be wrong • 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

15. 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 Population 1 Sample of MSGR cratered terrains more densely cratered than Mar. 10 avg. ~25% older than Caloris plains the smooth exterior plains Population 2 Secondaries Deficit w.r.t. Pop. 1 due to intercrater plains?