Clark R. Chapman ( SwRI ), R.G. Strom, C.I. Fassett, L.M. Prockter, J.W. Head III, S.C. Solomon, M. E. Banks, D. Baker,

1. Clark R. Chapman (SwRI), R.G. Strom, C.I. Fassett, L.M. Prockter, J.W. Head III, S.C. Solomon, M. E. Banks, D. Baker, W.J. Merline Cratering on Mercury: Insights from the MESSENGER Flybys 73rd Annual Meeting of the Meteoritical Society #5325, 11:45 a.m., Tuesday, 27 July 2010 New York City, NY USA

2. Meteoritical Context for Studying Craters on Mercury • Other Issues Addressed by Mercury’s Craters • Large craters penetrate deep within the crust, revealing layers of different, deep geological units. • Crater densities (esp. of primary craters) reveal stratigraphic sequences in emplacement of geological units expressed on the surface. • Cratering physics: why does basin morphology begin for smaller craters on Mercury than on other bodies like the Moon? • Chronology. Was the history of cratering on Mercury the same as on the Moon and Earth? • Impactor Populations. Was the same mix of asteroids and comets responsible for Mercury cratering, or was there a later bombardment by “vulcanoids”? • Style of Cratering on Mercury. Does the higher impact velocity on Mercury or other factors (e.g. those responsible for the prominence of large secondary craters on Mercury) help or hinder the launch of meteorites from Mercury that could reach the Earth? • Composition. Does the variety of compositional units revealed by penetration of Mercury’s crust by large craters provide clues about how to identify meteorites that might have come from Mercury?

3. Mercury has Many Basins! • Mercury has many more basins than the Moon • Yet the spatial density of large basins per unit area is about the same as the Moon • This apparent contradiction is resolved by realizing that basin morphology appears at a considerably smaller diameter on Mercury than on the Moon • Data are (so far) from images with limited lighting and viewing geometries. Once we have good global coverage, and global laser altimetry, we can search for large quasi-circular features, like those found on Mars and the Moon Rembrandt

4. There are Multiple Potential Sources for Mercury’s Craters • Primary craters from the same populations of Near Earth Asteroids (NEAs) and comets that crater the Earth, Moon, Mars, and Venus today. • Primary craters from the same population/s that caused the Late Heavy Bombardment (LHB) on the Moon ~3.9 Ga. (These may have been main-belt asteroids and outer solar system planetesimals, if the Nice Model is correct. Or other collisionally and dynamically processed remnants from accretion.) • Vulcanoids (remnants of hypothetical population of planetesimals interior to Mercury’s orbit, which would not have appreciably cratered other planets). • Secondary craters from larger craters and basins. • Endogenic craters (volcanic vents, subsidence craters, etc.) • Other rare causes (e.g. tidally split SL9-like bodies)

5. Our Working Hypothesis • Mercury was saturated with craters and basins during the Late Heavy Bombardment by the same population of impactors that struck the Moon. • Mercury has since been cratered by the same population of NEAs that still crater other inner solar system bodies. • Many-to-most craters <10 km diameter on widespread plains units are secondary craters. • Endogenic craters are a small but important contribution to negative relief features.

6. Interpretational Framework: Cratering Components 25

7. Populations 1 and 2 (R. Strom) Schematic diagram of possible LHB scenario • Lunar highlands are prototype for Pop. 1 produced by LHB • Young Mars plains are prototype for Pop. 2 (current NEAs derived by size-dependent Yarkovsky plus resonances from main belt) plus secondaries at D<1 km • Caloris exterior plains are dominantly Pop. 2 (plus large secondaries D<10 km)

8. Caloris Basin Relative Ages Calorisis a relatively young basin, with about half the density of superimposed craters as the general cratered highlands of Mercury, but it was cratered during the LHB, while the interior/exterior plains mainly post-date the LHB. Important result: If exterior plains are even younger than the Caloris interior plains, then they are certainly volcanic flows. Thus Mariner interpreta-tions of the knobby textured Odin Formation as Cayley-Plains-like Caloris ejecta are wrong. Caloris Basin

9. Variability of Intercrater Plains • Bottom panel shows MESSENGER crater spatial densities (R values) in a part of Mercury that resembles the average highlands measured from favorably observed regions by Mariner 10. The distinct deficiency of craters on Mercury <30 km diameter was ascribed to “intercrater plains”. • New studies of other regions show considerably variability. Top panel shows deficiency extending to craters 150 km in diameter, implying a thick sequence of intercrater plains (i.e. volcanism). • Middle panel shows a modest deficiency extending to 100 km diameter, but a prominent secondary crater branch appears at an unusually large diameter, ~20 km.

10. “Twin” Young Basins on Mercury • Both basins ~250-300 km diam. • Similar inner peak rings • Lightly cratered floors with circumferential extensional troughs • Similar rim morphologies Raditladi Basin Seen on M1 Flyby Rachmaninoff Basin Revealed on M3 Flyby

11. A Closer Look at the Recently Discovered Rachmaninoff Basin • Compare very low crater density inside peak ring with slightly higher crater density between peak ring and rim • Lighter colored interior floor has breached peak ring on the bottom • Both basins have fairly young ejecta blankets and many surround-ing secondary craters (next slide)

12. Ejecta and Secondary Craters of Raditladi and Rachmaninoff…and a Recently Volcanically Active Region Raditladi Basin Rachmaninoff Basin Note “orange” color within peak ring, like other young volcanic plains on Mercury. Also note the proximity of Rachmaninoff to what may be a large volcanic vent (in the very bright region northeast of the basin). 100 km

13. Relative Ages: Basin Rims and Plains within Basins Note: Very low crater densities and small areas of counting units cause poor statistics…but it’s the best we can do! • (A) Inner plains and annular plains of Rachmaninoff: Inner plains are clearly younger than annular plains, but apparently older than Raditladi plains (but size distribution is not the same shape, confusing the comparison) • (B) Rachmaninoff rim and ejecta suggests an older basin formation age than for Raditladi

14. Basins and Plains: Approximate Relative Stratigraphy by Crater Density Relative Crater Density(varies by factor >30!) • 1.0: Highlands craters • 0.5: Caloris rim = Rembrandt rim • 0.35: Floor of Rembrandt • 0.2: Interior Caloris plains (volcanic) • 0.15: Caloris exterior plains (volcanic) • 0.1 Rachmaninoff basin = annular plains • 0.05 Rachmaninoff inner plains • 0.03: Floor of Raditladi = rim of Raditladi (is floor impact melt prompt volcanism?)

15. Mercury’s Absolute Chronology: Raditladi Example (applying lunar chronology) • Sequence: Heavily cratered highlands → Caloris basin → Exterior plains → Raditladi basin/plains • If lunar chronology applies, then • If exterior plains formed early (3.9 Ga), then Raditladi is 3.8 Ga (red arrows) • If smooth plains formed ~3.75 Ga then Raditladi’s age is <1 Ga! (green arrows) Preferred!

16. Two Chronologies for Mercury Age before present, Ga 4.5 4 3.5 3 2.5 2 1.5 1 0.5 NOW Formation to magma ocean/crustal solidification CALORIS Bombardment, LHB, intercrater plains formation Smooth plains volcanism Raditladi Cratering, rays Lobate scarps, crustal shortening Classical (Lunar) Chronology Vulcanoid Chronology Example Formation to magma ocean solidification CALORIS Bombardment, LHB Vulcanoid bombardment, intercrater plains Smooth plains volcanism Raditladi Cratering, ray formation Lobate scarps, crustal shortening

17. Conclusion: We must wait for orbital mission for good stratigraphic studies • Mariner 10 imaged 45% of surface? (I don’t think so.) • MESSENGER has almost completed coverage? Not YET for robust geological analysis Mariner 10 Image & Shaded Relief MESSENGER image

18. Abstract Introduction: During its three Mercury flybys, MESSENGER imaged most regions unseen by Mariner 10 and viewed some previously seen regions under more favorable lighting. The surface density of impact craters and basins on Mercury with diameters D>200 km is comparable with that of the Moon, though possibly there are fewer large basins. The largest basin mapped from Mariner 10 (Borealis) has not been reliably recognized in MESSENGER images. Two smaller peak-ring basins (Raditladi and Rachmaninoff) are comparatively young. Large craters and basins have numerous secondary craters, which generally dominate Mercury’s crater populations at D<10 km. Extensive volcanism apparently modified Mercury’s crater populations at D <100 km, to variable degrees in different regions, but was as powerfully destructive of craters D<40 km as the many degradation processes that affected Martian highlands. Large Craters and Basins: The morphologies of dozens of peak-ring basins have illuminated the transition from smaller complex craters to basins. Caloris and Rembrandt basins are fairly well preserved and formed during the later part of the Late Heavy Bombardment (LHB); craters on their rims follow the Population-1 size-frequency distribution (SFD) characteristic of LHB cratering throughout the terrestrial planet region (believed to be the result of direct scattering of main-belt asteroids). Volcanic plains formation within Caloris ended well after the basin formed, close to the end of the LHB: its interior plains are dominated by the later Population-2 craters typical of near-Earth asteroids today, chiefly derived from the main belt by size-dependent processes such as the Yarkovsky effect. Volcanic plains formation exterior to Caloris continued afterwards, based on a lower density of almost purely Population-2 craters. These plains clearly postdate formation of the Caloris basin by a substantial interval and are not ejecta deposits like the lunar Cayley Plains, as had been hypothesized after Mariner 10. SFD’s for Mercury’s craters with D>10 km in various cratered regions of Mercury differ widely, more than was appreciated from Mariner 10. In some regions, voluminous intercrater plains obliterated all craters with D>100 km, whereas elsewhere plains buried only smaller craters so that many with D>40 km remain from older eras. Intercrater plains and younger, often more spatially restricted, smooth plains both formed by volcanic emplacement. Small Craters, Secondaries, and Young Plains: In some places (e.g. in regions near Raditladi) Mercury’s craters are dominated by secondaries for D<20 km. In general, the upturn of the SFD at smaller sizes occurs at D<8 km, a much larger diameter than the few km typical on the Moon and Mars. Perhaps larger secondaries are formed on Mercury than on other bodies. The temporally sporadic and spatially clustered nature of secondaries hinders studies of relative ages of small and/or recent units. Nevertheless, the extremely sparse densities of small craters within Raditladi and Rachmaninoff suggest that these basins are unusually young. In the case of Rachmaninoff, volcanism continued within its inner plains until comparatively recently, long after basin formation, and thus those plains cannot be impact melt.