Clark R. Chapman Southwest Research Inst. Boulder, Colorado, USA (member, MESSENGER Science Team)

1. Review of Mariner 10 Observations: Mercury Surface Impact Processes Clark R. Chapman Southwest Research Inst. Boulder, Colorado, USA (member, MESSENGER Science Team) Invited Oral Presentation Session PS02: The Exploration of Mercury 2nd Annual AOGS Meeting Singapore, 21 June 2005

2. Introduction to Cratering on Mercury • Only direct evidence is from Mariner 10 images of mid-70s (and recent radar) • Theoretical and indirect studies • Comparative planetology (Moon, Mars, …) • Calculations/simulations of impactor populations (asteroids, comets, depleted bodies, vulcanoids) • Theoretical studies of cratering mechanics, ejecta distributions, regolith evolution, etc. • Clearly, impact cratering dominates Mercury today, was important in the past • Impact processes range from solar wind and micrometeoroid bombardment to basin-forming impacts • MESSENGER will address cratering issues

3. Mercury’s Craters: Early Observations • Craters seen by Mariner 10 look superficially like Moon/Mars • But morphologies differ (high g, fewer erosive processes, etc.);see chapters by Spudis & Guest, Pike, and Schultz in Mercury (U. Ariz. Press) • Stratigraphy based on old Tolstoj and more recent Caloris basins • Recent, fresh craters affect albedo (e.g. rays)

4. Origins for Mercury’s Craters • Primary impact cratering • High-velocity comets (5x lunar production rate) • Sun-grazers, other near-parabolic comets • Jupiter-family comets • Crater chains may be solar-disrupted comets (Schevchenko & Skobeleva 2005, COSPAR) • Near-Earth, Aten, and Inter-Earth asteroids • Ancient, possibly depleted, impactor populations • Late Heavy Bombardment • Outer solar system planetesimals (outer planet migration) • Main-belt asteroids (planetary migration, collisions) • Trojans and other remnants of terrestrial planet accretion • Left-over remnants of inner solar system accretion • Vulcanoids (bodies that primarily impact Mercury only) • Secondary cratering • Craters <2 km diam. from larger impacts • Basin secondaries up to 30 km diam. (Wilhelms) • Endogenic craters (volcanism, etc.)

5. Terrestrial Planet Cratering (Robert Strom) • Old Mercury, Mars, & Moon similar…but: • Mars <40 km diam. depleted by erosion, filling (climate) • Mercury <40 km depleted by “intercrater plains”…but what are they? (Volcanic plains?) • Mercury “Post-Caloris” • Strom argues that shape is similar to highlands • Error bars are large; may be shallower • Recent cratering (Moon, Mars) horizontal • Strom interpretation • LHB produced highlands • NEAs made recent craters • Neukum interpretation: cratering population invariant in time and location

6. Role of ‘Late Heavy Bombardment’ The basin-forming epoch on the Moon (LHB) was of brief duration compared with the period when lunar rock ages were re-set, or the still longer period of bombardment apparently recorded in the HED meteorites (Bogard 1995). Chapman, Cohen & Grinspoon (2004) argue that the different histograms may reflect sampling biases. But if taken literally, the differences might instead mean that different populations of bodies and/or dynamical processes affected different planets. Was the lunar LHB responsible for Mercury’s cratered terrains? • LHB (whatever its cause) probably cratered Mercury similarly to the Moon and Mars • What happened before…and after…is not clear

7. Possible Role of Vulcanoids ? • Zone interior to Mercury’s orbit is dynamically stable (like asteroid belt, Trojans, Kuiper Belt) • If planetesimals originally accreted there, they may or may not have survived mutual collisional comminution • If they did, Yarkovsky drift of >1 km bodies in to Mercury could have taken several Gyr (Vokroulichy et al., 2000) and impacted Mercury alone long after LHB • Telescopic searches during last 20 years have so far failed to set stringent limits on current population of vulcanoids (but absence today wouldn’t negate earlier presence) • Vulcanoids could have cratered Mercury after the Late Heavy Bombardment, with little leakage to Earth/Moon zone; that would compress Mercury’s geological chronology toward the present (e.g. thrust-faulting might be still ongoing)

8. Images Suggesting Secondary Cratering on Mercury Cluster? Rays Secondaries 90m/pix Primary

9. Secondary Craters on Europa and the Moon) (Bierhaus et al., Nature, in press 2005) • From studies of spatial clustering and size distributions of ~25,000 craters on Europa, Bierhaus concludes that >95% of them (consistent with all of them) are secondaries! • Simple extrapolation to the Moon (if craters in ice behave as in rock) shows that secondaries could account for all small craters on the “steep branch” of the size-frequency relation!

10. Crater Production Function • Shoemaker first proposed steep branch as secondaries • Neukum (and most others eventually) considered it an attribute of primaries • Evidence from Europa and Mars now suggests Shoemaker was right after all • Another question: Big, secondaries from basins? (Wilhelms) “Secondary Branch” T.P. Highlands

11. Secondaries Dominate Mars(McEwen et al. 2005) “The Rayed Crater Zunil and Interpretations of Small Impact Craters on Mars” (Alfred S. McEwen, Brandon S. Preblich, Elizabeth P. Turtle, et al.,2005) • Zunil produced enough secondaries to account for 1 Myr of Neukum production function • Zunil may have made a billion craters >10m diam

12. Small and Microscale Impact and Regolith Processes • Potential ice deposits in near-polar shadows may be blanketed to some depth by regolith deposition • Competing processes of ice deposition, impact erosion, regolith deposition • Mercury’s surface is bombarded by micrometeorites and, periodically, by solar wind particles • Optical properties (albedo and color) are modified (“space weathering”) rendering compositional inferences suspect

13. Conclusion: MESSENGER Will Help Resolve Cratering Puzzles • MESSENGER’s high resolution will reveal many small craters (secondaries?) • Probably they will be less far-flung from their primaries than is true on Europa • Are multi-10s-of-km diameter craters secondaries from Mercury’s dozens of basins (as Wilhelms believes is true for the Moon)? • We should be cautious about tying Mercury’s geological history to the lunar LHB and cautious about relative age-dating of smaller units • Mercury’s geology may be old, with contraction/compression closing off the surface from the internal activity below • Or geology may be young, active today

14. The End

15. Supplementary Slides Follow

16. Mercury: an extreme planet Mercury’s size compared with Mars • Mercury is the closest planet to the Sun • Mercury is the smallest planet except for Pluto • Mercury is like a “Baked Alaska”: extremely hot on one side, extremely cold at night • Mercury is made of the densest materials of any planet: it is mostly iron

17. Mercury is Difficult (but Possible) to See for Yourself Tonight, Mercury is to the lower right of Jupiter at dusk • Mercury is visible several times a year • just after sunset (e.g. tonight, but it will be tough!) • just before sunrise (the week after Labor Day weekend is best); Mercury will be near Regulus in Leo • It is always close to the Sun, so it is a “race” between Mercury being too close to the horizon and the sky being too bright to see it…use a star chart to see where it is with respect to bright stars and planets • Through a telescope, Mercury shows phases like the Moon http://messenger.ciw.edu/WhereMerc/WhereMercNow.php

18. MESSENGER: A Discovery Mission to Mercury MErcury Surface, Space ENvironment, GEochemistry and Ranging • MESSENGER is a low-cost, focused Discovery spacecraft, built at Johns Hopkins Applied Physics Laboratory • It will be launched within days • It flies by Venus and Mercury • Then it orbits Mercury for a full Earth-year, observing the planet with sophisticated instruments • Designed for the harsh environs Important science instruments and spacecraft components

19. MESSENGER’s Trajectory

20. Is there or isn’t there: ferrous iron?Or is Mercury’s surface reduced? • Putative 0.9μm feature appears absent • Other modeling of color/albedo/near-to-mid-IR-spectra yield FeO + TiO2 of 2 - 4% (e.g. Blewett et al., 1997; Robinson & Taylor, 2001) Warell (2002): SVST data (big boxes) compared with earlier spectra Vilas (1985): all glass

21. Concluding Remarks • MESSENGER’s six science goals • Why is Mercury so dense? • What is the geologic history of Mercury? • What is the structure of Mercury's core? • What is the nature of Mercury's magnetic field? • What are the unusual materials at Mercury's poles? • What volatiles are important at Mercury? • But I think that serendipity and surprise will be the most memorable scientific result of MESSENGER • The history of past planetary spacecraft missions teaches us to expect surprise • MESSENGER has superb instruments, it will be so close to Mercury, and it will stay there a full year