Clark R. Chapman Southwest Research Inst. Boulder, Colorado

1. Cratering on Mercury: New Thoughts about Secondary Cratering Clark R. Chapman Southwest Research Inst. Boulder, Colorado MESSENGER Geology Subgroup Telecon: 11 June 2004 Repeated /Augmented for Science Team Meeting KSC, Florida, 31 July 2004

2. Mercury’s Craters: Introduction • Craters seen by Mariner 10 look superficially like Moon/Mars • But morphologies differ (high g, fewer erosive processes, etc.) • Origins of craters • Near-Earth/Inter-Earth asteroids • Comets (some focused toward Sun) • Vulcanoids (hypothetical: could have cratered Mercury post-LHB & moved observable history closer to the present, which would be compatible with still-active interior) • Secondary cratering

3. Possible Role of Vulcanoids ? • Zone interior to Mercury’s orbit is dynamically stable (like asteroid belt, Trojans, Kuiper Belt) • If planetesimals originally accreted there, it is unclear if they survived mutual collisional comminution • Searches during last 20 years have so far failed to set stringent limits on current population of vulcanoids • Vulcanoids could have cratered Mercury after the Late Heavy Bombardment, with little leakage to Earth/Moon zone; would compress Mercury’s geological chronology toward the present (e.g. thrust-faulting might be ongoing)

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

5. Secondary Craters on Europa(and the Moon) (Beau Bierhaus PhD, 2004) • 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!

6. 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 suggest Shoemaker was right after all • Another question: Big, secondaries from basins? (Wilhelms)

7. Secondaries Dominate Mars(McEwen et al. 2004) “The Rayed Crater Zunil and Interpretations of Small Impact Craters on Mars” Alfred S. McEwen, Brandon S. Preblich, Elizabeth P. Turtle, et al. • Zunil produced enough secondaries to account for 1 Myr of Neukum production function • Zunil may have made a billion craters >10m diam

8. Conclusion: Uncertain Roles of Vulcanoids & Secondaries • 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

9. MESSENGER Geology Discipline Group31 July 2004 (per Jim Head) • a) lunar calibration issues and sequence. • b) coordination of MDIS observations with VIRS. • c) coordination of science analysis with other instruments. • d) Venus flyby science. • e) Mercury encounter planning, coordination and analysis. • f) Mercury orbit planning, coordination and analysis. • g) preparatory science analyses. • h) science topics for future GDG telecons.