Clark R. ChapmanSouthwest Research Inst.

Boulder, Colorado

Clark R. ChapmanSouthwest Research Inst.

Boulder, Colorado

MESSENGER Geology SubgroupMESSENGER Geology SubgroupTelecon: 11 June 2004Telecon: 11 June 2004

Repeated /Augmented for Repeated /Augmented for Science Team MeetingScience Team Meeting

KSC, Florida, 31 July 2004KSC, Florida, 31 July 2004

MESSENGER Geology SubgroupMESSENGER Geology SubgroupTelecon: 11 June 2004Telecon: 11 June 2004

Repeated /Augmented for Repeated /Augmented for Science Team MeetingScience Team Meeting

KSC, Florida, 31 July 2004KSC, Florida, 31 July 2004

Cratering on Mercury:Cratering on Mercury:New Thoughts about Secondary CrateringNew Thoughts about Secondary Cratering

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

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)

?

Images Suggesting Secondary Cratering on Mercury

Rays

Secondaries 90m/pix

Primary

Rays

Secondaries 90m/pix

Primary

Cluster?

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!

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)

Secondaries Dominate Mars(McEwen et al. 2004)

Zunil produced enough secondaries to account for 1 Myr of Neukum production function

Zunil may have made a billion craters >10m diam

“The Rayed Crater Zunil and Interpretations of Small Impact Craters on Mars”

Alfred S. McEwen, Brandon S. Preblich, Elizabeth P. Turtle, et al.

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

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.