Clark R. ChapmanSouthwest Research Inst.Boulder, Colorado, USA

(member, MESSENGER Science Team)

Clark R. ChapmanSouthwest Research Inst.Boulder, Colorado, USA

(member, MESSENGER Science Team)

Invited Oral PresentationInvited Oral PresentationSession PS02: The Exploration of MercurySession PS02: The Exploration of Mercury

22ndnd Annual AOGS Meeting Annual AOGS MeetingSingapore, 21 June 2005Singapore, 21 June 2005

Invited Oral PresentationInvited Oral PresentationSession PS02: The Exploration of MercurySession PS02: The Exploration of Mercury

22ndnd Annual AOGS Meeting Annual AOGS MeetingSingapore, 21 June 2005Singapore, 21 June 2005

Review of Mariner 10 Observations: Mercury Surface Impact Processes

Review of Mariner 10 Observations: Mercury Surface Impact Processes

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

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)

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.)

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

Role of ‘Late Heavy Bombardment’

LHB (whatever its cause) probably cratered Mercury similarly to the Moon and Mars

What happened before…and after…is not clear

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?

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)

?

Images Suggesting Secondary Cratering on Mercury

Rays

Secondaries 90m/pix

Primary

Rays

Secondaries 90m/pix

Primary

Cluster?

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!

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

Secondaries Dominate Mars(McEwen 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

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

(Alfred S. McEwen, Brandon S. Preblich, Elizabeth P. Turtle, et al.,2005)

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

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

The End

Supplementary Slides Follow

Mercury: an extreme planet

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

Mercury’s size compared with MarsMercury’s size compared with Mars

Mercury is Difficult (but Possible) to See for Yourself

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

Tonight, Mercury is to the lower right of Jupiter at dusk

MESSENGER: A Discovery Mission to Mercury

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

MErcury Surface, Space ENvironment, GEochemistry and Ranging MErcury Surface, Space ENvironment, GEochemistry and Ranging

Important science instruments and spacecraft components

MESSENGER’s Trajectory

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

Warell (2002): SVST data (big boxes) compared with earlier spectra

Vilas (1985): all glassVilas (1985): all glass

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