Clark R. Chapman

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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
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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
Mercurys 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 Mercurys 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 lt2 km diam. from larger impacts
• Basin secondaries up to 30 km diam. (Wilhelms)
• Endogenic craters (volcanism, etc.)

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Terrestrial Planet Cratering (Robert Strom)

• Old Mercury, Mars, Moon similarbut
• Mars lt40 km diam. depleted by erosion, filling
  (climate)
• Mercury lt40 km depleted by intercrater
  plainsbut 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

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Role of Late Heavy Bombardment

• LHB (whatever its cause) probably cratered
  Mercury similarly to the Moon and Mars
• What happened beforeand afteris 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 Mercurys
cratered terrains?
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Possible Role of Vulcanoids

• Zone interior to Mercurys 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 gt1 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
  wouldnt negate earlier presence)
• Vulcanoids could have cratered Mercury after the
  Late Heavy Bombardment, with little leakage to
  Earth/Moon zone that would compress Mercurys
  geological chronology toward the present (e.g.
  thrust-faulting might be still ongoing)

?
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Images Suggesting Secondary Cratering on Mercury
Cluster?
Rays Secondaries 90m/pix Primary
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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 gt95 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!

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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
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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 gt10m diam

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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
• Mercurys surface is bombarded by micrometeorites
  and, periodically, by solar wind particles
• Optical properties (albedo and color) are
  modified (space weathering) rendering
  compositional inferences suspect

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Conclusion MESSENGER Will Help Resolve
Cratering Puzzles

• MESSENGERs 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 Mercurys dozens of basins (as Wilhelms
  believes is true for the Moon)?
• We should be cautious about tying Mercurys
  geological history to the lunar LHB and cautious
  about relative age-dating of smaller units
• Mercurys geology may be old, with
  contraction/compression closing off the surface
  from the internal activity below
• Or geology may be young, active today

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• The End

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Supplementary Slides Follow
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Mercury an extreme planet
Mercurys 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

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Mercury is Difficult (but Possible) to See for
Yourself
Tonight, Mercury is to the lower right of Jupiter
at dusk
http//messenger.ciw.edu/WhereMerc/WhereMercNow.ph
p

• 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 ituse a star
  chart to see where it is with respect to bright
  stars and planets
• Through a telescope, Mercury shows phases like
  the Moon

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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
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MESSENGERs Trajectory
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Is there or isnt there ferrous iron?Or is
Mercurys surface reduced?

• Putative 0.9µm feature appears absent
• Other modeling of color/albedo/near-to-mid-IR-spec
  tra 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
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Concluding Remarks

• MESSENGERs 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