Transcript www.boulder.swri.edu

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
Introduction to Cratering on
Mercury
 Only direct evidence is from Mariner 10
images of mid-70s (and recent radar)
 Theoretical and indirect studies
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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 basinforming 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
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High-velocity comets (5x lunar production rate)
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Sun-grazers, other near-parabolic comets
Jupiter-family comets
Crater chains may be solar-disrupted comets (Schevchenko
& Skobeleva 2005, COSPAR)
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Near-Earth, Aten, and Inter-Earth asteroids
Ancient, possibly depleted, impactor populations
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Late Heavy Bombardment
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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
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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:
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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”
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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’
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
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
Cluster?
Rays
Secondaries
Primary
90m/pix
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
T.P. Highlands
Europa and Mars
now suggests
Shoemaker was
right after all
 Another question:
Big, secondaries
from basins?
(Wilhelms)
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
Small and Microscale Impact
and Regolith Processes
 Potential ice deposits in near-
polar shadows may be
blanketed to some depth by
regolith deposition
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Competing processes of ice
deposition, impact erosion, regolith
deposition
 Mercury’s surface is bombarded
by micrometeorites and,
periodically, by solar wind
particles
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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?)
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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 agedating 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
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 The End
Supplementary Slides Follow
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
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.php
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Mercury is visible several
times a year
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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
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
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
Vilas (1985): all glass
Concluding Remarks
 MESSENGER’s six science goals
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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
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