What we Know (and Don’t Know)
about Asteroid Surfaces
Clark R. Chapman
Southwest Research Institute
Boulder, Colorado, USA
Workshop on Scientific Requirements
for Mitigation of Hazardous Comets
and Asteroids
Arlington VA Rosslyn Hyatt, 3 Sept. 2002
Surfaces: Aspects Relevant
for Mitigation Issues
 There is much of scientific interest about
asteroid surfaces, but here I concentrate on
aspects relevant to mitigation and other
human-scale interactions with asteroids.
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Are surfaces solid so we could anchor to them?
Are regoliths deep or thin, patchy or ubiquitous?
Are surfaces covered with huge boulders? Dusty?
How reliably do remote-sensing observations and
pictures tell us about interior properties?
Are asteroid surfaces likely to be similar or the same,
or must we study the particular body to be deflected?
In particular, is our knowledge from Eros relevant to
NEOs maybe a million times less massive?
Deflection Options
 Attach a thrusting device: e.g. powerful
rocket, ion engine, solar sail, mass driver
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Practical issues: e.g. how to you get through
regolith to “bedrock”?
Is there real bedrock, so that if you try to drag or
push the asteroid while anchored at one place, the
whole thing will move? (What is internal
structure?)
 Make the object thrust itself: e.g. stand-off
neutron bomb evaporates surface, vaporize
ice within a comet so it develops a strong,
“nongravitational” thrust, Yarkovsky effect
What is surface layer like? How will it respond?
 What is structure of surface/interior layers that
will shape the body’s own thrust?
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Sources of Information about
Asteroid Surfaces
 Telescopes. Hemispherically
averaged, ambiguous info on particle
size, mineralogy, and not much else
 Radar. Roughness on a human-scale
and reflectivity (ice, metal, rock…),
especially for NEAs that pass close
 Inferences from Meteorites. Better
than nothing, but meteorites are not
regolith samples
 Spacecraft. Only a few studied to
date; best data by far for Eros (NEAR)
“Ponds” from Low-Altitude Flyover
“Ponds”, “Beaches”
& Debris Flows
 “Ponds” are flat, level, and are
sharply bounded
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“Beaches” (not always seen)
surround some ponds and are
relatively lacking in either craters or
boulders
 Although stratigraphically younger,
ponds may have more small craters
than typical terrains, suggesting that
boulders may armor crater
production
 How are ponds formed?
Electrostatic levitation, seismic
shaking? If mass-wasting, why
don’t lunar ponds exist?
Upper smooth area may be shaped
by “debris flow” mechanics
(Cheng et al., Aug. 2002 MAPS)
NEAR-Shoemaker’s Landing
Spot on Eros
Inset shows Himeros
Estimated positions of
last images end within a
50 meter diameter crater,
which may have a “pond”
on its floor
 How typical is the edge of Himeros of Eros?
 How typical is Eros of other asteroids?
Fifth Last Image
(largest
boulders are 3 meters across)
Measuring (Big) Craters and
(Small) Boulders
Sparse craters,
tens to hundreds
of meters across,
are measured in
whole image
 Boulders, mostly
less than 15
meters across, are
more-than-well
sampled in onequarter of image
 This image is from
NEAR-Shoemaker
Low Altitude
Flyover (10/00)
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Eros is Covered with Rocks
Final Landing Mosaic
Closest Image of Eros
The Relative Plot (R Plot)
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Shows spatial densities of craters
as function of size relative to
saturation
R Plot: Eros Craters & Boulders
Eros is NOT Like the Moon!
Eros has rocks.
The Moon has craters.
Eros/Moon
Comparisons
The Surface of Eros is NOT
like the Lunar Regolith!
 Ejecta is very widespread on
Eros, much lost to space, few generations of churning
 Lunar ejecta is repeatedly churned in situ, becomes very mature
 Rocks (ejecta blocks from far-away large impacts and
exhumed from below) remain in place, cover the surface
of Eros
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Lunar rocks are fragmented and eroded; surface is covered by craters
 Flat, pond-like deposits (of fines) common in
depressions -- few rocks or craters
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Electrostatically levitated dust on Moon does not form ponds, at least
not commonly
Other Asteroids and Inexact
Asteroid Analogs (Mars Moons)
Ida
Mathilde
Gaspra
(Eros)
Phobos
and
Deimos
Mathilde and Its Huge Craters
C-type asteroids may be
very different places, at
all scales, compared
with what we have
found at Eros
“Steep”, Undersaturated Size
Distribution for Gaspra Craters
Ida Craters Saturated < 1 km Diameter
Ida Looks Much Like the Moon
(and Eros)...
…at scales of 100’s of meters and larger, but it may be
just like Eros (and not like the Moon) at human scales
Dactyl: Best Analog for an
NEO we May Want to Deflect?
 Dactyl: 1.6 km diam.
 1st asteroidal moon
 Sub-spherical shape:
broke up, reaccreted?
 Up to 29 craters: satur-
ated? Crater chain?
We can’t
tell what
Dactyl’s
surface is
really like,
even if it
were
relevant!
False color image
Asteroid Surfaces: Comparisons
 GASPRA Big craters absent (except
“facets”?); small craters undersaturated.
Young and/or made of strong metal, not rock.
 IDA Saturated with large craters. Old,
lunar-like megaregolith (2-chunk rubble pile?);
small-scale surface like Eros. Anchor to what?
 MATHILDE Supersaturated by giant craters
(small scales unknown). Low-density
materials and/or voids, perhaps compressible
or loosely bound. Analogs: mud, sand,
styrofoam?
 EROS Shattered shard, only source of data
at hi-res scales. Amazing! Surface character
Conclusions: We Need to
Learn More About Asteroids!
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Are surfaces solid so we could anchor to them? Not for those
we’ve seen, but small NEAs could well be “bare rocks”.
Are regoliths deep or thin, patchy or ubiquitous? Interiors could be
megaregoliths; surface regoliths (if they exist) could be rocky or
dusty, but lunar regolith is a poor analog.
Are surfaces covered with huge boulders? Dusty? Probably.
How reliably do remote-sensing observations and pictures tell us
about interior properties? Better than nothing, but very poorly.
Are asteroid surfaces likely to be similar or the same, or must we
study the particular body to be deflected? Great diversity;
studying them helps us prepare for the unexpected, but we must
study the particular body, if at all possible.
In particular, is our knowledge from Eros relevant to NEOs maybe a
million times less massive? Not very. NEOs are smaller than Eros
just as much as Eros is smaller than the Moon.
We must study: (a) diversity of surfaces; (b) interiors