[meteorite-list] Jeff Bell's Eros Thesis

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:11 2004
Message-ID: <200102202342.PAA18842_at_zagami.jpl.nasa.gov>

Forwarded from David Morrision (dmorrison_at_mail.arc.nasa.gov)

Jeff Bell's Eros Thesis

Dear Friends and Students of NEOs:

This is an unusual edition of NEO News. The NEAR-Shoemaker mission
has been a tremendous success, and the spacecraft continues to
collect data on the surface of the asteroid. We now know that this
asteroid is a monolithic (or nearly so) rock with relatively
primitive composition, with a surface sculpted by innumerable
impacts. But the size distribution of the craters on Eros is
different from anything we have seen before, with a remarkable
deficit of craters with diameters below 100 m, as well as a great
many rocks and boulders on the surface. Most of the interpretations
discussed by the NEAR science team involve the filling in of the
small craters by surface dust (a mobile regolith). This is the
"orthodox" opinion. What is printed below is an unorthodox new
interpretation by Jeff Bell of the University of Hawaii, submitted to
the Lunar and Planetary Science Conference (LPSC) to be held in
Houston next month. I certainly would not claim that Jeff is right,
but I do think his ideas are provocative and interesting, and that
they should contribute to a spirited technical debate about the
geological history of both Near Earth and Main Belt Asteroids.

David Morrison

=======================================================

EROS: A COMPREHENSIVE MODEL. Jeffrey F. Bell,
Hawaii Institute of Geophysics and Planetology, Univ. of
Hawaii, 2525 Correa Rd., Honolulu HI 96822
(bell_at_pgd.hawaii.edu).

Introduction: The NEAR spacecraft has provided a
variety of information about the planet-crossing S-class
asteroid Eros. However, most interpretations to date
have relied heavily on earlier experience on the Moon
and asteroids which were viewed only distantly during
spacecraft flybys. The close approach of NEAR to Eros
in October 2000 revealed surprising new facts that
suggest that impact processes and regolith evolution on
asteroids are very different from any other planetary
objects.

Craters and Boulders: Early crater abundance
curves [1] based on low-resolution NEAR images
suggested a "normal" crater curve in the 1000m to
100m diameter range, i.e. a high crater density, rising
toward saturation levels at the smaller sizes. Later
NEAR images from closer distances have revealed an
extraordinary crater distribution: the crater density
declines sharply below 100m until at 4m diameter the
craters are about 200 times less abundant than expected.
Post-cratering modification seems inadequate to explain
this situation. In particular, any geological process such
as regolith migration that obscures craters should also
obscure boulders; yet the size spectrum of boulders on
Eros shows an inverse correlation with the craters,
being highly biased toward smaller sizes. It seems much
more likely that this "anomalous" crater distribution
reflects the actual size spectrum of incoming projectiles.
This must be very different from the normal inner solar
system projectile size spectrum we see reflected in the
crater records of the Moon, Mercury, and Mars. I
propose that Eros exhibits the normal cratering function
on main-belt asteroids, previously concealed from us by
the low resolution of the images obtained in distant
flybys of Gaspra, Ida, and Mathilde.

Yarkovsky to the Rescue: The Yarkovsky Effect
[2] is a mechanism for orbital evolution due to asymmetric
emission of thermal IR photons from a rotating
object that is warmer on the "afternoon" region than on
the "morning" region. This effect is strongest for objects
around a few meters in size (the force/mass ratio is too
small for larger objects, while smaller ones cannot
maintain the necessary morning/afternoon thermal
assymetry due to internal conduction). It has been
proposed [3,4] that the Yarkovsky Effect provides an
efficient mechanism for moving meteorites from any
location in the asteroid belt to the narrow Jupiter
resonance zones, from which they are rapidly perturbed
out of the belt. If so, the asteroid belt should be strongly
depleted in objects smaller than a few meters, which
would naturally produce a strong depletion in craters
smaller than about 100m on all main-belt objects. Since
the collisional cascade in the belt is prematurely cut off
by the Yarkovsky Effect, even particles too small to be
directly affected will be underabundant. Since there is
a population of dust derived from asteroids (e.g. the dust
bands associated with the Hirayama families), there
must be some mechanism for limited replenishment of
very small particles, probably direct generation of dust
from larger objects.

The underabundance of small impactors also provides
a natural explanation for the size spectrum of
boulders on Eros. Boulders are created by ejection from
larger impacts and gradually eroded by smaller ones.
This can be easily seen on the Moon, where fields of
jagged boulders are seen around fresh craters. The
absence of smaller impactors allowed boulders to
accumulate on Eros without being broken up or eroded.
The fact that this unique signature of the main-belt
environment is still visible on Eros implies that it has
undergone little cratering since its orbital evolution
decoupled it from the asteroid belt. In its current orbit,
it should be experiencing a roughly lunar-like bombar-
dment environment. There is no trace of this late phase
of Eros history on its surface.

Elemental Abundances: The increasing abundance
of boulders visible in the close-approach images suggests
that at sizes somewhat smaller than the current
resolution limit, the surface may be mostly covered with
rocks. This implies that the published X-ray data [5]
(which samples material to ~100 microns depth) is
mostly sampling the outer layers of large rocks, not
fine-grained weathered regolith as expected before the
mission [6]. (In meteoritical terms, the data mostly
samples clasts instead of matrix) The XGRS team has
suggested that impact volatilization of sulfur in the
weathered regolith could account for the grossly
nonchondritic level of sulfur (<1%) observed in the X-
ray data. It appears more likely that their alternate
hypothesis of early partial differentiation (in which
sulphur is mobilized early) is the correct explanation.
Gamma-ray data, with a sampling depth of ~10cm,
should almost entirely represent weathering-free rock
interiors and resolve this ambiguity.

Color and Spectrum: A striking "anomaly" on
Eros is the close similarity of all areas on the surface in
color. While there are many regions of higher albedo in
exactly the locations we expect to find fresh,
unweathered material (steep slopes where downslope motion is
likely), the spectra curves of these areas are very similar
to those of darker (presumably older) areas. In particular,
there is no region that looks at all like ordinary
chondrites, the most publicized meteorite analog for
Eros. The advocates of a primitive Eros have been
forced to argue that "space weathering" is so rapid there
that even the most recent crater interiors, ejected blocks,
and landslides have been weathered almost to maturity.
Little attention has been given to the alternate hypothesis:
that even the oldest surfaces are so young and
immature that they are almost identical to the youngest
craters and slumps. This is the logical result of a
Yarkovsky-controlled main-belt bombardment
environment: an intense (~1000 times lunar) bombardment of
large, low-velocity (~6 km/sec) projectiles constantly
excavating fresh bedrock, and completely swamping the
weathering (impact melting and volatilization) effects
produced by the small component of high-velocity dust
entering the belt from other sources (mostly long-period
comets).

Of course, in the time since Eros became decoupled
from the asteroid belt, it has been in a more familiar,
approximately lunar, bombardment environment. But
spectral weathering effects on the moon simply do not
occur fast enough to account for the absence of any
chondrite-like areas on Eros. Planet-crossing asteroids
are ephemeral phenomena. On average, their survival
time against sun impact or ejection by Jupiter encounters
is 5-10My [7]. From the limited orbital modeling
that has been done for Eros, it appears that it may
currently be in a special dynamical environment that
would lengthen its lifetime to perhaps 50-100My [8].
Even this is about a factor of 10-20 too short. On the
Moon, we know that crater Tycho (age 110My) is almost
pristine spectroscopically, while Copernicus (age
~800My) is well along toward maturity. Unless Eros
has been specially preserved for ~1By in some "cosmic
lockbox", it has not been out of the belt long enough for
its spectral properties to reflect its new environment.

Summary: The history of Eros may be summarized as:
1) Condensation and accretion of a larger
parent body; 2) Heating and limited partial melting; 3)
Migration of the sulfur-rich melt to another region of
the parent body; 4) Progressive collisional fragmentation
of the parent body in which the current shape and
surface of Eros was produced; 5) Intense cratering in
the asteroid belt by a projectile population strongly
depleted in small objects by the Yarkovsky Effect; 6)
Recent perturbation onto a planet-crossing orbit; 7)
Short exposure to more lunar-like bombardment and
solar-wind environments which have had insufficient
time to significantly alter the surface.

References:

[1] Veverka J. et al. (2000) Science, 289, 2088-2097.
[2] Opik E. J. (1951) Proc. R. Irish Acad., 54, 165.
[3] Hartmann W. K. et al. (1997) LPS XXVIII, 517.
[4] Farinella P. et al. (1998) Icarus, 132, 378.
[5] Trombka J. I. et al. (2000) Science, 289, 2101.
[6] Bell J. F. (1997) LPS XXVIII, 83-84.
[7] Gladman et al. (1997) Science, 277, 197.
[8] Michel et al. (1998) Astron. J., 116, 2023.

-----------------------------------------------------

ADDED COMMENTS AFTER NEAR LANDING. (Jeffrey F. Bell, Univ. of Hawaii)

       The model for Eros described in my LPSC abstract (above) has
been fully confirmed by the high resolution images acquired during
the "landing phase" of the NEAR mission on Feb. 12. These reveal a
surface dominated by rocks, so many in some areas that the surface is
nearly saturated with cm- to m- sized rocks. There is no sign of
small craters or "zap pits" on the rocks, and most of them appear
angular and un-eroded, indicating that the population of mm- to cm-
sized projectiles in the asteroid belt is still depleted well below
the 1 to 100 meter size range directly affected by the Yarkovsky
Effect. This suggests that "filling-in" of the size spectrum below
the Yarkovsky Gap by further collisional evolution is negligible.
Furthermore, it is now clear that a large fraction of the X-ray
photons detected by the XGRS instrument came from solid rock
surfaces, not fine-grained "weathered" regolith. The >90% depletion
of sulfur abundance relative to chondrites observed by this
instrument cannot be explained by weathering processes and must be
intrinsic to the bedrock of Eros, implying a significant degree of
partial melting and migration of S (and Fe?) to some other part of
the Eros Parent Body.
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Received on Tue 20 Feb 2001 06:42:16 PM PST