Crater Chains on the Earth and Moon
Summary of Paper (Icarus, in press):
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Recent reports suggest that crater chains exist on Earth, possibly formed by
weak asteroids or comets tidally stretched apart by the Moon (analogous to
comet Shoemaker-Levy 9 at Jupiter) with the resulting fragment trains
proceeding directly to strike the Earth. By modeling tidal disruption by the
Earth and Moon of particulate bodies held together by self-gravity, we find
that the formation rate of crater chains on the Moon is ~10 times the
corresponding terrestrial rate. The number of known lunar crater chains
(~1) and the relative youth of the Earth's surface together suggest that
terrestrial crater chains, if they exist, form by another process.
Possible Crater Chains on the Earth?
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A crater chain is a regularly spaced row of three or more impact craters with
similar sizes and apparently identical ages. A crater chain is formed when an
asteroid or comet with low tensile strength is pulled apart by tides during a
close approach to a planet, separating into a train of fragments which hit a
moon of the planet rather than escaping to interplanetary space. Comet
P/Shoemaker-Levy-9 provided a striking demonstration of
tidal disruption during its penultimate encounter with Jupiter in 1992, and
chains of craters probably formed by the impacts of fragment chains similar to
that of SL9 have been found on Jupiter's Galilean satellites
and on the Earth's Moon.
Possible crater chain stretching across Missouri:
- Recent work indicates that crater chains may exist
on Earth as well. Eight circular depressions (3-17 km wide)
distributed along a 700 km line across Kansas, Missouri, and Illinois
may comprise part of a crater chain (M.R. Rampino and T. Volk (1996)
Geophys. Res. Lett. 23, p. 49.) Two of the eight
structures (Decaturville and Crooked Creek in Missouri) are known from
field studies to be impact craters ~300 Myr old.
Possible crater chain located by radar in Chad:
- A second study using radar imagery of the 17 km diameter,
360 Myr old Aorounga impact
crater in northern Chad discovered a ~10 km wide circular
structure adjacent to the Aorounga crater and a second, less distinct
feature somewhat displaced from the line defined by the other two
{S.C. Ocampo and K.O. Pope (1996) Lunar Planet. Sci. XXVII p. 977).
Field work to verify that the new structures are
impact craters has not been attempted, but it is possible that they and
Aorounga are members of a crater chain.
Crater Chains on the Moon:
The Davy crater chain:
- Two crater chains on the Moon are thought to have
been formed by the impact of tidally disrupted bodies. The first
is the Davy chain, a relatively fresh (i.e. post-Imbrian) chain 47 km long
containing 23 craters ranging in size between 1-3 km. Even though its
orientation is roughly radial to the Orientale basin, its morphology
and setting imply it may not be a secondary crater chain.
(H.J. Melosh and E.A. Whitaker (1994) Nature 369, p. 713)
The Abulfeda crater chain
- The second is the Abulfeda chain, probably formed at least 3.8 Gyr
years ago. It is 200-260 km long and has 24 craters ranging in size between
5--13 km. Its orientation is not radial to any large impact structure on the
Moon.
Tidal Disruption of an Asteroid Encountering Earth
- We created a model which
numerically simulates the tidal breakup process for weak asteroids encountering
the Earth and Moon. We explored the
regions of parameter space that give rise to catastrophic SL9-type
tidal disruptions and use these to predict the relative efficiency of
crater chain production on the Earth and Moon.
An example of a Shoemaker Levy-9 type disruption (I. Pre-Disruption)
- This figure shows
an asteroid just before it makes a close approach to the Earth.
Shoemaker Levy-9 type disruption (II. Disruption)
- This figure shows the asteroid slightly after the close encounter with Earth;
Note that tidal disruption is already well underway.
Shoemaker Levy-9 type disruption (II. Final Outcome)
- Final outcome; the asteroid now bears a strong resemblance to
comet P/Shoemaker Levy-9 after it encountered Jupiter. If this chain
were to strike the Moon, it would produce a crater chain!
Conclusions
- Intergrating our results over the 3.8 Gyr that have passed since the Late Heavy
Bombardment of the Moon, we find that the Moon has a 60% change of having
a single crater chain on it, matching observations. Although this result is
encouraging, especially given the uncertainties in many of our
parameters, we caution that a datum of one is certainly within the
realm of small number statistics. Our model also predicts that there
is only a 6% chance that a single crater chain will be formed on the
Earth over 3.8 Gyr, which is inconsistent with survey results that show
one or two terrestrial crater chains younger than 360 Myr. Moreover,
the probability of getting a single crater chain on Earth over the last
360 Myr is only 1 in 1000. Finally, to eliminate any unknown
systematic effects in our parameter estimates, we have taken the ratio of the
crater chain production frequency on the Earth and Moon. We find that,
on average, the production rate of crater chains on the Moon should be
10 times higher than the rate on the Earth. Thus, it seems extremely
unlikely that the terrestrial crater chains, if real, can be produced
by the mechanism tested here.
- Since few other plausible mechanisms are known which could create
terrestrial crater chains, we suggest that these unusual features
and/or their provenance should be reexamined. First, the structures
filling in the gaps in the Missouri crater chain between Decaturville
and Crooked Creek may not be of impact origin, implying this chain may
not be a chain at all. Second, the Aorounga impact crater and the
~10 km circular structure adjacent to it may be an example of a
doublet crater, produced by the impact of a binary asteroid
(Bottke and Melosh 1996). The third structure in the chain may be an unassociated
crater or not an impact structure at all. Finally, if it turns out
that these chains are real, we suggest that these objects may be
secondaries from a large terrestrial impact crater which has
disappeared through erosion or subduction processes.