Transcript Implications for small-body binaries from doublet craters

Implications for Small-Body Binaries
from Doublet Craters
Clark R. Chapman
Southwest Research Inst.
Boulder, Colorado
1st Workshop on Binaries in the Solar System
Steamboat Springs, Colorado, USA
23 August 2007
Purpose of this Talk
 I’m presenting no new data and few,
if any, new ideas…
 This is a review of the literature on
doublet/paired craters, a relevant
topic that I found was not otherwise
represented in the program…
 …with the purpose of generating
some discussion
History of the Topic
 Title: Martian doublet craters.
Authors: Oberbeck, V.R., Aoyagi, M.
Publication: J. Geophys. Res., Vol. 77, p. 2419 - 2432
Publication Date: 00/1972
 1978: Alex Woronow debated Oberbeck about
whether O & A correctly modeled spatial
randomness. Result is inconclusive. Mars may well
have an overabundance of paired craters, but
nobody really knows.
 Topic resurrected in 1991 by Melosh and Stansberry
who argued that 3 doublets on Earth must have been
formed by impact of binary asteroids (this is before
any asteroid satellites had been discovered).
 Considerable research in 1990s by Melosh, Bottke,
Cook, and others re-examined Martian doublets and
extended the analysis of doublets to Venus.
 Since the mid-1990s, thinking about doublet craters
has been in the context of Dactyl and SL-9.
Methods of Forming Doublets
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Random impacts (unavoidable)
Very oblique impacts, ricochet
(Messier, Messier A)
Endogenic crater formation
(volcanoes, collapse pits, etc.)
Atmospheric break-up, explosion
(Henbury)
Tidal break-up (Shoemaker-Levy 9)
Spatially clustered secondaries
Impact of binary asteroid or comet
How to Recognize Doublets
 The certain way
Adjacent craters with same
measured ages (Earth only)
 Overlapping craters with
shared walls (septum)
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 The very likely way
Adjacent craters with similar
relative ages
 Other unusual similarities
indicating, e.g., same oblique
impact angle
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 The statistical approach
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Find a greater abundance of
doublets than predicted by
chance (doesn’t say which
ones are the true doublets,
unless the characteristics are
very unusual)
Observed Frequencies of
Double Craters on Earth
 3 pairs of 28 craters >20 km
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Ries/Steinham would not be
recognized on many other
bodies: on Earth, spatial density
of craters is very low, so even
this distant pair of craters of
very dissimilar sizes stands out
Ries/Steinham also have
identical dated ages
Kara/Ust Kara had been
considered to be a pair, but
appear to have very different
ages
Problem: statistics of small
numbers
Observed Frequencies of
Doublets on Other Planets
 Mars
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Melosh et al. (1996) studied 133 craters
on northern plains, 5-100 km diam., and
found 3 likely pairs with separations
exceeding random expectations  2.3%
doublets, less than Earth and Venus
 Venus
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Cook, Melosh & Bottke (2003) found
2.2% of 10-150 km diam. craters were
doublets, but that “splotches” (due to
smaller impactors unable to make it
through the Venus atmosphere) imply
~14% doublets on Venus
 Moon, Mercury, satellites
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I’ve found no definitive studies
Doublets have been found, however
Geometry of NEA Binaries and
Opportunity to Make Doublets
Main Issue: Impacting NEAs form craters 10 – 20 times
their own diameter. Most NEA pairs are so close that,
even with favorable geometry, they form a single crater.
How can there be so many doublet craters?
 Separation larger
for oblique impacts
 Separation of
craters can be zero
if pair are unfavorably aligned,
even if widely
separated
 Tidal forces can
affect separation
Opportunities: Outer Solar System
and Elsewhere
There’s a fine PhD
thesis here!
 Classic doublet found in
Cassini image of Tethys
[right, thanks to Paul Schenk]
 Binary TNOs (hence binary comets?) are
widely separated, increasing the
chances for finding doublet craters.
 Some satellite surfaces are very youthful
with small crater densities (Europa,
Enceladus, Miranda, Triton), so
possibilities for confusion with random
pairing are reduced. (But confusion with
secondaries and sesquinaries may be
heightened in planetary systems with
many moons and rings.)