Clark R. Chapman

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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
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Purpose of this Talk

• Im 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

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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.

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Methods of Forming Doublets

• 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

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How to Recognize Doublets

• The certain way
• Adjacent craters with same measured ages (Earth
  only)
• Overlapping craters with shared walls (septum)
• The very likely way
• Adjacent craters with similar relative ages
• Other unusual similarities indicating, e.g., same
  oblique impact angle
• The statistical approach
• Find a greater abundance of doublets than
  predicted by chance (doesnt say which ones are
  the true doublets, unless the characteristics are
  very unusual)

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Observed Frequencies of Double Craters on Earth

• 3 pairs of 28 craters 20 km
• 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

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Observed Frequencies of Doublets on Other Planets

• Mars
• 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
• 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
• Ive found no definitive studies
• Doublets have been found, however

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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
  un-favorably aligned, even if widely separated
• Tidal forces can affect separation

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Opportunities Outer Solar System and Elsewhere
Theres 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.)