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Clark R. Chapman Southwest Research Inst. Boulder, Colorado

Implications for Small-Body Binaries from Doublet Craters. Clark R. Chapman Southwest Research Inst. Boulder, Colorado . 1 st Workshop on Binaries in the Solar System Steamboat Springs, Colorado, USA 23 August 2007. Purpose of this Talk.

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Clark R. Chapman Southwest Research Inst. Boulder, Colorado

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  1. 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

  2. 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

  3. 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.

  4. 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

  5. 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 (doesn’t say which ones are the true doublets, unless the characteristics are very unusual)

  6. 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

  7. 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 • I’ve found no definitive studies • Doublets have been found, however

  8. 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

  9. 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.)

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