In previous work we argued that the abundance of doublet craters
on Earth, made by the near-simultaneous impact of two
well-separated projectiles, implies that a substantial fraction
(about 15%) of Earth-crossing asteroids are well-separated pairs
[1]. A search for doublet craters on Venus showed that, when
proper account is taken of the Venusian atmospheres screening
potential, a similar proportion of doublet craters is found there
[2]. Although an excess of doublet craters has been reported for
Mars [3], the authors of that work chose to seek doublets in the
heavily cratered terrain, and serious questions of the statistical
validity of their work have been raised [4]. In our work we chose
to examine the most lightly cratered plains units on Mars so that
chance associations of crater pairs are unlikely. So far, we have
examined about 2 million km2 of terrain in Vastitas Borealis.
Out of 133 craters greater than 5 km in diameter, only 3 are good
candidates for true doublets. This implies that the proportion of
doublet craters on Mars is about 2-3%, considerably smaller than
that observed on Earth or Venus. This lower abundance of doublet
craters on Mars does, in fact, agree well with the predictions of a
model for double asteroid formation [5].
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