A recent article at BBC (and reported elsewhere as well) indicates that evidence for double-asteroid impacts on earth is growing more credible.
According to astronomers, around 15% of asteroids are binaries, with a smaller “moon” orbiting a larger body. These are formed when a rubble asteroid begins spinning so fast as the result of solar pressure that a portion of its substance is ejected, forming a small satellite.
Several candidates for double impacts exist on earth – Clearwater Lakes in Quebec, Canada are one potential pair (below).
It is obvious that any pair of craters must be the same geological age to be identified as a double-strike candidate. Now, “Dr. Jens Ormo and his colleagues studied two craters called Lockne and Målingen, which lie about 16km apart in northern Sweden. Measuring about 7.5km wide, Lockne is the bigger of the two structures; Målingen, which lies to the south-west, is about 10 times smaller.”
Dr. Ormo’s team dated the craters using chitnozoan fossils and determined that the Målingen structure and the Lockne crater are the same age, roughly 458 million years; it is hypothesized that these were formed as the result of a twin strike in the Ordovician Period.
Click through for another article at i09.
The more we know, the more we know that we don’t know. Science!
The Old Wolf has spoken.
 See the additional information provided in a comment by engastrobd below.
YORP spin-up of asteroids followed by fission is one way of forming asteroid satellites, but it is also possible that close binaries form by tidal disruption during a close encounter with a planet, especially in the near-Earth asteroid population, see Bottke and Melosh (1996; Formation of asteroid satellites and doublet craters by planetary tidal forces; Nature 381, 51-53). Anyway, thanks for reporting this, it’s particularly fun to hear about Lockne and Målingen since I’m a Swede 🙂
And tack so mycket for the additional information!
Var så god! (You’re welcome!)