A crater chain is an alignment of three or more impact craters, with the same apparent age, thought to be caused by the fragmentation and sequential collision of large bodies with a planet. But finding such chains on the Earth's surface is complicated by the process of plate tectonics which, over geological time, will disperse the craters. Elsewhere in this issue (Nature 392, 171–173; 1998), John G. Spray and co-workers describe how they have identified just such a chain. It consists of at least five craters, spread over more than 4,000 km, which were produced about 214 million years ago. The illustration here shows the three main craters in the chain — St Martin, SM; Manicouagan, M; and Rochechouart, R — which line up at constant latitude when the Earth's tectonic plates are reconfigured to their arrangement at the time of the collisions.

Credit: DAVID ROWLEY

In 1994, the break-up of comet Shoemaker-Levy 9 by the tidal forces of Jupiter, and the subsequent collision of the fragments with the planet, dramatically showed how such crater chains might be formed. The fact that the craters observed by Spray et al. lie at a constant latitude indicates that the fragmented body that produced them had likewise been captured into an Earth orbit, with the distance between craters giving the angle through which the Earth rotated between impacts. But the large difference in mass between Jupiter and the Earth (over 300 to 1), and the Earth's closer proximity to the Sun, mean that it seems unlikely that the Earth would have captured and, through tidal forces, fragmented a comet or asteroid in the same manner as Jupiter.

An alternative view of events is this — a comet or asteroid makes a grazing passage through the Earth's atmosphere; it is fragmented and the pieces are captured into an Earth orbit through the forces involved in ‘aerobraking’; the fragments then collide with the planet at their next pass-by (see W. F. Bottke et al. Icarus 126, 470-474 (1997); S. G. Love et al. Lunar Planet. Sci. Conf. XXVIII, 837-838 (1997)). Such a process would still happen only infrequently. But it is estimated to have affected roughly 1 out of every 1,000 objects that have hit the Earth, and so could have been responsible for the crater chain observed by Spray and colleagues.