【惑星科学】対で旅する小惑星

Two craters in central Sweden were most likely caused by a pair of asteroids in a binary system that broke off from the Main Asteroid Belt around 470 million years ago according to a study in Scientific Reports this week. The connection between the two craters had been suspected, but new study adds evidence to support this theory. The findings provide insights into impacts caused by closely interacting objects, and analyses of the craters gives clues about the properties of binary asteroid systems.

Approximately 470 million years ago, the disruption of a 200 km-wide asteroid in the Main Asteroid Belt triggered an increased shower of fragments into Earth-crossing orbits. Previous analyses have linked the 7.5 km-wide Lockne crater in central Sweden to this family of projectiles, and the recent discovery of the nearby 0.7 km-diameter Malingen crater suggests that the two may have been produced by closely interacting projectiles.

Geological analyses of the material in the craters indicate that they occurred simultaneously, Jens Ormo and colleagues report. Moreover, the authors argue that shape of the craters are consistent with the impact of a fragmented projectile, leading them to propose that the binary system may have been a ‘rubble pile’ asteroid (made of many fragments held together by gravitational forces).

Observations of near-Earth asteroids indicate that around 16% travel in pairs; however, only a handful of potential crater pairs have been identified, of which most are still disputed. The evidence presented here seems to demonstrate that the Lockne and Malingen craters are companions produced by a pair of asteroids.