A 320-metre asteroid may have smoothed Deimos

When Hera flew past the Martian moon Deimos, the probe’s cameras captured an unusually smooth target. An analysis by the European Space Agency (ESA) now suggests that an asteroid may have struck the smaller and outer Martian moon, covering it with ejected material. The impactor that best fits the data was 320 metres in size.

Sabina Raducan of the University of Bern and her research team tested around 100 simulations involving asteroids of different sizes and various impact angles. The best fit was a high-speed impact at the south pole. This would have created a large crater, while the ejected material would have spread across Deimos. Its outline is still visible in the Hera images.

The images tell a second story as well. Crater traces that must have formed before the massive impact have survived. On a more solid body, the shock waves would probably have erased these older structures. To the researchers, this points to an interior that is “highly porous and fractured”. Deimos may therefore have formed from material blasted out of Mars by earlier impacts.

And then? For space research, the scenario offers practical clues: future probes could search for further traces of the impact. It could also explain why the moon differs so markedly from Phobos. Alternative explanations have not yet been ruled out, and the simulations alone do not prove that the collision occurred.

Hera passed Deimos a year and a half ago. In November, the ESA probe is due to reach its actual destination: the Didymos and Dimorphos binary asteroid system. The complete study was published in Nature Astronomy.