320-Meter Impact Explains Deimos’s 10-Km Polar Depression and Regolith

3 articles · Updated · Nature.com · Aug 18

Simulations point to a 320±25-meter projectile striking Deimos obliquely at about 8.2 km/s as the best match for the moon’s 10-km-wide south-polar depression and smooth, debris-covered surface.
That impact would have redistributed 10%–20% of Deimos’s mass into a global regolith blanket—typically a few meters thick and up to about 200 meters on the Mars-facing side—while less than 1% escaped.
Crater-collapse modeling indicates Deimos’s upper layers are extremely weak, with cohesion no higher than a few hundred pascals and likely near 1 Pa, allowing rims to slump and older craters to be infilled.
Shock-survival tests and Deimos’s low 1.5 g/cm3 bulk density suggest a highly porous, energy-dissipating interior more like rubble-pile asteroids such as Bennu and Ryugu than lunar regolith.
The study says such a subcatastrophic collision would have changed Deimos’s orbit only slightly and offers testable predictions for future observations, including Japan’s MMX mission.