Mars has a doomed moon, and one big dent may say where it came from

Phobos is the inner moon of Mars. It's small, lumpy, and slowly falling toward the planet. In a few tens of millions of years Mars will tear it apart. That's enough to make me read anything with its name in it.

Nobody is sure where it came from. Either Mars captured an asteroid, or a huge impact threw debris off Mars and that debris later clumped back together into a moon.

A new paper by Benjamin Haser and Thomas Andert in Monthly Notices of the Royal Astronomical Society goes at the question in a way I hadn't thought of. Phobos has one enormous crater called Stickney, big enough that the impact nearly broke the moon apart. Their argument is that a hit like that would have squashed the rock underneath it and left a denser, compacted zone buried under the crater floor.

And buried density is something you can measure from orbit. It shows up in the moon's gravity field, in how its mass is spread out, and in the small wobble it makes as it turns. So they built voxel models of the interior and worked out what each version would look like to a spacecraft flying past. The models favour a very porous rubble pile, possibly with water ice inside, plus that denser patch under Stickney.

The timing is the part I find interesting. If Phobos is a captured asteroid, Stickney could have happened much later, around 2.6 billion years ago, and that fits the debris story a lot less well. Japan's MMX mission is on its way there to measure this and bring samples home. So an old dent may end up telling us where the whole moon came from, which I think is a neat way to work.

Source: https://www.sciencedaily.com/releases/2026/09/260919031040.htm