"You would assume that such an impact would have shattered Phobos, unless it has a very low homogeneous density, like a sponge that can absorb that kind of impact," Haser said. "And at that impact region, there must be very high temperature that melted and compressed the stone beneath it."

How do you weigh a small moon?

Phobos is small and strangely shaped. Its mean diameter is only 22.2 km (13.8 miles), and it races around Mars once every 7 hours and 39 minutes. Haser cautions against viewing it as simply a "rock in orbit."

"Phobos is not only a record of the past, but also an actively evolving geophysical system," he said. Its orbit lies extremely close to Mars, and the moon is slowly spiraling inward. Eventually it is expected either to break apart or collide with the planet.

Current estimates point to a porous interior that may contain water ice, along with a denser concentration of material near the moon's equatorial region. Mapping Phobos' gravity in detail would reveal how mass is distributed inside it — and that distribution would, in turn, constrain how it formed.

In the paper, Haser and Andert model how a compressed mass beneath Stickney Crater affects three measurable quantities:

• the moon's gravitational signal;
• its moments of inertia;
• its libration amplitude — essentially how much Phobos wobbles and oscillates.

Those tiny changes in movement could reveal how material is arranged inside the moon.

Does Phobos look like a rubble pile?

Several characteristics fit the captured-asteroid explanation well, Haser said. The moon's irregular appearance resembles what scientists call a rubble pile asteroid — an object made of many pieces of rock held loosely together by gravity.

But no single explanation accounts for everything. Phobos' gravity field, shape, density, spectral properties, and changing orbit must all be reconciled within one geophysical model. Its highly irregular shape and extreme proximity to Mars make interpreting its gravity especially hard. "Phobos' gravity field is strongly overshadowed by Mars' gravity field," Haser said.

For Haser, the deepest puzzle is not which materials make up the moon, but what kind of internal structure could explain all of its unusual characteristics at once. The answer must distinguish among asteroid capture, formation from Mars impact debris, or possibly a more complicated origin combining elements of both scenarios.

What will MMX actually do?

The MMX spacecraft will attempt to operate in a quasi-stable orbit around Phobos — an unusually difficult maneuver, because Phobos offers nothing like the stable orbital environment found around larger worlds. As Haser points out, there is truly no stable orbit around the moon.

Despite that challenge, the spacecraft is expected to gather surface material with two systems:

• a core sampler that will collect material from depths of up to 2 cm;
• a pneumatic sampler, contributed by NASA, that will fire pressurized gas at the surface to "loft material into a sample container," according to the Japanese Space Agency (JAXA).

The samples are expected to return to Earth by mid-2031, inside a capsule designed to survive atmospheric re-entry. If all goes to plan, the physical evidence from Phobos' surface — combined with gravity measurements and the models presented in Vienna — could finally resolve the question of where Mars' doomed moon came from.