A single asteroid impact may have dramatically reshaped Deimos, the smaller of Mars’ two moons, creating both a huge depression near its south pole and the dusty surface that covers the moon today.
An international team of researchers reached this conclusion after running about 100 detailed computer simulations and comparing the results with observations from the European Space Agency’s Hera spacecraft.
The study, published in Nature Astronomy, offers a possible answer to a long-standing mystery about the strange appearance of Deimos.
Mars has two small moons, Phobos and Deimos. While Phobos has a heavily cratered appearance, Deimos looks noticeably smoother.
Its surface is covered by a loose layer of dust, broken rock and other debris known as regolith. It also has a large depression near its south pole.
Scientists have struggled to explain how these two features formed.
The new research suggests they may have been created by the same violent event.
The researchers used a powerful computer program developed at the University of Bern to simulate collisions involving asteroids, comets and planets. The program can divide objects into millions of virtual particles and calculate how they behave during an impact.
The team tested different possible scenarios by changing the size and speed of the incoming asteroid, the angle at which it struck and the internal structure of Deimos. Each simulation took about a week to complete.
The results pointed toward a particularly convincing explanation.
Around 100 million years ago, an asteroid roughly 320 meters wide may have slammed into Deimos at an angle of about 45 degrees. The collision was powerful enough to create the large depression near the moon’s south pole.
But it didn’t stop there.
The impact blasted huge amounts of material across Deimos. This debris eventually fell back onto the surface, covering older craters and other features. In some areas, the layer of material may be more than 200 meters thick.
This could explain why Deimos appears much smoother than Phobos.
Despite the enormous collision, the moon survived. According to the simulations, the impact was powerful enough to rearrange material across nearly the entire surface but not powerful enough to break Deimos apart.
The findings also provide clues about what’s inside the little moon.
The researchers believe Deimos has extremely weak surface material and a highly porous interior. Rather than being a solid chunk of rock, it may behave more like a loosely assembled pile of rocky debris. That structure could have helped absorb some of the impact’s energy.
However, this doesn’t necessarily mean Deimos began life as a captured asteroid. Another possibility is that it formed from debris thrown into space by ancient impacts on Mars.
Future missions could help settle the question.
Japan’s Martian Moons eXploration mission, or MMX, is expected to study both Martian moons in detail and return samples from Phobos to Earth.
The new research provides predictions about Deimos’ surface and internal structure that future observations can test.
If those predictions prove correct, one well-placed asteroid strike may explain much of what we see on Deimos today—a tiny moon whose entire appearance could bear the scars of one enormous collision.
