The Capture Conundrum
Deimos, the smaller and more distant of Mars's two moons, is a puzzle to planetary scientists. It is a small, lumpy body, with dimensions of roughly 15 by 12.2 by 11 kilometers. At first glance, it appears to be a captured asteroid. Its physical characteristics, such as its irregular shape and low albedo—it reflects very little light—are consistent with C-type or D-type asteroids commonly found in the outer asteroid belt. These asteroids contain carbonaceous material, a composition that early spectral analysis also suggested for Deimos.
The primary problem with this capture theory lies in Deimos's orbit. It travels around Mars in a path that is remarkably stable, nearly circular and lies almost perfectly in the plane of the planet's equator. Its orbital eccentricity, a measure of how much an orbit deviates from a perfect circle, is a mere 0.00033. A captured object would almost certainly have entered Mars's gravity well with a much more eccentric and inclined orbit. While mechanisms like atmospheric drag or tidal forces can circularize an orbit over time, it is unclear if there was enough time for these forces to shape the orbit of a small body like Deimos into its current state. The current Martian atmosphere is far too thin to accomplish such a task.
Impact, Debris, and a New Moon
An alternative and increasingly supported explanation is the giant impact hypothesis. This model suggests that, billions of years ago, a large object—perhaps the size of a large asteroid—collided with Mars. The immense energy from this impact would have blasted a vast quantity of rock and debris into orbit around the planet, forming a ring. From this debris disk, one or more moons could have accreted. This formation scenario naturally accounts for the circular, equatorial orbits of both Deimos and its sibling moon, Phobos.
Recent data has added weight to this theory. In 2023, the United Arab Emirates' Hope spacecraft performed close flybys of Deimos, analyzing its composition with spectrometers. Preliminary results suggest Deimos is made of rocky material similar to Mars itself, rather than the carbon-rich material of a captured asteroid. Another variation of this model suggests Phobos and Deimos are the sole survivors of a larger population of moons that formed from the impact debris; the others having long since crashed back onto the Martian surface. Definitive answers may come from the Japanese Aerospace Exploration Agency's (JAXA) Martian Moons eXploration (MMX) mission. Scheduled for launch in 2026, MMX will study both moons and aims to return a sample from Phobos, which will provide direct evidence of its composition and, by extension, its origin.