Mars's small, smooth outer moon
Deimos
Moon of Mars · discovered 1877 by Asaph Hall
Deimos is the smaller and more distant of Mars's two moons, an irregular lump of rock only about 15 kilometres long. It orbits roughly 23,500 kilometres from the planet's centre, and from the Martian surface it would look like little more than a bright star.
Unlike its battered sibling Phobos, Deimos looks oddly smooth, its craters softened under a thick blanket of dust. That quiet appearance hides a genuine puzzle: nobody yet knows for certain whether Deimos is a captured asteroid or a fragment of Mars itself.
Key facts
- Diameter
- 12.4 km0% of Earth's Moon
- Mass
- 1.48 × 10¹⁵ kg
- Surface gravity
- 0.003 m/s²0.0003 g
- Density
- 1,471 kg/m³
- Distance from Mars
- 23,463 km≈ 6.9 Mars radii
- Orbital period
- 30.3 hoursSame as its rotation (tidally locked)
- Discovered
- 1877Asaph Hall
- Surface
- Smooth, dust-mantled regolith with softened craters
- Surface temperature
- Roughly −40 °C in sunlight, far colder at night
- Rotation
- Tidally locked, keeping one face towards Mars
- Notable feature
- Craters Swift and Voltaire, named after authors
- Named after
- Deimos, Greek personification of dread
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Deimos is very dark, reflecting less than 10 per cent of the sunlight that reaches it, and its surface is covered with fine regolith that may be tens of metres deep in places. This loose material has filled in many craters, giving Deimos a smoother look than Phobos in spacecraft images.
Only two features have official names: Swift and Voltaire, craters a few kilometres across. Both honour 18th-century writers who, long before the moons were discovered, speculated that Mars might have two satellites. Viking images also revealed brighter patches where fresh material may have slid downslope.
Interior and composition
Like Phobos, Deimos has a low density, suggesting a porous interior that may be partly rubble with significant empty space. Its surface spectrum has long been compared with dark, primitive asteroids, but observations by the United Arab Emirates' Hope orbiter in 2023 hinted at material more similar to Martian rock, a finding researchers are still interpreting.
Orbit and rotation
Deimos takes about 30.3 hours to circle Mars, only slightly longer than a Martian day. As a result it creeps very slowly across the sky, rising in the east and taking around two and a half Martian days to set again, during which it runs through its phases.
Because it orbits just outside synchronous orbit, Mars's tides push Deimos very gradually outwards, the opposite fate to Phobos. Its orbit is almost perfectly circular and lies close to Mars's equatorial plane.
Origin
Two main ideas compete. In one, Deimos and Phobos are asteroids captured by Mars's gravity long ago; in the other, they coalesced from a disc of debris created when a large body struck Mars. The tidy, equatorial orbits of both moons are difficult to achieve through capture, which has pushed many scientists towards the impact scenario.
Some researchers have even proposed that Deimos and Phobos are pieces of a single larger moon that broke apart. Detailed observations and samples from upcoming missions should help decide between these possibilities.
Exploration
Deimos has been studied mostly in passing, by spacecraft whose main target was Mars or Phobos. Even so, a handful of close encounters have revealed its surprisingly smooth face.
Viking 2 (1977)
The Viking 2 orbiter swooped to within about 30 kilometres of Deimos, capturing images sharp enough to show boulders and the deep regolith layer.
Hope (2023)
The Emirates Mars Mission flew within about 100 kilometres of Deimos in 2023, imaging its far side and gathering spectra that challenge the captured-asteroid idea.
Hera (2025)
ESA's Hera spacecraft, en route to the asteroid Didymos, used a Mars flyby in March 2025 to observe Deimos from its rarely seen far side.
MMX (launch planned around 2026)
Japan's Martian Moons eXploration mission plans to make observations of Deimos during flybys while focusing its landing and sample collection on Phobos.
The moons of Mars
| Moon | Diameter | Distance | Orbit |
|---|---|---|---|
| Phobos | 22.5 km | 9,376 km | 7.7 hours |
| Deimos | 12.4 km | 23,463 km | 30.3 hours |
Surprising facts about Deimos
- From the surface of Mars, Deimos appears no bigger than a bright star and would be easy to mistake for one.
- Its two named craters honour Jonathan Swift and Voltaire, whose fiction described two Martian moons more than a century before they were found.
- Deimos takes so long to cross the Martian sky that it can stay above the horizon for about 2.5 Martian days at a time.
- Its escape velocity is only a few metres per second, so a well-hit ball or a hard jump could, in principle, send something away from Deimos for good.
- While Phobos spirals inwards, Deimos is very slowly moving away from Mars.