Deimos
Deimos is the smaller and more distant of the two natural satellites of Mars. It orbits about 20,000 km above the planet's surface and completes one revolution in just over 30 hours. Deimos is comparable in size to a small city. Its dark surface appears noticeably smoother than that of Phobos.[4][2]
Size and physical characteristics
Deimos has an irregular shape and measures approximately 15 × 12 × 11 km. Its volumetric mean radius is about 6.2 km and its mass approximately 1.4 × 1015 kg. Phobos is roughly seven times more massive.[2][1]
The mean density is about 1.47 g/cm³, substantially lower than that of most solid rocks. This is consistent with a porous interior containing voids between separate blocks and particles.[1][7]
Surface gravity is only a few thousandths of a metre per second squared. It is several thousand times weaker than Earth's gravity and varies across the uneven surface. Particles ejected by impacts can therefore escape easily, although some material may later settle on the moon again.[1][2]
Orbit and rotation
Deimos orbits approximately 23,500 km from the centre of Mars. Its path is nearly circular and lies close to the planet's equatorial plane. One revolution takes 30 hours 18 minutes, slightly longer than a Martian day.[4]
Its rotation is synchronous, so Deimos constantly keeps the same side facing Mars. During its 2025 flyby, the Hera spacecraft photographed the less familiar hemisphere that faces away from the planet.[5]
Unlike Phobos, Deimos travels around Mars more slowly than the planet rotates. To an observer on the surface, the moon therefore moves gradually from east to west. Its position in the sky changes much more slowly than that of the inner moon.[4]
Surface and regolith
Deimos is covered with regolith, a loose mixture of dust and fragmented rock. This material partly fills many craters, leaving their rims and outlines less distinct than those on Phobos. Deimos also lacks the dense network of long grooves seen on its larger companion.[2]
Weak gravity allows some impact ejecta to leave the surface. Not all of it immediately escapes the Martian system: debris can remain in orbit around Mars before falling back onto Deimos. The gradual accumulation and redistribution of this material smooths the relief.[2]
Images reveal boulders, bright streaks on slopes and the subdued outlines of buried craters. The moon's comparatively smooth appearance does not imply a quiet impact history; some older structures are hidden beneath a younger covering layer.[8]
Craters and the southern depression
The two named craters on Deimos are Swift and Voltaire. They commemorate writers who described moons of Mars before their discovery. Jonathan Swift, for example, mentioned two Martian satellites in Gulliver's Travels in 1726.[6]
A broad southern polar depression about 10 km wide is a major component of Deimos's overall shape. It is a wide topographic low rather than an ordinary small crater with a sharp rim.[7]
The depression may have formed in an oblique collision with an object about 320 m across. Some of the ejected debris subsequently fell back and blanketed Deimos with loose material. This process could have created the depression while also smoothing the surrounding terrain, although other resurfacing mechanisms remain possible.[7][8]
Internal structure
The low density and impact modelling indicate that Deimos is probably a porous, fragmented body. Its interior may resemble an aggregate of debris held together by weak gravity rather than a monolithic rock.[7]
In impact models, porous material dampens shock waves and helps older craters survive a major collision. This conclusion depends on the assumed properties of the material; the internal structure of Deimos has not yet been measured directly.[8]
Origin
The origin of Deimos, like that of Phobos, remains uncertain. The principal explanations are the capture of an asteroid and formation from material placed in orbit by a large impact on Mars.[9]
Another model begins with an asteroid disrupted by Mars's gravity during a close encounter. Some fragments would have remained near the planet, while repeated collisions gradually formed a disc from which the moons could accumulate. Their composition and internal structure will help distinguish between these possibilities.[9]
Discovery and name
Asaph Hall discovered Deimos at the United States Naval Observatory during the night of 11–12 August 1877. The observation occurred on 11 August local time and 12 August UTC. Hall discovered Phobos less than a week later.[6]
The name Deimos comes from Greek mythology and means “dread” or “terror”. Deimos and Phobos accompany the war god Ares, the Greek counterpart of the Roman god Mars.[2]
Spacecraft exploration
Mariner 9 photographed Deimos while operating at Mars in 1971–1972. The Viking orbiters later returned more detailed images in which individual boulders could be distinguished.[10][6]
Mars Global Surveyor imaged the moon in July 2006, and the HiRISE camera on Mars Reconnaissance Orbiter obtained detailed colour views in February 2009. These observations reveal its shape, relief and variations across the surface.[6][11]
In 2023, the United Arab Emirates' Hope spacecraft began close observations of Deimos with instruments operating at visible, infrared and ultraviolet wavelengths.[12]
On 12 March 2025, Hera passed about 1,000 km from Deimos. Its cameras and spectrometers examined the surface, including the less familiar hemisphere. Some observations were coordinated with Mars Express.[5]
Deimos is also observed from the Martian surface. On 19 January 2024, Perseverance recorded the moon crossing the Sun as a small dark silhouette.[13]
MMX mission
Japan's Martian Moons eXploration—MMX mission will study both moons of Mars. Sampling operations are planned for Phobos, while Deimos will be examined remotely during flybys.[14]
An H3 rocket successfully launched MMX on 20 October 2026 Japan Standard Time, or 19 October UTC. The spacecraft separated from the upper stage about 71 minutes after liftoff and entered its planned interplanetary trajectory. Observations of Deimos will allow its properties to be compared with those of Phobos and help test models for the origin of the Martian moons.[15][14]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Planetary Satellite Physical Parameters // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Deimos // NASA Science.
- ↑ Solar Eruptions Could Electrify Martian Moons // NASA. 2017.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Planetary Satellite Mean Elements // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 5.0 5.1 5.2 Hera asteroid mission spies Mars's Deimos moon // European Space Agency. 13 March 2025.
- ↑ 6.0 6.1 6.2 6.3 6.4 Deimos! // NASA Science. 11 August 2006.
- ↑ 7.0 7.1 7.2 7.3 Deimos's shape and geology explained by a subcatastrophic impact // Nature Astronomy. 2026.
- ↑ 8.0 8.1 8.2 Hera's Mars flyby guided impact study of Deimos moon // European Space Agency. 18 August 2026.
- ↑ 9.0 9.1 Making Mars' Moons: Supercomputers Offer 'Disruptive' New Explanation // NASA. 2024.
- ↑ Mariner 9 // NASA Science.
- ↑ Deimos-MRO — image description, NASA/JPL-Caltech/University of Arizona // Wikimedia Commons.
- ↑ Emirates Mars Mission Unveils New Deimos Observations at EGU23 // UAE Space Agency. 24 April 2023.
- ↑ Perseverance Views a Transit of Deimos // NASA Science. 2024.
- ↑ 14.0 14.1 Mission Overview / Mission Flow // JAXA, Martian Moons eXploration.
- ↑ Launch result of the Martian Moons eXploration spacecraft aboard H3 Launch Vehicle flight No.10 // JAXA. 20 October 2026.
