Deimos () is the smaller and outer of the two natural satellites of Mars, the other being Phobos. Deimos has a mean radius of and takes 30.3 hours to orbit Mars. It is named after Deimos, the Ancient Greek god and personification of dread and terror.

Discovery and etymology

thumb|left|Asaph Hall III, discoverer of Deimos

Deimos was discovered by Asaph Hall<!--- there is an Asaph Hall, Junior, less well known ---> at the United States Naval Observatory in Washington, D.C., on 12 August 1877, at about 07:48 UTC. Hall, who also discovered Phobos shortly afterwards, had been specifically searching for Martian moons at the time.

The moon is named after Deimos, a figure representing dread in Greek mythology. The name was suggested by academic Henry Madan, who drew from Book XV of the Iliad, where Ares (Greek counterpart of the Roman god Mars) summons Dread (Deimos) and Fear (Phobos).

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek delta (the initial of Deimos) combined with Mars's spear as the symbol of Deimos (16px). This symbol is not widely used.

Origin

The origin of Mars's moons is unknown and the hypotheses are controversial. The main hypotheses are that they formed either by capture or by accretion.

Because of the postulated similarity to the composition of C- or D-type asteroids, one hypothesis is that the moons may be objects captured into Martian orbit from the asteroid belt, with orbits that have been circularized either by atmospheric drag or tidal forces, The main alternative hypothesis is that the moons accreted in the present position. Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a planetesimal.

In 2021, Amirhossein Bagheri (ETH Zurich), Amir Khan (ETH Zurich), Michael Efroimsky (US Naval Observatory) and their colleagues proposed a new hypothesis on the origin of the moons. By analyzing the seismic and orbital data from the Mars InSight Mission and other missions, they proposed that the moons were born from the disruption of a common parent body around 1 to 2.7 billion years ago. The common progenitor of Phobos and Deimos was most probably hit by another object and shattered to form Phobos and Deimos.

Physical characteristics

left|thumb|Size comparison between Phobos, Deimos and the [[Moon (right)]]

Deimos is a gray-colored body. Like most bodies of its size, Deimos is highly non-spherical with triaxial dimensions of , corresponding to a mean diameter of which makes it about 57% the size of Phobos. It is cratered, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters with regolith. The regolith is highly porous and has a radar-estimated density of only .

Escape velocity from Deimos is 5.6&nbsp;m/s. This velocity could theoretically be achieved by a human performing a vertical jump. The apparent magnitude of Deimos is 12.45.

Named geological features

<!--- thumb|200px|The two named craters on Deimos --->

Only two geological features on Deimos have been given names. The craters Swift and Voltaire are named after writers who speculated on the existence of two Martian moons before Phobos and Deimos were discovered.

{| class="wikitable" style="width: 100%"

! style="width:10em" | Craters !! style="width:10em" | Coordinates !! Diameter<br />(km) !! Approval<br />Year !! class="unsortable" | Eponym !! class="unsortable" | Ref

|-id="Swift"

| Swift || || align=right | 1 || align=center | 1973 || Jonathan Swift; Irish writer (1667–1745) ||

|-id="Voltaire"

| Voltaire || || align=right | 1.9 || align=center | 1973 || Voltaire; French writer (1694–1778) ||

|}

Orbital characteristics

frame|right|Orbits of [[Phobos (moon)|Phobos and Deimos (to scale)]]

Deimos's orbit is nearly circular and is close to Mars's equatorial plane. Deimos is possibly an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed. it is not clear that sufficient time was available for this to have occurred for Deimos. At its brightest ("full moon") it would be about as bright as Venus is from Earth; at the first- or third-quarter phase it would be about as bright as Vega. With a small telescope, a Martian observer could see Deimos's phases, which take 1.2648 days (Deimos's synodic period) to run their course.

Deimos's orbit is slowly getting larger, because it is far enough away from Mars and because of tidal acceleration. It is expected to eventually escape Mars's gravity.

Solar transits

thumb|Deimos [[transit (astronomy)|transits the Sun as viewed by the Mars rover Opportunity (4 March 2004)]]

Deimos regularly passes in front of the Sun as seen from Mars. It is too small to cause a total eclipse, appearing only as a small black dot moving across the Sun. Its angular diameter is only about 2.5 times the angular diameter of Venus during a transit of Venus from Earth. On 4 March 2004 a transit of Deimos was photographed by Mars rover Opportunity, and on 13 March 2004 a transit was photographed by Mars rover Spirit.

Exploration

thumb|Deimos and Phobos as seen from Mars, compared in [[angular size|apparent size to the Moon as seen from Earth. If they would be as far away from Mars as the Moon from Earth, they would appear as faint star-like features in the Martian sky.]]

Overall, its exploration history is similar to those of Mars and of Phobos. Deimos has been photographed close-up by several spacecraft whose primary mission has been to photograph Mars, including in March 2023 during a rare close encounter by the Emirates Mars Mission. No landings on Deimos have been made.

In 1997 and 1998, the proposed Aladdin mission was selected as a finalist in the NASA Discovery Program. The plan was to visit both Phobos and Deimos, and launch projectiles at the satellites. The probe would collect the ejecta as it performed a slow flyby (~1&nbsp;km/s). These samples would be returned to Earth for study three years later. The principal investigator was Carle M. Pieters of Brown University. The total mission cost, including launch vehicle and operations was $247.7 million. Ultimately, the mission chosen to fly was MESSENGER, a probe to the planet Mercury.

In 2008, NASA Glenn Research Center began studying a Phobos and Deimos sample-return mission that would use solar electric propulsion. The study gave rise to the "Hall" mission concept, a New Frontiers-class mission currently under further study.

Also, the sample-return mission called Gulliver has been conceptualized and dedicated to Deimos, in which 1 kilogram (2.2 pounds) of material from Deimos would be returned to Earth.

In March 2014, a Discovery class mission was proposed to place an orbiter in Mars orbit by 2021 and study Phobos and Deimos. It was called Phobos And Deimos & Mars Environment (PADME).

Human exploration of Deimos could serve as a catalyst for the human exploration of Mars. Recently, it was proposed that the sands of Deimos or Phobos could serve as a valuable material for aerobraking in the colonization of Mars. See Phobos for more detail.

ISRO's Mars Orbiter Mission captured pictures of the far side on Deimos. The far side was rarely pictured because most Mars missions orbit Mars closer than Deimos, so that they only see the Mars-facing side of the moon. But the highly elliptical orbit of MOM allowed it to capture the far side.

In April 2023, astronomers released close-up global images, for the first time, of Deimos that were taken by the Mars Hope orbiter. Observations reported by this mission contravene the captured asteroid hypothesis and indicate basaltic planetary origin of Deimos.

During its gravity assist from Mars en route to 65803 Didymos, the ESA's Hera took observations of Deimos in March 2025, approaching at a distance of 300 km (190 mi).

The JAXA MMX Mission to Phobos and Deimos is planned for launch in October 2026. It will make flybys of Deimos to investigate its composition and structure, as well as performing a sample return on Phobos and placing a rover on the surface of Phobos.

See also

Notes

References

</references>

  • Deimos Profile by NASA's Solar System Exploration
  • Deimos rotation movie
  • Animation of Deimos
  • 3D model of Deimos
  • USGS Deimos nomenclature