Deimos (moon)

Deimos is the smaller and outer of the two natural satellites of Mars, the other being Phobos. With a mean radius of about 6.2 kilometers, it is much smaller than its companion moon. Deimos orbits Mars in about 30.3 hours, traveling a long distance from the planet at roughly 23,460 kilometers away. Its name comes from Deimos, an Ancient Greek god who represented feelings of dread and terror, reflecting its distant and somewhat mysterious presence in the Martian system.

Discovery and etymology

Main article: Moons of Mars § Discovery

Deimos was found by Asaph Hall at the United States Naval Observatory in Washington, D.C. on August 12, 1877. Hall was looking for moons around Mars and also found another moon called Phobos not long after.

The moon is named after Deimos, a figure from Greek mythology that stands for dread. The name was suggested by a scholar named Henry Madan, based on a story from an old book called the Iliad. In that story, the god Ares, who is the same as the planet Mars, calls on Dread (Deimos) and Fear (Phobos).

Origin

Scientists do not know exactly how Mars got its two small moons, Phobos and Deimos. There are a few ideas about their origin. One idea is that these moons were once asteroids from the asteroid belt that got pulled into orbit around Mars. Another idea is that the moons formed right where they are now, maybe from bits of rock that came together after a collision near Mars long ago.

In 2021, some scientists looked at data from the Mars InSight Mission and suggested that Phobos and Deimos might have come from a single larger object that broke apart after being hit by another space rock billions of years ago.

Physical characteristics

Deimos is a gray-colored body that is not perfectly round. Its dimensions are about 16.1 km by 11.8 km by 10.2 km, which gives it a mean diameter of 12.5 km. This makes Deimos about 57% the size of Mars's other moon, Phobos. Deimos is made of rock that contains a lot of carbon, similar to certain types of rocks from space called asteroids and carbonaceous chondrite meteorites. The surface of Deimos has many craters, but it is smoother than Phobos because some of the craters are filled with a loose layer of rocks and dust, known as regolith. This regolith is very loose and light, with a density of about 1.471 g/cm³.

It would take a jump of only 5.6 meters per second to leave Deimos, a speed a person might reach by jumping straight up. Deimos shines very faintly from Earth, with an apparent magnitude of 12.45.

Named geological features

Only two places on Deimos have special names. The craters Swift and Voltaire are named after writers who guessed that Mars might have two moons before these moons were discovered.

Orbital characteristics

Deimos has an orbit that is almost round and lies close to the middle line of Mars. Scientists think Deimos might have once been an asteroid pulled into its orbit by the planet Jupiter, but this idea is still being discussed. Both Deimos and Phobos have orbits that are very round and lie in the same middle line as Mars. For Deimos to have been captured by Mars, its orbit would need to have changed shape and angle, possibly due to atmospheric drag and tidal forces.

From Mars, Deimos looks very small, like a bright star. At its brightest, it shines about as brightly as the planet Venus does from Earth. With a telescope, people on Mars could watch Deimos go through phases, which take about 1.26 days. Unlike Phobos, Deimos rises in the east and sets in the west, but it does so more slowly than Mars turns. Because Deimos is far enough from Mars, its orbit is slowly moving farther away, and it might eventually leave Mars's pull completely.

Solar transits

Main article: Transit of Deimos from Mars

Deimos often moves in front of the Sun as seen from Mars. It is too small to block out the Sun completely, so it looks like a tiny black dot moving across the Sun. On 4 March 2004, the Mars rover Opportunity took a picture of Deimos doing this, and on 13 March 2004, the rover Spirit also captured such an image.

Exploration

Deimos, like its sister moon Phobos, has been studied by many spacecraft that were mainly sent to observe Mars. In March 2023, the Emirates Mars Mission captured rare close-up images of Deimos. No spacecraft has landed on Deimos yet.

Several missions have been planned or considered to visit Deimos. In the late 1990s, a mission called Aladdin was proposed to visit both Phobos and Deimos and collect samples. Another idea was the Hall mission, which would bring back samples using special propulsion. There are also ideas like Gulliver to return just a small amount of material from Deimos, and OSIRIS-REx 2 which would build on a previous mission. In 2014, a mission called Phobos And Deimos & Mars Environment (PADME) was suggested to orbit Mars and study both moons.

Future missions include the JAXA MMX Mission set to launch in October 2026, which will fly by Deimos to learn about its makeup and return a sample from Phobos, plus place a rover there. In April 2023, the Mars Hope orbiter provided the first detailed global images of Deimos, showing it may have formed from the same material as Mars. In March 2025, the ESA's Hera will observe Deimos from a very close distance during its journey to 65803 Didymos.