Deimos Moon Facts

The planet Mars is one of the four terrestrial rocky planets and is the only one to have more than a single moon.

Deimos Moon Profile

Discovered By:Asaph Hall August 12, 1877
Diameter:12.4 km
Mass:1.48 × 10^15 kg (0.000002% Moon)
Orbit Distance:23,458 km
Orbit Period:1.3 days
Surface Temperature:-40 °C

Deimos (pronounced DEE-MOS), is the smaller of Mars’ two moons and both of the moons of Mars are smaller than our moon.

Both Phobos and Deimos looks more like an asteroid than our moon. Deimos is only 12.4 km/7.7 mi across and both moons seem to be made up of the same Type I or II carbonaceous chondrites that a lot of asteroids are made of.

Different Shape

While most moons, including our own, are more spherical or circular in shape, Deimos has more of an asteroid shape.

The combination of these characteristics have made scientists question how Phobos and Deimos were formed.

For many years, most scientists didn’t think the Mars had any moons.

Who discovered Deimos?

Johannes Kepler was the first to guess that Mars might have two moons, however, it was Asaph Hall, an American astronomer that studied the planet and discovered that it had two small bodies orbiting closely to it.

Hall decided to name the two satellites for the mythological sons of Ares, the Greek god of war.

The Romans used the name Mars for their god of war. The names of the twin sons were Deimos (for panic or dread) and Phobos (for fear).

Deimos is small

Deimos is so small that if you saw it in the sky it would look like a star. Even when it eclipses the sun it only appears like a tiny little dot.

However, when it is in a full moon condition, it is as bright as the planet Venus, which is one of our solar system’s brightest objects. Deimos has a longer orbit than Phobos, taking 30.3 hours to orbit around Mars.

Both of the moons on Mars have a lot of impact craters on their surface, however, Deimos has fewer than Phobos.

Craters on the Moon

The two largest craters are Swift, measuring a diameter of 1 km, and Voltaire, measuring 1.9 km in diameter.

Swift is named after the writer, Jonathan Swift, how authored “Gulliver’s Travels.” He wrote about two moons on the planet Mars 151 years before they were ever discovered.

Voltaire is named after François-Marie Arouet, the French writer known by the name of “Voltaire.”

Both Deimos and Phobos have stable orbits that almost totally circular. If they had been asteroids that had been captured or pushed into the orbit of Mars, their orbit would be more erratic. These two moons have given scientists a lot to think about.

How did Deimos form?

Some have a theory that they may have been formed from leftover Mars debris, while others think that maybe they were formed like our own moon, as a result of a collision.

Another idea blends both of these theories together stating that the collision debris could have turned into rings that eventually fused together to create the moons.

Both Phobos and Deimos have similar temperatures, from 25 degrees F/-4 C on the side that is lit by the sun, to -170 degrees F/-112 C on the side that is facing away from the sun.

While the moon Phobos is slowly inching closer to Mars, Deimos is moving further away from its parent planet. Eventually Deimos will leave the gravitational pull of Mars and enter into space.

Facts about Deimos

  • Quite a few spacecraft have done flyby explorations of Deimos, although none have focused on Deimos or Phobos as a primary target.
  • Both Deimos and Phobos have dusty material on their surfaces called “regolith.”
  • Neither of the Mars moons, Phobos or Deimos, have atmospheres due to gravity too low to keep any atmosphere.
  • Deimos and Phobos have the characteristics of a C-type asteroid.
  • The surface of Deimos is a lot smoother than that of Phobos.
  • There are future missions that are being discussed to visit both of the moons of Mars.
  • There is a possibility that one or both of the moons of Mars could be considered for use as “staging areas” for future manned space missions.


  • What are the names of the two major craters on Deimos?
    Swift and Voltaire
  • What is the diameter of Deimos?
    12.4 km/7.7 mi
  • What celestial object’s characteristics do both of the Mars moons share?
  • Who is Deimos named after?
    one of the sons of the Greek mythological god Ares
  • What important future space purpose are scientists considering for one or both of Mars’ moons?
    taging area for manned mission to Mars
  • What is happening to Deimos’ orbit?
    it’s moving away from Mars