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The Mysterious Moons of Mars: Deimos and Phobos

Updated on September 09, 2016

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Mars' Mysterious Satellites

The Red Planet has two satellites in its orbit. The innermost satellite is "Phobos", named after the Greek god of Horror. And the outermost satellite is "Deimos" and is named after the Greek god of Terror.

The definition of a satellite in astronomy is any celestial object in orbit around a planet. So while the Moon is Earth's satellite and it is inaccurate to call another planet's satellite a "moon" it has become a common practice and is used in this article to be understood.

The surfaces of both of Mars' moons are dented with craters. Their low gravity keeps the satellites from forming into a spheres and instead they are both elongated irregular shaped objects, similar to asteroids.

Deimos and Phobos are some of the smallest satellites in the solar system and their origins are highly speculated. Even their composition is not entirely certain. But despite how little we know about them they have become a hot topic for the future of human exploration of Mars.

Martian Moons: Phobos and Deimos
Martian Moons: Phobos and Deimos | Source

Discovery and Exploration

Since the Mars moons are so small and difficult to see from Earth their discovery was much later than the nearby planets and their satellites. Many scientists predicted there were objects orbiting Mars and they were finally identified in 1877 by Asaph Hall.

Not much could be determined about the satellites just by examining them from Earth, except their approximate size and orbit. Many Mars orbiters and landers have recorded data and images from Deimos and Phobos on their way to or from the Red Planet. But still, scientists have many unanswered questions about the two moons.

Phobos: the God of Horror


Phobos has a large crater (Stickney) on its surface.
Phobos has a large crater (Stickney) on its surface. | Source

About Phobos

Phobos is the larger of Mars' two satellites (14 miles across at the widest) but is still very small compared to most satellites in the solar system. At first glance at the moon an obvious crater stands out. This is Stickney, a 5.6 mile wide crater probably formed when a slightly smaller body slammed into the satellite.

Phobos is closer to Mars than Deimos and is actually getting gradually closer and closer to Mars. It is predicted that the moon will crash into the surface of Mars in the next 50 million years if it does not break up before then.

Monolith on Phobos: Natural or Artificial?
Monolith on Phobos: Natural or Artificial? | Source

Phobos Monolith

A monolith, or large rock protrusion, was discovered on Phobos' surface in images sent back from the Mars Reconnaissance Orbiter (MRO) mission. The strange object was large and square enough to make many believers of life on Mars claim it was built by Martians. Scientists claim is a natural occurrence and cite similar objects on Earth.

Deimos: the God of Terror

Deimos has two small craters: Voltaire and Swift.
Deimos has two small craters: Voltaire and Swift. | Source

About Deimos

Deimos is the smaller of the two moons at about 8 miles across. It is 14,500 miles from Mars and its orbital period lasts more than an Earth day. While Phobos is gradually being pulled in by Mars' gravity, Deimos is drifting away from Mars.

Deimos has fewer and smaller craters than its brother Phobos. The two most notable craters are named Voltaire and Swift and can be seen in the image above. Like Phobos, Deimos is thought to be a C or D type asteroid based on what we can see from Earth and the evidence returned from various fly-by missions. But we would need an actual soil sample in order to know for sure the origins of the moons.

The Origin of the Martian Moons

Scientists do not yet know enough about either moon to determine their how they formed or how they came into orbit around the Red Planet. A sample return or a craft that can analyze a sample on the surface of the satellite and send data back to Earth could give us enough information to determine how it formed. The most accepted theory of their origins is that both Deimos and Phobos used to be a part of the asteroid belt and were dislodged and captured into Mars orbit. The commonalities between the moons and C-type asteroids support this theory:

  • Numerous craters on the surface
  • Small size
  • Elongated, irregular shape

Compare Earth and the Moon to Mars, Phobos and Deimos

Diameter (miles)
Gravity (m/s^2)
Rock, Water
Rock, Water
Rock (?)
Rock, Water (?)

Phobos and Deimos Explained Video

This very short animated video below gives a good idea of the scale of the moons and presents the popular asteroid belt theory to describe their origins.

Moons of Mars

Stepping Stones to Mars Exploration

Many experts are pushing NASA and its international partners to use Deimos and Phobos in their plans for future human exploration of Mars. It may be a long time before man sets foot on Mars but in the meantime we can send robots and landers to set up power stations on the Martian Moons for when humans arrive.

Deimos may be a better choice for sending landers to support future crewed missions to Mars because it is further from Mars and thus spacecraft are less affected by its gravity during maneuvers. Also there are regions of Deimos that are continuously sunlit for an entire Deimos season, ideal for power generation.

Phobos' proximity to Mars puts it in a good position to be an intermediate stop for cargo, crew and messages to and from Earth.


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    • Greensleeves Hubs profile image

      Greensleeves Hubs 4 months ago from Essex, UK

      It would be interesting to send a probe to one of these two moons Katy, perhaps to sample the surface and explore its origins. I wonder also if such a mission would be (relatively) inexpensive in space mission terms as it could perhaps be combined with a Mars lander or Mars orbiter detaching a probe to Phobos or Deimos, much as the Cassini mission to Saturn released the Huygens probe to land on Titan.

    • KCO profile image

      Katy 4 months ago from Denver, CO

      Greensleeves, thanks for your comment and hopefully accurate prediction of the future! The idea of piggy backing a CubeSat sized probe on the next launch to Mars to sample one or more of the moons is getting more attention now that NASA and experts are considering the moons as key assets in human exploration of Mars.

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