Mars I

Phobos (“FOH bus”) is the larger and innermost of Mars’ two moons. Phobos is closer to its primary than any other moon in the solar system, less than 6000 km above the surface of Mars. It is also one of the smallest moons in the solar system.

        orbit:    9378 km from the center of Mars
        diameter: 22.2 km (27 x 21.6 x 18.8)
        mass:     1.08e16 kg

In Greek mythology, Phobos is one of the sons of Ares (Mars) and Aphrodite (Venus). “phobos” is Greek for “fear” (the root of “phobia”).

Discovered 1877 August 18 by Hall; photographed by Mariner 9 in 1971, Viking 1 in 1977, and Phobos in 1988.

Phobos orbits Mars below the synchronous orbit radius. Thus it rises in the west, moves very rapidly across the sky and sets in the east, usually twice a day. It is so close to the surface that it cannot be seen above the horizon from all points on the surface of Mars.

And Phobos is doomed: because its orbit is below synchronous altitude tidal forces are lowering its orbit (current rate: about 1.8 meters per century). In about 50 million years it will either crash onto the surface of Mars or (more likely) break up into a ring. (This is the opposite effect to that operating to raise the orbit of the Moon.)

Phobos and Deimos may be composed of carbon-rich rock like C-type asteroids. But their densities are so low that they cannot be pure rock. They are more likely composed of a mixture of rock and ice. Both are heavily cratered. New images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine dust about a meter thick, similar to the regolith on the Earth’s Moon.

The Soviet spacecraft Phobos 2 detected a faint but steady outgassing from Phobos. Unfortunately, Phobos 2 died before it could determine the nature of the material; water is the best bet. Phobos 2 also returned a few images (right).

The most prominent feature on Phobos is the large crater named Stickney, the maiden name of Hall’s wife (above). Like Mimas’ crater Herschel (on a smaller scale) the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact.

Phobos and Deimos are widely believed to be captured asteroids. There is some speculation that they originated in the outer solar system rather than in the main asteroid belt.

Phobos and Deimos may someday be useful as “space stations” from which to study Mars or as intermediate stops to and from the Martian surface; especially if the presence of ice is confirmed.

How was Phobos discovered?

Asaph Hall discovered Phobos in the United States Naval Observatory in Washington DC. It was found six days after its smaller sibling Deimos was found.

The astronomer used a 26-inch refractor to study the region around Mars. Studying closer to the red planet than any another scientist, he found the smaller moons orbiting the planet’s equator.

How did Phobos get its name?

Just like many planets and moons in our Solar System, the small satellite gets its name from Greek mythology. The moon is named after Phobos, the god of fear in Homer’s ancient poem, “The Iliad”. He is the son of Ares (Mars) and Aphrodite (Venus).

Geological Facts

  • The dimensions of Phobos are 27 km × 22 km × 18 km and just like Deimos, it is not a round shape like Earth’s moon.
  • Phobos does not have an atmosphere due to its low gravity.
  • The moon is one of the least reflective objects in the Solar System.
  • Both Deimos and Phobos are made up of the same material as C-type and D-type asteroids and dawrf planets.
  • The Viking probe has shown that the moon is not made of solid rock but instead has a significant porosity.
  • Phobos is heavily cratered and all
  • Phobos has an impact crater which is 9 km wide, called Stickney. The crater is named after Asptha Hall’s wife, Chloe Angeline Stickney Hall and was discovered in 1973.

Orbital Facts

  • Phobos orbit distance is 9,376 km from Mars; which this makes it the closest satellite to a planet in the Solar System.
  • The moon orbits Mars at 2.138 km/s.
  • Phobos orbit period is 7.7 hours and is below the synchronous orbit radius — which means it moves around Mars faster than the planet moves.
  • The moon orbits the planet so fast that it would rise in the west and set in the east twice per day.
  • It took so long to discover Phobos due to how close it was to Mars.
  • In approximately 50 million years scientists have predicted Phobos will be destroyed due to the tidal forces of the Red Planet. It will slowly enter the atmosphere of the planet and break up, potentially creating a ring.

Origin Facts

  • When it was first discovered, scientists believed that Martians created Phobos.
  • Just like Deimos, scientists believe that Phobos was an asteroid from our Solar System’s asteroid belt.
  • Another theory is that Phobos used to be a ring around the planet, formed by an object violently collied with Mars. The ring was slowly formed into the two moons of Mars, Phobos and Deimos.

Expeditions to Deimos

  • Phobos has been photographed by numerous spacecraft whose mission was to photograph and study Mars.
  • The first spacecraft to explore Phobos was the Mariner 9 in 1971, which became the first man-made spacecraft to orbit a planet.
  • A number of missions have proposed to study the moons of Mars. One mission is designated Phobos and Deimos and Mars Environment (PADME). Another mission is called OSIRIS-REx 2, a followup to OSIRIS-REx, a NASA asteroid study and sample study currently in development.
  • In 2011, Russia attempted a mission to Phobos called Phobos-Grunt, which was supposed to bring back a sample of the moon. However, due to a technical malfunction, it crashed into the Pacific Ocean.
  • NASA is considering a Phobos Survey mission which would deploy hedgehog-like probes to the surface of the moon.
  • Japan Aerospace Exploration Agency (JAXA) intends to launch the Mars Moons eXploration (MMX) mission to visit Phobos and Deimos, to collect samples and return in 2029.

More about Phobos

Open Issues

  • It seems likely that Phobos and Deimos formed elsewhere and were subsequently captured by Mars. But how did the capture occur? Was it made possible by a thicker Martian atmosphere long ago?