Phobos 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.

Phobos Moon Profile

Discovered By:Asaph Hall August 17, 1877
Diameter:22.2 km
Mass:1.07 × 10^16 kg (0.00001% Moon)
Orbit Distance:9,376 km
Orbit Period:7.7 hours
Surface Temperature:-40 °C

Phobos is the larger of Mars’ two moons and it has a surface that shows streaks on its sides and many grooves.

Both of the moons of Mars are smaller than our moon and each one looks more like an asteroid than our moon.

Phobos is only 22 km/14 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.

Elongated Shape

While most moons, including our own, are more spherical or circular in shape, Phobos is elongated like many asteroids.

The combination of these characteristics have made scientists question how Phobos and Deimos (the other Mars moon) were formed. Phobos is closer in orbit to Mars and its surface seems to be covered in debris and material that may be from impacts on Mars.

Fast Moving

It moves really fast, traveling around Mars three times per day, going west to east. For many years, most scientists didn’t think the Mars had any moons.

While Johannes Kepler guessed that Mars might have two moons, 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.

Phobos is named after

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

The orbit of Phobos is changing so that it is slowly moving toward Mars at 6 ft (1.8 meters) each 100 Earth years.

It’s estimated that in 50 million years Phobos will either break up and become a ring of debris around Mars or just crash into Mars.

What do scientists think?

NASA scientists have said that the grooves, which are thought to be “stretch marks” found on Phobos are already signs of the moon’s failure.

Unlike Phobos, Deimos is drifting away so in millions of years, Mars may not have any moons at all.

Both of the moons 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.

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.

Facts about Phobos

  • There have been a few spacecrafts that have taken pictures of both Phobos and Deimos as part of the flybys and several others have accomplished long-range observations, but none have had these moons as their primary target.
  • The Mars Rovers devoted years on the surface of the planet and have taken both videos and pictures of the moons. One of the most notable is when the rover Curiosity snapped an image of Phobos eclipsing the sun.
  • The orbit of Phobos is so fast that if you were watching it on Mars it would look like it set in the east two times each day.
  • Phobos casts a shadow across Mars and pictures of the shadow have been taken by quite a few spacecraft.
  • There is a very fine layer of dust on the surface of Phobos called “regolith” that is believed to drift off and leave a kind of “tail”
  • The shape of Phobos is kind of like a potato.
  • Many of the features on Phobos are named after places in the Jonathan Swift book “Gulliver’s Travels.”
  • There is conversation of using Phobos as a staging area for human missions to Mars.


  • What is the shape of Phobos?
    a potato
  • What celestial object do astronomers say Phobos looks like?
    an asteroid
  • Who is Phobos named after?
    one of the sons of the Greek mythological god Ares
  • What Rover took pictures of Phobos from the surface of Mars?
  • What is happening to the orbit of Phobos?
    it is drifting closer to Mars
  • What surface signs on Phobos do scientists think show that the moon is failing?
    grooves,stretch, marks