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The Two Moons of the Planet Mars: Phobos and Deimos

Updated on June 14, 2021
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My writing interests are general, with expertise in science, history, biographies, and “how-to” topics. I have written over seventy books.

Moons of Mars

A composite image of the two moons of Mars.
A composite image of the two moons of Mars. | Source


Phobos and Deimos are the names of the two natural satellites, or moons, of the planet Mars. The planet itself is named after Mars, the Roman god, supposedly the Father of Romulus and Remus, and Guardian of the Roman people; the Greeks called the same god Ares, attributing war-like characteristics to him. When it came time to name the moons of Mars, it seemed only natural to name them after two of Ares’ numerous mythical children: Phobos (fear) and Deimos (Terror).

Superficially, you couldn’t describe these two moons more aptly than to reach into a bag of potatoes and pull out two random samples. Any photograph will make that abundantly clear. This is further supported by the legend that the SFX teams for the original Star Wars movies had used potatoes for background asteroids during the Millennium Falcon’s escape from the Imperial Star Destroyer.


For a moon, Deimos is positively minuscule at only 7.8 miles (12.6 kilometres) in diameter. There are much bigger rocks in the Asteroid field that lies between Mars and Jupiter, and it is often conjectured that the Martian moons were captured from there.

Deimos orbits at a distance of 14,577 miles (23,460 km), which is 3.5 times the diameter of Mars itself, and takes 30 hours to complete a circuit around Mars. Mars, on the other hand, takes about 24 hours and 40 minutes to complete a rotation. This has the effect of making the moon appear to rise in the east, but only crawl across the sky very slowly. To an observer standing at the equator on Mars it would take about five-and-a-half days to see Deimos return to its starting point.

Astronomically it is known as Mars II, and was discovered by Asaph Hall on August 12th, 1877. Its gravitational force is so small that, if you accidentally travelled faster than 12.5 mph (20km/h) while on its surface, you would attain escape velocity and leave the surface forever to drift in space.

It constantly surprises me that it was discovered at all back in the late 1800s. Even in large modern sky-watching telescopes these moons are mere pinpricks of light. At closest approach to us, Mars is still over half of an Astronomical Unit (AU, the distance Earth is from the Sun) away. That would be about four light-minutes – say thirty-five million miles (55 million km) away. Finding a needle in a haystack would be child’s play in comparison. Imagine trying to find a grain of sand sitting next to a pea at the far end of a football field with a pair of low-power binoculars. Most people would have given up, and Hall nearly did but for the existence of an ardent supporter.

Deimos’ surface area (~300 mi2/500 km2) is approximately one quarter of the area of Rhode Island, and not nearly as interesting, unless you like really big, oddly shaped rocks. Its two largest craters (Swift and Voltaire) are both less than 2 miles across. They were named in honor of Jonathon Swift the author who mentioned the two moons of Mars in 1726 in his book Gulliver’s Travels and by François-Marie Arouet (known popularly as Voltaire because the repression of the time forced him to conceal his identity) who wrote a story called Micromégas in 1750. Voltaire’s story is about an alien visitor to Earth who spoke of the two moons of Mars. Neither of the moons had been discovered at the time!

Close up look at Deimos

Image of Deimos taken by the Viking I space probe in 1977.
Image of Deimos taken by the Viking I space probe in 1977. | Source


So what is interesting about Phobos? Well, it scoots across the sky with perceptible motion! This .gif image shows the view from NASA’s Curiosity Rover on August 1st, 2013 as Phobos occulted the smaller, distant moon Deimos.

Looking at the animation you might wonder if it is the wrong way around. But it’s not! This moon is so close and travels so fast that it rises in the west and sets in the east. This speedy fellow is so quick and spry that you can see two moonrises every night!

Phobos is in such a low orbit (only 1.4 times the diameter of Mars, as if our earthly Moon was only 12,000 miles away instead of a quarter of a million) that it is almost in areostationary orbit. This is the equivalent of geostationary orbit for Earth (Geo=Earth; Areo=Mars). Its orbital period is actually 30 hours long. If it moved a tiny bit faster, in a slightly lower orbit, it would stay in exactly the same place all the time.

Phobos surface area is 960 mi2 (1550 km2) or about eighty percent of Rhode Island’s area. It is known astronomically as Mars I, and has a rather large crater in one end, nearly six miles (nine kilometers) across. It is named Stickney Crater by the discoverer of Phobos (Aug/18/1877), Asaph Hall, in honor of his wife, Angeline Stickney Hall. He said that without the encouragement and support of his wife, he would never have discovered the Martian moons. Angeline Stickney Hall herself was a force to be reckoned with; she was not from a wealthy family; she was a suffragette, abolitionist and a mathematician, who obtained her degree by working at the college, teaching while she studied. She is believed to have helped Asaph with his astronomical calculations.

Phobos is not large by any stretch of the imagination, but is almost twice as large as its sibling Deimos. It would be a lot safer to be on Phobos rather than Deimos since you would have to accidentally achieve 25 miles per hour (41 km/h) in order to inadvertently obtain escape velocity and float off into space forever.

It orbits Mars at a distance of 3,721 miles (~6,000 km) which is 1.4 times the diameter of Mars itself. Again, about the shape and coloration of a potato, this moon outweighs Deimos by seven times the other’s mass. Having double Deimos’ outer dimensions doesn’t give it seven times its volume, so why is it so much heavier? Little Deimos is “fluffy” (for being made of rock) because it is speculated to possesses many interstices (small spaces) inside. These might derive from the original material forming from an aggregate of ice and rock, and in the intervening time, the ice has dissipated leaving gaps. Some suspect that Phobos still possesses a more solid icy interior.

Close Look at Phobos

The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took this image of Phobos on March 23, 2008.
The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took this image of Phobos on March 23, 2008. | Source

Where did the Moons come from?

Two theories prevail currently: that they were formed by accretion or acquired by capture. Their composition makes them look very much like common asteroids. The Asteroid Belt is about 80% Carbonaceous and Carbonaceous Chondrites, which are dark, low albedo (ability to reflect light) objects and the moons do seem to match this description.

Others contend that they “condensed out” of interspatial material “in place” after Mars itself had formed. Either explanation could adequately encompass the morphology we suspect is present. We won’t have any hard data until the arrival of a Russian probe headed to Phobos with the Boomerang Mission in 2020. There are many national and international ventures headed to Mars, including for the first time an Indian probe, constructed entirely of homegrown components and technology that will make India a member of a very exclusive club of Martian explorers! But it looks like we’ll be relying on the Russians to get us some good solid data on the moons.

Mars Moons Video

Where are they going?

Interestingly, these two moons have very different fates ahead of them. This is attributable to their significantly different orbits.

Poor Phobos is losing speed every day, and approaching Mars at about one inch per year because of tidal deceleration. To understand this you must grasp that even rocky surfaces bulge like our tides do here on Earth. The problem is that Phobos orbits so quickly that the tidal bulge is always behind it. It increases gravitational drag, slowing the moon fractionally. It is destined to be crushed into rubble by tidal forces when it reaches one Mars diameter from the surface. At that point there are two possible outcomes:

  1. It may dissolve into relatively small particles and form into a ring surrounding the planet
  2. It will shatter into big pieces and crash into the surface in about 40 million years

Deimos, on the other hand, is like our own Moon. It has a much longer orbital period than Phobos. The planet Mars turns significantly faster than Deimos orbits. This means that the tidal bulge that it creates is pulled along ahead of Deimos, which acts to speed it up, forcing it into a higher and higher orbit until finally it will achieve escape velocity and simply drift off into the void. Again, like our own Moon, this is millions of years into the future.


  • Fix, John D. Astronomy: Journey to the Cosmic Frontier. Third Edition. McGraw Hill Higher Education. 2004.

© 2014 Doug West


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