Phobos & Deimos: Impossible Moons of Mars

~5 min

145 years ago, in August 1877, two moons orbiting Mars — Phobos and Deimos — were officially discovered. They turned out to be among the weirdest natural satellites in the whole Solar System. Let’s see what’s so special about these moons.


Interesting facts

  • If you spent a few days on Mars, you’d be surprised by the weird motion of its moons. During Deimos’ slow 66-hour journey across the sky toward the west, Phobos rapidly swishes in the opposite direction more than six times.

  • Phobos orbits its planet closer than any other moon in the Solar System — only 6,000 km (3728 miles) above the Martian surface (in comparison, our Moon is 384,400 km above the Earth). Deimos is in the second place among Solar System planets’ moons.

  • Mars will lose its moons. Phobos that slowly approaches Mars will crash into the planet or break up into a ring. Deimos, on the contrary, moves away from Mars and eventually will leave the planet’s orbit.

  • Although Phobos is only 1% of our Moon’s diameter, it appears about half as wide as viewed from Mars. Deimos, in its turn, seems similar to a star in size from the Red Planet.

  • Phobos and Deimos’ origin is still controversial and doesn't correspond with how planetary moons are typically formed.

How many moons does Mars have?

To date, Phobos and Deimos remain the only two moons discovered in Mars’ orbit. There are theories that the planet has even more moons, about 50-100 meters (164-328 ft) in size, but none has been found yet.

Phobos (Mars I)

  • Radius: 11.2 km (6.9 miles)
  • Mass: 1.0659×10^16 kg
  • Orbital speed (sidereal): 7 h 39 m 12 sec
  • Surface temperature: from −4 °C to −112 °C (24.8 °F to −169.6 °F)
  • Orbital distance: 9235,6 km (5739.5 miles)
  • Rotation: synchronous
  • Apparent magnitude: 11.8
  • Named after: Greek god of fear and panic

Deimos (Mars II)

  • Radius: 6.2 km (3.8 miles)
  • Mass: 1.4762×10^15 kg
  • Orbital speed (sidereal): 30.3 hours
  • Surface temperature: -40.15°C (-40.27 °F)
  • Orbital distance: 23,458 km (14576 miles)
  • Rotation: synchronous
  • Apparent magnitude: 12.89
  • Named after: Greek god of dread and terror

Theories of Martian moons’ origin

There are a few theories explaining the Martian moons’ origin. Let’s take a closer look at them.

They’re ex-asteroids

Irregularly shaped and heavily-cratered Phobos and Deimos may be asteroids captured by the gravitational pull of Mars (just like Saturn’s Phoebe or Neptune’s Triton). But the Martian moons’ near-circular orbit is unlikely possible in this scenario. Computer simulations show that if Phobos and Deimos truly were asteroids, they would have more irregular orbits.

They’re results of a giant impact

Martian moons may have formed as a result of a massive impact (as it likely happened with the Earth’s Moon and Pluto’s moons). That impact sent rock and debris into orbit around the Red Planet. But, as with the asteroid hypothesis, computer simulations disprove this theory. Apparently, there is no combination of collision parameters that would lead to the formation of two small, low-mass moons, such as Phobos and Deimos.

They have formed from rock and dust around Mars

Phobos and Deimos could have formed from a giant disk of rock and dust that orbited Mars in the early Solar System. The same happened with most of the moons in the Jovian, Saturnian, and Uranian systems. But scientists say that this requires a large, massive disk, and if one ever existed around Mars, a formation of a single larger moon instead of two small ones would be more likely.

They’re remnants of a shattered moon

Because the three previous theories that explained the formation of other natural satellites can't explain the Martian moons, scientists came up with a new idea.

Recent research showed that the orbits of Phobos and Deimos might have intersected between 1 and 2.7 billion years ago, suggesting that their predecessor was a larger moon that likely broke apart due to a giant impact. The debris of this moon could have fallen down on Mars, which explains the multiple craters on its surface.

This theory needs more data to be proved. Scientists pin their hopes on the upcoming Martian Moons eXploration mission from Japan’s space agency that aims to survey the two moons and bring samples from Phobos. Clarifying the origin of the two moons will help us understand more about the formation of the Solar System.

Who discovered Mars’ moons?

American astronomer Asaph Hall discovered two Martian moons in 1877. He found the first moon that he named Deimos on August 12, 1877; the second one he saw six days later, on August 18, 1877, and called it Phobos.

Astronomers first suggested that Mars had moons in the 17th century, around the time when Jupiter’s natural satellites were discovered. But they came up with this idea not because of calculations or observations but due to a simple mistake.

To keep privacy, Galileo Galilei, in his letters to Johannes Kepler, used anagrams (scrambled texts). When Galileo saw something curious near Saturn, he sent Kepler an anagram meaning, “I have observed the highest planet to be triplets.” Galileo meant Saturn and his rings (which he mistook for moons because they looked like dots through his primitive telescope). However, Kepler interpreted his anagram as “Mars has two moons”, which, of course, was true, but none of the astronomers knew about this.

Telescopes back then were too weak to show the small Martian moons orbiting extremely close to the planet. After this curious coincidence, astronomers spent two more centuries trying to find Martian natural satellites.

The names of Mars’ moons

Asaph Hall named Martian moons after the Greek mythological twins — Phobos, the god of fear and panic, and Deimos, the god of terror and dread. Phobos and Deimos were sons of Ares, the Greek god of war. To Romans, Ares was known as Mars.


What are the two moons of Mars?

Mars has two known Moons — Phobos and Deimos, designated as Mars I and Mars II, respectively. They’re both very small: Phobos has a radius of 11.2 km (6.9 miles), and Deimos — only 6.2 km (3.8 miles).

Will Mars lose its moons?

Calculations show that Deimos is gradually moving away from Mars and eventually will leave the planet’s orbit; Phobos, on the contrary, is moving closer to the planet and one day will likely collide with Mars.

What exactly will happen to Phobos?

Mars will destroy Phobos. The Red Planet’s gravity is drawing Phobos in by about 1.8 meters (5.9 ft) every hundred years. At this rate, in 50 million years, the moon will either crash into Mars’ surface or break up into a ring.

Who discovered the two moons of Mars, Phobos and Deimos?

American astronomer Asaph Hall discovered Phobos and Deimos in August 1877. Hall first saw Deimos on August 12, 1877, and Phobos on August 18, 1877. History says that he was about to give up on his searches for Mars’ moons just a day before finding Deimos, but Hall’s wife, Angelina urged him to continue observations.

Bottom line: The two moons of Mars, Phobos and Deimos, are among the weirdest moons in the Solar System. They’re tiny, resemble asteroids in shape, and orbit the Red Planet closer than any other moons. Moreover, their origin doesn’t match the formation of other moons and seems almost impossible at first glance.