Phobos is one of the two moons that orbits Mars. It is only 3,700 mi/6,000 km above the surface and this is one of the main reasons that astronomers of the past had a difficult time seeing it. The fact is – Phobos is actually getting closer to Mars by 6 ft/1.8 m every century.

Moon Phobos Introduction

Scientists believe that in around 50 million years Phobos will either break up and create a ring around Mars or simply crash into the Red Planet. No other moon in the solar system orbits that close to it parent planet.

Mars has some of the smallest moons in the entire solar system. The two moons, Phobos and Deimos race around the planet at incredible speeds and completes three full orbits each day. Phobos is a bit larger than its brother moon, Deimos and is also slower in its orbit at 30 hours.

Looking at Phobos from the surface of Mars you would see that the moon takes up a huge part of the sky. Both of the moons are tidally locked, with only one face showing as they circle around Mars.

Phobos and Deimos share some similarities in appearance as they are each filled with craters from a lot of impacts, are lumpy looking, and are covered in loose rocks and dust. They are some of the darkest objects in our solar system and scientists think they are made up of carbon-rich rock and ice that may be left over from asteroids.

Phobos Statistics:

  • Orbits: Mars
  • Discovered By: Asaph Hall
  • Discovery Date: August 17, 1877
  • Diameter: 22.2 km
  • Mass: 1.07 × 10^16 kg (0.00001% Moon)
  • Orbital Period: 7.7 hours
  • Orbit Distance: 9,376 km
  • Surface Temperature: – 40 degrees C


Johannes Kepler was a German astronomer in the early 17th century. He based a proposal about the moons of Mars on the fact that it was positioned between Jupiter and Earth and they were known to have from 1-4 satellites. His theory was that Mars had two moons. During that time no evidence could be presented to prove Kepler’s theory and some astronomers thought that Mars had no moons.

In 1877 Asaph Hall, an American astronomer had been studying Mars from the U.S. Naval Observatory in Washington, D.C. He was almost ready to given up on his research to find a moon orbiting around Mars, but his wife, Angelina encouraged him to continue.

He discovered the first moon, Deimos and then six nights later he discovered Phobos. Both moons were so close to the planet that they were often hidden by the glare of Mars. The two Martian moons were the size of asteroids, and Phobos was only 7.24 times bigger than Deimos.

Hall decided to name the moons for the sons of Ares, the mythological Greek god of War. Mars is the Roman god of war.  Phobos means panic or fear, and we are familiar with the use of the word in “phobia.” Deimos means running away or fleeing after a defeat.

Stickney mro Phobos History

During the 1950s and 60s, astronomers discovered that the orbit of Phobos was decaying and some thought that Phobos was artificial. One of these individuals was President Dwight D. Eisenhower’s science advisor and they based the theory on both odd appearance of Phobos and its orbit.

This means that at one time, prominent scientists in the American government thought that Phobos was created by aliens, possibly, Martians. Other scientists examined images of Phobos to realize that it looked more like a rubble pile and assumed it was a natural formation.

It took 94 years before NASA could get closer images of Phobos with the Mariner 9 spacecraft. Mariner 9 took images of both moons, noting a major feature on Phobos of the 6 mi/10 km wide crater that took up almost half of the moon’s width. The crater was given the name “Stickney,” which was Angelina’s maiden name.

Formation, Structure and Surface:

Phobos’ surface looks like it has been beaten up by hundreds of impacts from meteorites and small asteroids. It is gouged and has an appearance as if it was almost shattered. It’s the larger of the two moons of Mars and is moving fast and close to the surface of Mars.

Both of the moons have strange, non-spherical shapes and are considered to be some of the darkest and least reflective objects in our solar system.

It seems to be made up of C-type rocks which are similar to the carbonaceous chondrites with a black color that make up asteroids and dwarf planets. The Mars Global Surveyor has made observations that show that the surface of this little moon has been pounded for thousands of years by meteor impacts.

Many of these have left dark trails and have created huge craters. The largest 6 mi/9.7 km wide crater, Stickney, seems to be filled with fine dust and has boulders sliding down the sloped crater surface.

Temperatures on Phobos are extreme in both the night and day sides. High temperatures are around 25 degrees F/- 4 degrees C and the lower temperatures hit -170 degrees F/-112 degrees C.

The cold is often worse that an Antarctic night on Earth. Scientists believe that the loss of heat may be caused by the fine surface dust which can’t retain heat.

Due to the composition and appearance, some scientists think that both moons may have originated in the asteroid belt but that Jupiter’s gravity pushed them into Mars’ orbit. Other scientists aren’t convinced that the two moons were from the asteroid belt because they have almost circular orbits which is not normal for captured objects.

Mars has only a thin atmosphere which wouldn’t be enough to hold Phobos and Deimos in such a perfect orbit.

The other argument against the asteroid belt origination is that they aren’t as dense as the objects found in the asteroid belt, and seem to instead be made up of rocks and dust that gravity drew together.

As astronomers continue to try to figure out where Phobos and Deimos came from they have developed another theory that they could be the result of a collision with Mars.

Globe of Phobos Statistics

An impact could have sent pieces into the area within the gravity of Mars where they came together to create the moons. This concept has led to another alternative that actually brings a number of the theories together.

Scientists have proposed that a collision could have scattered debris into the Mars orbit to create a ring, which then came together to create the moons.

A NASA-led study of Phobos in 2015 took a closer look at the long grooves that cover the surface. They believe that these grooves may be the preliminary signs that the moon is beginning to break apart due to the tidal stresses that are caused by the gravity of Mars.

The specialists at NASA’s Goddard Space Flight Center in Maryland believe that the grooves or stretch marks show that Phobos is failing.

Other scientists are offering the idea that instead, the grooves are what is left of the impact that created the Stickney crater or that they are from the left over material from Mars that hit the surface of Phobos.

Atmosphere and Magnetosphere:

Phobos doesn’t have any atmosphere or magnetosphere.

Could Life Exist?

Phobos doesn’t have any of the requirements needed to encourage life as we know it.

Interesting Information:

  • As we continue to explore space, scientists are currently considering using either Phobos or Deimos to set up a base for astronauts to use for observing Mars as well as launching robots to the surface of Mars. The miles of rock on the moons would help to protect the astronauts and equipment from solar radiation and cosmic rays for around two-thirds of each orbit.
  • Phobos is moving around Mars so fast that it crosses the Martian sky twice a day. It only takes Phobos 4 hours and 15 minutes to cross the visible sky. It rises in the west and sets in the east. It moves around Mars faster than Mars is moving.
  • Scientists need to continue to study Phobos so that they can confirm if it’s a captured asteroid or formed from rock and dust left over from an impact on Mars. It may be similar to an asteroid, but its circular orbit makes that seem unlikely.
  • As Phobos continues to have a decayed orbit it will spiral closer to Mars and either be torn apart to create a Martian ring or crash into Mars in 50 million years.
  • The impact that created the Stickney crater almost tore Phobos apart.
  • In the 1950s and 1960s, Iosif Samuilovich Shklovsky, a Russian astrophysicist, offered the idea that Phobos might be hollow and covered in a thin sheet of metal. This went along with the same idea that some American scientists had that Phobos was artificial.
  • Phobos is so small and has such low gravity, that if you weigh 150 lbs on Earth you would only weigh 2 ounces on Phobos.

Patraw Exploration


The Mariner 9 mission of 1971 was the first manmade satellite to ever orbit any other planet and it sent a lot of pictures back to Earth of both Phobos and Deimos.

The 1977 Viking 1 and 1988 Phobos took images of Phobos and added to the original information that the moon was oddly shaped and rather lumpy.

Exploring Mars is one of the top priorities for many countries and that often includes both of Mars’ moons. Additional information about Phobos has been picked up from the satellites that are orbiting Mars. In the late 1970s the Viking orbiters did a flyby, the Soviet Phobos 2 mission, the Mars Global Surveryor from NASA, and the European Mars Express have each added new information about the moons.

The Mars Rovers, Spirit, Opportunity, and Curiosity have all added new data about Phobos by sending images from the surface.

The NASA MAVEN (Mars Atmosphere and Volatile Evolution Mission) has taken pictures of Phobos to help scientists learn more about what it’s made up of.

Facts about Phobos for Kids:

  • Phobos looks somewhat like a potato and has a lot of craters.
  • Some of the larger features on Phobos were named after places in the novel “Gulliver’s Travels.”
  • The shadow cast by Phobos has been photographed by a number of spacecraft.
  • The fine dust that covers the surface of Phobos is called “regolith.”
  • Some astronomers have predicted that the fine dust or regolith that is all over the surface of Phobos drifts off into space and leaves a very faint “tail.”
  • Scientists are continuing to recommend that either Phobos or Deimos be used as a staging areas for future missions to Mars.

Bottom of Form

Pop Culture:

Phobos has been used as a center topic by many books, movies, and games.

Jonathan Swift’s novel Gulliver’s Travels, part 3, chapter 3, was published in 1726 and featured astronomers of Laputa describing their discovery of two satellites orbiting Mars. This is notable because this is many years before Asaph Hall discovered Phobos, in 1877.

Edgar Rice Burrough’s Barsoom novels included a moon around Mars that was named Thuria.

Arthur C. Clarke’s novel, The Sands of Mars, includes terraforming the planet Mars and igniting Phobos as a second sun. In another of his short stories, Hide and Seek, Clarke has a character evading capture by landing on Phobos.

In the television series, Doctor Who, the Eighth Doctor & Lucie Miller land on Phobos.

The television show Captain Scarlet and the Mysterons has Phobos featured in the episode Shadow of Fear.

A Donald A. Wollheim 1955 science fiction novel entitled Secret of the Martian Moons has both Phobos and Deimos as interstellar spacecraft that are only disguised as moons and that were constructed by a race of humanoid-looking aliens.

The Paul Capon novel Phobos the Robot Planet (aka Lost: A Moon), Phobos is really a giant computer that is the last relic of a race of Martians that have long vanished.