Facts About Phobos' Origin, Orbit, Composition And More


Phobos - imaged by the Mars Reconnaissance Orbiter on 23 March 2008
Phobos - imaged by the Mars Reconnaissance Orbiter on 23 March 2008

Phobos is the largest of the two potatoes like Martian moons. Discovered by an American astronomer Asaph Hall in 1877, it is named after a son of the Greek god of war, Ares. The 27 km × 22 km × 18 km Phobian surface is highly cratered and it is one of the darkest object in the solar system. The similar characteristics to asteroids led some scientists to hypothesize that it came from the asteroid belt. However, its birth is still controversial. Several missions have been proposed for its exploration that may reveal its origin.

Facts About Phobos

Phobos Profile

Name Phobos
Moon of Mars
Distance from Mars (avg) 5,989 km
Mean Radius 11.26 km
Volume 5783 km3
Mass 1.065 × 1016 kg
Surface area 1548 km2
Gravity 0.0057 m/s2
Temperature (avg) -40 °C or -40 °F
Length of day 7 h 39.2 min (earth time)
Discovered by Asaph Hall
Discovered on 18 August 1877

Quick Facts About Phobos

  1. Geological features on Phobos has been named after places in Gulliver's Travel Novel and astronomers who studied it.
  2. It is one of the least reflective body in the solar system.
  3. The most significant feature is Stickney crater which covers a substantial area of about 9 km, named after the discoverer wife, Angeline Stickney Hall.
  4. Phobos gravity is not strong enough to round itself into a spherical body.
  5. You need a speed of just 41 km/h to escape into space from Phobian surface.
  6. It is seven times more massive than the Mars second moon Deimos.
  7. Its indicated that Phobos is 20 to 35 per cent porous.
  8. The porous body suggests that it might have a reservoir of ice.
  9. The temperature on Phobos ranges from −4 °C or 25 °F in day to −112 °C or −170 °F at night.
  10. In 2011, the Russians sent a mission named Phobos-Grunt to collect samples from Phobos. However, it was failed to escape the earth's orbit and crashed back to the earth.
  11. The spacecraft was carrying some selected organism developed by the Planetary Society to test whether they could survive by flying them through interplanetary space.
  12. Phobos has been photographed by many spacecraft whose primary mission was to explore Mars.
  13. The first close-up image of Phobos was taken by Mariner 7 in 1969.
  14. Missions are proposed to send humans on Phobos.
  15. However, Intense solar wind can produce hundreds of volt, so scientists need to design spacesuits that prevent any charging hazard.
  16. The composition of Kaidum meteorite, which fell on a Soviet military base in1980, suggested that it originated from Phobos.

Detailed Facts About Phobos

  • Phobos was discovered six days after Deimos

Asaph Hall
Asaph Hall

On on 18th of August in 1877, Asaph Hall discovered the largest moon of Mars Phobos. Although, in the early 17th century, Johannes Kepler predicted that Mars should have two moons. However, later observation of Mars could not found any existence of moons. But in August of 1877, Hall was deliberately searching for Martain moons, using a 26-inch (66 cm) refractor telescope at the United States Naval Observatory or USNO in Washington, D.C.

Doing more precise observation than ever before, on 12th August, Hall saw a small faint object moving across the planet. The object was later named Deimos. Due to cloudy whether he had to stop his observation. After six-day, he was waiting for Deimos to re-appear and found Phobos as an object that was closer to Mars than Deimos.
  • Phobos is named after one of the sons of Mars

After the discovery of the moons, Asaph Hall preferred the name given by Henry Madan. He suggested the name "Phobos" for the bigger moon after the Greek God of fear. In Greek mythology, Phobos was the son of Ares (the Roman equivalent of Mars), the Greek God of war, and Aphrodite (Venus for Romans). Phobos with his brother Deimos helped their father into the battle by driving the chariot and spreading fear.
  • Phobos is orbiting closer to the planet than any other moon in the solar system

Phobos is revolving around Mars on an average altitude of only 5,980 km or 3,720 miles. Orbiting in an equatorial orbit and too close to mars, an observer standing at a latitude higher than 70.4° may not see Phobos above the horizon.
  • Phobos will disintegrate into a ring system around Mars

k Martian rings will probably be formed from its largest moon Phobos
Martian rings will probably be formed from its largest moon Phobos
© Flicker

With that close distance to Mars, it is coming more closer to the red planet. It is descending at a rate of approximately 2 centimeters every year. Astronomers have found many grooves and stretch marks that can be only formed by the tidal forces when Phobos is a rubble pile. New observation found more younger grooves suggesting that the process is ongoing.

With these pieces of evidence, it is likely to be torn apart rather than crashing into Mars. Researchers have estimated that in 30 to 40 million years, it will reach so close to the mars that the tidal forces will break it into debris, forming a ring system around Mars. It is calculated that the ring will last from one million to one hundred million years.
  • Phobos completes its orbit much faster than Mars itself rotates

Orbits of Phobos and Deimos
Orbits of Phobos and Deimos

Phobos orbits around Mars is below synchronous orbit that is its orbit period is less than the rotational period of Mars. It completes its orbit in about 7 hours and 40 minutes with an average orbital speed of about 2.1 km/s. So it can be seen twice a Martian day as rising in the West, and after remaining for only 4 h 15 min or less in the sky, it sets in the East.

The difference between its closest point (perihelion) and farthest point (aphelion) from the Mars is around 480m or 0.290 miles, making its orbit circular rather than elliptical.
  • Phobos frequently causes solar eclipses on Mars

Phobos shadow on Martian surface, captured by Mars Global Surveyor in 1999
Phobos shadow on Martian surface, captured by Mars Global Surveyor in 1999

Eclipses on Mars can be seen on most of the days of the year due to Phobos too close orbit. However, it cannot cause a full solar eclipse because of its small size. An observer at any particular area can see dozens of solar eclipse. Although observer at higher than 70.4° cannot see any transits but observer close to 70.4° will see Phobos covering a small portion of the sun's disk.

Shadows of Phobos on the Martian surface were photographed by many of the space-crafts and rovers. During a transit, Phobos can be seen as a black disk moving quickly across the sun. Due to the fast orbit, a transit of Phobos only lasts for thirteen seconds.
  • The origin of Phobos is a mystery

Stickney crater on Phobos imaged by the Mars Reconnaissance Orbiter in 2008

Its less reflective body and density similar to C or D types of asteroids led many scientists to think that it might be a captured asteroids. After Jupiter's gravity had pushed it from the asteroid belt, the martian gravity might have caught it. But settling from a highly eccentric orbit into an almost circular orbit needs much time and a thicker atmosphere. Since Mars has a fragile atmosphere that cannot provide the required atmospheric drag, so some scientists think that it has some other origin.

Now astronomers think that it is formed from Mars when a meteorite hits the Martian surface (similar to our moon's origin hypothesis). The dust that was ejected from the collision was later formed Phobos. JAXA, the Japanese Aerospace Exploration Agency, is sending a mission named Martian Moons eXploration or MMX to explore martian moons. The primary aim of the mission to bring back samples from Mars' largest moon Phobos and investigate whether the moons are captured or formed from a collision.

More Phobos Facts

Those are the interesting facts about Phobos. Check out these links to explore more about Phobos:

Contact Form

Send