The Hubble Space Telescope Spots Martian Moon Phobos

NASA’s Hubble has seen the Martian Moon Phobos Orbiting the Red Planet, Hubble Space's sharp eye has captured the tiny world appearing star-like in the Hubble pictures because the moon is so small. Hubble managed to take 13 separate exposures over the course of 22 minutes, which enable astronomers to create a time-lapse video showing the diminutive moon's orbital path. The Hubble observations were intended to photograph Mars, and the moon's cameo appearance was a bonus.

This moon is a football-shaped object measuring just 27 km × 22 km × 18 km and it is one of the smallest moons in the solar system, completing an orbit in just 7 hours and 39 minutes, which is faster than Mars rotation. Because of that it is the only natural satellite in the solar system that circles its planet in a time shorter than the parent planet's day. 

Phobos was discovered by Asaph Hall on August 17, 1877 at the U.S. Naval Observatory in Washington, D.C., six days after he found the smaller, outer moon, named Deimos. Hall was deliberately searching for Martian moons. Both moons are named after the sons of Ares, the Greek god of war, who was known as Mars in Roman mythology. Phobos (panic or fear) and Deimos (terror or dread) accompanied their father into battle.

Credits: NASA, ESA, and Z. Levay (STScI), Acknowledgment: J. Bell (ASU) and M. Wolff (Space Science Institute)

The origin of Phobos and Deimos is still being debated. Scientists concluded that the two moons were made of the same material as asteroids. This composition and their irregular shapes led some astrophysicists to theorize that the Martian moons came from the asteroid belt.

However, because of their stable, nearly circular orbits, other scientists doubt that the moons were born as asteroids. Such orbits are rare for captured objects, which tend to move erratically. An atmosphere could have slowed down Phobos and Deimos and settled them into their current orbits, but the Martian atmosphere is too thin to have circularized the orbits. Also, the moons are not as dense as members of the asteroid belt.

Phobos may be a pile of rubble that is held together by a thin crust. It may have formed as dust and rocks encircling Mars were drawn together by gravity. Or, it may have experienced a more violent birth, where a large body smashing into Mars flung pieces skyward, and those pieces were brought together by gravity. Perhaps an existing moon was destroyed, reduced to the rubble that would become Phobos.

Sources: NASA, Wikipedia