Phobos (moon)

Phobos is the larger and closer of the two moons of Mars, the other being Deimos. It was discovered in 1877 by American astronomer Asaph Hall. Phobos is named after the Greek god of fear and panic, who is the son of Ares (the Greek equivalent of Mars) and the twin brother of Deimos.

Phobos is a small, irregularly shaped object with a mean radius of 11 km. It orbits only 6,000 km from the Martian surface, which is closer than any other known moon to its planet. Phobos completes an orbit around Mars in just 7 hours and 39 minutes, much faster than Mars itself rotates. Because of this quick orbit, Phobos appears to rise in the west and set in the east twice each Martian day.

The moon’s surface is very dark, with an albedo of 0.071, meaning it reflects very little sunlight. Temperatures on Phobos range from about −4 °C on the sunlit side to −112 °C on the side in shadow. A major feature on its surface is a large impact crater named Stickney. The surface also has many grooves, and scientists have several ideas about how these formed.

Studies suggest that Phobos might be a rubble pile held together by a thin crust. It is slowly getting closer to Mars, moving about 2 centimetres closer every year due to tidal interactions.

Discovery and etymology

Main article: History of the moons of Mars

Phobos was found by the American astronomer Asaph Hall on 18 August 1877. He was at the United States Naval Observatory in Washington, D.C.. Hall had found Deimos, Mars's other moon, a few days before. They used the world's largest refracting telescope, called the 26-inch "Great Equatorial".

The names Phobos and Deimos were suggested by a British teacher named Henry Madan. He taught science at Eton College. He chose the names from Greek mythology, where Phobos is a companion to the god, Ares.

Physical characteristics

Phobos is a small, oddly shaped moon of Mars, about 26 by 23 by 18 kilometres. It has very little gravity, so it cannot keep an atmosphere. Phobos does not reflect much sunlight and looks dark. Its surface is covered with a loose layer of dust and rocks, and it may have some ice underneath.

Phobos has many craters, with its largest one, Stickney, almost 10 kilometres wide. This big crater almost broke Phobos apart. The moon also has long, narrow lines on its surface, which scientists believe came from pieces thrown up by impacts on Mars. These lines are grouped together, showing they came from different impacts.

Orbital characteristics

Phobos is the larger and closer of Mars's two moons. It orbits Mars very quickly, moving across the sky in just a few hours. Because it is so close to Mars, it appears to rise in the west and set in the east, opposite to most moons.

From Mars, Phobos looks much larger when it is directly overhead compared to when it is near the horizon. It also passes in front of the Sun, creating shadows on the Martian surface. Scientists believe that due to its close orbit, Phobos is slowly getting closer to Mars.

Main article: Transit of Phobos from Mars

Origin

The origin of Phobos and its sister moon Deimos is still being studied. Both moons look like some types of asteroids, so some think they were caught by Mars's gravity. But their paths around Mars are almost round and match Mars's equatorial plane, which makes this idea hard to believe. Another idea is that Phobos formed after Mars, from bits of material that came together in orbit.

Recent studies using data from the Mars InSight Mission suggest that Phobos and Deimos might have come from the same object that broke apart a very long time ago. The surface of Phobos has different colors, maybe because materials have moved across it due to Mars's gravity.

Exploration

Many spacecraft have taken pictures of Phobos while studying Mars. The first was Mariner 7 in 1969, and others followed, such as Mariner 9 and Viking 1. In 2005, the Spirit rover on Mars captured images of both Phobos and its companion moon, Deimos.

The Soviet Union tried to send two probes to Phobos in 1988 as part of the Phobos program. One probe, Phobos 1, stopped working on its way to Mars. The other, Phobos 2, reached Mars but stopped sending data before it could start detailed studies of Phobos. More recent missions, like Mars Reconnaissance Orbiter and the Mars orbiter of the Indian Space Research Organisation, have continued to capture images of Phobos from far away.

Phobos is a target for future missions. The Japanese Aerospace Exploration Agency plans to launch the Martian Moons eXploration (MMX) mission in 2026 to collect samples from Phobos and study its surface. Other ideas for missions to Phobos include using it as a base for future human exploration of Mars or even building a space elevator to help spacecraft travel to and from Mars more easily.