Topical Tens: 18th August: Phobos, Moon of Mars

Monday 14 August 2017

18th August: Phobos, Moon of Mars

On this date in 1877 American astronomer Asaph Hall discovered the moon of Mars, which he named Phobos.

Phobos is the innermost moon of Mars and is seven times as massive as the other moon, Deimos.
Phobos orbits 6,000 km (3,700 miles) from the Martian surface. It is, in fact, the moon which is closest to its planet than any other in the Solar System, and it orbits faster than the planet rotates. An orbit takes 7 hours and 39 minutes. If people ever go to Mars, they will see it rise in the west, move across the sky in four hours, fifteen minutes before setting in the east - twice a day.
They would also see it transit the Sun, casting a shadow on the planet's surface. It's not big enough to cause a Solar Eclipse.
It's too small to have much in the way of gravity, either and hence has no atmosphere. It's lack of mass is also a reason why it's basically a pile of rubble held together by a thin crust - it never had enough gravity to become rounded.
In the late 1950s and 1960s there was speculation that Phobos might be hollow and its crust made of a "thin sheet metal", ie that it was made by aliens. Subsequent studies have pretty much ruled that out.
Phobos is named after the Greek god Phobos, son of Ares (Mars) and Aphrodite (Venus). He is the personification of horror, the origin of the word phobia, meaning an irrational fear.
People may land on Phobos before they land on Mars. It could be a stepping stone to the red planet, because it would be easier and cheaper to land on Phobos than on Mars itself.
Phobos gets closer to Mars by about 2 meters every hundred years, Eventually, and we're talking 30-50 million years, it will either collide with the planet or break up to form a planetary ring.
We don't know much about the composition of Phobos. There have been several attempts to send a probe but things always seem to go wrong and even the one probe which managed to get there failed fairly quickly. Observations of Phobos in the thermal infrared suggest a composition of mainly phyllosilicates. The unique Kaidun meteorite that fell on a Soviet military base in Yemen in 1980 might be a part of Phobos, but because we don't know exactly what Phobos is made of we cannot be certain.
Surface features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver's Travels. The biggest feature is an impact crater called Stickney, after Chloe Angeline Stickney Hall, Asaph Hall's wife, who encouraged him not to give up the search for the moons of Mars. Stickney is 9 km (5.6 mi) in diameter and has a smaller crater inside it - Limtoc, after a character in Gulliver's Travels, which is about 2 km (1.2 miles) in diameter. Any people who get to Mars may well be able to see Stickney with the naked eye.