Mars' Moon Phobos
Written by Fraser Cain
Mars has two moons, Phobos and Deimos. The larger Mars moon Phobos, is named after the greek god (his name means "fear").
If you saw Phobos in space, you would think that it's nothing more than an asteroid. And astronomers think that it might actually be a captured asteroid. In fact, it's one of the least reflective objects in the Solar System, sharing many characteristics with D-type asteroids.
Phobos is only 22 km across, and orbits Mars at a distance of 9,377 kilometers. Its density is too low to be solid rock, so scientists think that it might be porous in nature. In fact, there could be large deposits of ice underneath its surface covering of dust.
Since Phobos orbits Mars so low, it actually completes an orbit every 7 hours and 39 minutes. Seen from the ground, Phobos would cross the sky in about 4 hours, and appear about 1/3rd the size of the Earth's moon.
Phobos orbits Mars faster than Mars takes to complete a day. And this is bad news for the tiny moon. The gravity of Mars is slowing down its orbit, and drawing it down slowly. Over the next 10-50 million years, Mars' moon Phobos will gradually descend until it passes within the Roche limit of Mars' gravity. When this happens, Phobos will be torn apart into a string of debris. And then a few million years later, this debris will rain down on Mars creating a line of craters. There are other lines of craters like this on Mars, showing that this process has probably happened before.
NASA's Mars Exploration rovers have seen Phobos pass directly in front of the Sun several times. Although we call this an eclipse on Earth, it's technically a transit, since the moon doesn't block the disk of the Sun.
Want more information on Phobos? Here's a story about how it might only have 10 million years to live. And here's a wonderful image of Phobos, captured by the Mars Reconnaissance Orbiter.
I'd also recommend the Wikipedia entry on Phobos, and the Solar Views article on Phobos.
Finally, if you'd like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.
Filed under: Astronomy
