The Moons of the Planet Mars: Phobos and Deimos
Since the last post focused on the Planet Mars, I thought it would be worth highlighting the two tiny moons that light up the Martian night sky. The names of the moons of Mars are Phobos and Deimos.
Let’s start with the moon that is closest to Mars, Phobos. Phobos is small – it’s radius is 11.1 km, deeply cratered, and it is not a sphere! It is closer to the surface of it’s primary than any other moon that we know of in the solar system. This causes some interesting effects. If you were to look at Mars from the surface of Phobos, it would appear 6,400 times larger than the full moon appears to us on Earth. Phobos also circles Mars faster than the planet rotates which means that it rises in the west, moves quickly across the the sky and sets in the east twice each day! Because Phobos is so close to the surface of Mars, it’s orbit is slowly decelerating. It is estimated that it will either impact the surface or break up into a planetary ring in about 11 million years.
The companion moon to Phobos is the smaller of the pair, Deimos. It is also irregularly shaped and deeply cratered. It has a radius of 6.2 km. Unlike Phobos, Deimos’ orbit is nearly circular and is close to the Martian equatorial plane. As seen from the surface of Deimos, Mars would look 1,000 times larger than the full moon as seen from Earth. Since Deimos is so small, it would look much like Venus does from the surface of the Earth. Deimos rises in the east and sets in the west. It’s orbital period is slightly longer than the Martian day, so 2.7 days elapse between it’s rising and setting.
The origin of the Martian moons is not clear. Some scientists speculate that they are captured asteroids, other scientists speculate that they were formed when the planet Mars was formed. Given their similar composition and Mars’ proximity to the asteroid belt, I’d bet they are captured asteroids. What do you think?