Mars Moons

Mars is the fourth planet from the Sun and is visible through the naked eye. This planet has no rings but only two moons: Phobos and Deimos.

The two moons of Mars were not formed in the same way as the Earth's moon. They are believed to be the fragments of larger objects orbiting around the planet broken apart in a collision, or could be captured gravitationally in the past. They are also considered to be asteroids that were captured by the planet's gravity which could not escape later. They are small in size and have high eccentricity, i.e. they deviate from a perfect circle.

Phobos

Out of the two Mars moons, Phobos is the larger and closer moon, named after the Greek God which means 'fear'. It was discovered by astronomer Asaph Hall, Sr, on August 18, 1877, at the United States Naval Observatory in Washington, D.C. It is non-spherical in nature and appears like an asteroid. Due to its shape, the gravity on its surface varies by about 210% and the tidal forces raised by Mars double this variation to 450%. Phobos is one of the least reflective objects in the solar system, having characteristics similar to C or D-type asteroids. It is composed of rocks which are rich in carbonaceous material much like C-type asteroids and carbonaceous chondrite meteorites.

Phobos orbits Mars at a distance of 9,377 km and is only 22 km across. Its density is low, and according to some scientists it could be porous in nature. There might be huge deposits of ice underneath its surface covering the dust. As it orbits slowly, it takes 7 hours and 39 minutes to complete one full orbit. But it orbits faster than Mars, as Mars takes a day to complete an orbit. If viewed from the Earth, it would cross the sky in 4 hours and appears ⅓ the size of the Earth's moon. Since the gravity of Mars is slowing down its orbit, it is estimated that in the coming 10-15 million years, Phobos will slowly descend until it passes within the Roche limit of Mars' gravity. If this happens, this moon will be torn apart into a string of debris. A few million years after that, these debris will rain down on Mars creating a line of craters. There are already many lines of craters on Mars, indicating that it is most likely that this process has happened before.

Deimos

Deimos is smaller than Phobos and orbits further away from Mars. It is named after a Greek mythological figure which represents 'dread'. It was discovered by Asaph Hall, Sr on August 12, 1877. Deimos has most of its characteristics and shape similar to an asteroid and was believed to be captured by Mars millions of years ago. Like Phobos, it has spectra, albedos, composition and densities similar to C or D-type asteroids. It is also non-spherical, measures 12 km across and orbits Mars at a distance of 23,460 km. Deimos is cratered, but due to the partial filling of the craters with regolith, it has a more smoother surface than Phobos. It takes about 30 days or 1.2 Martian days to complete its orbit. From Mars, it appears like a star to an unaided eye. Unlike Phobos, it will not crash into the surface of the Mars.

NASA's Mars Exploration rovers have observed these moons passing directly in front of the Sun. When Phobos passes, it is commonly known as an eclipse on the Earth, but technically it is just a pass through, as the moon doesn’t block the disk of the Sun. On the other hand, Deimos is too small to cause an eclipse, and looks like a black dot moving across the face of the Sun.