Satellites and Rings (Part 1)

Phobos and Deimos are two small, irregularly shaped moons orbiting Mars, discovered in 1877.

In mythology, their names mean "fear" and "panic," representing either Mars's sons or his war chariot horses.

Phobos:

• Inner and larger moon, potato-shaped and heavily cratered.

• Surface is covered in dust about 1 m thick from impacts.

• Has a very low density, suggesting it is highly porous (up to 30% empty space).

• Orbits Mars 3 times in 1 Martian day, rising in the west and setting in the east.

• Descending at 1.8 m/Earth year and is predicted to either crash into Mars or form a ring within 50 million years due to tidal forces.

Deimos:

• Smaller and less cratered than Phobos.

• Orbits slower, rising in the east and taking about 3 Earth days to cross the sky.

Both moons orbit in the same direction as Mars's rotation.

Origins:

• One theory suggests they are captured asteroids, though their nearly circular, equatorial orbits are hard to explain.

• Another theory proposes they formed from debris ejected after Mars was struck by a massive asteroid, similar to Earth's Moon formation.