Mars is the fourth planet from the Sun and the most Earth-like planet in the solar system. It is about half the size of Earth and has a dry, rocky surface covered in iron-rich dust. Mars once had rivers, streams, and lakes, and possibly an ocean, but its atmosphere slowly depleted into outer space causing surface water to permanently evaporate. Mars has two small moons called Deimos and Phobos that are much smaller than most towns on Earth and have weak gravity causing them to not be rounded in shape. Phobos is doomed to eventually crash into Mars within about 50 million years as it gets closer to the planet every hundred years due to Mars' gravity.