How Many Moons Does Mars Have?
QUICK ANSWER
Mars has two moons: Phobos and Deimos. Both are small, irregularly shaped (more potato than sphere), and were likely captured asteroids or formed from impact debris. Phobos is about 14 miles across and Deimos about 8 miles, much smaller than Earth's Moon.
Mars has two moons, both small enough to look like flying potatoes and named after the Greek figures of fear and panic. They're nothing like Earth's Moon, but they have their own strange stories.
What are Mars's two moons?
Mars has two moons named Phobos and Deimos, both discovered in 1877 by American astronomer Asaph Hall. According to NASA, Phobos is the larger and closer of the two, about 14 miles across at its widest, orbiting only 3,700 miles above Mars's surface. Deimos is smaller (about 8 miles across) and farther out at roughly 14,580 miles. Both are tiny compared to Earth's Moon, which is over 2,000 miles across.
Where do Phobos and Deimos come from?
Their origin is still debated. The leading theory is that both are asteroids captured by Mars's gravity sometime in the planet's history. Their composition resembles asteroids from the outer asteroid belt, and their irregular shapes suggest they weren't massive enough to form into spheres under their own gravity. Another theory proposes they formed from debris ejected by a massive impact early in Mars's history, similar to how Earth's Moon formed. Future missions may help settle the question.
Why are the moons named after Greek figures?
Phobos and Deimos were the sons of Ares, the Greek god of war (the equivalent of the Roman god Mars). Their names translate to fear and panic, the two things their father Ares was said to bring to battle. Asaph Hall, who discovered them, named the moons in keeping with the war-god theme that gives Mars its own name. The naming is one of the most darkly fitting in astronomy: a planet of war attended by personifications of dread.
Is anything happening to Phobos?
Yes, slowly and dramatically. Phobos is spiraling inward toward Mars at a rate of about 6 feet every century. In roughly 50 million years, it will either crash into Mars's surface or be pulled apart by tidal forces and form a ring around the planet. Mars may eventually have rings made of Phobos debris. Deimos, by contrast, is slowly drifting outward and will eventually escape Mars's gravity. The two moons are heading in opposite directions in geological time.
Mars's two moons, Phobos and Deimos, are small and strange and probably captured asteroids. Phobos is doomed to crash into the planet (or fragment into rings) in about 50 million years; Deimos is slowly drifting away. Earth's Moon will probably outlast both of them by a wide margin.
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