Moons of Mars

The two moons of Mars, Phobos and Deimos, are small and irregular in shape. They were discovered by American astronomer Asaph Hall in August 1877 and are named after twin characters from Greek mythology, Phobos (fear and panic) and Deimos (terror and dread), who followed their father Ares, the counterpart of Mars.

Phobos is slightly larger, with a diameter of about 22 kilometers, while Deimos is smaller, measuring about 12.6 kilometers across. Phobos orbits very close to Mars, completing a circle around the planet in just under 8 hours, whereas Deimos takes about 30 hours for one orbit.

Scientists have two main ideas about where these moons came from. One idea is that they were created when a large object hit Mars a long time ago. The other idea is that Mars pulled these moons from space, capturing them as its own. Both ideas match what we currently know, and future space missions might help us learn which explanation is correct.

History

People have wondered if Mars had moons since the moons of Jupiter were found. The writer Jonathan Swift mentioned two moons of Mars in his story Gulliver's Travels in 1726, even though they were not discovered until much later. These moons were finally discovered by Asaph Hall in August 1877 at the US Naval Observatory in Washington, D.C..

Hall found Deimos on August 12, 1877, and Phobos on August 18, 1877, using a big telescope. The moons are named after characters from an old story about the god Mars.

Characteristics

If you were standing on Mars near its equator, the moon Phobos would look about one-third as big as our Moon looks from Earth. Deimos, however, would appear more like a bright star or planet, similar to how Venus looks from our planet. Neither moon is big enough to cause a total solar eclipse on Mars, but Phobos can cause total lunar eclipses almost every night.

Phobos moves very quickly across the sky, rising in the west and setting in the east, and it rises again in just eleven hours. Deimos rises more slowly in the east but takes much longer to set in the west. Both moons always show the same face to Mars because they are tidally locked. In the far future, Phobos might break apart and either crash into Mars or form a ring around the planet.

Origin

The origin of Mars’s moons, Phobos and Deimos, is still a mystery. One idea is that they might be captured asteroids from the asteroid belt, but their orbits are almost perfectly circular, which makes this hard to explain. Another idea is that they formed from material thrown into orbit after a huge collision with another space rock. Recent studies suggest they may have broken apart from a single parent moon long ago, though this theory also has challenges. Whatever their origin, these moons give us clues about the history of Mars and its place in our solar system.

Exploration

Main articles: Phobos (moon) § Exploration, Deimos (moon) § Exploration, and List of missions to Mars § Missions to the moons of Mars

Many probes have studied the moons of Mars, but only a few were meant to visit them directly. In 1988, two Soviet probes were launched to study Phobos and Deimos, but they did not land as planned. In 2011, a Russian probe failed to leave Earth’s orbit and fell back into the Pacific Ocean.

New missions are being planned. Japan plans to launch the Martian Moons eXploration mission in 2026 to collect samples from Phobos and bring them back to Earth in 2029. This mission will also study Deimos with flybys.