Mars is the only terrestrial planet to host multiple moons. The smaller of the two, the lumpy moon Deimos, bears more resemblance to an asteroid than to most of the moons in the solar system, a similarity that raises questions about its formation.
Discovery and nomenclature
On August 12, 1877, the focused search for Martian moons by American astronomer Asaph Hall resulted in the discovery of Deimos. Six days later, he identified the second Martian moon, Phobos.
The existence of the moons had been suggested years before, when Johannes Kepler proposed that since Earth hosted one moon and Jupiter four (as only the Galilean moons were known at the time), Mars might have two moons in orbit around it. However, no signs of such moons existed until Hall undertook his careful search.
Using a 26-inch refractor at the U.S. Naval Observatory in Washington, D.C., Hall made a methodical study of the region around the red planet. Peering closer to Mars than previous astronomers, he found Deimos circling only 14,576 miles (23,458 kilometers) from the center of the planet, traveling around its equator. Phobos orbited even closer in. Their close proximity and small size had kept them hidden in the glare from the planet.
Like many objects in the solar system, the Martian moons take their names from Greek mythology. In Homer's ancient poem, "The Iliad," Deimos (Flight) and Phobos (Fear) were the twin sons of Mars (Ares to the Greeks), and accompany him into battle.
Exploring the moons
It took almost another century for scientists to begin to understand the two tiny Martian moons. In 1971, NASA's Mariner 9 spacecraft became the first manmade satellite to orbit another planet. Images from the craft revealed that both Deimos and Phobos have lumpy, potato-like shapes, rather than being spherical like Earth's moon. Observations of Deimos were limited by the tidal locking of the moon to the planet, resulting in the same side always facing outward.
As the exploration of continued, scientists were able to glean more information about the two tiny moons. The Viking Orbiters flew by in the late 1970s, with the second orbiter passing within 19 miles (30 km) of Deimos. The Soviet Phobos 2 mission, NASA's Mars Global Surveyor, and the European Mars Express all provided more clues about the two curious moons. Rovers from the planet's surface even got in on the act, with Spirit and Opportunity and Curiosity all providing images from the ground.
Formation and composition
All of this information combined created a puzzling picture. The dark moons are made up of material similar to Type I or II carbonaceous chondrites, the substance of asteroids and dwarf planets such as Ceres. They are tiny, with the smaller Deimos having a radius of only 3.9 miles (6.2 km). This, combined with their potato-like shape, hints that both moons might be asteroids, pushed by Jupiter from the asteroid belt and snatched up by the gravity of Mars.
But this is far from conclusive. The close orbit of Deimos is nearly circular. It travels around the equatorial plane of Mars in 30 hours, a little over a Martian day. To reach such a stable orbit would require braking by the atmosphere, but the atmosphere on the red planet is thinner than on Earth.
Another possible origin for the moons is that dust and rock could have accreted, or drawn together, while in orbit around Mars. A third possibility includes a collision, much like the one that formed Earth's moon, with most of the large debris being shed from the planet's orbit, leaving behind only Deimos and Phobos.
From the surface of Mars, the tiny moon appears star-like. At full moon, Deimos shines about as brightly as Venus. When the moon eclipses the sun, it appears as a small dot crossing its surface.
But the pair won't shine in the sky forever. Within 100 million years, the closer Phobos will collide with the red planet. Deimos will suffer the opposite fate. Its orbit is slowly drawing it away from Mars, and eventually the moon will be cast off into space.
Deimos has a number of craters caused by impact from meteorites. But craters on Deimos look different from those on most bodies in the solar system. When a rock collides with another body, material from the impact tends to fly up in the air and fall back to the surface, creating ejecta deposits. But the small size of the moon means that objects only need to travel 13 miles (20 km) per hour to fly off into space. Although the moon is covered with regolith that may lie as deep as 328 feet (100 meters), it is created by meteorites pulverized by impact, rather than by castoff material.
Only two of the craters on Deimos are named. In 1726, English author Jonathan Swift cited Kepler when he referred to two Martian moons in his fictional work, Gulliver's Travels. A few years later, the French writer Voltaire referenced two moons in a short story. The two craters bare the names of these authors.
Facts about Deimos:
- Radius of moon: 3.9 miles (6.2 km)
- Semi-major axis around Mars (distance from planet's center): 14,576 miles (23,458 km)
- Closest approach: 14,576 miles (23,458 km)
- Farthest approach: 14,576 miles (23,458 km)
- Orbit eccentricity: 0.0002
- Orbit inclination: 1.788 degrees
- Time to make one orbit: 30 hours
- Mass: 1.4762 x 10^15 kg
- Density: 1.471 g/cm^3
- Surface gravity: 0.003 m/s^2
- Escape velocity: 13 mph (20 km/h)
— Nola Taylor Redd, SPACE.com Contributor