NASA's Mars Reconnaissance Orbiter took this image of the larger of Mars' two moons, Phobos, from a distance of about 6,800 km (about 4,200 miles).
NASA's Mars Reconnaissance Orbiter took this image of the larger of Mars' two moons, Phobos, from a distance of about 6,800 km (about 4,200 miles).
Credit: NASA/JPL/University of Arizona

Of all the known moons in the solar system, the Martian moon Phobos orbits closest to its primary, hovering only a few thousand miles above the surface. Its proximity to its planet is one of the reasons astronomers were unable to see the satellite until the late 19th century.

Robotic probes on Mars' surface can see Phobos passing overhead:

Discovery and nomenclature

In the early 17th century, German astronomer Johannes Kepler proposed that Mars might host two moons, given that it lay between the Earth and Jupiter, which were known to have one and four satellites. No evidence of such moons could be found. Although most people thought that Mars had no moons, American astronomer Asaph Hall performed a methodical study from the U.S. Naval Observatory in Washington, D.C., searching closer to the planet than previous surveys.

After searching without success, a frustrated Hall was about to give up but his wife, Angelina, urged him to continue. The next night, August 12, 1877, he discovered the moon that would later be known as Deimos. Six days later, he found Phobos as well. The two moons lay so close to their primary that they were hidden by the planet's glare. Asteroid-sized, they are also two of the smallest moons in the solar system, with the larger Phobos 7.24 times as massive as its companion, Deimos.

NASA's Mars rover Curiosity snapped this photo of the largest Martian moon Phobos during a Mars sky observing session. Phobos is Mars' largest moon, but only 14 miles across. Image released Sept. 26, 2012.
NASA's Mars rover Curiosity snapped this photo of the largest Martian moon Phobos during a Mars sky observing session. Phobos is Mars' largest moon, but only 14 miles across. Image released Sept. 26, 2012.
Credit: NASA/JPL

In Greek mythology, Phobos was one of the sons of the god of war Ares (Mars to the Romans). The twin sons attended their father in battle. Phobos means Fear, while Deimos means Flight.

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 Phobos and Deimos have lumpy, potato-like shapes, rather than spherical like most moons. Observations of Phobos 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 learn more information about the satellites circling Mars. The Viking Orbiters flew by in the late 1970s. 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, Opportunity, and Curiosity all providing images from the ground.

NASA may not be finished with Phobos yet. The space agency is considering the Phobos Surveyor mission, which would deploy small, hedgehog-like probes to the surface of the moon.

Formation and composition

The examination of Phobos and its companion revealed more than their odd, nonspherical shapes. The two moons are dark gray in color, and heavily cratered. The moons are some of the darkest, least reflective objects in the solar system.

After observing the pair, scientists concluded that they were made of material similar to Type I or II carbonaceous chondrites, the material that makes up asteroids and dwarf planets. The composition and odd shape led some scientists to conclude that Phobos and Deimos came from the asteroid belt, with Jupiter's gravity long ago nudging them into orbit around Mars.

However, scientists aren't certain that the asteroid belt is the source of the moons' birth. Both have a nearly circular orbit unusual for captured objects. The thin atmosphere of Mars would have a hard time providing the necessary braking to settle the pair into their present-day orbits. Similarly, the moons are not as dense as objects in the asteroid belt.

Instead, the moons may have formed as satellites around Mars, with dust and rock drawn together by gravity.

A more violent birth may have occurred by collision. A large impactor smashing into the red planet could have sent pieces flying into the air, where gravity may have drawn them together. An existing moon might also have been destroyed, creating the rubble that later formed Phobos and Deimos.

Traveling only 3,700 miles (6,000 kilometers) above the Martian surface, Phobos flies around the red planet three times a day. Crossing the sky in about four hours, the moon appears to rise in the west and set in the east. Phobos is so close that in some places, the curvature of the planet would block it from the sight of an observer on the surface.

Many science fiction films feature large moons dominating alien skies, but despite its proximity to its planet, Phobos is only a third as wide as the full moon seen from Earth. On the other hand, Mars dominates the horizon of Phobos, taking up a fourth of the sky.

But Phobos won't zip around the red planet forever. The doomed moon is spiraling inward at a rate of 1.8 centimeters (seven-tenths of an inch) per year, or 1.8 meters (about 6 feet) each century. Within 50 million years, the moon will either collide with its parent planet or be torn into rubble and scattered as a ring around Mars.

The unusual characteristics of Phobos, including its decaying orbit, led some scientists in the 1950s and 1960s to conclude that it was artificial. One prominent proponent was the science advisor to United States President Dwight D. Eisenhower. Further examination revealed that the moon bears stronger resemblance to a rubble pile than an artificial satellite, and images sent back from orbiting craft show that it formed in nature.

On the surface

The larger of the two moons, Phobos has a diameter of 10 by 14 by 11 miles (17 by 22 by 18 km). The surface is covered with a dusty powder 3 feet (1 meter) thick, likely caused by meteor bombardment.

A large impact crater dominates the moon. Stretching nearly 6 miles (9.5 km), Stickney crater covers most of the surface. The impact that formed it likely caused many secondary impacts, as rocks flew up and fell back to Phobos. The crater bears the maiden name of Hall's encouraging wife.

The moon is marred by long grooves that may have formed when ejecta from impacts on the Martian surface flew upward and struck the satellite.

Temperatures vary on Phobos depending whether one stands on the perpetual dayside or the side facing Mars. On the sunlit side, temperatures resemble a pleasant winter day in Chicago, reaching highs of 25 degrees Fahrenheit (minus 4 degrees Celsius), while the dark side can be as cold as minus 170 degrees F ( minus 112 degrees C).

The moon is so small that a 150-pound person standing on its surface would weigh only two ounces.

Facts about Phobos:

  • Radius of moon: 6.9 miles (11.1 km)
  • Semi-major axis around Mars (distance from planet's center): 5,826 miles (9,376 km)
  • Closest approach: 5,738 miles (9,234 km)
  • Farthest approach: 5,914 miles (9,518 km) 
  • Orbit eccentricity: 0.0151
  • Orbit inclination: 1.075 degrees
  • Time to make one orbit: 7.65 hours
  • Mass: 1.0659 x 10^16 kg
  • Density: 1.872 g/cm^3
  • Surface gravity: 0.0057m/s^2
  • Escape velocity: 25 mph (41 km/h)

— Nola Taylor Redd, Contributor