Mars' moons are among the smallest in the solar system. Phobos is a bit larger than Deimos, and orbits only 6,000 km (3,700 miles) above the Martian surface. No known moon orbits closer to its planet. It whips around Mars three times a day, while the more distant Deimos takes 30 hours for each orbit. Phobos is gradually spiraling inward, drawing about 1.8 m closer to the planet each century. Within 50 million years, it will either crash into Mars or break up and form a ring around the planet.
Mars' Moon Deimos
This illustration provides a comparison for how big the moons of Mars appear to be, as seen from the surface of Mars, in relation to the size that Earth's moon appears to be when seen from the surface of Earth. Earth's moon actually has a diameter more than 100 times greater than the larger Martian moon, Phobos. However, the Martian moons orbit much closer to their planet than the distance between Earth and Earth's moon. Image Credit & Copyright: NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M Univ.
The Discovery of Phobos
In the early 17th century, German astronomer Johannes Kepler proposed that Mars might host two moons. However, no evidence of such moons could be found. American astronomer Asaph Hall tried searching closer to the planet than previous surveys. After searching without success, a frustrated Hall was about to give up when his wife 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.
Exploration of Phobos
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. 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.
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.
Image Credit & Copyright: NASA/JPL-Caltech/University of Arizona
Mars' Moon Phobos
Phobos' Profile:
Age: Phobos is about 4.5 billion years old, about the same age as Mars.
Distance from Jupiter: Phobos is the first moon from Mars, traveling only 3,700 miles (6,000 kilometers) above the Martian surface. Phobos flies around the red planet three times a day.
Size: 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.
Temperature: Temperatures vary on Phobos depending whether one stands on the perpetual dayside or the side facing Mars. On the sunlit side, temperatures reach 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).
This movie clip shows the larger of Mars' two moons, Phobos, passing in front of the smaller Martian moon, Deimos, as observed by NASA's Mars rover Curiosity. The series of 41 images is shown at increased speed. The actual elapsed time is 55 seconds. Image Credit & Copyright: NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M Univ.
The Discovery of Phobos
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. 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.
Exploration of Phobos
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. 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.
These color-enhanced views of Deimos, the smaller of the two moons of Mars, result from imaging on Feb. 21, 2009, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Image Credit & Copyright: NASA/JPL-Caltech/University of Arizona
Deimos is a small lumpy, heavily cratered object. The smaller of the two moons, the 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.
Deimos' Profile:
Age: Deimos is about 4.5 billion years old, about the same age as Mars.
Distance from Jupiter: Deimos is the second moon from Mars, traveling 14,576 miles (23,458 km) from the moon's surface.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.
Size: Deimos, the smaller of the two moons, has a radius of only 3.9 miles (6.2 km). From the surface of Mars, the moon appears star-like.
Temperature: The average temperature of Deimos is minus 40.15 decrees. This is a similar temperature to that of Phobos, ranging from around −4°C (25°F) on the sunlit side of the moon, down to around −112°C (−170°F) on the shadowed side.