Findings: Phobos — a closer look

On March 28, NASA’s Mars Reconnaissance Orbiter captured two stunning images of Mars’ moon, Phobos.

Phobos (pronounced FOH-bus) is an interesting, if somewhat unfortunate, object. It is small, weighing only 1.07 x 10¹⁶ kilograms, or 1.8 billionths of Earth. In addition, Phobos has less than one-thousandth the gravitational force of Earth.

This is not enough gravitational force to hold Phobos in a spherical shape. Thus, Phobos is oblong. Phobos has been battered and bruised for eons, after suffering thousands of collisions with asteroids and meteorites. Actually, both Phobos and Deimos, Mars’ other moon, are thought to be captured asteroids.

The biggest scar on Phobos, a six-mile wide impact crater called the Stickney crater, dominates one side of the moon. The impact crater nearly destroyed Phobos, causing streaks to permeate the moon’s surface in the same way a lodged baseball causes streaks to radiate from the point of impact on a broken windshield.
Other lines are the result of landslides down the steep sides of the enormous crater.

But Phobos’ troubles do not end with its bruises. The moon also orbits dangerously close to Mars — closer, in fact, that any moon in the solar system is to its planet. At certain times, Phobos (which orbits Mars three times a day), is so close to Mars that it cannot always be seen from the surface of the red planet. This orbital proximity to Mars has bleak implications for Phobos. Currently, Phobos’ orbit is drawing the moon closer to Mars at a rate of 1.8 meters every hundred years. At this rate, Phobos will slam into Mars in roughly 50 million years.

Luckily, the HiRISE (High Resolution Imaging Science Experiment) camera aboard the Mars Reconnaissance Orbiter has captured some of the most detailed images of Phobos to date. The images (available in high resolution at HiRISE) were taken 6800 kilometers above the moon’s surface and show Phobos in striking and vivid detail.

In the images, Phobos looks like a glistening, pockmarked marble covered with untold grooves, striations, and canyons. Two of the images on the HiRISE website were taken 10 minutes apart from each other, and at roughly the same angle. Because of this, the two images can be combined to form a stereo view.

“Phobos is of great interest because it may be rich in water, ice, and carbon-rich materials,” said Alfred McEwen, the principal investigator of the HiRISE project at the Lunar and Planetary Laboratory at the University of Arizona, in an official NASA press release.

Both Phobos and Deimos are also composed of C-type rock, which is similar in composition to blackish carbonaceous chondrites, stony meteorites with high levels of water and organic compounds.

The photos should also help shed light on the somewhat murky history of Phobos. Phobos looks remarkably like a boulder, leading to usual speculations that it might have been ejected from Mars after the planet’s encounter with another asteroid-like object. Phobos and Deimos could also be the by-products of a much larger moon that once circled Mars.

The images of Phobos were obtained by collecting information from the camera’s blue-green, red, and near-infrared color filters. By combining this information, scientists were able to deduce that the material surrounding the Stickney crater, Phobos’ most noticeable feature, appears bluer than the rest of the surface.

“Based on analogy with material on our own moon, the bluer color could mean that the material is fresher, or hasn’t been exposed to space as long as the rest of Phobos’ surface has,” Bridges said.

Before the HiRISE images were sent back, the most detailed images of Phobos were taken by the High Resolution Stereo Camera (HRSC) on board the European Space Agency’s Mars Express spacecraft in 2004.

Launched in August 2005, the $720 million Mars Reconnaissance Orbiter, built by Lockheed Martin, is a flying armada of cameras, spectrometers, radar devices, and engineering instruments. Although its unofficial mission is Mars exploration and reconnaissance, the tools aboard the MRO have serendipitously shed some spectacular light on Mars’ closest neighbor as well.