Eye-popping image of Phobos
The Mars Reconnaissance Orbiter with its powerful HiRISE camera onboard came within 6000 km of one of Mars’s tiny moons, Phobos, providing some of the best images yet of this moon.
The large crater on the right, called Stickney, is comparable to the size of the moon, suggesting that had the impactor that formed the crater been a little more massive (or traveling a little faster) it would have shattered the moon, leading to a debris ring around Mars. Phobos survived that impact, but its fate is sealed. Because Phobos orbits Mars in less than a Martian day (only 7 hours and 40 minutes compared to the Martian day of about 24 hours and 40 minutes), it races ahead of the tidal bulge that its gravity produces on Mars. This tidal bulge then produces a torque that, unlike that of the Earth on our Moon which is causing it to move slowly away from the Earth (because the Moon orbits the Earth much slower than the Earth rotates, causing it to lag behind the tidal bulge it raises on the Earth, expressed by the ocean tides) retards the motion of Phobos causing it to spiral inward toward the planet. This will cause Phobos to hit Mars in less than 100 million years. Before that happens it is likely to be torn apart by tidal forces making a debris ring around Mars which itself will quickly decay into Mars’s atmosphere.
Phobos, like Mars’s other moon Deimos, is likely a former asteroid, captured into orbit around Mars through a combination of tidal dissipation and perhaps atmospheric drag from an earlier, denser Martian atmosphere. Although Phobos orbits quite close to Mars, its small physical size means that it is not able to fully block the Sun as seen from the surface of Mars. Nevertheless, the Mars rover Opportunity took this series of images showing Phobos transiting in front of the disk of the Sun. Phobos’s aspherical shape can be clearly seen in silhouette.
Credit: NASA/JPL/Cornell.