The moon that is slowly falling into the Red Planet

Science · Published 1 November 2026 Swipe more like this →

In short: Phobos orbits Mars so closely that it completes three revolutions every single day. Because of its proximity, there are spots on the Martian surface where the moon remains hidden from view entirely. But this intimacy comes with a terminal cost: Phobos is on a one-way collision course.

Measuring roughly 17 by 14 by 11 miles (27 by 22 by 18 kilometers) across, Phobos represents the biggest satellite of Mars. Its composition appears to be C-type rock, which is similar to blackish carbonaceous chondrite asteroids. While some scientists suggest it may be a captured asteroid, other evidence contradicts this theory. The moon's surface tells a violent history; it was nearly shattered by a giant impact and remains scarred by thousands of smaller meteorite strikes that have pounded the surface into fine powder.

The most striking geological feature is the Stickney Crater, named for mathematician Chloe Angeline Stickney Hall. Covering almost fifty percent of the moon's exterior, this huge crater spans around 6 miles (9.7 kilometers) in width. NASA’s Mars Global Surveyor has captured images showing that this crater is filled with fine dust, with visible evidence of boulders sliding down its steep slopes. Because the moon lacks an atmosphere, it experiences extreme temperature swings: the sunlit side can reach 25 degrees Fahrenheit (-4 degrees Celsius), while the dark side plunges to -170 degrees Fahrenheit (-112 degrees Celsius).

This thermal disparity is driven by the fine dust on the surface, which fails to retain heat. Furthermore, Phobos is moving toward Mars at a rate of about six feet (1.8 meters) every hundred years. At this current pace, the moon faces two possible fates in roughly 50 million years: it will either crash directly into the Martian surface or be torn apart by tidal forces to form a ring system around the planet.

Discovered on August 17, 1877, by astronomer Asaph Hall, Phobos was named after the Greek personification of fear. Hall chose the name to reflect the mythological sons of Ares, the Greek counterpart to the Roman god Mars.

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