Previously unpublished photos of Mars' moon Phobos hint that
the mysterious satellite may actually be a trapped comet — or perhaps just a piece of one, along with its twin moon Deimos.

© NASA/JPL-Caltech/GSFC/Univ. of ArizonaA composite photo of Mars with its twin moons Phobos and Deimos. New research suggests the pair may in fact be two halves of an ancient comet captured by Mars long ago.
Mars' moon Phobos may actually be a comet — or at least part of one — that was gravitationally captured by the Red Planet long ago, a new preprint study based on previously unpublished photos suggests.For years, researchers have puzzled over the origins of Phobos and its twin, Deimos. Some have theorized that the moons are former asteroids lured in by
Mars' gravity, because their chemical composition is similar to that of certain rocks in the main asteroid belt between Mars and Jupiter. However, computer models simulating this capture process have not been able to replicate the pair's near-circular paths around Mars.
Another hypothesis suggests that a
giant impact, like that which created our
moon, gouged the duo out of the Red Planet; but Phobos has a different chemical composition from Mars, making this scenario unlikely, too.
Figuring out exactly how Phobos was born is one of the aims of the Japan Aerospace Exploration Agency's
Martian Moons eXploration (MMX) mission, slated to launch in 2026.
Sonia Fornasier, an astronomy professor at the Paris Cité University and lead author of the new study, is an instrument scientist for the MMX mission. While she and other scientists were analyzing images to fine-tune the spacecraft's planned path, Fornasier stumbled upon unpublished photos.
Comment: There appears to be an increase in rare, and even previously unknown, phenomena occurring in and around Earth: