London, Oct 25 : A new study has suggested that an unusual pair of craters on Mars formed when a moon broke apart before crashing into the planet's surface about a billion years ago.
According to a report in New Scientist, the alignment of two oval-shaped craters on Mars - one spanning 10 km and the other, 3 km - hints they might have been gouged out by a fallen moon that broke apart in the Red Planet's atmosphere.
The two craters, which lie about 12.5 kilometers apart, share the same oval shape and nearly the same west-east alignment.
Similar crater pairs are seen elsewhere, including a duo called "Messier" on the Moon.
The Messier craters may have formed from a pair of orbiting asteroids that crashed to the surface together at a low impact angle.
But, John Chappelow and Rob Herrick of the University of Alaska, Fairbanks, said that there is only a 2 percent chance that the two craters on Mars formed that way.
They said that the original asteroids in such a pair could have orbited each other in any configuration, making the craters' observed alignment unlikely.
"In such a case, the craters should be oriented randomly," Chappelow told New Scientist.
Instead, their calculations suggests that a moonlet about 1.5 km wide was pulled into a 'death spiral' by the planet's gravity.
It then broke apart in the atmosphere, where atmospheric drag separated the pieces so that they struck the ground at different points.
The scientists said that the pieces probably hit the surface at an oblique angle of 10 degrees or less.
The craters could hint at what lies in store for Phobos, a potato-shaped moon that is expected to smash into Mars millions of years from now.
Chappelow and Herrick think Phobos and the lost moonlet once circled Mars together, but that the moonlet fell in first because its orbit was closer to the planet.