An unusual pair of craters on Mars formed when a moon broke apart before crashing into the planet鈥檚 surface about a billion years ago, a new study suggests.
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.
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The two craters, which lie about 12.5 kilometres apart, share the same oval shape and nearly the same west-east alignment.
Similar crater pairs are seen elsewhere, including a duo called 鈥淢essier鈥� on the Moon (scroll down for image). 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, say there is only a 2% chance that the two craters on Mars formed that way.
They say the original asteroids in such a pair could have orbited each other in any configuration, making the craters鈥� observed alignment unlikely. 鈥淚n such a case, the craters should be oriented randomly,鈥� Chappelow told 麻豆传媒.
Instead, their calculations suggests that a moonlet about 1.5 km wide was pulled into a 鈥榙eath spiral鈥� by the planet鈥檚 gravity. It then broke apart in the atmosphere, where atmospheric drag separated the pieces so that they struck the ground at different points. They say the pieces probably hit the surface at an oblique angle of 10掳 or less.
Not random
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.
But Jay Melosh, a crater expert at the University of Arizona in Tucson is sceptical of that scenario.
For one thing, the craters are located at a latitude of about 40掳 in the planet鈥檚 northern hemisphere. But nearby moons should settle into orbits above the planet鈥檚 equator due to Mars鈥檚 gravitational tugs. 鈥淎ny close natural satellite must, like Phobos, orbit in Mars鈥檚 equatorial plane,鈥� Melosh told 麻豆传媒.
The researchers contend that the moonlet may have fallen before it could stabilise into an equatorial orbit. 鈥淲e don鈥檛 know the details of the [moonlet鈥檚] capture mechanism, so I don鈥檛 know that we can definitively say that the object must have moved to an equatorial orbit before spiralling in,鈥� Herrick said.
Melosh also says observations show that most double asteroids do not, in fact, orbit each other in random orientations. Instead, sunlight tends to make them orbit each other in the same plane as the planets.
鈥淭here is even a known near-Earth binary asteroid, 1999 KW4, that has precisely the characteristics that, if it were to strike Mars at an angle of 10掳, would produce a doublet closely resembling [the Martian] pair,鈥� he says.
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