It is dust to dust for Mars鈥檚 moon Phobos. There is new evidence that the misshapen little moon has repeatedly gone through a cycle of being smashed up and then spread into a ring around Mars before coalescing into a solid moon again.
This ring-moon cycle was proposed in 2017, and new simulations by Matija 膯uk at the SETI Institute in Mountain View, California, and his colleagues show that such a cycle for Phobos may have forced Mars鈥檚 other moon, Deimos, into the orbit it occupies now. 膯uk presented this work at a virtual meeting of the American Astronomical Society on 2 June.
Deimos orbits Mars in a near-perfect circle, but its path is tilted by two degrees, and astronomers haven鈥檛 been able to figure out why. 鈥淭wo degrees doesn鈥檛 seem like much, but for this kind of thing it鈥檚 a lot 鈥� moon systems tend to be pretty flat,鈥� says 膯uk.
Advertisement
His team鈥檚 simulations showed that if Phobos used to be much larger 鈥� about 20 times as massive as it is now 鈥� its gravity could have dragged Deimos onto its current path and caused that orbital tilt. The simulations suggest that Phobos could have been this big two cycles聽ago.
After tilting Deimos鈥� orbit, this massive proto-Phobos would have fallen towards Mars until it got so close that it was ripped apart by the planet鈥檚 gravity, with some of the debris falling to the surface and some forming a ring around the Red Planet. Then, over time, some of the dust and pebbles of the ring could clump together and eventually go on to form a new, smaller moon.
鈥淚t鈥檚 a cycle,鈥� says 膯uk. 鈥淭he moon forms a ring and the ring forms a moon.鈥� Two of these cycles would give us the Phobos we see today, which was a ring for almost 2 billion years before becoming a moon again, according to 膯uk. If that is actually what happened, Phobos may fall apart and become a ring again in about 40 million years, he says.
To test this theory, we have to figure out how old these moons are. 鈥淲hile Deimos might be old, about 4 billion years or so, Phobos should be much younger,鈥� says 膯uk. 鈥淚t was put back together only a couple of hundred million years ago.鈥� The Japanese space agency is planning a mission to Phobos in 2024 that will return samples of the moon, so we may have our answer within the next decade.
Reference:
Sign up to our free Launchpad newsletter for a monthly voyage across the galaxy and beyond
