Pity poor Cressida 鈥� not the tragic figure in Shakespeare, but a small moon orbiting Uranus.聽It looks like it will crash into another of the planet鈥檚 moons in a mere million years.
The Voyager 2 spacecraft discovered Cressida in 1986. It is just 82 kilometres across, dark in colour and orbits close to Uranus but beyond most of its rings. It belongs to the most tightly packed group of satellites in the solar system, nine moons whose orbits all lie within 18,000 kilometres of one another.
Now, at the University of Idaho and his colleagues have deduced the small moon鈥檚 mass 鈥� and from it discovered the probable shape of its demise.
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The team started by investigating one of the planet鈥檚 rings, called Eta, and found that its orbit is slightly triangular rather than perfectly circular.聽 鈥淲e didn鈥檛 really expect to find that,鈥� Chancia says.
Distorted ring
They believe Cressida鈥檚 gravity is behind the distortion, as it keeps pace with the moon鈥檚 orbit of Uranus. 聽In contrast, the individual particles that make up the ring revolve faster than the moon, completing three orbits in the same time the moon takes to complete two orbits.
This link enabled the team to use the ring to deduce the moon鈥檚 mass, the first time anyone has weighed such a small moon of Uranus. They found that Cressida is about 1/300,000th as massive as Earth鈥檚 moon and about 86 per cent as dense as water.聽That鈥檚 much denser than the small moons of Saturn, which are mostly made of water ice but are porous, making them lighter than ice.
Cressida is probably porous as well. 鈥淚t鈥檚 too small to crush out its porosity through self-gravity, so there鈥檚 got to be more stuff than ice there,鈥� says at Washington University St Louis, Missouri, who wasn鈥檛 involved in the work.
鈥淭here鈥檚 got to be rock, probably carbonaceous stuff, and that fits in with the fact that all those small moons and the rings themselves are so dark, so different compared to Saturn鈥檚 rings,鈥� says McKinnon.
Desdemona鈥檚 doom
The findings spell trouble for Cressida.聽The denser Uranus鈥檚 closely packed moons, the more their gravity tugs at one another, raising the spectre that one will swerve into the wrong lane.聽鈥淪ome of these moons are probably going to crash into each other,鈥� says team member , also at the University of Idaho.
In only about a million years, Cressida will probably strike Desdemona, a moon that orbits just 900 kilometres outside Cressida鈥檚 path, says the team.聽A similar fate awaits the moons Cupid and Belinda, which will hit each other.
Furthermore, says Chancia, 鈥渢here鈥檚 actually evidence of past collisions.鈥澛燭wo diffuse rings near the moons could be made of debris from previous smash-ups.
ArXiv
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