\n<\/p>\n
Mimas photographed by NASA\u2019s Cassini spacecraft<\/p>\n
NASA\/JPL-Caltech\/Space Science I<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
Saturn\u2019s moon Mimas appears to have a vast global ocean underneath its icy shell, according to close measurements of its orbit. If other icy worlds have similar oceans, it could increase the number of planets that are hospitable to life.<\/p>\n
Mimas is the smallest of Saturn\u2019s seven major moons. It was long thought to be mostly composed of solid ice and rock, but in 2014 astronomers observed that its orbit around Saturn was unexpectedly wobbling, which could only be explained by either a rugby ball-shaped core or a liquid ocean.<\/p>\n
Many astronomers rejected the ocean explanation because the friction needed to melt the ice should also have produced visible marks on Mimas\u2019s surface. However, recent simulations have suggested that this ocean could exist without such marks.<\/p>\n
To look for more clues, Val\u00e9ry Lainey at the Paris Observatory in France and his colleagues analysed observations of Mimas\u2019s orbit made by NASA\u2019s Cassini spacecraft. They found that its orbit around Saturn has drifted around 10 kilometres over 13 years.<\/p>\n
According to the team\u2019s calculations, this orbital drift could only have been produced by wobbles from an icy shell sliding over an ocean, or a core with a physically impossible pancake shape.<\/p>\n
<\/p>\n The moon\u2019s oval-shaped orbit and lack of surface marks also suggest that the ocean is around 30 kilometres deep and formed less than 25 million years ago. \u201cIt\u2019s very, very recent,\u201d says Lainey. \u201cWe are more or less seeing the birth of this global ocean.\u201d<\/p>\n As well as explaining the lack of surface marks, this recent activity could help explain why the moon is so markedly different from neighbouring moons. Enceladus, which has a similar shape and orbit to Mimas, has a global ocean but also a very active surface and a giant water spout. This difference might just be one of time, says Lainey, and in millions of years Mimas\u2019s melting ice could make it look similar to Enceladus.<\/p>\n \u201cIt\u2019s remarkable if it\u2019s true,\u201d says William McKinnon at Washington University in St. Louis, Missouri. But there are still things that don\u2019t quite add up, he says, like the vast 139-kilometre-wide Herschel crater, which was formed from an enormous impact. If Mimas\u2019s icy shell really is only tens of kilometres deep, then we would have seen evidence of this in the impact and aftermath, like a warped crater floor, says McKinnon. Also, it is unlikely that we would have a front-row seat for such a short and unique time in Mimas\u2019s long history, he says. \u201cI remain a Mimas ocean sceptic,\u201d says McKinnon.<\/p>\n But if Mimas does have a hidden ocean, then it could suggest that other icy planets and moons in our solar system or elsewhere could be similar, which also expands the possibility for life. \u201cIt\u2019s extending our vision of what is a habitable world and what is not,\u201d says Lainey. \u201cMimas shows you that even a dead body that doesn\u2019t look like it\u2019s harbouring anything could have life one day.\u201d<\/p>\n Topics:<\/p>\n<\/section><\/div>\n