The moons of Uranus may have short-lived atmospheres every time the seasons change. The seasons there are so intense that these tenuous atmospheres, called exospheres, could exist briefly twice every Uranian year before freezing and falling back down to the surface.
Uranus鈥檚 poles are extremely tilted with respect to the planet鈥檚 orbit around the sun, which, along with its powerful magnetic field, makes the seasons there particularly extreme. at the Southwest Research Institute in Texas and his colleagues used laboratory experiments on how carbon dioxide reacts to similar seasonal changes, as well as computer modelling, to examine how they could affect the ices that are abundant on the Uranian moons. They presented this work at the .
The researchers found that the radiation from the sun is strong enough that it could cause ices on the surfaces of these moons to sublimate, transforming directly from solids into gases. These gases would then float around the moon, forming an exosphere several billionths the density of Earth鈥檚 atmosphere, before freezing again at the end of the warm season and snowing back down to the ground.
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鈥淥n Uranus鈥檚 moons, the polar night zones spend 42 [Earth] years in darkness with the surface looking up at this black sky, and it gets so cold that atmospheric particles literally freeze to the surface,鈥� says Teolis. 鈥淭he entirety of the atmosphere gets sucked down to the surface and freezes to the pole, and then it surges back up again, and at the end of the season it freezes back down to the other pole.鈥�
If you were able to walk on the surface of one of Uranus鈥檚 moons, you wouldn鈥檛 be able to detect this seasonal exosphere, and not just because of its low density. 鈥淚f you were to just step on the surface, the heat from your body, from the equipment in the space suit, would probably be鈥� enough to start vaporising the ground below you,鈥� says Teolis. 鈥淵ou would generate your own exosphere over the whole planetary body.鈥�
