Scientists could be wrong too, Curious Case of Dione

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Space Demystified

In 1980, the Voyager probe photographed an unusual phenomenon on the moon Dione. The whole of its trailing hemisphere appeared to be covered with large wispy features, initially christened the "wispy terrain".

At first, it was puzzling as to what these features actually were and scientists put forward various theories to explain them. 
One hypothesis was that shortly after its formation Dione was geologically active, and some process such as cryovolcanism resurfaced much of its surface, with the streaks forming from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.

This hypothesis was proven wrong by the Cassini spacecraft flyby of December 13, 2004, which produced close-up images of these things, even though the mission was planned to study Gravity and not the Structure! These revealed that the 'wisps' were, in fact, not ice deposits at all, but rather bright gigantic ice cliffs created by tectonic fractures (chasmata). Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere. A subsequent closer flyby from Cassini was able to image them from a different angle, showing that some of these cliffs are around 200 miles high.


Does Dione Have an Atmosphere?

The Cassini probe comes into its own once again. In 2010, it detected a thin layer of molecular oxygen around the moon. However, it is so extremely tenuous that scientists refuse to call it an actual "atmosphere", preferring the term "exosphere".

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