When a planet or a moon or any celestial body is spinning around an axis – the line between its north and south poles – it will tend to stay at it. The axis itself might wander a little, tracing tiny gradual circles in the firmament, but the planet (or what have you) will follow it, by and large.
This is because all rotating objects have what’s called angular momentum, and angular momentum is conserved (which means it doesn’t change without some good reason, such as an intervention by an outside force).
At a deep level, angular momentum is conserved because all directions are the same from the point of view of the universe – this is called rotational symmetry – and the German mathematician Emmy Noether showed that symmetries like this lead to the conservation of things like energy and momentum. At a more familiar level, the conservation of angular momentum is what holds up spinning tops, keeps gyroscopes steady and provides that twinge of resistance when you try to tilt a fidget spinner.
All of this is a long way to explain why NASA scientists were surprised to see evidence from the Cassini mission that Saturn’s icy moon Enceladus may have tipped over somewhere in the distant past, with the direction of its axis of spin shifting by as much as 55 degrees.
“We found a chain of low areas, or basins, that trace a belt across the moon’s surface that we believe are the fossil remnants of an earlier, previous equator and poles,” said Radwan Tajeddine, a Cassini imaging team associate at Cornell University, Ithaca, New York.
Tajeddine and colleagues speculate that an outside force in the form of an incoming asteroid may have struck the region in the past. “In order to drive such a large reorientation of the moon, it’s possible that an impact was behind the formation of this anomalous terrain.”
Michael Lucy is a former features editor of Cosmos.
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