Space and Physics
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Titan, the largest moon of Saturn, is truly a remarkable world. It has a thick atmosphere as well as rivers, lakes, and seas of methane and hydrocarbons. It is too cold for liquid water, but it is the only other world in the Solar System with liquid on its surface. And of course it rains too. If all of this were not extraordinary, researchers have discovered that the atmosphere wobbles in a different way from the moon.
Celestial bodies spin. It's easy to compare them to a gyroscope, the scientific version of a spinning top. The angular momentum of the celestial body defines its rotational axis, and a lot of different things depend on that. The rotational axis of the Earth, for example, is tilted 23.45 degrees from the plane of its orbit around the Sun. That’s the reason for the seasons. As the planet moves around the Sun, it's always pointing towards the same direction – that’s how we got the North Star.
It seems that the angular momentum axis of the atmosphere is offset from that of the solid body.
Dr Lucy Wright
Titan too has a tilt, but it's very small compared to its orbit around Saturn, just 0.3 degrees. Saturn, on the other hand, has a tilt closer to Earth’s own – 26.7 degrees relative to its orbital plane. Saturn and Titan have seasons too, for this reason. But observations suggest that the atmosphere of Titan rotates with a different tilt altogether.
“It seems that the angular momentum axis of the atmosphere is offset from that of the solid body. Over time, the rotation axis of the atmosphere is changing relative to the planet's spin-axis,” lead author Dr Lucy Wright from the University of Bristol, told IFLScience.
Using 13 years of observations from the Cassini mission, the team studied the motion of the atmosphere with respect to the surface. It appears that the atmospheric temperature field of Titan is not centered on its poles as expected.
“The behaviour of Titan’s atmospheric tilt is very strange. Titan’s atmosphere appears to be acting like a gyroscope, stabilising itself in space,” Wright said in a statement. “We think some event in the past may have knocked the atmosphere off its spin axis, causing it to wobble. Even more intriguingly, we’ve found that the size of this tilt changes with Titan’s seasons.”
Exactly what event might have caused this discrepancy is unknown. And the current observations are not clarifying what’s going on.
“What’s puzzling is how the tilt direction remains fixed in space, rather than being influenced by the Sun or Saturn,” said Professor Nick Teanby, co-author and planetary scientist at Bristol. “That would’ve given us clues to the cause. Instead, we’ve got a new mystery on our hands.”
This research will be useful to the Dragonfly mission, which will bring a flying vehicle to Titan in 2034.
A paper describing the discovery is published in the Planetary Science Journal.
