Scientists watching Saturn’s moon Titan have finally gotten evidence that the planet experiences seasons like Earth. It’s subtle, but a shimmering region in the higher latitudes indicates that summer has started in Titan’s northern hemisphere. This discovery has been years in the making because of Titan’s exceedingly long orbit, but we have the dearly departed Cassini probe to thank.
We often talk about how much Titan and Earth have in common. They’re the only two objects in the solar system with permanent bodies of liquid on their surfaces. Titan also has a thick atmosphere just a little more dense than Earth’s. Venus also has an atmosphere, but it’s about 90 times denser. Titan also has a stable axial tilt of 27-degrees, a bit more than Earth’s 23.5-degree angle. However, Earth and Titan aren’t exactly twins. Those lakes and oceans are liquid hydrocarbons instead of water, and the atmosphere is mostly nitrogen and methane. Still, the arrival of the Cassini probe gave scientists a chance to prove Titan experiences Earth-like seasonal weather.
Confirming seasons on Titan has been a challenge because the year is so much longer. Saturn’s massive gravity ensures that Titan is tidally locked. Thus, its day is 15.9 Earth days (the same as its orbit around Saturn), and a year is 29.5 Earth years (Saturn’s orbit around the sun). With the healthy axial tilt, scientists predicted that Titan should experience seasons about 7.5 Earth years in length.
Cassini arrived in 2004, and in 2009 Titan reached its spring equinox. In 2011, the probe spotted atmospheric changes in the southern hemisphere that looked like the onset of winter. However, corresponding hydrocarbon rains in the northern hemisphere remained elusive — there weren’t even any storm clouds.
That brings us to the latest discovery. Cassini dropped into Saturn’s atmosphere in 2017, but its data is still being analyzed. Researchers from the University of Arizona and the University of Idaho spotted a telltale shimmer in a Cassini photo from June 7, 2016. According to the new paper, the shiny area covering 46,332 square miles (120,000 square kilometers) indicates rainfall on the surface.
The image comes from Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) instrument, which can peer through the moon’s thick cloud layer. The shimmer was missing in all previous and subsequent images, supporting the idea that it was freshly fallen liquid methane. After falling, it would have drained into Titan’s lakes like the nearby Kraken Mare.
This is the missing piece of the puzzle. Together with weather patterns in the southern hemisphere, the newly detected rainfall seemingly confirms predictable seasonal weather on Titan. This could help in the planning of future surface missions to the moon as well as remote observations.
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