Saturn is now in the "September" of its lengthy year, and an iconic space telescope is watching the ringed planet's change of seasons.
Spring and autumn officially got underway here on Earth's Northern and Southern Hemispheres, respectively, just a few days ago with the arrival of the vernal equinox. The coming and going of seasons is possible when a planet has a tilted axis — and both Earth and Saturn have a lean in their spins.
But there's a big difference in their length of seasons. Earth takes one year to orbit the sun, during which our planet experiences the cold and dreary, the sunny and delightful, and every seasonal middle-ground in between. But Saturn, the sixth planet from the sun, takes 29 Earth years to go around our star once. Earth moves through all its seasons repeatedly all the while Saturn remains in just one season; it takes roughly seven Earth years for Saturn to begin and end one of its seasons.
On March 18, Hubble team members based out of NASA's Goddard Space Flight Center in Greenbelt, Maryland published new images that show subtle shade differences to the appearance of Saturn's clouds over the last several Earth years. In these images taken by Hubble during 2018, 2019 and 2020, Saturn's northern hemisphere is experiencing summertime and approaching its autumn. Meanwhile, like on Earth, it was winter (ang going on spring) in Saturn's southern hemisphere.
"What we found was a slight change from year-to-year in color, possibly cloud height, and winds — not surprising that the changes aren't huge, as we're only looking at a small fraction of a Saturn year," according to Amy Simon, planetary scientist at Goddard, who commented in the NASA press release that describes the new images. "We expect big changes on a seasonal timescale, so this is showing the progression towards the next season."
The Hubble data show that, from 2018 to 2020, the equator brightened by about 5% to 10%, and some cloud bands that were visible during certain years do not appear at other times.
Wind speeds also changed, according to NASA. Back when Cassini observed Saturn during the years 2004 to 2009, winds near the equator were roughly 800 miles per hour (roughly 1,300 kilometers per hour), according to the release.
Hubble's 2018 observations show equatorial wind speeds became much faster, about 1,000 mph (roughly 1,600 kph). By 2019 and 2020, however, Hubble detected that those wind speeds dropped back to the Cassini speeds from more than a decade ago.
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