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As the Earth is about to experience its northward, or vernal, equinox on March 20, heralding the start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere, we are reminded that the changing seasons are not exclusive to our planet. Over the last few years, Hubble has been keeping an eye on Saturn as the planet goes through its seasons, and in a newly released GIF, you can see the changes with the naked eye.
The data and analysis published in The Planetary Science Journal covered the period from 2018 to 2020. Saturn has a tilt relative to its orbit of 27 degrees, close to Earth’s 23 degrees. But it takes a longer time to orbit the Sun so each Saturnian season is about 7.5 years instead of the four months we experience on Earth.
The northern summer solstice on the ringed planet took place in May 2017, which means the Hubble observations between 2018 and 2020 caught midsummer in Saturn's Northern Hemisphere. The observations show the equator got 5-10 percent brighter over this 3-year period. The color of the atmosphere also changed due to chemical reactions induced by sunlight in the planet’s atmosphere.
“These small year-to-year changes in Saturn’s color bands are fascinating,” Dr Amy Simon, a planetary scientist at NASA’s Goddard Space Flight Center said in a statement. “As Saturn moves towards fall in its northern hemisphere, we see the polar and equatorial regions changing, but we are also seeing that the atmosphere varies on much shorter timescales.”
The team also report changes to the speed of winds around the equator. In 2018, it appeared that the equatorial winds were going at 2,900 kilometers (1,800 miles) per hour. This is was more than double the measurement conducted around the planet by the Cassini spacecraft. These faster winds appeared to calm down back to the Cassini speeds seen during 2019 and 2020.
The measurements are based on the motion of the clouds, so the team considers that the change in speed was due to clouds being visible from a different altitude of the atmosphere. If this is the case, the clouds seen in the Hubble data would be 60 kilometers deeper in the atmosphere compared to those observed by Cassini.
“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,” added Simon. “We expect big changes on a seasonal timescale, so this is showing the progression towards the next season.”
Simon is the principal investigator for the Hubble’s Outer Planets Atmospheres Legacy (OPAL) program that has delivered important insights into the giant planets of the Solar System, including keeping an eye on the mysterious dark storms of Neptune.
