Astro Bob: Watching the seasons change at Saturn
New Hubble photos show the ringed planet transitioning from summer to fall.
Any planet with a tipped axis like the Earth has seasons. On Earth, the northern hemisphere angles steeply sunward from May through August, so the sun shines high in the sky. As a consequence, daylight hours are long, heat builds up and summer happens. In winter, when the Earth moves to the other side of its orbit, the northern hemisphere is tipped away from the sun. It shines low in the sky, resulting in shorter hours of daylight and falling temperatures.
It's rather incredible to think that the profound differences between the seasons stem from a single cause: the varying amount of solar energy each hemisphere receives across a year. This fact holds for any planet with a significant nod to its axis.
Other planets with pronounced seasons include Mars (axis tipped 25.2°), Saturn (26.7°), Uranus (97.8°) and Neptune (28.3°). All the others rotate nearly straight up and down with little to no seasonal variation. Like Earth, the four "seasonal" planets have four seasons, but the length of their length depends on how long it takes the planet to go around the sun.
On Earth as on any tilted planet, the start of each new season is defined by the four key spots in its orbit — the winter and summer solstices and spring and fall equinoxes. Because it takes our planet only a year to complete solar revolution, each season lasts 1/4 of that time or about three months. More distant Mars needs 687 days to circle the sun, making each season about twice as long as those on Earth. Saturn orbits the sun every 29 years at a distance of 886 million miles (1.4 billion km). There each season lasts more than 7 years and each sees changes in the planet's climate.
Summer began in Saturn's northern hemisphere in May 2017. Four years later we're starting to see the transition to fall, which starts in May 2025. Astronomers have been busy tracking changes in the planet's equatorial and polar regions as well as its atmosphere with the Hubble Space Telescope Because seasons are long-lasting affairs at the ringed planet, changes occur more slowly than they do on Earth.
The Hubble data show that from 2018 to 2020 the equator got 5 to 10 percent brighter, and the winds shifted. In 2018, winds measured near the equator were about 1,000 miles per hour (1,600 kph), some 200 mph faster than those measured by NASA’s Cassini spacecraft during 2004-2009. Then in 2019 and 2020 they decreased back to the Cassini speeds. Saturn’s winds also vary with altitude, so the change in speeds could possibly mean the clouds in 2018 were around 37 miles (60 km) deeper than those measured during the Cassini mission.
A drop in the intensity of seasonal sunlight appears to be behind these variations. You can read more about the observations in this free, scientific paper published earlier this month in the Planetary Science Journal. There's also a cool animation of the changing tilt and appearance of Saturn from 2018 to 2020 — click here to see it.
Speaking of atmosphere, that's pretty much all Saturn is. — gas. Like Jupiter, Saturn's air is mostly hydrogen and helium with traces of methane and other gases. Both planets are massive enough to hold onto these light gases. On Earth they routinely escape from the atmosphere into space because gravity here is so much less.
When you look at Saturn in a telescope or photograph the first thing you notice are its bright, pale-white rings, which are composed primarily of water ice. In contrast, the planet's globe shines pale yellow from clouds made of ammonia and ammonium hydrosulphide ice crystals. Enormous storms , some almost as large as Earth, occasionally erupt from deep within the atmosphere. They occur about every 30 years and are called Great White Spots.
The last one started in late 2010 and lasted well into the following year. Since many of the planets discovered around other stars are gas giants as well, astronomers look to Saturn and Jupiter to teach us more about giant-planet atmospheres work.
Right now, Saturn shines low in the southeastern sky at the start of dawn. An hour before sunrise is the best time to find it. The ringed planet is joined by Jupiter, which is brighter and easier to see. In case you spot Jupiter first, Saturn is located one horizontal fist held at arm's length to the upper right of the planet. Both Jupiter and Saturn rise higher and get easier to see as we approach the end of March.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .