[♪ INTRO] June 21st marks the northern  summer solstice on Earth, meaning the longest day of the  year for the northern hemisphere.

But Earth is not the only planet that  experiences solstices… or seasons. Some of the most dramatic seasons in  the solar system can be found on Saturn, and the Cassini probe spent almost a decade in orbit around that gas giant  so that it could study them.

The probe’s ringside seat gave it  an unrivaled view of the planet as the Saturnian solstice  approached, and helped to transform our understanding of the solar  system’s second largest planet. As a ringed gas giant 1.4 billion kilometers  from the sun, there’s not much that Saturn has in common with the Earth,  but there is one noticeable similarity. Both planets are tilted on their  axes by roughly the same amount: twenty-three and a half degrees for  Earth; twenty-seven degrees for Saturn.

That means that Saturn experiences  seasons just like the Earth. It’s summer when the hemisphere  is pointed toward the sun, and winter when it’s tilted away. And, just like on Earth, the summer solstice  marks the point when the sun appears to reach its highest point in the sky,  marking the astronomical start of summer.

But Saturn’s summers are just  a little bit longer than ours. Because the planet takes about 29 Earth  years to complete a single orbit, each of its four seasons lasts about seven  years compared to Earth’s three months. So when the Cassini probe first arrived  at the sixth planet from the sun in 2004, it was around the winter solstice  in Saturn’s northern hemisphere.

But by the time its main mission was completed, four years later in 2008,  it was barely even spring. The probe was still in good condition, though, so NASA extended Cassini’s  service in the Equinox Mission. This mission extension ran for an  additional two years from 2008 to 2010, allowing Cassini to hang  around and study the gas giant as it went through its spring  equinox in August 2009.

An equinox marks the moment when the sun  is directly above a planet’s equator, which on Saturn means something very special. Since Saturn’s incredible rings  are in line with the equator, during the equinox the sun  is striking them edge on. In most places, the rings are only  about ten meters thick, so the equinox would reduce them to a razor thin  line from the sun’s point of view.

But as it orbited Saturn, Cassini was  able to spot brand new structures that protruded up to 2.5 kilometers above the disk, based on the shadows that  they cast across the rings. Scientists believed they were caused by moonlets, around one kilometer or more in diameter, causing the smaller ring particles to  sort of ‘splash’ up as they streamed past. But there were more discoveries  in store for Cassini as the spring turned to summer.

After the Equinox mission  was completed successfully, the probe’s journey was extended again,  this time for the Solstice Mission. Cassini flew for a further seven years  from 2010 to 2017, to witness the changes from the equinox to the  northern summer solstice in May of 2017. Over 155 orbits of the gas giant, the  extended mission allowed scientists to study the global changes that took  place over an entire Saturnian season.

First, the probe spotted a gigantic  storm in the northern hemisphere in 2011. It was so big that the disturbance  it caused in the clouds completely encircled the planet, and it  overtook its own tail after 12 weeks. Researchers think that such a violent  storm is related to the fact that more of the sun’s heat is being absorbed  in the northern hemisphere as the planet moves towards its solstice.

But storms aren’t the only  feature to be seen on the surface. The planet is usually striped with vivid  bands of color, which are thought to result from the presence of different  chemicals at different latitudes. And throughout its long solstice mission, Cassini got to watch how these  bands changed over the seasons.

For one, the probe monitored the  north polar region and watched as the area gradually turned from  blue to orange between 2012 and 2016. Scientists believe that this happens  because the north pole region is exposed to more sunlight throughout the early summer, leading to the production  of more photochemical haze. This is a kind of smog that’s  formed when sunlight breaks down certain chemicals in the atmosphere,  creating new gaseous compounds that are more orange and more opaque.

Normally, the polar jet  stream keeps the area cut off from the hazes that are found  elsewhere on the planet. But as the pole basks in sunlight  as Saturn approaches the solstice, it’s able to produce its own haze,  turning the whole region orange. Cassini saw other color changes  through Saturn’s banded atmosphere, but surprisingly these happened  suddenly, instead of gradually, at different points in time  as the solstice approached.

This was thought to be caused by different  compounds in the planet’s atmosphere reacting to changing sun  intensity at different rates. And there were more discoveries  to be made in Saturn’s rings as it approached the solstice, too. As the rings tilt to their maximum angle,  the sunlight is able to penetrate deeper, heating them to the warmest temperatures  seen throughout the Cassini mission.

And the radio waves that Cassini emits  in order to image the rings as it orbits Saturn are able to pass more cleanly  through the rings when they’re more face-on. This gives us the highest possible resolution data about the particles that make up the disk. These unique imaging conditions revealed  how the particles clump together, and how the composition  varies throughout the rings.

The probe was able to spot details  like the propeller shaped gaps caused by moonlets, and waves in the  ring edges caused by larger moons. The twice-extended Cassini mission  finally ended in September of 2017, not long after the Saturnian solstice, as it dove dramatically into the  planet it had studied for 13 years. But its data are still  revealing how the giant planet, which is so similar and yet  so different to the Earth, experiences its seasons, equinoxes and solstices.

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