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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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