Cassini spacecraft still has a few months to go before it completes its mission in September. But the Saturn explorer reaches a new breakthrough today.

Taken on June 2004 by NASA’s Cassini Spacecraft.

Saturn’s solstice – the longest day of summer in the northern hemisphere and the shortest day of winter in the southern hemisphere, arrives today for the planet and its moons. This event on Saturn occurs about every 15 Earth years as the planet and its entourage slowly orbit the sun, with the north and south hemispheres alternating their roles as the summer and winter poles.

“During Cassini’s Solstice Mission, we have witnessed, up close for the first time, an entire season at Saturn,” said Linda Spilker (the Cassini Project Scientist at the Jet Propulsion Laboratory in Pasadena). “The Saturn system undergoes dramatic transitions from winter to summer, and thanks to Cassini, we had a ringside seat.”


During its seven-year Solstice Mission, Cassini watched as a huge storm erupted and encircled Saturn. Scientists think storms like this are related, in part, to seasonal effects of sunlight on Saturn’s atmosphere.
Credits: NASA/JPL/Space Science Institute

During its Solstice Mission, Cassini watched a giant storm erupt and encircle the planet. The spacecraft also saw the disappearance of blue hues that had lingered in the far north as springtime haze began to form there.

Data showed how the formation of Saturn’s haze is related to the seasonally changing temperatures and chemical composition of Saturn’s upper atmosphere. Cassini researchers have found that some of the trace hydrocarbon compounds there, react more quickly than others to the changing amount of sunlight over the course of Saturn’s year. 

Following equinox and continuing toward northern summer solstice, the sun rose ever higher above the rings’ northern face. And as the sun rises higher, its light penetrates deeper into the rings, heating them to the warmest temperatures seen there during the mission. The solstice sunlight helps reveal to Cassini’s instruments how particles clump together. Also, whether the particles buried in the middle of the ring plane have a different composition or structure than the ones in the rings’ outer layers.

Saturn’s changing angle with respect to the sun also means the rings are tipped toward Earth by their maximum amount at the solstice.


Cassini has watched Saturn’s largest moon, Titan, change with the seasons, with occasional dramatic outbursts of cloud activity. The spacecraft observed methane storm clouds around Titan’s south pole in 2004. After that, Cassini also watched giant storms transition to Titan’s equator in 2010.

In 2013, Cassini observed a sudden and rapid buildup of haze and trace hydrocarbons in the south. They were previously observed only in Titan’s high north. This indicated to scientists that a seasonal reversal was underway, in which Titan’s main atmospheric circulation changes direction. This circulation was apparently channeling fresh hydrocarbon chemicals from closer to the equator toward the south pole, where they were safe from destruction by sunlight as that pole moved deeper into winter shadow.


The most important seasonal change in Enceladus was the onset of winter darkness in the south. Cassini could no longer take sunlit images of the geologically active surface. However, the spacecraft could more clearly observe the heat coming from within Enceladus itself. With the icy moon’s south pole in shadow, Cassini scientists have been able to monitor the temperature of the terrain there without concern for the sun’s influence. These observations are helping researchers to better understand the global ocean that lies beneath the surface.

On the way to the Final Breakthrough

Cassini is currently in the final phase of its long mission, called its Grand Finale. Over the course of 22 weeks, the spacecraft is making a series of dramatic dives between the planet and its icy rings. The mission is returning new insights about the interior of the planet and the origins of the rings. Also, images from closer to Saturn than ever before are on their way. The mission will end with a final plunge into Saturn’s atmosphere on Sept. 15.