A planet's solstice is the longest day of summer in the northern hemisphere and the shortest day of winter in the southern hemisphere.
One Saturn, it 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.
Reaching the solstice, and observing seasonal changes in the Saturn system along the way, was a primary goal of Cassini's Solstice Mission - the second extended mission of the spacecraft.
"We have witnessed - up close for the first time - an entire season at Saturn," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in the US. "The Saturn system undergoes dramatic transitions from winter to summer, and thanks to Cassini, we had a ringside seat," said Spilker.
Cassini watched a giant storm erupt and encircle the planet. The spacecraft also saw the disappearance of bluer hues that had lingered in the far north as springtime hazes began to form there. The hazes are part of the reason why features in Saturn's atmosphere are more muted in their appearance than those on Jupiter.
Data from the mission showed how the formation of Saturn's hazes is related to the seasonally changing temperatures and chemical composition of Saturn's upper atmosphere.
Researchers have found that some of the trace hydrocarbon compounds there - gases like ethane, propane and acetylene - react more quickly than others to the changing amount of sunlight over the course of Saturn's year.
They were also found that the changes Cassini observed on Saturn did not occur gradually. They saw changes occur suddenly, at specific latitudes in Saturn's banded atmosphere.
"Eventually a whole hemisphere undergoes change, but it gets there by these jumps at specific latitude bands at different times in the season," said Robert West, a Cassini imaging team member at JPL.
During the northern summer solstice, the Sun rose ever higher above the rings' northern face. Light penetrates deeper into the rings, heating them to the warmest temperatures seen there during the mission.
The solstice sunlight helps reveal how particles clump together and 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.
Cassini was launched in 1997 and arrived at Saturn in 2004 for its four-year primary mission to study Saturn and its rings and moons. It is currently in the final phase of its long mission, called its Grand Finale.
Over the course of 22 weeks from April 26 to September 15, the spacecraft is making a series of dramatic dives between the planet and its icy rings. The mission will end with a final plunge into Saturn's atmosphere on September 15.
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