by Staff Writers
Pasadena CA (JPL) May 26, 2017
NASA's Cassini spacecraft still has a few months to go before it completes its mission in September, but the veteran Saturn explorer reaches a new milestone today. Saturn's solstice - that is, 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. The Saturnian solstice 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 name of Cassini's second extended mission.
Cassini arrived at Saturn in 2004 for its four-year primary mission to study Saturn and its rings and moons. Cassini's first extended mission, from 2008 to 2010, was known as the Equinox Mission. During that phase of the mission, Cassini watched as sunlight struck Saturn's rings edge-on, casting shadows that revealed dramatic new ring structures. NASA chose to grant the spacecraft an additional seven-year tour, the Solstice Mission, which began in 2010.
"During Cassini's Solstice Mission, 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, Pasadena, California. "The Saturn system undergoes dramatic transitions from winter to summer, and thanks to Cassini, we had a ringside seat."
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. Cassini 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.
Researchers were also surprised that the changes Cassini observed on Saturn didn't 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.
Saturn's changing angle with respect to the Sun also means the rings are tipped toward Earth by their maximum amount at solstice. In this geometry, Cassini's radio signal passes more easily and cleanly through the densest rings, providing even higher-quality data about the ring particles there.
"Observations of how the locations of cloud activity change and how long such changes take give us important information about the workings of Titan's atmosphere and also its surface, as rainfall and wind patterns change with the seasons too," said Elizabeth Turtle, a Cassini imaging team associate at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
In 2013, Cassini observed a sudden and rapid buildup of haze and trace hydrocarbons in the south that were previously observed only in Titan's high north. This indicated to scientists that a seasonal reversal was under way, 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.
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. From the moon's south polar region, that hidden ocean sprays a towering plume of ice and vapor into space that Cassini has directly sampled.
Toward the Final Milestone
Pasadena, Calif. (UPI) Dec 29, 2016
Cassini is now ring-grazing, but its orbital path earlier this year sent the NASA probe high above Saturn's north pole. In late September, the vantage revealed the pole fully illuminated by the sun's rays. The probe used the opportunity to photograph the sun-soaked north pole with its wide-angle camera. Recently, NASA shared the image online. The summer sun shining on Saturn's no ... read more
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