NASA’s Cassini spacecraft will be making its closest swoop over the surface of Saturn’s moon Dione and scrutinizing the atmosphere of Titan, Saturn’s largest moon. The closest approach to Dione, about 61 miles (99 kilometers) above the surface, will take place at about 1:39 a.m. PST (4:39 a.m. EST) on December 12. One of the questions Cassini scientists will be asking during this flyby is whether Dione’s surface shows any signs of activity. Understanding Dione‘s internal structure will help address that question, so Cassini‘s radio science instrument will learn how highly structured the moon’s interior is by measuring variations in the moon‘s gravitational tug on the spacecraft. The composite infrared spectrometer instrument
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will also look for heat emissions along fractures on the moon‘s surface. Cassini will also be probing whether Dione, like another Saturnian moon, Rhea, has a tenuous atmosphere. Scientists expect a Dionean atmosphere, if there is one, to be much more ethereal than even Rhea‘s. On Cassini‘s journey out from Dione toward Titan, the imaging science subsystem will turn back to look at Dione’s distinctive, wispy fractures and a ridge called Janiculum Dorsa. Cassini will approach within about 2,200 milles (3,600 kilometers) of the Titan surface, at about 12:11 p.m. PST (3:11 PM EST) on December 13. At Titan, the composite infrared spectrometer will be making measurements to understand how the seasonal transition from spring to summer affects wind patterns in the atmosphere near Titan’s north pole. It will also search for mist.