As 2015 comes to a close, NASA releases another jaw-dropping image from the Cassini spacecraft.
Wispy fractures brighten the mostly icy surface of Dione. And Saturn’s majestic rings instantly make any image better. Cassini captured this image on August 15, 2015 as it continues its 10+ year journey around Saturn and its many moons.
The spacecraft was just over 1 million miles away from Dione when it imaged the 698 mile-across moon.
Slate’s Phil Plait points out that if Cassini were ‘above’ Saturn’s rings, the rings would appear much brighter. That’s because summer is coming in Saturn’s northern hemisphere. And sunlight is shining down from above the rings.
The story behind Dione’s wisps
Dione’s wispy features were first spotted in 1980 by the Voyager space probe. Scientists were puzzled. All they knew for sure was the material had a high albedo (brightness), and the features didn’t appear to obscure the surface features beneath. One theory suggested cryovolcanism resurfaced much of Dione’s surface with the streaks corresponding to eruptions along cracks on the moon’s surface. Not a bad theory. But it was ultimately wrong.
Cassini solved the mystery during a close flyby in December 2004. It’s not resurfacing. What we are seeing are deep ice cliffs dropping hundreds of meters. These icy cliffs are geologically young too. How do we know? Some of these ice-cliffs run across several craters. That tells scientists the icy cliffs are at least as young as the craters.
Cassini’s goodbye to Dione
On August 17, 2015, Cassini soared by Dione for its last time. It came within 295 miles of the moon. Here’s a view of Dione’s surface from an altitude of 600 miles during the August flyby.
We are actually looking at Dione’s ‘night’ side. The surface is being entirely lit by sunlight reflecting off Saturn. It’s dubbed Saturnshine.
During the August flyby, Cassini trained its cameras and instruments on Dione’s north pole. Cassini’s Composite Infrared Spectrometer also took a look at several areas on the icy moon with unusual thermal anomalies.