Cassini image of Dione and Saturn's rings

A Lovely Portrait of a Saturn Moon and Backlit Rings

A Lovely Portrait of a Saturn Moon and Backlit Rings

Bad Astronomy
The entire universe in blog form
Dec. 29 2015 9:30 AM

Dione, From Afar

Dione and rings
Dione, a moon of ice, shines before the rings of Saturn. Click to encronusenate.

Photo by NASA/JPL-Caltech/Space Science Institute

As the year winds down, can we take a moment to remember how beautiful our Universe can be? Especially our little local piece of it.

That image above was taken by the Cassini spacecraft, which has been orbiting Saturn since—and I still have a hard time believing this—2004. In more than a decade it has taken countless observations of Saturn, its atmosphere, the rings, and its ridiculously photogenic moons.

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The photo shows Dione, a mostly-ice-but-with-some-rock moon, some 1,120 kilometers across (our Moon is roughly three times that diameter). One hemisphere of the moon contains the wispy features you can see in the shot; they were discovered during the Voyager probes in the early 1980s. From those images they were thought to be material extruded from beneath the surface, but Cassini revealed them to be fractures in the surface, interconnecting canyons with bright walls, probably made of nearly pure water ice.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

From 1.7 million kilometers away, Cassini got an interesting overview of the moon, with the wisps and a few large craters visible. Highlighting the shot are Saturn’s rings, seen very nearly edge-on, behind the moon. Cassini was “below” the rings, south of Saturn’s equator, looking “up” (north), in this shot; you can tell because the rings look dark gray. Right now it’s approaching summer in Saturn’s northern hemisphere (the solstice for Saturn is in May 2017), so the Sun shines down from the north on the rings. If Cassini were north of Saturn’s equator, the rings would look far brighter as the tiny ice particles reflect sunlight toward the spacecraft.

This image was taken on Aug. 15, 2015, just two days before Cassini flew past Dione, dipping down to an astonishing 500 km of its surface, and sending back pictures so incredible they almost stopped my heart. That was the very last flyby of Dione; there are no more scheduled between now and the time the Cassini mission ends in 2017.

That’s going to be a tough one, I’ll admit. Cassini has been nothing short of a triumph, a phenomenal paean to what humans can do when we look up to the sky in wonder instead of at each other in hatred. It is the best of us, and has shown us how devastatingly beautiful, how intricately woven, and how magnificently displayed are the results of nature’s laws. 

I have friends who have worked on Cassini for decades. It will be incredibly sad to see it end, fittingly plunging into the atmosphere of Saturn. There are no plans by any nation to visit Saturn again … but we have a Europa mission on the boards, and I hope, with recent news of a healthy increase to NASA’s budget, that we may venture this way again, and learn more about the giants that populate the outer solar system. Even after all this time, there is still a Universe worth of knowledge to gain out there.