Cassini’s epic mission to Saturn is coming to a close with a spectacular finale of daring flybys and swooping encounters. The beginning of the end happened today with the last targeted flyby of Dione, the icy moon of towering cliffs and dizzying canyons.




Dione against the backdrop of Saturn in blue, green, and infrared light on October 11, 2005 from 39,000 kilometers (24,200 miles) away for 2 kilometer (1 mile) per pixel resolution. Image credit: NASA/JPL/Space Science Institute


Cassini came to within just 474 kilometers (295 miles) of Dione at 11:33 am on Monday, August 17, 2015. The images are currently being sent back to Earth via the Deep Space Network — you can check out the raw images as they’re released here. This was the final targeted flyby of the ice moon; from here on out the spacecraft will be making a final tour of the Saturn system with no time for detours before going ring-diving in 2016 and crashing into the gas giant in September 2017.

What have we learned about Saturn’s fourth largest moon during the Cassini mission?

Dione as seen on June 16, 2015 from 77,000 kilometers (48,000 miles) away from the gas giant, cutting across Saturn’s rings. Image credit: NASA


Dione was discovered in 1684 by Giovanni Cassini, and was named for the Greek titan Dione, daughter of Tethys and Oceanus, and mother of Aphrodite.


Dione [upper] and Tethys [lower] on April 4, 2015. Image credit: NASA

The first flyby of the little moon was in November 1980 as part of Voyager 1’s cruise through the solar system. The probe passed within 161,520 kilometers (100,364 miles) at closest approach, producing images of Eurotas Chasmata at a resolution of roughly a kilometer (0.6 miles) per pixel.


Map of Dione constructed from Voyager flyby data. Image credit: NASA/USGS

Dione orbits Saturn at 377,400 kilometers (209,651 miles), roughly the same distance as the moon around the Earth. It has a mass of 1.10 x 1021 kilogram, or roughly the same as all the water in Earth’s oceans. With a diameter of 1,118 kilometers (695 miles), it is the fourth largest of Saturn’s moons. Dione has a density of about 1.43 grams per cubic centimeter. That’s high enough to suggest that Dione has a dense rock core muffled in a thick mantle of ice.


Raw image of Dione with Saturn in the background seen on the August 17, 2015 flyby. Image credit: NASA/JPL-Caltech/Space Science Institute


The first flyby of Dione since Voyager was almost a decade ago on October 11, 2005 when Cassini swooped 1,000 kilometers (621 miles) past the moon, getting closer with each subsequent flyby until the closest approach from just 99 kilometers (62 miles) away on December 12, 2011. Since then, it made another quick pass earlier this summer before today’s final flyby. In all those flybys, Cassini has pieced together a full map of the little moon. The moon map is at an array of resolutions: this final flyby involves a targeted high-resolution peek at Dione’s north pole to only a few meters per a pixel.


Map of Dione; places are named for Virgil’s Aeneid. Image credit: NASA/USGS

The Cassini spacecraft took gravity measurements during the flyby, allowing for investigating the moon’s internal structure. Only a few of Saturn’s 62 known moons have been prodded in this manner. Cassini also kept its Composite Infrared Spectrometer on and pointed at the small moon, looking for any thermal anomalies to identify places on the icy moon that are unusually good at trapping heat. As always, the Cosmic Dust Analyzer is continuously monitoring for any dust particles emitted by the moon.


Saturn and the main moons of its 62 known satellites. Image credit: NASA

Dione is tidally locked to Saturn so the same side always faces the massive gas giant. It has exerted its own lock on two smaller moons: Helene and Polydeuces hang out 60° ahead and behind the moon in the L4 and L5 Lagrange points. The moon is also in resonance with two other moons. Dione, Mimas, and Enceladus perform a complex dance where they speed up as they approach and slow down as they draw away; the result keeps Enceladus in an orbit exactly half the period of Dione’s orbit.


Map of Dione using infrared, green, and ultraviolet wavelengths with a resolution ranging from 200 meters to 1.4 kilometers per pixel. Image credit: NASA/Paul Schenk/LPI




Dione’s colour is decidedly lopsided: the leading hemisphere of the tidally-locked moon is constantly coated in fresh ice from the E-ring while the trailing hemisphere is more heavily cratered. This is a bit odd as normally, the leading hemisphere should be heavily cratered, so the current theory is that a recent, massive impact spun the moon around. Any of the many 35-kilometer (22-mile) diameter craters could be evidence of a sufficiently forceful to spin the moon around, although it’d still be strange to be a perfect 180° reversal.


Although Dione does not have indisputable evidence of recent geological activity, it shows traces of an active past in faults and crustal ridges.

Janiculum Dorsa, an 800 kilometer (500 mile) long sinuous mountain/range varying in height from 1 to 2 kilometers (0.6 to 1.2 miels), a crustal bend suggesting Dione’s icy crust was warm in the recent past. Image credit: NASA/JPL-Caltech/SSI/Brown


Younger light-coloured fractures cross the trailing hemisphere. The wispy lines are ice canyons with walls up to hundreds of meters tall, revealed as darker material sheds off the walls to expose bright water ice. The canyons are possibly formed by subsidence cracks; they might be a later stage of the same processes that create the tiger stripes on Enceladus.


Previous flybys have almost but not quite provided evidence for if Dione joins the ranks of geologically active small icy worlds. Cassini science team member Bonnie Buratti explains:

Dione has been an enigma, giving hints of active geologic processes, including a transient atmosphere and evidence of ice volcanoes. But we’ve never found the smoking gun. The fifth flyby of Dione will be our last chance.


Raw image of Dione taken during the final targeted flyby on August 17, 2015. Image credit: NASA/JPL-Caltech/Space Science Institute


After completing the Dione flyby, Cassini will continue its final tour of Saturn’s moons. It will swing out of the equatorial plane in late 2015, setting up next year for its grand finale by edging closer to the rings. The spacecraft will dive between the gas giant and its rings in late 2016 before crashing into Saturn’s atmosphere as the grand finale to the mission in 2017.


Cassini’s orbits from insertion around Saturn on June 30, 2004 through its planned end-of-mission on September 15, 2017. The orbit of Titan is marked in red, and six other moons are marked in white. Image credit: NASA/JPL-Caltech

You can watch a compilation of images of Dione taken by Cassini in this farewell video to the icy moon:

The images from today’s flyby will be sent home over the next few days. You can check out the raw images here.




[ NASA ]

Top image: Raw image of Dione as seen on August 17, 2015 with Saturn’s rings in the background. Credit: NASA/JPL-Caltech/Space Science Institute