Saturn’s Family Portrait

Wide angle view of five of Saturn’s moons. Click for larger view. The image was taken in visible red light on Aug. 1, 2004, at a distance of 7.8 million kilometers (4.8 million miles) from Saturn. The image scale is 464 kilometers (288 miles) per pixel.
Credit: NASA/JPL

A stately Saturn poses for a portrait with five of its moons in this Cassini spacecraft wide angle camera view.

Satellites visible in this image are (clockwise from upper left): Dione (1,118 kilometers or 695 miles wide), Enceladus (499 kilometers or 310 miles wide), Tethys (1,060 kilometers or 659 miles wide), Mimas (398 kilometers or 247 miles wide) and Rhea (1,528 kilometers or 949 miles wide).

Cassini is conducting a four-year orbital mission, circling Saturn 77 times and cruising by more than 50 close encounters (and another dozen or so more-distant encounters) with the planet’s moons. In all, Cassini will aim its instruments at 8 of Saturn’s 33 or more known moons. Cassini has already discovered a few that were unknown from ground observation and an earlier Voyager flyby.

Along with most of the other 33 satellites, the featured moons appear to have an icy crust. Condensed ices give the moons very high albedo (reflection coefficients) but some are mottled with darker regions that may be rich in organic chemicals like methane or ammonia. These building blocks for primitive biochemistry may offer insight into how a similar, but much warmer environment on Earth, might have given rise to primordial life.

Titan will get the lion’s share of attention: 45 close flybys are planned for the giant moon. Titan will also be the target of the Huygens probe, which will be released by Cassini on Christmas day (Christmas eve in the U.S.) for descent through Titan’s atmosphere 3 weeks later. Titan is of particular interest to scientists because, like Earth, it has an atmosphere that contains nitrogen and organic molecules such as methane. Some scientists speculate that Titan’s chemistry may offer a snapshot of what Earth’s chemistry was like before life took hold.

Saturn’s moon Enceladus.
Image Credit: NASA/JPL

"Cassini will do the [Titan] mapping from orbit, looking at large numbers of crater basins, to see if they’re filled with liquid; Huygens will get a local view of one area [on Titan] to see if there are on small scales lots of liquid patches", said Jonathan Lunine of the Lunar and Planetary Laboratory at the University of Arizona. Lunine explained to Astrobiology Magazine that "if either the imaging or VIMS system on the orbiter, or the Huygens probe while descending [to Titan in January 2005], takes images of liquid-filled crater basins–that, to me, would be very, very exciting, because that would sort of bring together all of the aspects of the chemistry. It would demonstrate that there’s a source and a sink on the surface, that chemistry’s been going on for a long time, and so it would really tie the story together. Finding life somewhere is the most interesting thing you can do anywhere, but in terms of the most immediate likely result that Cassini-Huygens could get, that would be the most exciting."

After Cassini releases the Huygens probe, it will spend about 10 exploring several of Saturn’s icy satellites. Cassini will make close flybys of Enceladus, Hyperion, Dione and Rhea during this grand lunar tour.

"One of our major objectives in returning to Saturn was to survey the entire system for new bodies," said Dr. Carolyn Porco, imaging team leader, Space Science Institute, Boulder, Colo. "It’s really gratifying to know that among all the other fantastic discoveries we will make over the next four years, we can now add the confirmation of two new moons, unnoticed around Saturn for billions of years until now," she added.

Moons surrounding the giant planets generally are not found where they originally formed because tidal forces from the planet can cause them to drift from their original locations. In drifting, they may sweep through locations where other moons disturb them, making their orbits eccentric or inclined relative to the planet’s equator.