Hello, Daphnis! On January 16, 2017, the Cassini spacecraft captured the best photo yet of Daphnis, a 5-mile-wide shepherd moon that orbits Saturn inside the Keeler Gap at the outermost edge of the A ring (and also just so happens to be my personal favorite moon of Saturn!) The raw image arrived on Earth today, and it’s just beautiful.
When you think of spacecraft landings on other worlds, you probably first think of Mars, the Moon, Venus, and comet 67P (if you’ve been following along over the past couple of years.) But—in addition to the asteroid Eros and hard impacts on a comet and Mercury—Saturn’s moon Titan was also visited by an alien (i.e., Earthly) spacecraft back in January of 2005. ESA’s Huygens probe, which traveled to Saturn onboard the Cassini orbiter, was deployed to the surface of Titan six months after Cassini arrived in orbit at Saturn. Huygens took 20 days to reach the cloud-covered moon, and during its two-and-a-half hour descent on Jan. 14, 2005 transmitted our first—and last—views from below Titan’s clouds and even from its methane-slush-covered surface. It was the first landing on a moon other than our own and the farthest landing from the Sun, but hopefully not the final time we’ll visit the fascinating surface of this icy moon.
Read more and watch the video of the Huygens probe landing on the Cassini mission site: Huygens: ‘Ground Truth’ from an Alien Moon
Saturn’s moon Dione (pronounced DEE-oh-nee) is a heavily-cratered, 700-mile-wide world of ice and rock, its surface slashed by signature “wispy lines” that mark the bright faces of sheer ice cliffs. But Dione has some strange colorations too, evident here in a global map created in 2014 from Cassini images. Its leading half—the side that faces “forward” as it moves around Saturn in its tidally-locked orbit—is pale and bright, while its trailing hemisphere is stained a brownish color, the result of surface interaction with charged particles in Saturn’s magnetic field.
After more than twelve years in orbit around Saturn, NASA’s Cassini spacecraft is now in its final year of operation…and newly-positioned in an orbit that will send it soaring high over the planet’s north pole as well as close by the outer edge of its glorious shining rings.
Over the course of 20 week-long “ring-grazing orbits” — the first of which was completed on Dec. 4 — Cassini will obtain close-up images and data on the composition and structure of Saturn’s rings and nearby shepherd moons.
It’s the mission’s penultimate phase, heralding the end of Cassini in September 2017.
“This is it, the beginning of the end of our historic exploration of Saturn,” said Cassini imaging team leader Carolyn Porco. “Let these images — and those to come — remind you that we’ve lived a bold and daring adventure around the solar system’s most magnificent planet.”
Round and round they go… the animation above, made from 14 raw images taken by NASA’s Cassini spacecraft on August 23, 2016, shows the moons Prometheus and Atlas orbiting Saturn within the Roche Division gap between its A (top right) and F (center) rings. The gravitational tug of Prometheus (92 miles / 148 km long) is strong enough to pull on the fine, smoke-like icy particles of the F ring, creating streamer and “clump” features that follow it along.
The much smaller Atlas (23 miles / 37 km wide) follows a path around Saturn just past the outer edge of the A ring. It was once thought to be a “shepherd moon” of the A ring, but it’s now known that the pull of the more distant Janus and Epimetheus are responsible for that.
Atlas does have its very own ring, though—a very faint (i.e., not visible above) band of material that runs along its orbit named R/2004 S 1, discovered by the Cassini mission in July 2004.
Image credit: NASA/JPL-Caltech/SSI. Animation by Jason Major.
The lines of Saturn’s rings appear to get sharply bent as their reflected light passes through the upper atmosphere of the planet before being captured by Cassini’s camera in this raw image, acquired on June 9, 2016.
Enormous Saturn of course is, by volume, nearly all atmosphere with a solid core only about the size of Earth. The bands of tans, blues, purples and pale greens we see in Cassini images are just the tops of clouds, and even more transparent layers of atmosphere exist above those, gradually fading out into space. Like what astronauts see from orbit when the Moon passes behind these upper layers, getting squished in the process, Saturn’s rings appear to be distorted by the same refractive process for Cassini. It’s literally an optical illusion, as obviously the rings aren’t changing shape at all!
The image above has been cropped and rotated for artistic effect; see the original raw image here.