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The Cassini orbiter has gotten its closest look yet at Saturn’s moon Mimas, which is nicknamed the “Death Star” because of its curious crater. The fresh imagery has generated new curiosities and yet another nickname for Mimas: the “Pac-Man” moon.
"After much deliberation, we have concluded: Mimas is not boring. Who knew?!" Cassini imaging team leader Carolyn Porco said in an e-mail announcing today's image release.
Linda Spilker, the Cassini project scientist at NASA's Jet Propulsion Laboratory, agreed: "Other moons usually grab the spotlight, but it turns out Mimas is more bizarre than we thought it was," she observed in an image advisory. "It has certainly given us some new puzzles."
The roughly 250-mile-wide (400-kilometer-wide) moon, whose name can be pronounced either like My-mass or Mee-muss, is already famous for a couple of features: First of all, its surface is dominated by an 88-mile-wide (140-kilometer-wide) pockmark known as Herschel Crater. That big circle makes the moon look eerily like the Death Star super-space station built by the evil Galactic Empire in the "Star Wars" saga.
Despite that impact crater, Mimas has kept its roundish shape - and that makes it the smallest astronomical object we know of that's round due to its self-gravity. It's less than a third as wide as Pluto, just to cite one example. If Mimas' principal orbit went around the sun rather than Saturn, it would be grouped along with Pluto as a dwarf planet. And that's led some astronomers to suggest that any worlds at least as massive as Mimas on the solar system's fringe should be considered dwarf planets - even if we can't see whether they're round.
Not so smooth
Being round doesn't necessarily mean you're smooth, and Mimas is a perfect example. The fresh imagery shows craters upon craters on the moon's predominantly icy surface. Some of the craters - and especially Herschel Crater - seem to have streaks and piles of darker stuff concentrated in valleys and at the base of crater walls.
The Cassini imaging team suspects that sunlight has been "baking" the ice for quite some time now, and as the ice evaporates out into space, darker impurities are left behind to pile up. Eventually those impurities fall to the bottom of the craters, creating avalanche streaks as they tumble down the walls. As the dark stuff falls, brighter ice is exposed on the walls themselves.
"These processes are not unique to Mimas," Cornell University's Paul Helfenstein, an imaging team associate, said in today's advisory, "but the new high-definition images are like Rosetta stones for interpreting them."
Cassini has been in Saturnian orbit since 2004, sending back thousands of pictures of the ringed planet and its moons. These latest high-definition images are based on data collected on Feb. 13, when the bus-sized probe came within about 5,900 miles (9,500 kilometers) of the moon. Today's findings come after weeks of data processing.
The Pac-Man moon
The findings took the form of surface temperature readings from Cassini's composite infrared spectrometer as well as pictures from Cassini's narrow-angle camera. The combination of those different types of data have given rise to a new puzzle ... and the "Pac-Man" nickname.
Because Mimas is mostly round, planetary scientists expected temperatures to vary evenly across the surface. Instead, they found a sharp boundary between warmer temperatures (around 294 degrees below zero Fahrenheit, or 92 Kelvin) and colder temperatures (-320 degrees F, or 77 K). A little warm spot (-310 degrees F, 84 K) was detected around Herschel Crater.
If you map the warmer temperatures in yellow and pink, and the colder temperatures in blue, you get a picture that looks exactly like the classic Pac-Man character eating a warm pinkish dot in Herschel Crater - as shown in the picture above.
The dot makes sense because Herschel's 3-mile-high (5-kilometer-high) walls can trap heat inside the crater. But scientists were baffled by the sharply defined, V-shaped pattern on the opposite side of Mimas' disk.
"We suspect the temperatures are revealing differences in texture on the surface," Southwest Research Institute's John Spencer, a member of the composite infrared spectrometer team, said in today's advisory. "It's maybe something like the difference between old, dense snow and freshly fallen powder."
Denser ice quickly conducts the heat of the sun away from the surface, keeping it cold during the day. Powdery ice is more insulating and traps the sun's heat at the surface, so the surface warms up.

	NASA / JPL / SSI </A>
	A red-green stereo images shows the interior of Herschel Crater on Mimas. Click on the image for a larger version.

What could be behind the difference in texture? The fact that Herschel Crater is in the middle of all this suggests that Mimas' surface was dramatically altered by the impact that created the crater. The blast might have turned surface ice into water ... which was then flash-frozen as a dense top layer over the crater's surroundings.
But all the impacts that have occurred on Mimas since Herschel Crater was created should have pulverized that dense layer by now, Spencer said. So the mystery remains: Why are there such sharp differences on Mimas' surface, and why do they endure?
Chances are that Mimas' Death Star crater will play a big role in any explanation that astronomers come up with. To get a full sense of the crater's dramatic look, pull out your red-blue glasses and gaze at a 3-D stereo image of the hole, produced from bits and pieces of Cassini imagery. And to get a full sense of Mimas' roundness as compared with the non-roundness of some other moons, keep your glasses on and take a look at Prometheus and Phobos.

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