You may have noticed an amusing new image floating around the blogosphere: the latest Cassini Equinox snapshot of one of Saturn‘s moons, Mimas, looking for all the world like a fluorescent-hued Pac-Man.
What are we looking at here? Well, these two images compare what Mimas looks like in ordinary visible light, while the other is shows the unexpected sharply varying temperatures across the moon’s surface, based on the latest data from Cassini’s composite infrared spectrometer. There’s a sharp v-shaped “hot zone” boundary that looks remarkably like Pac-Man, especially since it’s “mouth” is on the verge of gobbling the famed Herschel Crater.
The image is making headlines, not just because of the Pac-Man tie-in (which, let’s face it, is fun), but because nobody expected Mimas to be all that special. It’s the innermost major moon of Saturn. Astronomer William Herschel discovered Mimas in 1789, although to him it was just a tiny dot orbiting Saturn. We didn’t get a really good look at the object until 1980, when Voyagers I and II swung by to say hello and snap a few fuzzy candid shots. Cassini has given us the best images to date of this tiny moon, most notably its heavily cratered surface. More than one observer has noted its uncanny resemblance to a certain Death Star.
(Fun bit of historical trivia: Mimas got its name thanks to Herschel’s son, John, who suggested it made sense for the moons of Saturn to have names of the Greek mythical siblings of Kronus — the Greek name for Saturn. Mimas was one of the Titan gods slain by either Hercules, Ares or Zeus, depending on which version of myth you’re reading).
But to date, the most interesting scientific aspect of Mimas is the fact that its surface appears to be composed mostly of solidly frozen water ice, despite expectations that it should experience more tidal heating than, say, fellow moon Enceladus because Mimas is closer to Saturn. But Enceladus got the impressive geysers of water that one would expect to see on Mimas — which just gets that heavily cratered surface because it’s constantly bombarded by asteroids or whatever. (Say what you will, but that Mimas can take a punch.) There’s bound to be a bit of sibling resentment on that score. Mimas can take comfort in the fact that astronomers have a “Mimas test” to assess theories to explain the partially thawed water of Enceladus. Any theory has to also explain why the water of Mimas is entirely frozen.
Because of that frozen surface, astronomers expected smoothly varying temperatures across Mimas’ surface, not the sharp temperature contrasts Cassini’s instruments actually observed. They figure the colder portions occur because the surface materials in those areas are more thermally conductive. That means the sun’s rays soak through to the moon’s subsurface, rather than warm the surface. But why thermal conductivity should vary so wildly across the surface of Mimas is shrouded in mystery.
John Spencer, a member of Cassini’s composite infrared spectrometer team, compares it to “the difference between old dense snow and freshly fallen powder.” Another Cassini team member, Paul Helfenstein, explains the unusual pattern may be the result of how Mimas’ surface ages: slowly accumulating “a thin veil of silicate materials or carbon-rich particles.” Where do those particles come from? Again, we’re not sure, but likely culprits include meteor dust scattering across Mimas, impurities already embedded in the surface ice.
Apparently the actual processes that lead the contrasts aren’t unique to Mimas — it’s just a bit of a surprise to see them where they weren’t expected to be seen. In honor of Cassini’s strange discovery, here’s a YouTube video tribute to Pac-Man. And if you want a more hip-hop kind of tribute check out this fun video (warning: NSFW language). Wocka-wocka-wocka, space fans!