Half-Moons Rising

Saturn’s moon Mimas (left) compared to the fictional 1977 Star Wars ‘Death Star’, (right) which used its large depression as a ‘superlaser focus lens’ to exact planet-scale revenge. Image Credit: NASA/JPL

This Cassini image shows a nearly half-full Mimas (a moon that is 398 kilometers, or 247 miles, across) beyond Saturn’s rings. The image was contrast-enhanced to make visible the reflected light from Saturn that illuminates the dark side of Mimas and to improve the visibility of the faint F ring.

The enormous crater at the top of the right image, named Herschel, is about 130 kilometers (80 miles) wide and 10 kilometers (6 miles) deep. Deeper than the Grand Canyon, Herschel stretches across nearly a third of the tiny moon’s diameter. The central mountain shown at the center of Hershel is the height of Mount Everest on Earth.

This impact probably came close to disintegrating the moon. Traces of fracture marks can be seen on the opposite side of Mimas, suggesting that the destruction nearly split the satellite into two pieces.

Mimas has a low density, meaning it probably consists mostly of ice. Because Mimas has such a low temperature of about -200° C (-328°F), the impact features may date back to the time of the moon’s creation. Mimas is named from a mythological Titan who was slain by Hercules. The moon was discovered in 1789 by William Herschel, for whom the huge crater is named after today.

Mimas Dawn, acrylic painting of what an astronaut might see standing on Mimas looking back towards Saturn.
Credit:William K. Hartmann, FIAAA

One prominent gap in the rings of Saturn–in particular the one known as the Cassini division– is caused by Mimas, and a resonance condition set up between Saturn’s dust halo and the orbital period of the tiny moon.

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.

Today Cassini will get its first real glimpse of the surface geology of Saturn’s largest moon, Titan, and digest its first gulp of rich Titan air.

Cassini imaging cameras will photograph Titan every 15 minutes or so during approach, said Alfred S. McEwen, a member of the Cassini imaging team. "We’ll get a movie of Titan’s very interesting clouds. They form and dissipate and blow in the wind. Some of them are strange shapes and streaks and things we really don’t understand.

Icy patch in southern hemisphere in Titan. Image Credit: NASA/JPL

"Then, as we get closer, we’ll start mapping. We’ll make a full disk, four-color mosaic. We’ll see the surface, we’ll see the limb hazes, we’ll see whatever clouds there are," McEwen said. "These are things we’ll make posters of, and that everyone will have on their walls."

"As we get closer and closer, we map specific regions at higher and higher resolution. This includes a mosaic over the Huygens landing site. It should be our best look at that location," McEwen said.

"The Titan imaging atmosphere observations for the upcoming flyby have been planned by scientists at the Jet Propulsion Laboratory and sequenced in Boulder," said Carolyn Porco, University of Arizona adjunct professor of planetary sciences. "But the very close observations, those with the goal of mapping the Titan surface at between 50 and 200 meters per pixel, have all been planned, designed and sequenced by our team members at the Lunar and Planetary Laboratory. It’s a very challenging task to plan imaging sequences during a close flyby when the geometry is changing rapidly. And they’ve done an excellent job. We’re in for quite a show."

Robert H. Brown leads Cassini’s visual and infrared mapping spectrometer (VIMS) team, based at UA’s Lunar and Planetary Lab in Tucson. "We know VIMS will see through the haze to Titan’s surface," Brown said. "At closest approach – 1,200 kilometers (745 miles) – we’ll have 600-meter-pixel resolution. We’ll be able to see very small geologic features. We’ll get very high resolution looks at atmospheric phenomena, too. But from my perspective, the really important thing about this encounter is really digging down below the atmosphere and getting our first real glimpse of Titan geology.

"We don’t know what we’re going to encounter there. I suppose you can assume we’ll see common geologic forms like mountains and craters and tectonic faults, maybe even volcanism," Brown said.

Titan is possibly the land of a thousand hydrocarbon lakes. Arizona planetary sciences and physics Professor Jonathan I. Lunine theorized as a graduate student more than 20 years ago that Titan could have liquid hydrocarbon seas or lakes. Lunine is the only U.S. scientist selected by the European Space Agency for its three-member Huygens probe interdisciplinary science team. He and Ralph Lorenz of Arizona’s Lunar and Planetary Laboratory also are members of the radar team. Cassini will get its first radar images of Titan on tomorrow’s flyby.

Hubble surface maps from four global views. Titan’s planned landing ellipse is just to the north and west of the large white-yellow region, thought to be a continent about the size of Australia.
Image Credit: NASA/JPL

"If either the radar or VIMS system on the orbiter take images of liquid-filled crater basins, that to me would be very, very exciting," Lunine said. Scientists would then have evidence that surface lakes are a source and sink for methane in Titan’s hydrologic cycle. VIMS will see Titan’s hydrocarbon pools, if they exist and aren’t hidden by some low-lying fog or other strange phenomenon, Brown said.

VIMS team member Caitlin Griffith said, "Closest approach will give us the most exciting VIMS data because we have that clear view down to the surface. We want to isolate different terrain types and start seeing texture."

When the Cassini spacecraft flew within 339,000 kilometers (210,600 miles) of Titan in July, VIMS was so far away that everything it saw was smeared over 150 kilometers (93 miles), Griffith said. "That’s like taking a picture of Arizona but smearing all of Tucson with all of Phoenix and beyond, towards Flagstaff. This time, we’ll be close enough to isolate and identify lakes and mountains, and maybe see shadows cast at different illumination angles."

Cassini won’t just look at Titan next Tuesday. Cassini’s Ion and Neutral Mass Spectrometer (INMS) will taste mysterious, subtle flavors in Titan’s atmosphere, team member and UA planetary sciences Professor Roger Yelle said.

"Our instrument will scoop up a breath of Titan’s puffy atmosphere during the flyby," Yelle said. The experiment will measure how many molecules of different masses it got in the gulp of Titan’s mostly nitrogen, methane-laced atmosphere.

Carolyn Porco, Imaging Team Lead. Image Credit: NASA/JPL

"Scientists with telescopes have so far seen 19 different chemical molecules in Titan’s atmosphere — more than in any other solar system body’s atmosphere except Earth’s," Yelle said. Laboratory experiments show there are probably many more kinds of chemicals in Titan’s atmosphere, he added.

Yelle and other INMS scientists want to identify the big, complicated and interesting hydrogen-and-carbon-containing molecules because they are part of a planetary system that possibly rains methane and produces ethane ponds. "Titan is a big laboratory where you get to play with atmospheres on planetary scales," Yelle said.

In addition, Yelle said, he is fascinated by Titan chemistry as a scientist interested in the origins of life.

Learning more about how carbon-containing, or "organic," molecules form doesn’t explain how DNA came to be, Yelle said. "A single strand of DNA contains about 3 billion nucleotides that if stretched out, would be something like 1.7 meters long. We’re trying to understand molecules with just 10 or 12 atoms."

But Titan’s hydrocarbon chemistry holds clues that explain the very first steps of how nature assembled organic molecules, which are the precursors to amino acids, the building blocks of life, he said.