Minnesota may have a new rival for the label "land of 10,000 lakes" – but don't break out the jet skis. The rival is 944 million miles from Earth.
New images of Titan, Saturn's largest moon, reveal lakes that appear as odd-shaped splotches across the moon's far north. Lakes seen thus far range in size from just over half a mile to more than 60 miles long. Researchers say they contain liquid ethane and methane at a frosty –256 degrees F.
The find caps a month of jaw-dropping discoveries about Titan's Australia-size highlands, dubbed Xanadu, and insights into rainfall patterns on the moon – an active world sculpted by processes that are remarkably Earthlike.
"Looking at these images, they could be from Minnesota or Finland," says planetary scientist Ralph Lorenz, a member of the team announcing the results.
With the discovery, unveiled Friday, scientists heaved a sigh of relief. They had long suspected that Titan had a version of Earth's water cycle, but until now had no evidence of standing bodies of liquids that should be part of that cycle. Indeed, when NASA's Cassini spacecraft arrived at Saturn two years ago and began sending back radar images, it initially failed to turn up any convincing reservoirs.
"We were getting pretty puzzled," says Rosaly Lopes, a researcher on the Cassini radar team at NASA's Jet Propulsion Laboratory in California. Now, with the Cassini images taken July 22, the puzzle's pieces appear to be falling into place.
The lakes show up as dark patches on the surface. Several appear to be fed via liquid-carved channels. Several display what look like rims.
These are the darkest features seen so far on Titan's surface, indicating something very smooth. "It's really hard to say whether the lakes hold liquid or not," notes Dr. Lopes. "They're full of black stuff," she says, but it's unclear if the smoothness represents a glassy liquid surface or uniform deposits of hydrocarbon "meringue topping" left on lake bottoms after liquids evaporate.
Confirmation could come in October, when Cassini makes another pass close by Titan that will allow the radar to cover some of the same features from a different angle. This will enable scientists to build a 3-D image that could pick up the signatures of tiny wavelets, if they are there.
The nonliquid alternatives, such as hydrocarbon sand or "fluffy soot," would yield a poorer radar signal than the ones the team has already received, Dr. Lorenz says. He feels confident that the lakes are liquid-filled. If so, he adds, Titan could become the playground for a new breed of planetary scientists: astrolimnologists and astro-oceanographers.
"We're trying to find out how Titan works," then put the pieces together "in a cohesive story," says Steven Wall, deputy team leader for Cassini's radar group. The tantalizing part, he says, is that each time Cassini swings by Titan and beams data back, it's like turning a page in a novel that you can't put down. In this case, researchers have to wait weeks to turn a new page.
One of those pages bears the story of Xanadu, the subject of a Cassini "Kodak moment" in April. Images were released in mid-July. Researchers say they were stunned to see the array of Earthlike land forms – from vast stretches of dunes to mountains looking like kissing cousins of the Appalachians.
Xanadu's origin is perhaps the biggest puzzle.
"We don't know if began high and eroded down or was pushed up" above the surrounding terrain, Lorenz says. One crater could either be volcanic or an impact crater, researchers say.
The landform's bright color suggests that it might have been washed clean compared with the darker land surrounding Xanadu. Deep, rough-bottom channels criss-cross the region, suggesting they were carved by liquids flowing with flash-flood ferocity.
Those flash floods could be triggered by brief but heavy downpours that, some scientists say, Titan's climate can generate at high latitudes in winter. In the current edition of the journal Nature, one research team suggests that as much as 60 percent of the moon is shrouded by a thin cloud layer that rains a hydrocarbon drizzle – covering the surface with a layer of hydrocarbon mush. This may help explain why Europe's Huygens lander, which Cassini carried to Titan, landed on the surface with a squish rather than a crunch or a splash. A second team, independent of the first, conducted modeling studies of the atmosphere that reinforce the idea that Titan's wintertime poles experience hydrocarbon cloudbursts that can yield the equivalent of an Arizona flash flood.
Cassini's radar has studied only about 8 percent of Titan – with a goal of covering 20 percent by the end of the spacecraft's primary mission on July 1, 2008.