A new study of eight years of radar data collected by the Saturn-orbiting Cassini spacecraft shows that the planet's largest moon, Titan - the only other body in the solar system besides Earth where liquids pool on the surface - has a sea of pure methane.
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Before Cassini, scientists had expected Titan's seas to be dominated by ethane, since sunlight breaks apart methane and converts it into the more complex ethane hydrocarbon.
Instead, Alice Le Gall, a Cassini scientist at France's LATMOS research laboratory, and colleagues discovered that Ligeia Mare, Titan's second-largest sea, is almost pure methane.
Scientists suspect that methane rain may be regularly filling the sea, or that ethane is locked in the sea's crust, or flowing into the adjacent sea, according to a press release about the study, which was published in the March 11 issue of Journal of Geophysical Research Planets.
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The findings are based on radar observations made by Cassini between 2007 and 2015. Those measurements of heat given off by Ligeia Mare were combined with results of a 2013 experiment that bounced radar waves off the seafloor, which allowed scientists to estimate the sea's depth.
Ligeia Mare, which turns out to be as deep as 525 feet, also likely sports a layer of organic-rich sludge on its floor, the scientists said.
"It's a marvelous feat of exploration that we're doing extraterrestrial oceanography on an alien moon," Cassini scientist Steve Wall, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., noted in the press release.
ANALYSIS: Cassini Spies a Sunny Day on Titan's Seas
Cassini, which has been studying the Saturn system for almost 12 years, has revealed that almost 2 percent of Titan's 620,000 square miles of real estate are covered in liquid.
The moon has three large seas, all located in the northern polar region, that are surrounded by small lakes. So far, just one large lake has been found in Titan's southern hemisphere.
This image, taken by the Cassini spacecraft, shows the first flash of sunlight reflected off a hydrocarbon lake on Saturn’s moon Titan. The July 2009 image confirmed the presence of liquid in the moon’s northern hemisphere.
Ten years ago,
the European Huygens probe descended through Titan's atmosphere
and became the first ever robotic mission to touch down on a world in the outer solar system. During its daring 2 hour, 27 minute descent through the murky atmosphere of Saturn's largest moon, the probe revealed an unprecedented view of of the alien environment. On landing, Huygens survived on the hydrocarbon-rich surface for only 72 minutes before its batteries drained, but the data it transmitted via NASA's Cassini spacecraft was nothing short of revolutionary -- data that continues to be analyzed 10 years after that fateful day on Jan. 14, 2005.
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Here are just a few mind-blowing images from Huygens as it gave us our first intimate look at the solar system's only moon known to possess a thick atmosphere and vast liquid methane-ethane lakes -- a world that, like Jupiter's moon Europa, invokes exciting hypotheses of extraterrestrial biology.
After traveling with the Cassini mission for seven years during its interplanetary transit from Earth to Saturn orbit, the command was given for Huygens to detach from its mothership. For 21 days, the small disk-like probe was by itself, cruising toward Titan. As Huygens ripped through Titan's atmosphere, eventually slowing down enough for its heatshield to drop away and parachutes deploy, the probe got to work, rapidly photographing its descent and collecting atmospheric data. On Wednesday, The European Space Agency released the full series of
stunning processed descent images
, showing how the moon's dune-covered surface slowly came to view as Huygens slowly drifted to the surface below.
Looking down, Huygens also captured a slowly evolving view of its eventual landing spot. Shown here, a fish-eye view of the landscape below starts to detail some of the surface features the probe would be soon analyzing up-close. In the run-up to landing day, mission scientists were unsure whether Huygens would land on a solid surface or splash down in a methane/ethane puddle or lake. As it turned out, the probe "splatted down" in Titan's alien mud -- a mix of small grains of ice.
ANALYSIS: Titan's 'Magic Island' Appeared Mysteriously From the Depths
Photographs during descent gradually showed an alien, yet familiar, landscape. Titan is covered in dunes, valleys and lakes -- all shaped by erosion processes we'd find on Earth. The valleys are cut by liquid action and the thick atmosphere produces winds and weather systems that form dune fields of fine hydrocarbon sand. But these Titan weather systems are not driven by an Earth-like water cycle. As the atmosphere is too cold to support water in a liquid state, other chemicals, such as methane and ethane exist as a liquid, forming their own cycle. Methane rain is now known to fall onto the landscape, creating rivers that erode valleys and form deltas in vast liquid methane-ethane lakes.
By landing a probe onto Titan's surface, the joint NASA/ESA Cassini-Huygens mission was able to get "ground proof" of flyby imaging and radar. Last year, Cassini completed its 100th Titan flyby, so in the 10 years since Huygens landed, planetary scientists have been getting a front row seat of the moon. But in 2005, Titan science was as foggy as the moon's atmosphere, so by overlaying ground-based observations with Cassini images, a better interpretation of landscape features spotted by Cassini could be made.
Although low-resolution and grainy, the first images of the landscape surrounding Huygens after it landed at 13:34 CET (12:34 GMT) on Jan. 14, 2005, stunned the world. Rounded stones appeared to litter the grains of hydrocarbon sand and ice. The eroded rocks immediately reminded us of eroded pebbles -- rocks that have undergone liquid action for long periods of time. The landing zone resembled a dried-up lake bed and surrounding that area, evidence for rapid, transient flows of liquid could be seen.
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The Huygens lander, in its short solo mission, punched well above its weight, opening our eyes to an alien world within our solar system that is littered with prebiotic chemicals, a world that resembles a young Earth, beckoning our inquiring minds to return some day.