“Titan is just covered in carbon-bearing material — it’s a giant factory of organic chemicals.”
“We are carbon-based life, and understanding how far along the chain of complexity towards life that chemistry can go in an environment like Titan will be important in understanding the origins of life throughout the universe.”
~Ralph Lorenz -Johns Hopkins University Applied Physics Laboratory
Saturn’s orange moon Titan has hundreds of times more liquid hydrocarbons than all the known oil and natural gas reserves on Earth, according to new data from NASA’s Cassini spacecraft. The hydrocarbons rain from the sky, collecting in vast deposits that form lakes and dunes. Titan is a big laboratory where several of the world’s leading space scientists get to play with atmospheres on a planetary scale.
At an eye popping minus 179 degrees Celsius (minus 290 degrees Fahrenheit), Titan has a surface of liquid hydrocarbons in the form of methane and ethane with tholins believed to make up its dunes. The term “tholins,” coined by Carl Sagan in 1979, describe the complex organic molecules at the heart of prebiotic chemistry.
Before the first Cassini Mission flyby’s Robert Brown who led Cassini’s visual and infrared mapping spectrometer (VIMS) team, said: “We know VIMS will see through the haze to Titan’s surface. 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 was spot on with his predictions.
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
Cassini’s Ion and Neutral Mass Spectrometer (INMS) took a taste mysterious, subtle flavors in Titan’s atmosphere, team member and UA planetary sciences Professor Roger Yelle said, scooping up a breath of Titan’s puffy atmosphere during the flyby, The experiment measured how many molecules of different masses it got in the gulp of Titan’s mostly nitrogen, methane-laced atmosphere. Yelle and other Cassini scientists want to identify the big, complicated hydrogen-and-carbon-containing molecules because they are part of a planetary system that possibly rains methane and produces ethane ponds.
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.
Cassini to date has mapped about 20 percent of Titan’s surface with radar. Several hundred lakes and seas have been observed, with each of several dozen estimated to contain more hydrocarbon liquid than Earth’s oil and gas reserves. Dark dunes that run along the equator contain a volume of organics several hundred times larger than Earth’s coal reserves.
Proven reserves of natural gas on Earth total 130 billion tons, enough to provide 300 times the amount of energy the entire United States uses annually for residential heating, cooling and lighting. Dozens of Titan’s lakes individually have the equivalent of at least this much energy in the form of methane and ethane.
“This global estimate is based mostly on views of the lakes in the northern polar regions. We have assumed the south might be similar, but we really don’t yet know how much liquid is there,” said Lorenz. Cassini’s radar has observed the south polar region only once, and only two small lakes were visible. Future observations of that area are planned during Cassini’s proposed extended mission.
“We also know that some lakes are more than 10 meters or so deep because they appear literally pitch-black to the radar. If they were shallow we’d see the bottom, and we don’t,” said Lorenz.
The question of how much liquid is on the surface is an important one because methane is a strong greenhouse gas on Titan as well as on Earth, but there is much more of it on Titan. If all the observed liquid on Titan is methane, it would only last a few million years, because as methane escapes into Titan’s atmosphere, it breaks down and escapes into space. If the methane were to run out, Titan could become much colder. Scientists believe that methane might be supplied to the atmosphere by venting from the interior in cryovolcanic eruptions. If so, the amount of methane, and the temperature on Titan, may have fluctuated dramatically in Titan’s past.
A giant, glassy lake larger than Earth’s Lake Ontario occupies Titan’s south pole according to research from the University of Arizona’s Lunar and Planetary Laboratory. The lake which covers 20,000 square kilometers is filled mostly with methane and ethane, hydrocarbons that are gases on Earth but liquid on the bone-freezing surface of Titan -the only solar system moon known to support a planet-like atmosphere.
“We know the lake is liquid because it reflects essentially no light at 5-micron wavelengths,” Brown said. “It was hard for us to accept the fact that the feature was so black when we first saw it. More than 99.9 percent of the light that reaches the lake never gets out again. For it to be that dark, the surface has to be extremely quiescent, mirror smooth. No naturally produced solid could be that smooth.”
Before the Cassini mission, several scientists thought that Titan would be awash in global oceans of ethane and other light hydrocarbons, the byproducts of photolysis, or the action of ultraviolet light on methane over 4.5 billion years of solar system history. But 40 close flybys of Titan by the Cassini spacecraft show no such oceans exist.
Titan is also more squashed in its overall shape—like a rubber ball pressed down by a foot—than researchers had expected, said Howard Zebker, a Stanford geophysicist and electrical engineer involved in the work. The findings may help explain the presence of the large lakes of hydrocarbons at both of Titan’s poles, which have been puzzling researchers since being discovered in 2007.
“Since the poles are squished in with respect to the equator, if there is a hydrocarbon ‘water table’ that is more or less spherical in shape, then the poles would be closer down to that water table and depressions at the poles would fill up with liquid,” Zebker said. The shape of the water table would be controlled by the gravitational field of Titan, which is still not fully understood.
The next Cassini fly on August 25, 2009 in the spacecraft’s first close flyby of a moon since Saturn’s August 11 equinox. Highlights this time include a RADAR ‘scrub’ to get more detailed views of the Shangri-La dunes, unique southern equatorial magnetosphere measurements, and an opportunity for high-resolution Visible and Infrared Mapping Spectrometer (VIMS) observations of the southern hemisphere.
Posted by Casey Kazan.
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