NASA’s Cassini spacecraft snapped new photos of Saturn’s biggest moon Titan during a recent flyby over the moon’s northern pole that targeted the home of the cloudy moon’s largest methane lake.
Cassini made its closest approach to Titan on June 5 UTC (Friday afternoon, June 4 Pacific time), when it zoomed within approximately 1,300 miles (2,000 km) of the giant moon’s surface.
Mission scientists at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., hoped to get another good look at Kraken Mare, the biggest lake on Titan, which spans an area greater than the Caspian Sea on Earth.
The hydrocarbon lake, which covers about 150,000 square miles (nearly 400,000 square km) is filled with liquid methane and ethane ? molecules of carbon and hydrogen atoms.
Titan’s lakes are of particular interest to scientists because of the potential for these chemicals to create an environment that could support life. Still, no firm proof of life has been found on Titan.
Cassini took infrared scans of the Titan’s north polar region, which was cloaked in darkness for the first several years of the spacecraft’s tour around the Saturn system. As northern spring has started to dawn over the area, the lighting has improved, allowing Cassini to get a better look at the hazy moon.
During the flyby, the visual and infrared spectrometer were the instruments of choice during the closest approach, but Cassini’s imaging science subsystem cameras also snapped pictures.
Although this most recent flyby is dubbed “T69,” planning changes early in Cassini’s orbital tour actually made it the 70th targeted flyby of Titan.
Last month, Cassini got close-up glimpses of Titan and Enceladus, another of the ringed planet’s moons, in a double flyby. Cassini performed radio science experiments as it passed by Titan and collected data about the moon’s internal structure.
NASA launched Cassini in 1997 and the spacecraft arrived at Saturn in 2004. It dropped a European-built lander called Huygens on Titan as part of its joint mission between NASA and European and Italian space agencies. Cassini’s mission has been extended through 2017.
Strange Discovery on Titan Leads to Speculation of Alien Life
New findings have roused a great deal of hoopla over the possibility of life on Saturn’s moon Titan, which some news reports have further hyped up as hints of extraterrestrials.
However, scientists also caution that aliens might have nothing to do with these findings.
All this excitement is rooted in analyses of chemical data returned by NASA’s Cassini spacecraft. One study suggested that hydrogen was flowing down through Titan’s atmosphere and disappearing at the surface. Astrobiologist Chris McKay at NASA Ames Research Center speculated this could be a tantalizing hint that hydrogen is getting consumed by life.
“It’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth,” McKay said.
Another study investigating hydrocarbons on Titan’s surface found a lack of acetylene, a compound that could be consumed as food by life that relies on liquid methane instead of liquid water to live.
“If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth,” McKay said.
However, NASA scientists caution that aliens might not be involved at all.
“Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,” said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team. “We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results.”
“Both results are still preliminary,” McKay told SPACE.com.
To date, methane-based life forms are only speculative, with McKay proposing a set of conditions necessary for these kinds of organisms on Titan in 2005. Scientists have not yet detected this form of life anywhere, although there are liquid-water-based microbes on Earth that thrive on methane or produce it as a waste product.
On Titan, where temperatures are around minus 290 degrees Fahrenheit (minus 179 degrees Celsius), any organisms would have to use a substance that is liquid as its medium for living processes. Water itself cannot do, because it is frozen solid on Titan’s surface. The list of liquid candidates is very short — liquid methane and related molecules such as ethane. Previous studies have found Titan to have lakes of liquid methane.
The dearth of hydrogen Cassini detected is consistent with conditions that could produce methane-based life, but do not conclusively prove its existence, cautioned researcher Darrell Strobel, a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, Md., who authored the paper on hydrogen appearing online in the journal Icarus.
Strobel looked at densities of hydrogen in different parts of the atmosphere and the surface. Previous models from scientists had predicted that hydrogen molecules, a byproduct of ultraviolet sunlight breaking apart acetylene and methane molecules in the upper atmosphere, should be distributed fairly evenly throughout the atmospheric layers.
Strobel’s computer simulations suggest a hydrogen flow down to the surface at a rate of about 10,000 trillion trillion molecules per second.
“It’s as if you have a hose and you’re squirting hydrogen onto the ground, but it’s disappearing,” Strobel said. “I didn’t expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should ‘float’ to the top of the atmosphere and escape.”
Strobel said it is not likely that hydrogen is being stored in a cave or underground space on Titan. An unknown mineral could be acting as a catalyst on Titan’s surface to help convert hydrogen molecules and acetylene back to methane.
Although Allen commended Strobel, he noted “a more sophisticated model might be needed to look into what the flow of hydrogen is.”
Scientists had expected the sun’s interactions with chemicals in the atmosphere to produce acetylene that falls down to coat the Titan surface. But Cassini mapped hydrocarbons on Titan’s surface, it detected no acetylene on the surface, findings appearing online in the Journal of Geophysical Research.
Instead of alien life on Titan, Allen said one possibility is that sunlight or cosmic rays are transforming the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.
In addition, Cassini detected an absence of water ice on the Titan surface, but loads of benzene and another as-yet-unidentified material, which appears to be an organic compound. The researchers that a film of organic compounds are covering the water ice that makes up Titan’s bedrock. This layer of hydrocarbons is at least a few millimeters to centimeters thick, but possibly much deeper in some places.
“Titan’s atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again,” said Cassini team scientist Roger Clark based at the U.S. Geological Survey in Denver. “All that implies Titan is a dynamic place where organic chemistry is happening now.”
Speculation ‘Jumping the Gun’
All this speculation “is jumping the gun, in my opinion,” Allen said.
“Typically in the search for the existence of life, one looks for the presence of evidence — say, the methane seen in the atmosphere of Mars, which can’t be made by normal photochemical processes,” Allen added. “Here we’re talking about absence of evidence rather than presence of evidence — missing hydrogen and acetylene — and often times there are many non-life processes that can explain why things are missing.”
These findings are “still a long way from evidence of life,” McKay said. “But it could be interesting.”
Titan is the largest moon of Saturn. It is the only moon known to have a dense atmosphere, and the only object in space other than Earth where clear evidence of stable bodies of surface liquid has been found.
Titan is the sixth ellipsoidal moon from Saturn. Frequently described as a planet-like moon, Titan is 50% larger than Earth’s Moon, and it is 80% more massive. It is the second-largest moon in the Solar System, after Jupiter’s moon Ganymede, and is larger than the smallest planet, Mercury, but only 40% as massive. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite (after Earth’s Moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii. From Titan’s surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth.
Titan is primarily composed of water ice and rocky material. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan’s surface until new information from the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in Titan’s polar regions. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.
The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan’s methane cycle is analogous to Earth’s water cycle, at the much lower temperature of about 94 K (−179.2 °C).