Organic molecules in Titan's atmosphere are intriguingly skewed

An ALMA image of the distribution of the organic molecule HNC in the upper atmosphere of Saturn's moon Titan. 

The denser, brighter concentrations are shown near the moon's north and south poles. 

Their shifted, off-axis locations were unexpected and could help researchers better understand Titan's complex atmospheric processes. 

The globe outline represents Titan's orientation at the time of the observations. 

Credit: NRAO/AUI/NSF; M. Cordiner et al./NASA

While studying the atmosphere on Saturn's moon Titan, scientists discovered intriguing zones of organic molecules unexpectedly shifted away from its north and south poles.

These misaligned features seem to defy conventional thinking about Titan's windy atmosphere, which should quickly smear out such off-axis concentrations.

"This is an unexpected and potentially groundbreaking discovery," said Martin Cordiner, an astrochemist working at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the lead author of a study published online today in the Astrophysical Journal Letters.

"These kinds of east-to-west variations have never been seen before in Titan's atmospheric gases. Explaining their origin presents us with a fascinating new problem."

This discovery, made during a remarkably brief three-minute "snapshot" observation with the Atacama Large Millimeter/submillimeter Array (ALMA), may help astronomers better understand the processes that shape this world's complex chemistry.

Titan's atmosphere has long been of interest because it acts as a chemical factory, using energy from the Sun and Saturn's magnetic field to produce a wide range of organic molecules.

Studying this complex chemistry may provide insights into the properties of Earth's very early atmosphere, which may have shared many chemical characteristics with present-day Titan.

An ALMA image of the distribution of the organic molecule HC3N in the upper atmosphere of Saturn's moon Titan. 

The denser, brighter concentrations are shown near the moon's north and south poles. 

Their shifted, off-axis locations were unexpected and could help researchers better understand Titan's complex atmospheric processes. 

The globe outline represents Titan's orientation at the time of the observations. 

Credit: NRAO/AUI/NSF; M. Cordiner et al./NASA

The researchers used ALMA's extreme sensitivity and resolution to track the atmospheric distributions of hydrogen isocyanide (HNC) and cyanoacetylene (HC3N), which initially appeared to be concentrated evenly over Titan's north and south poles.

These findings were consistent with observations made by NASA's Cassini spacecraft, which found high concentrations of some gases over whichever pole is experiencing winter on Titan.


Recent observations of comet Lemmon provided a 3-D view of the inner coma, including detailed mapping of the molecule HCN (made of one hydrogen, one carbon and one nitrogen), shown here, as well as HNC and formaldehyde.

Visualization by Brian R. Kent/NRAOThe surprise came when the research

ers compared the gas concentrations at different levels in the atmosphere. At the highest altitudes, the pockets of organic molecules were shifted away from the poles.

These off-pole concentrations are unexpected because the fast-moving, east-west winds in Titan's middle atmosphere should thoroughly mix the molecules formed there.

The researchers do not have an obvious explanation for these findings yet.

"It seems incredible that chemical mechanisms could be operating on rapid enough timescales to cause enhanced 'pockets' in the observed molecules," said Conor Nixon, a planetary scientist at Goddard and a coauthor of the paper.

"We would expect the molecules to be quickly mixed around the globe by Titan's winds."

An ALMA image of the distribution of the organic molecule HC3N at intermediate-to-lower elevations in the atmosphere of Saturn's moon Titan. 

The denser, brighter concentrations are oriented more evenly about the poles than is observed for HC3N at higher elevations. 

The globe outline represents Titan's orientation at the time of the observations. 

Credit: NRAO/AUI/NSF; M. Cordiner (NASA) et al.

At the moment, the scientists are considering thermal or other effects tied to interaction with Saturn's powerful magnetic field, which extends far enough to engulf Titan, as potential sources of this skewed molecular concentration.

"Alternatively, I don't think we could rule out some kind of peculiar atmospheric circulation pattern," speculates Cordiner.

This marks ALMA's first foray into atmospheric studies of a major body in our Solar System.

Further observations are expected to improve our understanding of the atmosphere and ongoing processes on Titan and other objects throughout our Solar System.

Titan is in some ways the most Earthlike body in the Solar System, with a thick atmosphere and prominent lakes, rivers, and seas.

In place of water, however, Titan's frigid surface flows with liquid organic molecules, including methane (CH4) and ethane (C2H6).

"These ALMA observations give us new insights into how organic molecules, the building blocks of life, form and evolve in a planet-like environment," said Anthony Remijan, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Va., and coauthor on the paper.

"It is exciting to imagine the new discoveries ALMA will enable as we look more deeply at other interesting objects in our Solar System."

NASA Cassini: Saturn's moon Titan - Land of Lakes

Cassini spacecraft looked though layers of haze to the lakes of Titan's northern regions. 

Saturn’s moon, Titan (3,200 miles, or 5,150 kilometers across), has a hydrological cycle similar to Earth's, but instead of water, Titan's lakes and seas contain liquid methane and ethane. 

Lit terrain seen here lies on the leading hemisphere of Titan. North on Titan is up. 

The image was taken with the Cassini spacecraft wide-angle camera on Jan. 1, 2014, at a distance of approximately 114,000 miles (183,000 kilometers) from Titan.

NASA's Cassini Image: Titan's Ligeia Mare - Second largest liquid lake

Ligeia Mare represents the second largest known body of liquid on Saturn's moon, Titan, shown here in a false-colour image obtained by NASA's Cassini mission. 

Ligeia Mare contains liquid hydrocarbons, such as ethane and methane, and makes up one of the many seas and lakes located in Titan's north polar region. 

The image consists of a false-colour mosaic of synthetic aperture radar images obtained by the Cassini spacecraft between February 2006 and April 2007. 

In this image, liquids, dark to the radar, appear black and the solid surface of Titan, which appears bright to the radar, appears yellow. 

Image released May 22, 2013.

Saturn's Moon Titan: Methane Lakes Drying Up

This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan.

These lakes are liquid hydrocarbon (methane) and not water.

CREDIT: NASA/JPL

Today, methane sloshes around in pools on the surface of Saturn's Titan, but the hydrocarbon may eventually vanish from Saturn's giant moon, according to a new study.

Images and data from NASA's Cassini spacecraft show that the compound doesn't seem to be getting replenished fast enough on Titan's surface to keep the methane cycle sustainable, scientists say.

Besides Earth, Titan is the only known place in our solar system to have stable liquids on its surface. The huge moon's clouds, lakes and rain are made up of hydrocarbons, or molecules composed of hydrogen and carbon, such as methane and ethane.

Cassini images have revealed that Titan's hydrocarbon lakes stay remarkably consistent in size and shape over time.

This means that either the lakes evaporate at a crawling pace, or there are enough downpours to offset the evaporation.

Rain Clouds
Since scientists have observed only occasional bursts of rain on Titan, they believe that the lakes must have quite stable, slowly evaporating contents, and may be dominated by ethane, which doesn't vanish as quickly as methane.

Christophe Sotin

When methane floats high into Titan's soupy atmosphere, the compound is broken apart by sunlight. Many of its hydrogen atoms keep rising and disappear into space, while the remaining elements go on to make carbon-rich products like ethane.

Scientists say this process should eventually diminish the overall amount of methane in Titan's environment, as compounds pieced together out of methane's leftovers continue to get dumped on the moon's surface.

"We are seeing an active Titan whose active chemistry may come to an end in some tens of million years," said Christophe Sotin, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who has been analyzing Cassini measurements of Titan's lakes and seas.

That might sound like a long time away, but it could mean that Titan's "methane era" is somewhat short-lived, at least on a geological timescale.

NASA Cassini Titan Research: Ice On A Hydrocarbon Lake

This artist's concept envisions what hydrocarbon ice forming on a liquid hydrocarbon sea of Saturn's moon Titan might look like. 

A new model from scientists on NASA's Cassini mission suggests that clumps of methane-and-ethane-rich ice could float under some conditions. They are shown here as the lighter-colored clusters.

A new paper by scientists on NASA's Cassini mission finds that blocks of hydrocarbon ice might decorate the surface of existing lakes and seas of liquid hydrocarbon on Saturn's moon Titan.

The presence of ice floes might explain some of the mixed readings Cassini has seen in the reflectivity of the surfaces of lakes on Titan.

"One of the most intriguing questions about these lakes and seas is whether they might host an exotic form of life," said Jonathan Lunine, a paper co-author and Cassini interdisciplinary Titan scientist at Cornell University, Ithaca, N.Y.

"And the formation of floating hydrocarbon ice will provide an opportunity for interesting chemistry along the boundary between liquid and solid, a boundary that may have been important in the origin of terrestrial life."

Titan is the only other body besides Earth in our solar system with stable bodies of liquid on its surface. But while our planet's cycle of precipitation and evaporation involves water, Titan's cycle involves hydrocarbons like ethane and methane.

Ethane and methane are organic molecules, which scientists think can be building blocks for the more complex chemistry from which life arose.

Cassini has seen a vast network of these hydrocarbon seas cover Titan's northern hemisphere, while a more sporadic set of lakes bejewels the southern hemisphere.