New images from Cassini show icy particles in Saturn's F ring clumping into giant snowballs as the moon Prometheus makes multiple swings by the ring. The gravitational pull of the moon sloshes ring material around, creating wake channels that trigger the formation of objects as large as 20 kilometers (12 miles) in diameter.

"Scientists have never seen objects actually form before," said Carl Murray, a Cassini imaging team member based at Queen Mary, University of London. "We now have direct evidence of that process and the rowdy dance between the moons and bits of space debris."

Murray discussed the findings at the Committee on Space Research meeting in Bremen, Germany, and they are published online by the journal Astrophysical Journal Letters on July 14, 2010. A new animation based on imaging data shows how one of the moons interacts with the F ring and creates dense, sticky areas of ring material.

Saturn's thin, kinky F ring was discovered by NASA's Pioneer 11 spacecraft in 1979. Prometheus and Pandora, the small "shepherding" moons on either side of the F ring, were discovered a year later by NASA's Voyager 1. In the years since, the F ring has rarely looked the same twice, and scientists have been watching the impish behavior of the two shepherding moons for clues.

Prometheus, the larger and closer to Saturn of the two moons, appears to be the primary source of the disturbances. At its longest, the potato-shaped moon is 148 kilometers (92 miles) across. It cruises around Saturn at a speed slightly greater than the speed of the much smaller F ring particles, but in an orbit that is just offset.

As a result of its faster motion, Prometheus laps the F ring particles and stirs up particles in the same segment once in about every 68 days.

"Some of these objects will get ripped apart the next time Prometheus whips around," Murray said. "But some escape. Every time they survive an encounter, they can grow and become more and more stable."

Cassini scientists using the ultraviolet imaging spectrograph previously detected thickened blobs near the F ring by noting when starlight was partially blocked. These objects may be related to the clumps seen by Murray and colleagues.

The newly-found F ring objects appear dense enough to have what scientists call "self-gravity." That means they can attract more particles to themselves and snowball in size as ring particles bounce around in Prometheus's wake, Murray said. The objects could be about as dense as Prometheus, though only about one-fourteenth as dense as Earth.

What gives the F ring snowballs a particularly good chance of survival is their special location in the Saturn system. The F ring resides at a balancing point between the tidal force of Saturn trying to break objects apart and self-gravity pulling objects together.

One current theory suggests that the F ring may be only a million years old, but gets replenished every few million years by moonlets drifting outward from the main rings. However, the giant snowballs that form and break up probably have lifetimes of only a few months.

The new findings could also help explain the origin of a mysterious object about 5 to 10 kilometers (3 to 6 miles) in diameter that Cassini scientists spotted in 2004 and have provisionally dubbed S/2004 S 6. This object occasionally bumps into the F ring and produces jets of debris.

"The new analysis fills in some blanks in our solar system's history, giving us clues about how it transformed from floating bits of dust to dense bodies," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The F ring peels back some of the mystery and continues to surprise us."

The late Kevin Beurle was made the honorary first author on this paper because of his contributions in developing software and designing observation sequences for this research. He died in 2009.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

]]> Thu, 29 JUL 2010 14:33:31 AEST Pasadena CA (SPX) Jul 16, 2010 - On Earth, lake levels rise and fall with the seasons and with longer-term climate changes, as precipitation, evaporation, and runoff add and remove liquid. Now, for the first time, scientists have found compelling evidence for similar lake-level changes on Saturn's largest moon, Titan-the only other place in the solar system seen to have a hydrological cycle with standing liquid on the surface.

Using data gathered by NASA's Cassini spacecraft over a span of four years, the researchers-led by graduate student Alexander G. Hayes of the California Institute of Technology (Caltech) and Oded Aharonson, associate professor of planetary science at Caltech-have obtained two separate lines of evidence showing roughly a 1 meter per year drop in the levels of lakes in Titan's southern hemisphere.

The decrease is the result of the seasonal evaporation of liquid methane from the lakes-which, because of Titan's frigid temperatures (roughly minus 300 degrees Fahrenheit at the poles), are composed largely of liquid methane, ethane, and propane.

"It's really exciting because, on this distant object, we're able to see this meter-scale drop in lake depth," says Hayes. "We didn't know Cassini would even be able to see these things."

One of the lakes-Ontario Lacus (named after Earth's Lake Ontario, which is of comparable size) -is the southern hemisphere's largest lake, and was the first lake to be observed on the moon.

In a paper submitted to the journal Icarus, Hayes, Aharonson, and their colleagues report that the shoreline of Ontario Lacus receded by about 10 kilometers (6 miles) from June 2005 to July 2009, a period of time that represents mid-summer to fall in Titan's southern hemisphere. (One Titan year lasts 29.5 Earth years.)

Ontario Lacus and other southern-hemisphere lakes were analyzed using Synthetic Aperture Radar (SAR) image data from the Cassini spacecraft. In radar data, smooth features-such as lakes-appear as dark areas, while rougher features-such as mountain belts-appear bright. The intensity of the radar backscatter provides information about the composition and roughness of surface features.

In addition to the SAR data, radar altimetry-which measures the time it takes for microwave signals bouncing off a surface to arrive back at the spacecraft-was collected across a transect of Ontario Lacus in December 2008.

"The combination of SAR and altimetry measurements across the transect gave information about the absorptive properties of the liquid, and argues that the liquids are relatively pure hydrocarbons made up of methane and ethane and not a gunky tar," Aharonson says.

"The liquid is not highly attenuating," explains Hayes, "which means it is fairly clear to radar energy-that is, transparent, like liquid natural gas."

Because of this, radar can see through the liquid in Titan's lakes to a depth of several meters. "Then the radar hits the floor, and bounces back," he says. "Or, if the lake is deeper than a few meters, the radar is completely absorbed, producing a 'black' signature."

Once the liquid's optical properties were known, the researchers could use the radar data to "see" the lakebed underneath the liquid-at least, down to the depth where the signal is completely attenuated. "How far offshore you can see is determined by the local slope of the lakebed, or bathymetry," says Hayes.

"This gave us the ability to take changes in radar signals and convert them to depths," and thus to calculate the slope of the lakebed around the entire lake.

"We were able to determine the bathymetry of the lake out to a depth of about 8 meters," he says. The lake is shallowest and most gently sloped along its southern edge, in areas where sediment is accumulating. Along its eastern shore, the slope of the lake is somewhat steeper.

"This is what we are calling the 'beachhead,'" Hayes says. The slope is very steep along the lake's northern boundary, where it butts up against a range of mountains.

"The slope changes we see are consistent with the geology around the lake," Hayes says. The bathymetry measurements and their geologic correlations are discussed in a separate paper by Hayes, Aharonson, and colleagues, which has been accepted for publication in the Journal of Geophysical Research (JGR).

The researchers compared lake images obtained four years apart, and found that Ontario had shrunk. "The extent to which the lake has receded is related to the slope-i.e., where the lake is shallow, the liquid will have receded more," Hayes says. "This allows us to deduce the vertical height by which the lake depth has dropped, which is about 1 meter per year."

The researchers also analyzed the evaporation of methane from nearby lakes by comparing the radar signatures of these lakes as measured in December 2007 with data obtained in May 2009. Over that period, the "apparent darkness" of the lakes-indicating the presence of a radar-attenuating liquid-either decreased or disappeared entirely, which means that their liquid levels had been reduced.

The researchers were able to calculate the drop in lake depth, "and we got the same result: 1 meter per year of liquid loss," Aharonson says.

Lakes in Titan's northern hemisphere-which is now entering spring-have also been covered multiple times by radar instruments, but so far no analogous changes have been conclusively detected.

That doesn't mean the changes haven't occurred, however. "We would expect it will happen, but we don't know how it would manifest in the data if the lakes in the north are significantly deeper. We'll continue to look for this effect with future radar images, to disentangle the seasonal variations from longer-term climate variations we previously have proposed." Aharonson says.

The work described in the two papers-"Transient Surface Liquid in Titan's Polar Regions from Cassini," which was submitted to Icarus, and "Bathymetry and Absorptivity of Titan's Ontario Lacus," which was accepted by JGR-was supported by the Cassini Project and NASA's Graduate Student Researchers Program, and was carried out in collaboration with members of the Cassini RADAR Science Team. The Cassini mission is managed by the Jet Propulsion Laboratory in Pasadena, California.

]]> Thu, 29 JUL 2010 14:33:31 AEST Pasadena CA (SPX) Jul 16, 2010 - Ontario Lacus, the largest lake in the southern hemisphere of Saturn's moon Titan, turns out to be a perfect exotic vacation spot, provided you can handle the frosty, subzero temperatures and enjoy soaking in liquid hydrocarbon.

Several recent papers by scientists working with NASA's Cassini spacecraft describe evidence of beaches for sunbathing in Titan's low light, sheltered bays for mooring boats, and pretty deltas for wading out in the shallows.

They also describe seasonal changes in the lake's size and depth, giving vacationers an opportunity to visit over and over without seeing the same lake twice. (Travel agents, of course, will have to help you figure out how to breathe in an atmosphere devoid of oxygen.)

Using data that give us the most detailed picture yet of a lake on another world, scientists and animators have collaborated on a new video tour of Ontario Lacus based on radar data from Cassini's Titan flybys on June 22, 2009, July 8, 2009, and Jan. 12, 2010.

"With such frigid temperatures and meager sunlight, you wouldn't think Titan has a lot in common with our own Earth," said Steve Wall, deputy team lead for the Cassini radar team, based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "But Titan continues to surprise us with activity and seasonal processes that look marvelously, eerily familiar."

Cassini arrived at Saturn in 2004 when the southern hemisphere of the planet and its moons were experiencing summer. The seasons have started to change toward autumn, with winter solstice darkening the southern hemisphere of Titan in 2017. A year on Titan is the equivalent of about 29 Earth years.

Titan is the only other world in our solar system known to have standing bodies of liquid on its surface. Because surface temperatures at the poles average a chilly 90 Kelvin (about minus 300 degrees Fahrenheit), the liquid is a combination of methane, ethane and propane, rather than water.

Ontario Lacus has a surface area of about 15,000 square kilometers (6,000 square miles), slightly smaller than its terrestrial namesake Lake Ontario.

Cassini first obtained an image of Ontario Lacus with its imaging camera in 2004. A paper submitted to the journal Icarus by Alex Hayes, a Cassini radar team associate at the California Institute of Technology in Pasadena, and colleagues finds that the lake's shoreline has receded by about 10 kilometers (6 miles). This has resulted in a liquid level reduction of about 1 meter (3 feet) per year over a four-year period.

The shoreline appears to be receding because of liquid methane evaporating from the lake, with a total amount of evaporation that would significantly exceed the yearly methane gas output of all the cows on Earth, Hayes said.

Some of the liquid could also seep into porous ground material. Hayes said the changes in the lake are likely occurring as part of Titan's seasonal methane cycle, and would be expected to reverse during southern winter.

This seasonal filling and receding is similar to what occurs at the shallow lakebed known as Racetrack Playa in Death Valley National Park, Hayes said. In fact, from the air, the topography and shape of Racetrack Playa and Ontario Lacus are quite similar, although Ontario Lacus is about 60 times larger.

"We are very excited about these results, because we did not expect Cassini to be able to detect changes of this magnitude in Titan's lakes," Hayes said. "It is only through the continued monitoring of seasonal variation during Cassini's extended mission that these discoveries have been made possible."

Other parts of the Ontario Lacus' shoreline, as described in the paper published in Geophysical Research Letters in March 2010 by Wall, Hayes and other colleagues, show flooded valleys and coasts, further proof that the lake level has changed.

The delta revealed by Cassini radar data on the western shore of Ontario Lacus is also the first well-developed delta observed on Titan, Wall said. He explained that the shape of the land there shows liquid flowing down from a higher plain switching channels on its way into the lake, forming at least two lobes.

Examples of this kind of channel switching and wave-modified deltas can be found on Earth at the southern end of Lake Albert between Uganda and the Democratic Republic of Congo in Africa, and the remains of an ancient lake known as Megachad in the African country Chad, Wall said.

The radar data also show a smooth beach on the northwestern shore of Ontario Lacus. Smooth lines parallel to the current shoreline could be formed by low waves over time, which were likely driven by winds sweeping in from the west or southwest. The pattern at Ontario Lacus resembles what might be seen on the southeastern side of Lake Michigan, where waves sculpt the shoreline in a similar fashion.

"Cassini continues to take our breath away as it fills in the details on the surfaces of these far-off moons," said Linda Spilker, Cassini project scientist based at JPL. "It's exhilarating to ride along as it takes us on the ultimate cold-weather adventure."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the Cassini-Huygens mission for NASA's Science Mission Directorate. The Cassini orbiter was designed, developed and assembled at JPL. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the United States and several European countries.

]]> Thu, 29 JUL 2010 14:33:31 AEST Pasadena CA (JPL) Jul 09, 2010 - Scientists using NASA's Cassini spacecraft at Saturn have stalked a new class of moons in the rings of Saturn that create distinctive propeller-shaped gaps in ring material. It marks the first time scientists have been able to track the orbits of individual objects in a debris disk.

The research gives scientists an opportunity to time-travel back into the history of our solar system to reveal clues about disks around other stars in our universe that are too far away to observe directly.

"Observing the motions of these disk-embedded objects provides a rare opportunity to gauge how the planets grew from, and interacted with, the disk of material surrounding the early sun," said Carolyn Porco, Cassini imaging team lead based at the Space Science Institute in Boulder, Colo., and a co-author on the paper. "It allows us a glimpse into how the solar system ended up looking the way it does."

The results are published in a new study in the July 8, 2010, issue of the journal Astrophysical Journal Letters.

Cassini scientists first discovered double-armed propeller features in 2006 in an area now known as the "propeller belts" in the middle of Saturn's outermost dense ring, known as the A ring.

The spaces were created by a new class of moonlets - smaller than known moons, but larger than the particles in the rings - that could clear the space immediately around them. Those moonlets, which were estimated to number in the millions, were not large enough to clear out their entire path around Saturn, as do the moons Pan and Daphnis.

The new paper, led by Matthew Tiscareno, a Cassini imaging team associate based at Cornell University, Ithaca, N.Y., reports on a new cohort of larger and rarer moons in another part of the A ring farther out from Saturn. With propellers as much as hundreds of times as large as those previously described, these new objects have been tracked for as long as four years.

The propeller features are up to several thousand kilometers (miles) long and several kilometers (miles) wide. The moons embedded in the ring appear to kick up ring material as high as 0.5 kilometers (1,600 feet) above and below the ring plane, which is well beyond the typical ring thickness of about 10 meters (30 feet). Cassini is too far away to see the moons amid the swirling ring material around them, but scientists estimate that they are about a kilometer (half a mile) in diameter because of the size of the propellers.

Tiscareno and colleagues estimate that there are dozens of these giant propellers, and 11 of them were imaged multiple times between 2005 to 2009. One of them, nicknamed Bleriot after the famous aviator Louis Bleriot, has been a veritable Forrest Gump, showing up in more than 100 separate Cassini images and one ultraviolet imaging spectrograph observation over this time.

"Scientists have never tracked disk-embedded objects anywhere in the universe before now," Tiscareno said. "All the moons and planets we knew about before orbit in empty space. In the propeller belts, we saw a swarm in one image and then had no idea later on if we were seeing the same individual objects. With this new discovery, we can now track disk-embedded moons individually over many years."

Over the four years, the giant propellers have shifted their orbits, but scientists are not yet sure what is causing the disturbances in their travels around Saturn. Their path may be upset by bumping into other smaller ring particles, or responding to their gravity, but the gravitational attraction of large moons outside the rings may also be a factor.

Scientists will continue monitoring the moons to see if the disk itself is driving the changes, similar to the interactions that occur in young solar systems. If it is, Tiscareno said, this would be the first time such a measurement has been made directly.

"Propellers give us unexpected insight into the larger objects in the rings," said Linda Spilker, Cassini project scientist based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Over the next seven years, Cassini will have the opportunity to watch the evolution of these objects and to figure out why their orbits are changing."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

]]> Thu, 29 JUL 2010 14:33:31 AEST Pasadena CA (JPL) Jul 07, 2010 - As American schoolchildren head out to pools for a summer splash, NASA's Cassini spacecraft will be taking its own deep plunge through the Titan atmosphere this week.

The altitude for the upcoming Titan flyby, whose closest approach occurs in the evening of July 6, Pacific and Eastern time (or shortly after midnight on July 7, Coordinated Universal Time) will be about 125 kilometers (78 miles) higher than the super-low flyby of June 21. The altitude of this flyby - 1,005 kilometers (624 miles) - is still considered a low dip into Titan's atmosphere. Cassini will not go lower again until May 2012.

During closest approach, Cassini's ion and neutral mass spectrometer will be sniffing out the chemical composition of Titan's atmosphere to refine estimates of the densities of nitrogen and methane there.

The radar instrument will be mapping an area south of the dark region known as Senkyo and the Belet sand seas. It is an area that had not been well studied by radar until this flyby.

Because the geometry of this flyby is similar to the previous one, the magnetometer and other instruments measuring the magnetic bubble around Saturn will be conducting similar experiments.

Though the magnetometer will be too high to detect any whisper of an internal magnetic field from Titan - which was the focus of the search on the last flyby - scientists will be looking into the interaction of Titan's atmosphere with the magnetic bubble around Saturn.

This latest flyby is dubbed "T71," though planning changes early in the orbital tour have made this the 72nd targeted flyby of Titan.

]]> Thu, 29 JUL 2010 14:33:31 AEST Washington DC (SPX) Jul 05, 2010 - Complex interactions between Saturn and its satellites have led scientists using NASA's Cassini spacecraft to a comprehensive model that could explain how oxygen may end up on the surface of Saturn's icy moon Titan. The presence of these oxygen atoms could potentially provide the basis for pre-biological chemistry.

The interactions are captured in two papers, one led by John Cooper and another led by Edward Sittler, published in the journal Planetary and Space Science in late 2009. Cooper and Sittler are Cassini plasma spectrometer team scientists at NASA's Goddard Space Flight Center in Greenbelt, Md.

"Titan and Enceladus, another icy moon of Saturn, are chemically connected by the flow of material through the Saturn system," Cooper said.

In one paper, Cooper and colleagues provide an explanation for forces that could generate the Enceladus geysers that spew water vapor into space. In the other, published in the same issue, Sittler and colleagues describe a unique new process in which oxygen that circulates in the upper atmosphere of Titan can be carried all the way to the surface without further chemical contamination by being encased in carbon cages called fullerenes.

The work draws upon previous work by Sittler and others that model the dynamics of how particles, including water molecules, travel from Enceladus to Titan. At Enceladus the flow process begins with what they call the "Old Faithful" model, after the Old Faithful geyser in Yellowstone National Park.

In this model, gas pressure slowly builds up inside Enceladus, then gets released occasionally in geyser-like eruptions.

Unlike terrestrial geysers, or even geyser-like forces on Jupiter's moon Io, the model proposed by Cooper shows that charged particle radiation raining down from Saturn's magnetosphere can create the forces from below the surface that are required to eject gaseous jets.

Energetic particles raining down from Saturn's magnetosphere - at Enceladus, mostly electrons from Saturn's radiation belts - can break up molecules within the surface.

This process is called radiolysis. Like a process called photolysis, in which sunlight can break apart molecules in the atmosphere, energetic radiation from charged particles that hit an icy surface, like that of Enceladus, can cause damage to molecules within the ice.

These damaged molecules can get buried deeper and deeper under the surface by the perpetual churning forces that can repave the icy surface. Meteorites constantly crashing into the surface and splashing out material might also be burying the molecules.

When chemically altered icy grains come into contact beneath the surface with icy contaminants such as ammonia, methane and other hydrocarbons, they can produce volatile gases that can explode outward. Such gases can create plumes of the size seen by Cassini. Cooper and colleagues call such icy volatile mechanics "cryovolcanism."

What's unique about the "Old Faithful" model is that it "is a model for cryovolcanism that is based on not only liquid water, but also requires the production of gases by the radiolytic chemistry observed at Enceladus," said Sittler.

The plumes that emanate from Enceladus' south polar region consist of water, ammonia and other compounds. Scientists have known since the 1980s that Saturn's magnetosphere is inexplicably filled with neutral particles.

In the intervening decades, particularly since the discovery of plumes jetting out from the south pole of Enceladus, work has shown how some of the water molecules that escape from Enceladus get split up into neutral and charged particles and are transported throughout Saturn's magnetosphere.

Sittler's new model indicates that as these broken water molecules enter Titan's atmosphere, they may be captured by fullerenes-hollow, soccer-ball shaped shells made of carbon atoms. Although the heavy molecules Cassini has detected in the upper atmosphere of Titan may be other molecules, Sittler suggests they are likely fullerenes.

In Sittler's model, the fullerenes then condense into larger clusters that can attach to polycyclic aromatic hydrocarbons-chemical compounds also found on Earth in oil, coal and tar deposits, and as the byproducts of burning fossil fuels. The fullerene clusters form even larger aerosols that travel down to Titan's surface.

This process protects the trapped oxygen from Titan's atmosphere, which is saturated with hydrogen atoms and compounds that are capable of breaking down other molecules.

Otherwise, the oxygen would combine with methane in Titan's atmosphere and form carbon monoxide or carbon dioxide. Until now, scientists have not been able to explain how oxygen fits into the picture of the dynamics and chemistry of Saturn and its moons.

As the oxygen-rich aerosols fall to Titan's surface, they are further bombarded by products of galactic cosmic ray interactions with Titan's atmosphere. Cosmic rays bombarding the oxygen-stuffed fullerenes could produce more complex organic materials, such as amino acids, in the carbon-rich and oxygen-loaded fullerenes. Amino acids are considered important for pre-biological chemistry.

Scientists have been able to couple the new models that describe the generation of plumes at Enceladus and oxygen ion capture in fullerenes near the top of Titan's atmosphere to existing theories of the transport of oxygen across the magnetosphere. Taken together, Sittler and Cooper suggest a chemical pathway that allows the oxygen to be introduced to Titan's surface chemistry.

"Cooper and Sittler's work helps us understand more about the potential for chemical interactions among Saturn's moons," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"The Saturn system is indeed a dynamic place, with the Enceladus plumes creating the E ring and loading the magnetosphere with water which interacts with Titan and the other moons," Spilker said.

The Cassini mission is a joint effort of NASA, the European Space Agency, and the Italian space agency Agenzia Spaziale Italiana. The mission is managed for NASA by the Jet Propulsion Laboratory, a division of the California Institute of Technology. Partners include the U.S. Air Force, Department of Energy, and academic and industrial participants from 19 countries.

]]> Thu, 29 JUL 2010 14:33:31 AEST Moffett Field CA (SPX) Jun 08, 2010 - Recent results from the Cassini mission suggest that hydrogen and acetylene are depleted at the surface of Titan. Both results are still preliminary and the hydrogen loss in particular is the result of a computer calculation, and not a direct measurement. However the findings are interesting for astrobiology.

Heather Smith and I, in a paper published 5 years ago (McKay and Smith, 2005) suggested that methane-based (rather than water-based) life - ie, organisms called methanogens - on Titan could consume hydrogen, acetylene, and ethane.

The key conclusion of that paper (last line of the abstract) was "The results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface."

Now there seems to be evidence for all three of these on Titan. Clark et al. (2010, in press in JGR) are reporting depletions of acetylene at the surface. And it has been long appreciated that there is not as much ethane as expected on the surface of Titan. And now Strobel (2010, in press in Icarus) predicts a strong flux of hydrogen into the surface.

This is a still a long way from "evidence of life". However, it is extremely interesting.

Benner et al. (2004) first suggested that the liquid hydrocarbons on Titan could be the basis for life, playing the role that water does for life on Earth. Those researchers pointed out that "... in many senses, hydrocarbon solvents are better than water for managing complex organic chemical reactivity".

Two papers in 2005 followed up on this logic by computing the energy available for methanogenic life based on the consumption of both the organics in Titan's atmosphere along with the hydrogen in the atmosphere (McKay and Smith, 2005; Schulze-Makuch and Grinspoon, 2005). Both papers made the case that H2 on Titan would play the role that O2 plays on Earth. On Earth organisms (like humans) can react O2 with organic material to derive energy for life's functions.

On Titan organisms could react H2 with organic material to derive energy. The waste product of O2 metabolism on Earth is CO2 and H2O; on Titan the waste product of H2 metabolism would be CH4. As a result of the Cassini mission, there is now abundant evidence for CH4, even in liquid form, on Titan.

Organic molecules on the surface of Titan (such as acetylene, ethane, and solid organics) would release energy if they reacted with hydrogen to form methane. Acetylene gives the most energy. However this reaction will not proceed under ordinary conditions.

This is similar to our experience on Earth. Consider a chocolate bar in a jar full of air. The organics in the chocolate would release energy if they reacted with the oxygen in the air but the reaction does not proceed under normal conditions.

There are three ways to make it proceed: heat it to high temperatures (fire), expose it to a suitable metal catalyst that promotes the reaction, or eat it and use biological catalysts to cause the reaction. Biology can thrive in an environment that is rich in chemical energy but requires a catalyst for the chemical energy to be released. Such is the case on Titan.

McKay and Smith (2005) predicted that if there were life on Titan living in liquid methane then that life should be widespread on the surface because liquid methane is widespread on the surface. We have direct evidence that the surface of Titan at the landing site of the Huygens Probe near the equator was moist with methane, and radar and near-infrared imagery from Cassini have revealed extensive polar lakes on Titan, both north and south. Methane-based life would have a lot of environments in which to live.

Again, this is analogous to Earth. Life is widespread on Earth because it uses water and water is widespread on Earth.

Furthermore, because it is widespread, life on Earth, in turn, has a profound effect on the environment. For example, each spring the amount of CO2 in the atmosphere drops as plants consume it to form leaves; each autumn, the amount of CO2 in the atmosphere goes up as these leaves decompose. That is, because of the ubiquity of life, the Earth breathes: one breath in during the spring, one breath out during the autumn. Widespread life has observable effects.

Taking this logic to Titan, McKay and Smith (2005) predicted that Titanian life at the surface would consume near-surface hydrogen and that this might be detectable.

The depletion of hydrogen is key because all the chemical methods suggested for life to derive energy from the environment on Titan involve consumption of hydrogen (McKay and Smith 2005; Schulze-Makuch and Grinspoon 2005). Acetylene, ethane, and solid organic material could all be consumed as well. Acetylene yields the most energy, but all give enough energy for microorganisms to live.

A few notes about liquid methane based life on Titan.

First, while such life would produce CH4 it would not be a net source of CH4 but would be merely recycling C back into CH4 - undoing the photochemistry caused by sunlight in the upper atmosphere. It does not explain the persistence of CH4 on Titan over geological time.

Second, it is impossible to predict any isotopic effect that this life might have on C. On Earth, methanogens produce CH4 from CO2+H2, or from organic material derived from CO2. The net reaction is CO2 + 4H2 => CH4 + 2H2O and thus methanogens on Earth are a net source of CH4 in a world of CO2. The enzymes that mediate these reactions create methane with a large isotopic enrichment of 12C over 13C of ~5%.

On Titan, it has been predicted that methanogens would produce CH4 by C2H2 + 3H2 => 2CH4 (eg. McKay and Smith 2005). This is obviously not a net source of CH4: it merely recycles CH4, thereby undoing the photolysis of CH4 and there is no a priori reason to expect the resulting CH4 to exhibit an isotopic shift from these reactions. The C-C bond in acetylene is strong but this by itself does not imply a strong isotopic selectivity.

For example, life on Earth breaks the strong bond between the N atoms in N2 without leaving a clear isotopic effect. Thus, the istopic state of C on Titan is not relevant to the question of the presence of Titanian methanogens..

The data that suggests that there is less ethane on Titan than expected is well established (Lorenz et al. 2008). Photochemical models have predicted that Titan should have a layer of ethane sufficient to cover the entire surface to a thickness of many meters but Cassini has found no such layer. The new results of Clark et al. (2010) find a lack of acetylene on the surface despite its expected production in the atmosphere and subsequent deposition on the ground.

There was also no evidence of acetylene in the gases released from the surface after the Huygens Probe landing (Niemann et al. 2005, Lorenz et al. 2006). Thus, the evidence for less ethane and less acetylene than expected seems clear and incontrovertible.

The depletion of ethane and acetylene become significant in the astrobiological sense because of this latest report of a hydrogen flux into the surface This is the key that suggests that these depletions are not just due to a lack of production but are due to some kind of chemical reaction at the surface.

The determination by Strobel (2010) that there is a flux of hydrogen into the surface of Titan is not the result of a direct observation. Rather it is the result of a computer simulation designed to fit measurements of the hydrogen concentration in the lower and upper atmosphere in a self-consistent way. It is not presently clear from Strobel's results how dependent his conclusion of a hydrogen flux into the surface is on the way the computer simulation is constructed or on how accurately it simulates the Titan chemistry.

In conclusion, there are four possibilities for the recently reported findings, listed in order of their likely reality:

1. The determination that there is a strong flux of hydrogen into the surface is mistaken. It will be interesting to see if other researchers, in trying to duplicate Strobel's results, reach the same conclusion.

2. There is a physical process that is transporting H2 from the upper atmosphere into the lower atmosphere. One possibility is adsorption onto the solid organic atmospheric haze particles which eventually fall to the ground. However this would be a flux of H2, and not a net loss of H2.

3. If the loss of hydrogen at the surface is correct, the non-biological explanation requires that there be some sort of surface catalyst, presently unknown, that can mediate the hydrogenation reaction at 95 K, the temperature of the Titan surface. That would be quite interesting and a startling find although not as startling as the presence of life.

4. The depletion of hydrogen, acetylene, and ethane, is due to a new type of liquid-methane based life form as predicted (Benner et al. 2004, McKay and Smith 2005, and Schulze-Makuch and Grinspoon 2005).

References
+ Benner, S.A., A. Ricardo and M.A. Carrigan (2004) Is there a common chemical model for life in the universe? Current Opinion in Chemical Biology 8, 672-689. Clark, R. N., J. M. Curchin, J. W. Barnes, R. Jaumann, L. Soderblom, D. P. Cruikshank, R. H. Brown, S. Rodriguez, J. Lunine, K. Stephan, T. M. Hoefen, S. Le Mouelic, C. Sotin, K. H. Baines, B. J. Buratti, and P. D. Nicholson (2010) Detection and Mapping of Hydrocarbon Deposits on Titan. J. Geophys. Res., doi:10.1029/2009JE003369, in press.

+ Lorenz, L.D., H.B. Niemann, D.N. Harpold, S.H. Way, and J.C. Zarnecki (2006) Titan's damp ground: Constraints on Titan surface thermal properties from the temperature evolution of the Huygens GCMS inlet. Meteoritics and Planetary Science 41, 1705-1714.

+ Lorenz, R.D., K.L. Mitchell, R.L. Kirk, A.G. Hayes, O. Aharonson, H.A. Zebker, P. Paillou, J. Radebaugh, J.I. Lunine, M.A. Janssen, S.D. Wall, R.M. Lopes, B. Stiles, S. Ostro, G. Mitri, and E.R. Stofan (2008) Titan's inventory of organic surface materials Geophys. Res. Lett. 35, L02206, doi:10.1029/2007GL032118. McKay, C.P., Smith, H.D. (2005) Possibilities for methanogenic life in liquid methane on the surface of Titan. Icarus, 178, 274-276.

+ Niemann H. B., Atreya S. K., Bauer S. J., Carignan G. R., Demick J.E., Frost R. L., Gautier D., Haberman J. A., Harpold D. N., Hunten D. M., Israel G., Lunine J. I., Kasprzak W. T., Owen T.C., Paulkovich M., Raulin F., Raaen E., and Way S. H. (2005) The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe. Nature 438, 779-784.

+ Schulze-Makuch, D., and D.H. Grinspoon (2005) Biologically enhanced energy and carbon cycling on Titan? Astrobiology 5, 560-564.

+ Strobel, D.F. (2010) Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions. Icarus, in press.

]]> Thu, 29 JUL 2010 14:33:31 AEST Pasadena CA (JPL) Jun 07, 2010 - NASA's Cassini spacecraft will be eyeing the north polar region of Saturn's moon Titan this weekend, scanning the moon's land o' lakes.

At closest approach on early morning Saturday, June 5 UTC, which is Friday afternoon, June 4 Pacific time, Cassini will glide to within about 2,000 kilometers (1,300 miles) of the Titan surface.

Cassini will make infrared scans of the north polar region, which was in darkness for the first several years of Cassini's tour around the Saturn system. The lighting has improved as northern spring has started to dawn over the area.

The visual and infrared spectrometer will be prime during closest approach, but the imaging science subsystem cameras will also be taking pictures. Among the scientific bounties, Cassini team members are hoping to get another good look at Kraken Mare, the largest lake on Titan, which covers a greater area than the Caspian Sea on Earth.

Although this latest flyby is dubbed "T69," planning changes early in the orbital tour made this the 70th targeted flyby of Titan.

]]> Thu, 29 JUL 2010 14:33:31 AEST Pasadena CA (JPL) Jun 07, 2010 - Two new papers based on data from NASA's Cassini spacecraft scrutinize the complex chemical activity on the surface of Saturn's moon Titan. While non-biological chemistry offers one possible explanation, some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan's surface.

According to one theory put forth by astrobiologists, the signatures fulfill two important conditions necessary for a hypothesized "methane-based life."

One key finding comes from a paper online now in the journal Icarus that shows hydrogen molecules flowing down through Titan's atmosphere and disappearing at the surface. Another paper online now in the Journal of Geophysical Research maps hydrocarbons on the Titan surface and finds a lack of acetylene.

This lack of acetylene is important because that chemical would likely be the best energy source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA Ames Research Center, Moffett Field, Calif., who proposed a set of conditions necessary for this kind of methane-based life on Titan in 2005.

One interpretation of the acetylene data is that the hydrocarbon is being consumed as food. But McKay said the flow of hydrogen is even more critical because all of their proposed mechanisms involved the consumption of hydrogen.

"We suggested hydrogen consumption because it's the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth," McKay said.

"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."

To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere, though 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 90 Kelvin (minus 290 degrees Fahrenheit), a methane-based organism would have to use a substance that is liquid as its medium for living processes, but not water itself. Water is frozen solid on Titan's surface and much too cold to support life as we know it.

The list of liquid candidates is very short: liquid methane and related molecules like ethane. While liquid water is widely regarded as necessary for life, there has been extensive speculation published in the scientific literature that this is not a strict requirement.

The new hydrogen findings are consistent with conditions that could produce an exotic, methane-based life form, but do not definitively prove its existence, said Darrell Strobel, a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, Md., who authored the paper on hydrogen.

Strobel, who studies the upper atmospheres of Saturn and Titan, analyzed data from Cassini's composite infrared spectrometer and ion and neutral mass spectrometer in his new paper.

The paper describes densities of hydrogen in different parts of the atmosphere and the surface. Previous models 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 found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10,000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escape out of the upper atmosphere.

"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. The Titan surface is also so cold that a chemical process that involved a catalyst would be needed to convert hydrogen molecules and acetylene back to methane, even though overall there would be a net release of energy. The energy barrier could be overcome if there were an unknown mineral acting as the catalyst on Titan's surface.

The hydrocarbon mapping research, led by Roger Clark, a Cassini team scientist based at the U.S. Geological Survey in Denver, examines data from Cassini's visual and infrared mapping spectrometer. 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 detected no acetylene on the surface.

In addition Cassini's spectrometer detected an absence of water ice on the Titan surface, but loads of benzene and another material, which appears to be an organic compound that scientists have not yet been able to identify.

The findings lead scientists to believe that the organic compounds are shellacking over the water ice that makes up Titan's bedrock with a film of hydrocarbons at least a few millimeters to centimeters thick, but possibly much deeper in some places. The ice remains covered up even as liquid methane and ethane flow all over Titan's surface and fill up lakes and seas much as liquid water does on Earth.

"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," Clark said. "All that implies Titan is a dynamic place where organic chemistry is happening now."

The absence of detectable acetylene on the Titan surface can very well have a non-biological explanation, said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team.

Allen is based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. 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.

"Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed," Allen said.

"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 - for example, reactions involving mineral catalysts."

"These new results are surprising and exciting," said Linda Spilker, Cassini project scientist at JPL. "Cassini has many more flybys of Titan that might help us sort out just what is happening at the surface."

]]> Thu, 29 JUL 2010 14:33:31 AEST Pasadena CA (JPL) May 21, 2010 - NASA's Cassini spacecraft is on its way to a flyby of Saturn's largest moon, Titan, after capturing some stunning images of Enceladus. One view shows the hazy outline of Titan behind Saturn's rings, with the dark curve of Enceladus at the bottom.

In other images, Enceladus put its craggy face forward, exhibiting some of the fractures and cratering that have made the Saturnian moon a favorite of both planetary scientists and outer-planet mission groupies. A view of Enceladus' terminator was taken by NASA's Cassini spacecraft on May 18 from approximately 75,000 kilometers (46,500 miles) away.

Cassini sent back numerous images May 18, 2010, as it finished the first leg of its planned double flyby. Cassini passed within about 435 kilometers (270 miles) of the Enceladus surface.

Cassini is heading toward Titan for a flyby that occurs in the late evening May 19 Pacific time, which is in the early hours of May 20 UTC. Because of a fortuitous cosmic alignment, Cassini can catch glimpses of these two contrasting worlds within less than 48 hours, with no maneuver in between.

The main scientific goal at Enceladus was to watch the sun play peek-a-boo behind the water-rich plume emanating from the moon's south polar region. Scientists using the ultraviolet imaging spectrograph will be able to use the flickering light to measure whether there is molecular nitrogen in the plume. Ammonia has already been detected in the plume, and scientists know heat can decompose ammonia into nitrogen molecules. Determining the amount of molecular nitrogen in the plume will give scientists clues about thermal processing in the moon's interior.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

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