Most people are familiar with the piles of loose, granular material which make up sand dunes in coastal areas and deserts on Earth. Derived from the weathering and erosion of rocks over many millennia, these wind-blown, shifting dunes are capable of transporting huge amounts of material.

While Titan's dunes resemble in many ways the features found on Earth, they are made of tiny particles of organic (carbon-rich) material which have fallen to the surface as a never-ending "drizzle" from the dense, orange clouds above. As such, they are the largest known reservoir of organics on Titan, playing a key role in the moon's carbon and methane cycles.

With the exception of seemingly featureless plains, these dune fields are the most widespread landform on Titan. However, although they cover about 13 per cent of the surface, the dunes are confined to the tropical regions, between latitudes 30 degrees north and south.

The majority of Titan's dunes are linear in morphology, but their morphometry (width, length, spacing, thickness of the sand cover in the interdune area) seems to vary with location. Shaped by east-west zonal winds, they are typically 1-2 km wide, 1-4 km apart and perhaps 100 m high.

As on Earth, several conditions must be met in order to develop dunes: a supply of sand-sized sediments, winds strong enough to transport these sediments from their source areas, the absence of sediment removal or a trapping system (such as expanses of liquid, rough surfaces or collection basins) and climatic and topographic conditions favourable to sand deposition.

Furthermore, dune dimensions and shapes reflect the meteorological and geological boundary conditions under which they have formed and evolved.

Now, detailed studies by an international team, led by Alice Le Gall from the Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS-UVSQ), Paris, have attempted to explain the regional variations amongst these dunes. (Some of her research for the paper took place when Le Gall held a postdoctoral post at NASA's Jet Propulsion Laboratory in California.)

Radar studies of Titan's dunes
In a recent paper in the journal Icarus, the authors discuss correlations in dune morphometry with altitude and latitude. The regional variations are investigated by analysing the microwave electromagnetic signatures of dune fields obtained during passive and active observations by the radar instrument on Cassini.

The instrument, which operates at a wavelength of 2.2 cm, can be used as a synthetic aperture radar (SAR), a scatterometer, an altimeter or, in a passive mode, as a radiometer. Information can also be inferred from the radar data through direct altimetry and techniques such as SAR-derived topography.

By studying the radar backscatter and emissivity of Titan's dune terrains, up to and including Cassini's Titan 55 flyby (21 May 2009), the team discovered that, with the exception of its polar lakes, dune fields are the most emissive and least reflective features on Titan. However, towards the north they become less emissive and brighter at radar wavelengths.

"We believe that this increase in brightness (and concurrent decrease in emissivity) is caused by a larger proportion of interdune area in the radar footprint," explained Le Gall. "The corridors between the dunes are generally more radar-bright than the dunes themselves. As we go north, the dunes become thinner or/and wider apart."

The data suggest that the quantity of windblown sand tends to decrease towards the north. This could result from a gradual increase in surface moisture with latitude, possibly caused by the asymmetrical seasons associated with Titan's current orbital configuration.

"The eccentric orbit of Titan at the present time results in the southern summer being shorter, but warmer, than the northern summer," said Le Gall. "The less intense northern summers reduce evaporation relative to precipitation, probably resulting in increased soil moisture. This explanation was first advanced to explain why most of the lakes on Titan are at high northern latitudes.

"If the surface contains more moisture, the sand grains are more likely to stick together and the dunes are starved of material."

The team also discovered that the morphometry of the dunes varies with altitude. Using SAR-derived topography, they found that Titan's main dune fields (Shangri-La, Fensal, Belet and Aztlan) tend to occupy the areas of lowest elevation in equatorial regions, occurring at mean elevations between -400 and 0 metres (relative to the geoid, or mean altitude of Titan).

"Titan's dune fields tend to occupy the lowest regions in the equatorial belt and none of them are located in the most elevated areas," noted Le Gall. "On Earth, dune fields commonly form within topographic basins, where air entering the basin expands and decelerates, resulting in the deposition of sediment. It seems that this same aerodynamic condition also controls the location of dune field development on Titan. Seasonal streams may also carry sediment into the basins, which then dry out.

"The main exception is the large basin known as Xanadu, which may receive no sand or be swept clear of sand by wind currents. We also found that dunes do not occur in the lowest terrains on Titan, where the surface may be moist due to interaction with a potential subsurface reservoir of liquid hydrocarbons."

In elevated dune terrains, the data show a trend towards thinner dunes or/and a wider separation of the dunes, and possibly thinner sand cover in the interdune areas. This is consistent with the idea that sediment sources occur in lowlands, whereas the supply of sand is reduced in elevated regions. Increased rates of sediment erosion by the wind, rivers or rain at higher altitudes may also contribute.

How old are the dunes on Titan?
Although Titan's wind speeds are thought to be low - 0.5-2 metres per second - they are probably capable of transporting the light sand particles. However, the massive dunes observed by Cassini may have formed under the other climatic conditions.

Earth experiences dramatic climate changes due to changes in its orbit. Many linear dunes on Earth were formed during the Last Glacial Maximum, about 20,000 years ago.

In much the same way, the orbital cycle on Titan is expected to reverse over a period of 42,000 years, so it is possible that the distribution and morphometry of its dune fields were modified during the latest switch to longer northern summers. Alternatively, they could all be stabilised, representing fossil dunes that evolved in a very different climate.

"Radar images show that dunes cut across most of the other geological features suggesting that they are among the youngest geological features on Titan," said Le Gall.

"For example, when dunes encroach upon an impact crater, we know that they are younger than the crater."

"On the other hand, even though there is an altitudinal / latitudinal control of the dunes, processes which limit the sand supply, transport capacity or sand availability do not appear to vary that much from one location to another, suggesting that the dune fields were all built at the same time."

"Understanding how the dunes form, as well as explaining their shape, size and distribution on Titan, is of great importance in improving our knowledge of its climate and geology," said Nicolas Altobelli, ESA's Cassini-Huygens Project Scientist.

"In particular, as their material is made out of frozen atmospheric hydrocarbons, the dunes might provide us with important clues on the still puzzling methane/ethane cycle on Titan, comparable in many aspects with the water cycle on Earth."

A. Le Gall, et al., "Latitudinal and altitudinal controls of Titan's dune field morphometry", 2012, Icarus, 217, 231-242. DOI: 10.1016/j.icarus.2011.10.024. ;A. Le Gall, et al., "Cassini SAR, radiometry, scatterometry and altimetry observations of Titan's dune fields", 2011, Icarus, 213, 608-624. DOI: 10.1016/j.icarus.2011.03.026.

]]> Fri, 03 FEB 2012 13:10:03 AEST Paris (ESA) Jan 24, 2012 - A new analysis of radar data from the international Cassini spacecraft has revealed regional variations amongst Titan's sand dunes. The result yields new clues to the giant moon's climatic and geological history.

Dune fields are common on Titan, the largest moon of Saturn, second only to the seemingly uniform plains that cover most of the surface. They cover about 13% of Titan, stretching over 10 million sq km, roughly equivalent to the area of Canada. Thus they offer a large-scale insight into the moon's environment.

Though similar in shape to the linear sand dunes found in the deserts of Namibia or southern Arabia, Titan's dunes are gigantic by Earthly standards. They are on average 1-2 km wide, hundreds of kilometres long and around 100 m high.

However, their size and spacing vary across the surface, betraying the environment in which they have formed and evolved.

Another difference is that sand on Titan is not made of silicates as on Earth, but of solid hydrocarbons that precipitate out of the atmosphere. These then aggregate into millimetre-sized grains by a still unknown process.

Using radar data from the NASA-ESA-ASI Cassini spacecraft, Alice Le Gall, of LATMOS-UVSQ, Paris and NASA-JPL, California, and collaborators have discovered that the size of Titan's dunes is controlled by at least two factors: altitude and latitude.

The main dune fields on Titan are found in lowland areas. Dunes at higher elevations tend to be narrower and more widely separated, and the gaps between them appear brighter to Cassini's radar, indicating a thinner covering of sand.

This suggests that there is relatively little sand available at higher elevations to build dunes, while more is present in the lowlands.

In terms of latitude, the dunes on Titan are confined to its equatorial region, in a band between 30 degrees S and 30 degrees N.

However, they tend to become narrower and more widely spaced at northern latitudes. Dr Le Gall and colleagues think that this may be due to Saturn's elliptical orbit.

Titan orbits Saturn and so the moon's seasons are controlled by Saturn's path around the Sun. Because Saturn takes about 30 years to complete an orbit, each season on Titan lasts for just over seven years.

The slightly elliptical nature of Saturn's orbit means that the southern hemisphere of the moon has shorter but more intense summers.

As a result, in southern regions, surface wetness due to ethane and methane vapour in the soil is reduced. The drier the sand grains, the more easily they can be transported by the winds to make dunes.

"As one goes to the north, the soil moisture probably increases, making the sand particles less mobile and, as a consequence, the development of dunes more difficult," says Dr Le Gall.

Backing up this hypothesis is the fact that Titan's lakes and seas of liquid ethane and methane are predominantly found in the northern hemisphere, suggesting again that the soil may be moister towards the north, again making it harder to transport sand grains by the wind.

"Understanding how the dunes form as well as explaining their shape, size and distribution on Titan's surface is of great importance to understanding Titan's climate and geology," says Nicolas Altobelli, ESA's Cassini-Huygens project scientist.

"As their material is made out of frozen atmospheric hydrocarbons, the dunes might provide us with important clues on the still puzzling methane/ethane cycle on Titan, comparable in many aspects with the water cycle on Earth."

]]> Fri, 03 FEB 2012 13:10:03 AEST Pasadena CA (JPL) Jan 24, 2012 - Saturn's moon Mimas peeks out from behind the night side of the larger moon Dione in this Cassini image captured during the spacecraft's Dec. 12, 2011, flyby of Dione.

Dione is 698 miles, or 1,123 kilometers, across and its day side dominates the view on the right of the image. Smaller Mimas is on the left and measures 246 miles, or 396 kilometers, across.

Lit terrain seen here is on the Saturn-facing side of Mimas and in the area between the trailing hemisphere and anti-Saturn side of Dione. North on the moons is up and rotated 20 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera.

The view was obtained at a distance of approximately 58,000 miles (94,000 kilometers) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 42 degrees. Image scale is 1,833 feet (559 meters) per pixel on Dione.

The view was obtained at a distance of approximately 380,000 miles (611,000 kilometers) from Mimas and at a Sun-Mimas-spacecraft, or phase, angle of 41 degrees. Image scale is 2 miles (3 kilometers) per pixel on Mimas.

]]> Fri, 03 FEB 2012 13:10:03 AEST Paris (UPI) Jan 16, 2012 - Saturn's moon Titan may be more like Earth-like than previously thought with a layered atmosphere just like our planet, European researchers said.

Titan is the only moon in the solar system known to have a dense atmosphere, and its lowest layer, known as its boundary layer, is most influenced by its surface.

This is true of any world possessing a dense atmosphere, researchers said.

"This layer is very important for the climate and weather -- we live in the terrestrial boundary layer [on Earth]," study lead author Benjamin Charnay, a planetary scientist at France's National Center of Scientific Research, told SPACE.com

Earth's boundary layer, between 1,650 feet and 1.8 miles thick, is affected largely by solar heat warming the planet's surface.

The boundary layer on Titan, much further from the sun, might behave quite differently, researchers said.

To study Titan's atmosphere, scientists created a 3D climate model of how it might react under the influence of solar heat over time.

"The most important implication of these findings is that Titan appears closer to an Earth-like world than once believed," Charnay said.

The research could prove useful for further explorations beyond our solar system, he said.

Three-dimensional "models will be very useful in the future to explain the data we will get about the atmospheres of exoplanets," Charnay said.

]]> Fri, 03 FEB 2012 13:10:03 AEST Pasadena, Calif. (UPI) Jan 13, 2012 - NASA says it's conducting diagnostic tests on its Cassini spacecraft orbiting Saturn after its signal was not detected during a tracking pass in late December.

The spacecraft has been communicating with Earth using a backup radio system, the space agency said in a release Thursday.

The problem centers on the ultra-stable oscillator, used for radio science experiments and also as a means of sending data to Earth.

Cassini mission engineers say the spacecraft has shifted to an auxiliary oscillator, whose frequency stability is adequate for transmitting data from the spacecraft to Earth.

Tests this month will help mission managers decide whether it will be possible to bring the ultra-stable oscillator back into service, NASA said.

Age may by a reason for the problem, NASA said, noting Cassini has been in orbit around Saturn since 2004 and has had its mission extended twice.

Cassini was launched in 1997.

]]> Fri, 03 FEB 2012 13:10:03 AEST Pasadena CA (SPX) Jan 10, 2012 - Saturn's largest moon, Titan, is an intriguing, alien world that's covered in a thick atmosphere with abundant methane. With an average surface temperature of a brisk -300 degrees Fahrenheit (about 90 kelvins) and a diameter just less than half of Earth's, Titan boasts methane clouds and fog, as well as rainstorms and plentiful lakes of liquid methane. It's the only place in the solar system, other than Earth, that has large bodies of liquid on its surface.

The origins of many of these features, however, remain puzzling to scientists. Now, researchers at the California Institute of Technology (Caltech) have developed a computer model of Titan's atmosphere and methane cycle that, for the first time, explains many of these phenomena in a relatively simple and coherent way.

In particular, the new model explains three baffling observations of Titan. One oddity was that Titan's methane lakes tend to cluster around its poles and that there are more lakes in the northern hemisphere than in the south.

Secondly, the areas at low latitudes, near Titan's equator, are known to be dry, lacking lakes and regular precipitation. But when the Huygens probe landed on Titan in 2005, it saw channels carved out by flowing liquid-possibly runoff from rain. And in 2009, Caltech researchers discovered raging storms that may have brought rain to this supposedly dry region.

Finally, scientists uncovered a third mystery when they noticed that clouds observed over the past decade-during summer in Titan's southern hemisphere-cluster around southern middle and high latitudes.

Scientists have proposed various ideas to explain these features, but their models either can't account for all of the observations, or do so by requiring exotic processes, such as cryogenic volcanoes that spew methane vapor to form clouds.

The Caltech researchers say their new computer model, on the other hand, can explain all these observations-and does so using relatively straightforward and fundamental principles of atmospheric circulation.

"We have a unified explanation for many of the observed features," says Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering.

"It doesn't require cryovolcanoes or anything esoteric." Schneider, along with Caltech graduate student Sonja Graves, former Caltech graduate student Emily Schaller (PhD '08), and Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, have published their findings in the January 5 issue of the journal Nature.

Schneider says the team's simulations were able to reproduce the distribution of clouds that's been observed-which was not the case with previous models. The new model also produces the right distribution of lakes. Methane tends to collect in lakes around the poles because the sunlight there is weaker on average, he explains. Energy from the sun normally evaporates liquid methane on the surface, but since there's generally less sunlight at the poles, it's easier for liquid methane there to accumulate into lakes.

But then why are there more lakes in the northern hemisphere? Schneider points out that Saturn's slightly elongated orbit means that Titan is farther from the sun when it's summer in the northern hemisphere. Kepler's second law says that a planet orbits more slowly the farther it is from the sun, which means that Titan spends more time at the far end of its elliptical orbit, when it's summer in the north.

As a result, the northern summer is longer than the southern summer. And since summer is the rainy season in Titan's polar regions, the rainy season is longer in the north. Even though the summer rains in the southern hemisphere are more intense-triggered by stronger sunlight, since Titan is closer to the sun during southern summer-there's more rain over the course of a year in the north, filling more lakes.

In general, however, Titan's weather is bland, and the regions near the equator are particularly dull, the researchers say. Years can go by without a drop of rain, leaving the lower latitudes of Titan parched.

It was a surprise, then, when the Huygens probe saw evidence of rain runoff in the terrain. That surprise only increased in 2009 when Schaller, Brown, Schneider, and then-postdoctoral scholar Henry Roe discovered storms in this same, supposedly rainless, area.

No one really understood how those storms arose, and previous models failed to generate anything more than a drizzle. But the new model was able to produce intense downpours during Titan's vernal and autumnal equinoxes-enough liquid to carve out the type of channels that Huygens found. With the model, the researchers can now explain the storms.

"It rains very rarely at low latitudes," Schneider says. "But when it rains, it pours."

The new model differs from previous ones in that it's three-dimensional and simulates Titan's atmosphere for 135 Titan years-equivalent to 3,000 years on Earth-so that it reaches a steady state. The model also couples the atmosphere to a methane reservoir on the surface, simulating how methane is transported throughout the moon.

The model successfully reproduces what scientists have already seen on Titan, but perhaps what's most exciting, Schneider says, is that it also can predict what scientists will see in the next few years. For instance, based on the simulations, the researchers predict that the changing seasons will cause the lake levels in the north to rise over the next 15 years.

They also predict that clouds will form around the north pole in the next two years. Making testable predictions is "a rare and beautiful opportunity in the planetary sciences," Schneider says. "In a few years, we'll know how right or wrong they are.

"This is just the beginning," he adds. "We now have a tool to do new science with, and there's a lot we can do and will do."

The research described in the Nature paper, "Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle," was supported by a NASA Earth and Space Science Fellowship and a David and Lucile Packard Fellowship.

]]> Fri, 03 FEB 2012 13:10:03 AEST Moffett Field CA (SPX) Jan 10, 2012 - A team of scientists has traveled to remote Laguna Negra in the central Andes of Chile to test technologies that could one day be used to explore the lakes of Titan. The Planetary Lake Lander (PLL) project is led by Principal Investigator Nathalie Cabrol of the NASA Ames Research Center and the SETI Institute, and is funded by the NASA Astrobiology Science and Technology for Exploring Planets (ASTEP) program.

This three-year field campaign will design and deploy a lake lander at Laguna Negra, which is a particularly vulnerable system where ice is melting at an accelerated rate.

In addition to preparing us for Titan, the study will also help answer questions about how deglaciation affects life in glacial lakes. During the 2011 field campaign, Astrobiology Magazine's Expeditions Editor, Henry Bortman, is providing a first-hand account of the team's work through blogs and images.

Biology under Stormy Skies
The weather has taken a turn toward the dramatic. For the first few days we were here, the sky was practically cloudless all day long. Then for a couple of days, puffy white clouds would build up in the afternoon.

But in the past couple of days, the afternoon sky has gone dark, we've heard thunder in the distance, and the peaks of the mountains that surround the lake have been obscured in mist.

And it snowed. Not down here in base camp. Here we got a half an hour or so of light rain. But up on Cerro Echaurren to the north and even more so on Meson Alto to the east, there are new dustings of white powder.

That may not seem odd to those of you reading this in the northern hemisphere, where winter is approaching. But here below the Equator, we are only two weeks from the first day of summer.

Although some of us would prefer to sit on a rock overlooking the lake, listening to the wind and watching cloud formations roll through, the threat of thunderstorms has sent us scurrying to zip up the rain flies on our tents and has forced most of our activity indoors. It has also put a temporary halt to biological sampling on the lake.

But biologists are dedicated lot, and some members of the PLL team have plenty of samples, collected when the weather was better, that need to be filtered and prepared for laboratory analysis.

This process involves sucking up water, liters and liters of water collected from Laguna Negra and Laguna Lo Encanado, into a syringe, some 50 milliliters at a time, and then forcing the water through a small circular filter laced with pores tiny enough to trap microbial cells.

The water gets discarded, except a small volume that will be use for measuring dissolved cations and anions. Also of interest is the yellowish-brown stain left behind on the filter paper, comprised largely of microorganisms from the lake.

Some of these filters are preserved in ethanol, for later DNA extraction. Other filters are kept on dry ice until they can be transported down to Santiago de Chile University, where they will be freeze-dried for the trip back to Madrid, Spain.

Madrid is where microbial ecologists Yolanda Blanco and Luis A. Rivas work, at the Centro de Astrobiologia. They are part of a team that for the past several years has been developing a life-detection device, a device they hope will be sent one day to Mars, or perhaps to other worlds in our solar system, in search of evidence for extra-terrestrial life.

The device goes by the name SOLID (Signs Of LIfe Detector), which includes an antibody microarray in its sample analysis unit. This microarray crams hundreds of microscopic dots, each a distinct biological probe, onto a small glass slide.

The dots, printed onto the slide, contain antibodies. Some of these antibodies react to specific types of organisms. Others react to common biological molecules, such as amino acids, the building blocks of proteins; or the lipids found in cell walls.

Unlike past life-detection methods that look for biosignatures by heating a sample to high temperatures and sniffing at the released gases, the antibody-microarray approach is less destructive, more precise and capable of searching for hundreds of different biosignatures simultaneously.

The array is exposed to a sample, such as the material trapped by filtering water from Laguna Negra. If the organism or biomolecule that a particular antibody is designed to detect is present in the sample, the antibody binds to the sample material.

This captured material is revealed by using a fluorescent probe. Examined under the right type of light, these fluorescent tags appear as glowing dots. The position of the dot on the glass slide tells researchers which organism or molecule has been detected.

Blanco, Rivas and their colleagues previously tested detectors at Rio Tinto, in Spain, and in Chile's Atacama Desert, each time with a slightly different emphasis tuned to the environmental conditions of those field sites. One focus of the work at Laguna Negra will be looking for psychrophilic, or cold-loving, organisms.

The Garden World of the Northwest Shore
So far, our activities have focused on the waters along the southern shore of Laguna Negra, with occasional side trips to Laguna Lo Encanado to collect samples. But yesterday, for the first time, PLL team members Liam Pedersen and Chris Haberle struck out in the battery-powered Zodiac, Mariner 1, on a 40-minute, 4-kilometer journey to the northwest shore of the lake.

There, they discovered another world.

Laguna Negra is only 1.5 kilometers wide, but it is 6 kilometers long, the longer direction running north-south. The lake has two long "fingers" that stretch north, one to the northwest, the other to the northeast.

PLL Base Camp is situated at the center of the southern shore. The Echaurren glacier, however, sits high above the northwest finger. The team has been anxious to do research in this area, because it is where interaction between the glacier and the lake is likely to be greatest. That also makes it an ideal spot to consider as a summer home for the Planetary Lake Lander.

What a difference a few kilometers makes. Upon approaching the shoreline, the PLL advance team spotted underwater caves with denser vegetation than the scraggly assortment of plants along the southern shore. Then they saw the streams, lined with lush vegetation, and displays of wildflowers. And the massive waterfall, Victoria's Cascade, named by the expedition that explored the area a century and a half ago.

Most importantly, though, from a scientific point of view, was the turbidity of the northwest water where the waterfall meets Laguna Negra. Most of the lake is crystal clear, which makes for lovely sightseeing, but an indication that the lake is nutrient-poor. As a result of global warming, the glacier in recent years has retreated to a great extent.

There is no longer any direct contact between the glacier and the lake. And the water that spills down into the lake from the melting ice no longer carries much sediment. It is that sediment that makes lake waters cloudy. It is also that sediment that carries nutrients for life.

There is a small area on the northwest shore where this turbid water can be seen diffusing into the lake, but because it's colder than the lake water, it quickly sinks to the bottom, mixing very little with the otherwise transparent lake.

It is just this interaction between the glacier and the lake, however, and the difference between this area, which can support a distinct ecosystem, and other parts of the lake, that makes it scientifically appealing - and a potential long-term site for the Planetary Lake Lander.

Fortunately, communications tests conducted between the northwest-shore landing site and PLL Base Camp were successful, making the prospect of situating the lake lander there for the summer even more promising.

]]> Fri, 03 FEB 2012 13:10:03 AEST Pasadena CA (SPX) Jan 05, 2012 - Saturn's largest moon, Titan, is an intriguing, alien world that's covered in a thick atmosphere with abundant methane. With an average surface temperature of a brisk -297 degrees Fahrenheit (about 90 kelvins) and a diameter just less than half of Earth's, Titan boasts methane clouds and fog, as well as rainstorms and plentiful lakes of liquid methane. It's the only place in the solar system, other than Earth, that has large bodies of liquid on its surface.

The origins of many of these features, however, remain puzzling to scientists. Now, researchers at the California Institute of Technology (Caltech) have developed a computer model of Titan's atmosphere and methane cycle that, for the first time, explains many of these phenomena in a relatively simple and coherent way.

In particular, the new model explains three baffling observations of Titan. One oddity was discovered in 2009, when researchers led by Caltech professor of planetary science Oded Aharonson found that Titan's methane lakes tend to cluster around its poles-and noted that there are more lakes in the northern hemisphere than in the south.

Secondly, the areas at low latitudes, near Titan's equator, are known to be dry, lacking lakes and regular precipitation. But when the Huygens probe landed on Titan in 2005, it saw channels carved out by flowing liquid-possibly runoff from rain. And in 2009, Caltech researchers discovered raging storms that may have brought rain to this supposedly dry region.

Finally, scientists uncovered a third mystery when they noticed that clouds observed over the past decade-during summer in Titan's southern hemisphere-cluster around southern middle and high latitudes.

Scientists have proposed various ideas to explain these features, but their models either can't account for all of the observations, or do so by requiring exotic processes, such as cryogenic volcanoes that spew methane vapor to form clouds. The Caltech researchers say their new computer model, on the other hand, can explain all these observations-and does so using relatively straightforward and fundamental principles of atmospheric circulation.

"We have a unified explanation for many of the observed features," says Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering. "It doesn't require cryovolcanoes or anything esoteric." Schneider, along with Caltech graduate student Sonja Graves, former Caltech graduate student Emily Schaller (PhD '08), and Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, have published their findings in the January 5 issue of the journal Nature.

Schneider says the team's simulations were able to reproduce the distribution of clouds that's been observed-which was not the case with previous models. The new model also produces the right distribution of lakes.

Methane tends to collect in lakes around the poles because the sunlight there is weaker on average, he explains. Energy from the sun normally evaporates liquid methane on the surface, but since there's generally less sunlight at the poles, it's easier for liquid methane there to accumulate into lakes.

But then why are there more lakes in the northern hemisphere? Schneider points out that Saturn's slightly elongated orbit means that Titan is farther from the sun when it's summer in the northern hemisphere. Kepler's second law says that a planet orbits more slowly the farther it is from the sun, which means that Titan spends more time at the far end of its elliptical orbit, when it's summer in the north.

As a result, the northern summer is longer than the southern summer. And since summer is the rainy season in Titan's polar regions, the rainy season is longer in the north. Even though the summer rains in the southern hemisphere are more intense-triggered by stronger sunlight, since Titan is closer to the sun during southern summer-there's more rain over the course of a year in the north, filling more lakes.

In general, however, Titan's weather is bland, and the regions near the equator are particularly dull, the researchers say. Years can go by without a drop of rain, leaving the lower latitudes of Titan parched. It was a surprise, then, when the Huygens probe saw evidence of rain runoff in the terrain. That surprise only increased in 2009 when Schaller, Brown, Schneider, and then-postdoctoral scholar Henry Roe discovered storms in this same, supposedly rainless, area.

No one really understood how those storms arose, and previous models failed to generate anything more than a drizzle. But the new model was able to produce intense downpours during Titan's vernal and autumnal equinoxes-enough liquid to carve out the type of channels that Huygens found. With the model, the researchers can now explain the storms. "It rains very rarely at low latitudes," Schneider says. "But when it rains, it pours."

The new model differs from previous ones in that it's three-dimensional and simulates Titan's atmosphere for 135 Titan years-equivalent to 3,000 years on Earth-so that it reaches a steady state. The model also couples the atmosphere to a methane reservoir on the surface, simulating how methane is transported throughout the moon.

The model successfully reproduces what scientists have already seen on Titan, but perhaps what's most exciting, Schneider says, is that it also can predict what scientists will see in the next few years. For instance, based on the simulations, the researchers predict that the changing seasons will cause the lake levels in the north to rise over the next 15 years.

They also predict that clouds will form around the north pole in the next two years. Making testable predictions is "a rare and beautiful opportunity in the planetary sciences," Schneider says. "In a few years, we'll know how right or wrong they are.

"This is just the beginning," he adds. "We now have a tool to do new science with, and there's a lot we can do and will do."

]]> Fri, 03 FEB 2012 13:10:03 AEST Pasadena CA (JPL) Dec 23, 2011 - Saturn's third-largest moon Dione can be seen through the haze of its largest moon, Titan, in this view of the two posing before the planet and its rings from NASA's Cassini spacecraft.

The north polar hood can be seen on Titan appearing as a detached layer at the top of the moon here. See PIA08137 and PIA09739 to learn more about Titan's atmosphere and the north polar hood.

See PIA10560 and PIA07638 to learn more about and see a closer view of the wisps on Dione's trailing hemisphere, which appear as bright streaks here.

This view looks toward the anti-Saturn side of Titan (3200 miles, 5150 kilometers across) and Dione (698 miles, 1123 kilometers across). North is up on the moons. This view looks toward the northern, sunlit side of the rings from just above the ring plane.

Images taken using red, green and blue spectral filters were combined to create this natural color view.

The images were obtained with the Cassini spacecraft narrow-angle camera on May 21, 2011 at a distance of approximately 1.4 million miles (2.3 million kilometers) from Titan 2 million miles (3.2 million kilometers) from Dione. Image scale is 9 miles (14 kilometers) per pixel on Titan and 12 miles (19 kilometers) on Dione.

Cassini scientists regularly make observations such as this pictured here to study the ever-changing orbits of the planet's moons.

But even in these routine images, the Saturnian system shines. A few of Saturn's stark, airless, icy moons appear to dangle next to the orange orb of Titan, the only moon in the solar system with a substantial atmosphere.

Titan's atmosphere is of great interest because of its similarities to the atmosphere believed to exist long ago on the early Earth.

While it may be wintry in Earth's northern hemisphere, it is currently northern spring in the Saturnian system and it will remain so for several Earth years.

Current plans to extend the Cassini mission through 2017 will supply a continued bounty of scientifically rewarding and majestic views of Saturn and its moons and rings, as spectators are treated to the passage of northern spring and the arrival of summer in May 2017.

"As another year traveling this magnificent sector of our solar system draws to a close, all of us on Cassini wish all of you a very happy and peaceful holiday season, " said Carolyn Porco, Cassini imaging team lead at the Space Science Institute, Boulder, Colo.

]]> Fri, 03 FEB 2012 13:10:03 AEST Pasadena CA (JPL) Dec 14, 2011 - NASA's Cassini spacecraft successfully completed its closest-ever pass over Saturn's moon Dione on Monday, Dec. 12, slaloming its way through the Saturn system on its way to tomorrow's close flyby of Titan.

Cassini is expected to glide about 2,200 miles (3,600 kilometers) over the Titan surface on Dec. 13.

In the selection of the raw images obtained during the Cassini Dione flyby, Dione is sometimes joined by other moons. Mimas appears just beyond the dark side of Dione in one view.

In another view, Epimetheus and Pandora appear together, along with Saturn's rings.

This Dione encounter was intended primarily for Cassini's composite infrared spectrometer and radio science subsystem.

However, the imaging team did capture views of the distinctive, wispy fractures on the side of Dione that always trails in its orbit around Saturn.

It also obtained images of a ridge called Janiculum Dorsa on the hemisphere of Dione that always leads in its orbit around Saturn.

While other flybys produced more detailed views of the surface, the best resolved images from this flyby have scales ranging from about 1,100 feet (350 meters) to about 1,600 feet (500 meters) per pixel.

Janiculum Dorsa will be imaged by Cassini at higher resolution in May 2012.

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