Posted on by Nancy Atkinson

Cassini’s Cruise: Close Flybys of Two Moons in Less Than Two Days

On the left, Saturn's moon Enceladus is backlit by the sun, showing the fountain-like sources of the fine spray of material that towers over the south polar region. On the right, is a composite image of Titan. Image credit: NASA/JPL/SSI and NASA/JPL/University of Arizona

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It’s a space navigator’s dream! The Cassini spacecraft will perform close flybys of two of Saturn’s most enigmatic moons all within less than 48 hours, and with no maneuvers in between. Enceladus and Titan are aligned just right so that Cassini can catch glimpses of these two contrasting moons – one a geyser world and the other an analog to early Earth.

Cassini will make its closest approach to Enceladus late at night on May 17 Pacific time, which is in the early hours of May 18 UTC. The spacecraft will pass within about 435 kilometers (270 miles) of the moon’s surface.

The main scientific goal at Enceladus will be to watch the sun play peekaboo 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.

Then on to Titan: the closest approach will take place in the late evening May 19 Pacific time, which is in the early hours of May 20 UTC. The spacecraft will fly to within 1,400 kilometers (750 miles) of the surface.

Cassini will primarily be doing radio science during this pass to detect the subtle variations in the gravitational tug on the spacecraft by Titan, which is 25 percent larger in volume than the planet Mercury. Analyzing the data will help scientists learn whether Titan has a liquid ocean under its surface and get a better picture of its internal structure. The composite infrared spectrometer will also get its southernmost pass for thermal data to fill out its temperature map of the smoggy moon.

Cassini has made four previous double flybys and one more is planned in the years ahead.

For more information on the Enceladus flyby, dubbed “E10,” see this link.

For more information on the Titan flyby, dubbed “T68,” see this link.

Source: JPL

Posted on by Nancy Atkinson

Incredible! Cassini as Houdini Cuts Titan in Half

Saturn's rings, made dark in part as the planet casts its shadow across them, cut a striking figure before Saturn's largest moon, Titan. Credit: NASA/JPL/Space Science Institute

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There’s nothing up the sleeves of the Cassini imaging team in this image; it is real! Is the moon Titan being cut in half by Saturn’s rings? What is actually happening here is that the middle part of the rings are made dark as Saturn casts its shadow across them. Cassini was just in the right place at the right time, making it appear as though Titan is being sliced in half! The night side of the planet is to the left, out of the frame of the image. Illuminated Titan can be seen above, below and through gaps in the rings. Click the image for a larger version.

As an added benefit in this shot, Mimas (396 kilometers, 246 miles across) is near the bottom of the image, and Atlas (30 kilometers, 19 miles across) can barely be detected near the thin F ring just above the center right of the image. Lit terrain seen here is the area between the leading hemisphere and Saturn-facing side of Titan (5,150 kilometers, 3,200 miles across). This view looks toward the northern, sunlit side of the rings from just above the ringplane.

Below are a few more magical images from Cassini:

Here the moon Enceladus appears strung along a wispy ring of Saturn, likely the G ring. Look close and Enceladus’ plumes are visible, too.

Enceladus and a wispy ring. Credit: NASA/JPL/Space Science Institute
Pandora and Epimetheus sit on Saturn's rings. Credit: NASA/JPL/Space Science Institute

Two of Saturn’s small moons appear to be sitting on Satun’s thin F ring in this image.

From the CICLOPS website:

Pandora (81 kilometers, 50 miles across) is on the left, and Epimetheus (113 kilometers, 70 miles across) is on the right. This view looks toward the northern, sunlit side of the rings from just above the ringplane. Both moons are closer to Cassini than the rings are. Pandora is slightly closer to Cassini than Epimetheus here.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Nov. 23, 2009. The view was acquired at a distance of approximately 1.3 million kilometers (808,000 miles) from Pandora and Epimetheus. Image scale is 8 kilometers (5 miles) per pixel.

For more great images from Cassini (which I contend is actually an artist and not a magician!) go to the CICLOPS website and NASA’s Cassini website.

Posted on by Nancy Atkinson

Life on Titan Could Be Smelly and Explosive

Artist concept of Methane-Ethane lakes on Titan (Credit: Copyright 2008 Karl Kofoed). Click for larger version.

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Could there be life on Titan? If so, one astrobiologist says humans probably couldn’t be in the same room with a Titanian and live to tell about it. “Hollywood would have problems with these aliens” said Dr. William Bains. “Beam one onto the Starship Enterprise and it would boil and then burst into flames, and the fumes would kill everyone in range. Even a tiny whiff of its breath would smell unbelievably horrible. But I think it is all the more interesting for that reason. Wouldn’t it be sad if the most alien things we found in the galaxy were just like us, but blue and with tails?”

While giving an obvious nod to the recent movie “Avatar,” Bains’ research provides insight to the difficulties we might encounter – beyond cultural – if we ever meet up with alien life. There could be unintended harmful consequences for one species, or both.

Bains is working to find out just how extreme the chemistry of life can be. Life on Titan, Saturn’s largest moon, represents one of the more bizarre scenarios being studied. While images sent back by the Cassini/Huygens mission might make Titan look Earth-like and maybe even inviting, it has a thick atmosphere of frozen, orange smog. At ten times our distance from the Sun, it is a frigid place, with a surface temperature of -180 degrees Celsius. Water is permanently frozen into ice and the only liquid available is liquid methane and ethane.

So instead of water based-life (like us), life on Titan would likely be based on methane.

“Life needs a liquid; even the driest desert plant on Earth needs water for its metabolism to work. So, if life were to exist on Titan, it must have blood based on liquid methane, not water. That means its whole chemistry is radically different. The molecules must be made of a wider variety of elements than we use, but put together in smaller molecules. It would also be much more chemically reactive,” said Bains.

Additionally, Bains said a metabolism running in liquid methane would have to be built of smaller molecules than terrestrial biochemistry.

“Terrestrial life uses about 700 molecules, but to find the right 700 there is reason to suppose that you need to be able to make 10 million or more,” Bains said. “The issue is not how many molecules you can make, but whether you can make the collection you need to assemble a metabolism.”

Bains said doing such assembling is like trying to find bits of wood in a lumber-yard to make a table.

“In theory you only need 5,” he said. “But you may have a lumber-yard full of offcuts and still not find exactly the right five that fit together. So you need the potential to make many more molecules than you actually need. Thus the 6-atom chemicals on Titan would have to include much more diverse bond types and probably more diverse elements, including sulphur and phosphorus in much more diverse and (to us) unstable forms, and other elements such as silicon.”

Energy is another factor that would affect the type of life that could evolve on Titan. With Sunlight a tenth of a percent as intense on Titan’s surface as on the surface of Earth, energy is likely to be in short supply.

“Rapid movement or growth needs a lot of energy, so slow-growing, lichen-like organisms are possible in theory, but velociraptors are pretty much ruled out,” said Bains.

Whatever life may be on Titan, at least we know there won’t be a Jurassic Park.

Bains, whose research is carried out through Rufus Scientific in Cambridge, UK, and MIT in the USA, is presenting his research at the National Astronomy Meeting in Glasgow, Scotland on April 13, 2010.

Source: RAS NAM

Posted on by John Carl Villanueva

What Is A Moon?

Full Moon
Full Moon

Before the invention of the telescope in the early 1600’s, man just knew of the Moon — a round, mysterious astronomical object that people would gaze up to in the night sky. As time progressed however, astronomers discovered that the moon isn’t exactly unique to earthlings, and other planets had their own moons. So exactly what is a moon?

A moon is defined to be a celestial body that makes an orbit around a planet, including the eight major planets, dwarf planets, and minor planets. A moon may also be referred to as a natural satellite, although to differentiate it from other astronomical bodies orbiting another body, e.g. a planet orbiting a star, the term moon is used exclusively to make a reference to a planet’s natural satellite.

The first moons to be discovered outside of the Earth’s moon were the Galilean moons of Jupiter, named after astronomer and discoverer Galileo Galilei. The moons Io, Europa, Ganymede, and Callisto are Jupiter’s largest and only the first four to be revealed, as to date, the planet has 63 moons.

Other than the four Galilean moons, Saturn’s Titan and Neptune’s Triton are two other moons which are comparable in size to the Earth’s Moon. In fact, these seven moons are the largest natural satellites in the solar system, measuring more than 3,000 kilometers in diameter. Only the inner planets Mercury and Venus have no moons.

An interesting fact about some of the solar system’s largest moons that most people may not be aware of is that a few of them are geologically active. While we may not see the Moon spewing lava or displaying any evidence of tectonic activity, Jupiter’s Io and Europa, Saturn’s Titan and Enceladus, and Neptune’s Triton have been found to be volcanically active bodies.

If the moon count had a grand total of just one in the olden times, that number has ballooned to 336 as of July 2009, with 168 moons orbiting the six planets, while the rest are moons of dwarf planets, asteroids moons, and natural satellites of Trans-Neptunian objects.

As more and more discoveries are made however, astronomers may find it more difficult to put a really defining line on what can or what can’t be classified as a moon. For instance, can you consider a 10-inch rock that’s orbiting Jupiter a moon? If yes, then there could be thousands or even millions of moons out there. If not, then where do you draw the line? Obviously, even the size of an “official” moon is still up for debate, so other than the simple definition of it being a natural satellite of a planet, there really is no clear cut answer to the question, “What is a moon?”.

Here in Universe Today, we have a nice collection of articles that explain why the Moon landings could not have been faked. Here are some of them:

Moon Rocks – Discusses how the Moon rocks are one of the most tangible objects that prove the landings took place.

Moon Landing Hoax – An explanation that counters some of the points raised by skeptics

Apollo 11 Hoax – another point for point discussion by Jerry Coffey

TV – Alert: Mythbusters and the Moon Hoax Myth – a teaser for the Mythbusters episode featuring the so-called hoax. You’ll find the comments below that article equally interesting, by the way.

Here’s an article from NASA that debunks the hoax theory using the Moon rock arguments. Another article about Moon rocks from the same site.

Episodes about the moon from Astronomy Cast. Lend us your ears!

Shooting Lasers at the Moon and Losing Contact with Rovers
The Moon Part I

References:
NASA Solar System Exploration: Moons of Jupiter
NASA Solar System Exploration: Moons

Posted on by Nicholos Wethington

A Double-Dose of Cassini Goodness

The Cassini mission is just a non-stop faucet of fantastic images! Here are two that were released today, for your viewing pleasure. The first image, above, is an eclipse of Saturn’s moon Tethys, which lies in the background, by Dione. The three images were each taken one minute apart.

As you can see, from Cassini’s perspective Dione passes right in front of Tethys. Make no mistake in thinking that these two Saturnian companions are close together in this shot, however; Dione, the moon in the foreground, is 2.2 million kilometers (1.4 million miles) from the Cassini spacecraft, while Tethys is 2.6 million kilometers (1.6 million miles) away.

An interesting feature of the image is how Tethys appears brighter on the side of the moon opposite the Sun. This is because Saturn, which lies out of the image to the right, is reflecting light from the Sun back onto the moon. Dione is not being backlit by Saturn from the vantage point of Cassini, so its face that is opposite the Sun appears darker.

Visible on Tethys is the Odysseus Crater, which spans a whopping 400km (240 miles). Given that Tethys is only 1,062 kilometers, or 660 miles across, the crater appears very large in comparison to the moon. It also makes the moon very much resemble the Death Star from Star Wars, don’t you think? These images were taken using Cassini’s narrow-angle camera on Nov. 28, 2009.

This second image is a synthetic aperture radar image of the surface of Saturn’s moon Titan. In the lower right and upper center of the image, the two wrinkly features are actually small Titanian mountains. What exactly causes the grooves in these mountains has still to be determined.

On Earth, the shifting of tectonic plates can form such structures, as well as the processes of water flowing, freezing, and melting.

Since Titan has an atmosphere composed mostly of methane and ethane, and experiences rain much like here on Earth, it’s quite possible that these processes are the cause of such features.

Because the illumination of this image comes from the radar on Cassini, the peaks of these formations should be the brightest. As is visible, this isn’t the case. Notice how the left side of the upper mountain in the image, and right side of the lower-right mountain are brighter. The materials that make up the darker and lighter areas are the cause for this lighting effect.

The image represents a patch of Titan’s surface 250 km (155 miles) high and 285 km (180 miles) wide, and the resolution is about 350 meters (1,150 feet) per pixel, and it was taken on December 28th, 2009.

Source: Cassini Equinox Mission, here and here.

Posted on by Nicholos Wethington

Sailing the Seas of Titan

Titan's Ligeia Mare. Credit: NASA/JPL/USGS

The first interplanetary nautical craft may be a boat to explore the methane seas of Titan. A proposed mission to Titan would explore some of its largest seas, including Ligeia Mare (pictured) or the Kraken Mare, both of which are in the northern hemisphere of the foggy moon of Saturn. The concept has been studied for over two years by scientific team led by Ellen Stofan of Proxemy Research, Inc. in Washington DC, and has recently been submitted to NASA.

The concept is under consideration by NASA to be one of the Discovery Class missions – low-cost, high-return missions, which include the MESSENGER and Kepler missions. If chosen, the Titan Mare Explorer (TiME), could launch as early as January of 2015, and would make port at Titan in June of 2023. The total proposed cost of TiME is currently estimated at $425 million. Stofan described the proposal at this year’s American Geophysical Union meeting in San Fransisco, CA.

Lakes, seas, and rivers were discovered on Titan by the Cassini spacecraft in 2005. Since then, the weather and climate patterns of the moon have been scrutinized by scientists, leading to the discovery of both fog and rain.

Of course, the proposed boat wouldn’t be the first craft to land on Titan – that distinction is held by the Huygens probe, which as part of the Cassini mission landed on Titan on January 14th, 2005 and for three hours took images and scientific data which it sent back to Earth. Huygens touched down on dry land, though it was designed to operate on either land or ocean.

Proposed instruments for the boat include a mass spectrometer, sonar, cameras and meteorology instruments. TiME would investigate the chemical composition of the seas of Titan, as well as monitor the cycle of ethane and methane on the moon (called the “methane-ologic” cycle), a process that scientists are just beginning to understand. The sonar would be used just like it is on submarines and boats here on Earth – to map the depth of the seas, as well as get an accurate image of the sea bottom.

Since the cloudy and foggy surface of Titan sees little sunlight, the boat is proposed to be powered by an Advanced Stirling Radioisotope Generator. These types of engines, called Stirling engines after the inventor, Robert Stirling, use a radioactive source such as plutonium to heat a gas in one chamber, and as it flows to a cooler chamber the flow is turned into mechanical energy with a very high rate of efficiency.

If the boat is seaworthy, it may set a precedent to give us Earthlubbers a chance at understanding the only other body in our Solar System with lakes and seas on its surface (though Europa and Enceladus are thought to have watery oceans under their crusts). By comparing the methane-ologic cycle on Titan with the Earth’s hydrologic cycle, scientists could gain a more intricate knowledge of the large-scale impact of these cycles.

Source: Physorg, Ellen Stofan’s presentation (available here in PDF)

Posted on by Fraser Cain

Largest Moon of Saturn

The largest moon of Saturn is Titan, measuring 5,150 km across. In fact, Titan is the second largest moon in the Solar System, after Jupiter’s Ganymede. Titan is so big that it’s even larger than planet Mercury, which is only 4,879 km across. And it’s much bigger than the Earth’s moon at 3,474 km.

Astronomers used to think that Titan was actually the largest moon in the Solar System, but when NASA’s Voyager spacecraft first arrived at the moon in the 1980s, they were able to make detailed observations of the moon at its atmosphere. They proved that Titan’s atmosphere extended out for dozens of kilometers, and so the physical moon itself was actually smaller than previously thought, making it smaller than Ganymede.

Titan orbits Saturn at an average distance of 1,221,870 km, completing an orbit every 15.945 days. It’s tidally locked to Saturn, so it always presents the same face to Saturn. So a day on Saturn is also the same amount of time it takes to orbit Saturn.

Titan is the only moon in the Solar System known to have a thick atmosphere. In fact, the pressure of the atmosphere on the surface of Saturn is 1.5 times greater than the atmospheric pressure here on Earth. Of course, the atmosphere of Titan is almost entirely nitrogen, and the temperature is -179° C. So it wouldn’t be a comfortable place to visit without a spacesuit.

We’ve written many articles about Titan for Universe Today. Here’s an article about seasonal changes on Titan, and here’s an article about how Titan’s haze acts like an ozone layer.

If you’d like more info on Titan, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.

We’ve also recorded an episode of Astronomy Cast just about Saturn’s moons. Listen here, Episode 61: Saturn’s Moons.

Posted on by Nancy Atkinson

Cassini Captures Sunshine Gleaming off Lake on Titan


This image shows the first flash of sunlight reflected off a lake on Saturn’s moon Titan. Credit: NASA/JPL

Dear friend,
Ah, yes. Another gorgeous day here in the northern lake district. It warmed up to about 94 K (-179 °C, or -290 °F) and we sat and enjoyed the sunshine gleaming off the liquid lakes here on Titan. Wish you were here!

Liquid lakes? Gleaming sunshine? Titan?

Yes, it’s all true. The Cassini Spacecraft has captured the first flash of sunlight reflected off a lake on Saturn’s moon Titan, confirming the presence of liquid on the part of the moon dotted with many large, lake-shaped basins.

Cassini scientists had been looking for the glint, also known as a specular reflection, since the spacecraft began orbiting Saturn in 2004. But Titan’s northern hemisphere, which has more lakes than the southern hemisphere, has been veiled in winter darkness. The sun only began to directly illuminate the northern lakes recently as it approached the equinox of August 2008, the start of spring in the northern hemisphere. Titan’s hazy atmosphere also blocked out reflections of sunlight in most wavelengths. This serendipitous image was captured on July 8, 2009, using Cassini’s visual and infrared mapping spectrometer.

This image is being presented at the fall meeting of the American Geophysical Union in San Francisco.

“This one image communicates so much about Titan — thick atmosphere, surface lakes and an otherworldliness,” said Bob Pappalardo, Cassini project scientist, based at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “It’s an unsettling combination of strangeness yet similarity to Earth. This picture is one of Cassini’s iconic images.”

Titan, Saturn’s largest moon, has captivated scientists because of its many similarities to Earth. Scientists have theorized for 20 years that Titan’s cold surface hosts seas or lakes of liquid hydrocarbons, making it the only other planetary body besides Earth believed to harbor liquid on its surface. While data from Cassini have not indicated any vast seas, they have revealed large lakes near Titan’s north and south poles.

In 2008, Cassini scientists using infrared data confirmed the presence of liquid in Ontario Lacus, the largest lake in Titan’s southern hemisphere. But they were still looking for the smoking gun to confirm liquid in the northern hemisphere, where lakes are also larger.

Katrin Stephan, of the German Aerospace Center (DLR) in Berlin, an associate member of the Cassini visual and infrared mapping spectrometer team, was processing the initial image and was the first to see the glint on July 10th.

“I was instantly excited because the glint reminded me of an image of our own planet taken from orbit around Earth, showing a reflection of sunlight on an ocean,” Stephan said. “But we also had to do more work to make sure the glint we were seeing wasn’t lightning or an erupting volcano.”

Team members at the University of Arizona, Tucson, processed the image further, and scientists were able to compare the new image to radar and near-infrared-light images acquired from 2006 to 2008.

They were able to correlate the reflection to the southern shoreline of a lake called Kraken Mare. The sprawling Kraken Mare covers about 400,000 square kilometers (150,000 square miles), an area larger than the Caspian Sea, the largest lake on Earth. It is located around 71 degrees north latitude and 337 degrees west latitude.

The finding shows that the shoreline of Kraken Mare has been stable over the last three years and that Titan has an ongoing hydrological cycle that brings liquids to the surface, said Ralf Jaumann, a visual and infrared mapping spectrometer team member who leads the scientists at the DLR who work on Cassini. Of course, in this case, the liquid in the hydrological cycle is methane rather than water, as it is on Earth.

“These results remind us how unique Titan is in the solar system,” Jaumann said. “But they also show us that liquid has a universal power to shape geological surfaces in the same way, no matter what the liquid is.”

Source: JPL

Posted on by Nicholos Wethington

Lake Asymmetry on Titan Explained

This mosaic of Cassini, SAR, ISS, and VIS images data shows that there are many more lakes in the northern regions of Titan than in the south. The eccentric orbit of Saturn is thought to have caused this imbalance. Image Credit: NASA/JPL/Caltech/University of Arizona/Cassini Imaging Team

If you’ve wanted to take a swim in a lake on Titan, don’t: they’re not lakes like we have here on Earth, composed of methane and ethane instead of water. If you have somehow evolved lungs to breathe and swim in these chemicals, you should take your beach vacation in the northern hemisphere of Titan, where you’ll find many more lakes. Data taken by the Cassini mission has shown that there are more of these methane lakes concentrated in the northern hemisphere of Saturn’s moon than in the southern hemisphere. A recent analysis of the Cassini findings by a team at Caltech has shown that the cause of this asymmetry of lakes is due to the orbit of Saturn.

Because of the eccentricity of Saturn’s orbit around the Sun, there is a constant transfer of methane in Titan’s atmosphere from the south to the north. This effect is called astronomical climate forcing, or the Milankovitch cycle, and is thought to be the cause of ice ages here on Earth. We wrote about the Milankovitch cycles and their influence on climate change just earlier today.

Scientists originally thought that the northern hemisphere was somehow differently structured than the south. Imaging data from Cassini showed that ethane and methane lakes cover 20 times more area in the northern hemisphere than lakes in the south. There also are more half-filled and dried-up lake beds in the north. For example, if the composition of the surface of Titan somehow allowed for more methane and ethane to permeate the ground more in the north, this could have explained the difference. But further data from Cassini has confirmed that there is no great difference in topography between the two hemispheres of Titan.

The seasonal differences on Titan only partially explain the asymmetry of lake formation. One year on Titan is 29.5 Earth years, so about every 15 years the seasons of Titan reverse. In other words, the winter and summer seasons could have caused the evaporation and transfer of gas to the north, where it is cooled and is currently in the form of lakes until the seasons change again.

A team led by Oded Aharonson, associate professor of planetary science at Caltech found that there was much more to the story, though. The seasonal effect could only account for changes in lake depth for each hemisphere to vary by about one meter. Titan’s lakes are hundreds of meters deep on average, and this process is too slow to explain the depth changes we see today. It became apparent that the seasonal differences were only partly contributing to this difference.

“On Titan, there are long-term climate cycles in the global movement of methane that make lakes and carve lake basins. In both cases we find a record of the process embedded in the geology,” Aharonson said in a press release.

The Milankovitch cycle on Titan is likely the cause of the lake imbalance. Summers in the north are long and relatively mild, while those in the south are shorter, but warmer. Over thousands of years, this leads to a net movement of gas towards the north, which then condenses and stays there in liquid form. During southern summer Titan is close to the sun, and during northern summer it is approximately 12% further from the Sun.

Their results appear in the advance online version of Nature Geoscience for November 29th. Animations detailing the transfer are available on Oded Aharonson’s home page.

If Cassini would have been sent to Titan 32,000 years ago, the picture would have been reversed: the south pole would have many more lakes than the north. Conversely, any Titanian deep-lake divers in a few thousand years will fare much better in the lakes of the south.

Source: Eurekalert, Oded Aharonson’s Home Page

Posted on by Nancy Atkinson

New Evidence of Seasonal Change on Titan

Stereographic projection of Synthetic Aperture Radar (SAR) imagery of Titan’s south polar region obtained between Sep. 2005 and July 2009. The Cassini radar has observed 60% of this area and 9% has repeat coverage. Areas where changes have been detected are outlined in red. Credit: Alex Hayes and Jonathan Lunine

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New images of Titan’s surface from the Cassini spacecraft show changes which are evidence of seasonal change. Objects identified earlier as liquid hydrocarbon lakes are shrinking and disappearing over the course of one to several Earth years. Scientists say seasonal temperature variations causing evaporation is the most likely cause for the changes observed. Cassini’s Synthetic Aperture Radar (SAR) repeatedly peered through Titan’s thick atmosphere, and data show that the lakes exhibit more than an order of magnitude increase in radar return and have disappearing borders between observations, suggesting surface change. These changes cannot be explained without invoking temporal variability, scientists reported at the American Astronomical Society’s Division for Planetary Sciences meeting now under way in Fajardo, Puerto Rico.

Alex Hayes, of the California Institute of Technology, and Dr. Jonathan Lunine, of the University of Rome Tor Vergata shared images of several regions on Titan’s south pole. Ontario Lacus is the largest and best characterized lake on Titan. Between July 2004 and July 2009, the shorelines of Ontario Lacus have receded, consistent with liquid evaporation and/or infiltration. In June and July 2009, the Cassini radar acquired its first high-resolution SAR images of the lake. Together with closest approach altimetry acquired in December 2008, these observations provide a unique opportunity to study Ontario.

Areas where the Cassini radar has observed transient surface liquid in Titan’s south polar region. The top two images are located near (60S, 210W) and were obtained in December 2007 and May 2009. Empty lake features are outlined in red and filled lakes, observed in the 2007 image, are outlined in cyan. The lake features disappear between observations. The bottom row consists of images near (69S, 90W) obtained in Oct. 2007 and Dec. 2008. Empty lake features observed in Dec. 2008 are outlined in red. The empty lake features in the bottom-left section of the image are dark in Oct. 2007, consistent with liquid-filled lakes. In the Dec. 2008 image the brightness of these features are indistinguishable from the empty lakes in the upper-right section of the image (which are bright in both observations), suggesting surface change.
Areas where the Cassini radar has observed transient surface liquid in Titan’s south polar region. The top two images are located near (60S, 210W) and were obtained in December 2007 and May 2009. Empty lake features are outlined in red and filled lakes, observed in the 2007 image, are outlined in cyan. The lake features disappear between observations. The bottom row consists of images near (69S, 90W) obtained in Oct. 2007 and Dec. 2008. Empty lake features observed in Dec. 2008 are outlined in red. The empty lake features in the bottom-left section of the image are dark in Oct. 2007, consistent with liquid-filled lakes. In the Dec. 2008 image the brightness of these features are indistinguishable from the empty lakes in the upper-right section of the image (which are bright in both observations), suggesting surface change.

Evaporation is the most likely scenario for observed changes on Titan’s surface. Alternative explanations include freezing, cryovolcanism, and subsurface infiltration. Freezing is unlikely due to thermodynamic reasons during the summer season in Titan’s south pole, and there are no clearly observable cryovolcanic features in the study areas. However, liquids evaporating and becoming part of a static hydrologic system is inconsistent with the observations. But, the scientists said, infiltration into a dynamic hydrologic system with a regionally varying methane/ethane table is possible.

“If evaporation is responsible, model results suggest rates are about 1m/yr, similar to current GCM estimates of methane evaporation rates for the latitudes and season in question,” Hayes and Lunine wrote in their press release. “An analysis of the receding shorelines observed in Ontario Lacus also yield evaporation rates of about 1 m/yr and support the results of the two- layer model for the smaller lakes. These observations constrain volatile fluxes and hence, the evolution of Titan’s hydrologic system.”

Source: AAS Planetary Science Division