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.

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

Lake Asymmetry on Titan Explained

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

New Evidence of Seasonal Change on Titan

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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

Fog on Titan? Help Review Mike Brown’s Paper

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Titan is the only place in the solar system other than the earth that appears to have large quantities of liquid sitting on the surface. Granted, conditions on Titan are quite different than on Earth. For one thing, it’s a lot colder on Titan and the liquids there are various types of hydrocarbons. “Methane is to Titan what water is to the earth,” says astronomer Mike Brown (yes, that guy, of Pluto, Eris and Makemake fame.) But now Brown and his colleagues have discovered another similarity. Titan has fog. “All of those bright sparkly reddish white patches (shown in the image here) are fog banks hanging out at the surface in Titan’s late southern summer,” Brown wrote in his blog.

Wow.

But how does this happen? Fog only usually appears when 1.) there is liquid in the atmosphere (i.e., that means it must be “humid” on Titan) and 2.) the air temperature cools drastically. But Titan’s atmosphere is extremely thick, so it cools slowly. Plus the atmosphere is already really cold and making it colder would be difficult.

“If you were to turn the sun totally off,” said Brown, “Titan’s atmosphere would still take something like 100 years to cool down. And even the coldest parts of the surface are much too warm to ever cause fog to condense.”

So what is going on there?

To get the humidity in Titan’s atmosphere, Brown said the liquid methane must be evaporating.
“Evaporating methane means it must have rained,” he wrote. “Rain means streams and pools and erosion and geology. Fog means that Titan has a currently active methane hydrological cycle doing who knows what on Titan.”

Plus, the only one way to make the fog stick around on the ground for any amount of time is have both humidity and cool air. And the only way to cool the air on Titan is have it in contact with something cold: like a pool of evaporating liquid methane.

Brown said the fog doesn’t appear to be around the just the dark areas near the south pole that likely are hydrocarbon lakes. “It looks like it might be more or less everywhere at the south pole. My guess is that the southern summer polar rainy season that we have witnessed over the past few years has deposited small pools of liquid methane all over the pole. It’s slowly evaporating back into the atmosphere where it will eventually drift to the northern pole where, I think, we can expect another stormy summer season. Stay tuned. Northern summer solstice is in 2016.”

And here comes the fun part (as if fog on Titan wasn’t fun enough!) Brown is looking for a little citizen science help. You can read the paper on this by Brown and his colleagues here. Most peer review is done by one person, and brown would like a few more eyes to see this paper to look for any flaws, and to see if their arguments make sense and are convincing.

Brown says: “I thought I would try an experiment of my own here. It goes like this: feel free to provide a review of my paper! I know this is not for everyone. Send it directly to me or comment here (at his blog). I will take serious comments as seriously as those of the official reviewer and will incorporate changes into the final version of the paper before it is published.

Please, though, serious reviewers only.

Source: Mike Brown’s Blog

Plains of Titan to be Named for “Dune” Novels

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Titan’s mysterious dark plains will be named after planets in the series of “Dune” science fiction novels by author Frank Herbert. The US Geological Survey Astrogeology Science Center announced the first plain or “planitia” given a name will be designated as Chusuk Planitia. Chusuk was a planet from the Dune series, known for its musical instruments. Chusuk Planitia on Titan is located at 5.0S, 23.5W, and in the picture here is the small, dark area next to the “C” of Chusuk.

Download a large map of Titan with the named features (pdf file).

The Cassini spacecraft has enabled us to finally see these dark plains on Titan. This moon is enveloped by an orange haze of naturally produced photochemical smog that frustratingly obscured its surface prior to Cassini’s arrival. Since 2004, the spacecraft’s observations have taken the study of this unique world into a whole new dimension.

Crescent Titan. Credit: NASA/JPL/Space Science Institute


One of Cassini’s latest images of Titan looks down on the north pole of Titan, showing night and day in the northern hemisphere of Saturn’s largest moon.

This view is centered on terrain at 49 degrees north latitude, 243 degrees west longitude. The north pole of Titan is rotated about 23 degrees to the left and it lies on the terminator above and to the left of the center of the image. Titan is 5,150 kilometers, or 3,200 miles across.

This natural color image was created by combining images taken with red, green and blue spectral filters. The images were obtained with the Cassini spacecraft wide-angle camera on June 6, 2009 at a distance of approximately 194,000 kilometers (121,000 miles) from Titan. Image scale is 11 kilometers (7 miles) per pixel.

Titan is one of the most Earth-like world we have found in our solar system. With its thick atmosphere and organic-rich chemistry, Titan resembles a frozen version of Earth, several billion years ago, before life began pumping oxygen into our atmosphere.

Cassini has revealed that Titan’s surface is shaped by rivers and lakes of liquid ethane and methane which forms clouds and occasionally rains from the sky as water does on Earth. Winds sculpt vast regions of dark, hydrocarbon-rich dunes and plains around Titan’s equator and low latitudes.

Source: USGS, Cassini website.

Hat tip to Emily Lakdawalla!