Raw Cassini image of Titan and Enceladus backdropped by Saturn's rings. Image Credit: NASA/JPL/Space Science Institute
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An incredible set of images are beaming back from the Cassini spacecraft as it orbits Saturn, snapping away at the sights. The moons Titan and Enceladus snuggling up together in front of Saturn’s rings creates an amazing view, especially when they are all lined up together. These were taken on May 21, 2011. I’ve posted some of what I think are the most amazing, below, or you can see the whole set at the Cassini raw images page. When the Cassini imaging team gets a chance to process (and colorize) these, they’ll likely go down as some of the most representative images from the entire mission.
Titan snuggles up to Saturn and its rings. Image credit: NASA/JPL/Space Science Institute
Titan, Enceladus and an onside view of Saturn's rings. Credit: NASA/JPL/Space Science Institute
Ancient comets may have created Titan's nitrogen-rich atmosphere
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Titan is a fascinating world to planetary scientists. Although it’s a moon of Saturn it boasts an opaque atmosphere ten times thicker than Earth’s and a hydrologic cycle similar to our own – except with frigid liquid methane as the key component instead of water. Titan has even been called a living model of early Earth, even insofar as containing large amounts of nitrogen in its atmosphere much like our own. Scientists have wondered at the source of Titan’s nitrogen-rich atmosphere, and now a team at the University of Tokyo has offered up an intriguing answer: it may have come from comets.
Traditional models have assumed that Titan’s atmosphere was created by volcanic activity or the effect of solar UV radiation. But these rely on Titan having been much warmer in the past than it is now…a scenario that Cassini mission scientists don’t think is the case.
New research suggests that comet impacts during a period called the Late Heavy Bombardment – a time nearly 4 billion years ago when collisions by large bodies such as comets and asteroids were occurring regularly among worlds in our solar system – may have generated Titan’s nitrogen atmosphere. By firing lasers into ammonia-and-water-ice material similar to what would have been found on primordial Titan, researchers saw that nitrogen was a typical result. Over the millennia these impacts could have created enough nitrogen to cover the moon in a dense haze, forming the thick atmosphere we see today.
“We propose that Titan’s nitrogen atmosphere formed after accretion, by the conversion from ammonia that was already present on Titan during the period of late heavy bombardment about four billion years ago.”
– Yasuhito Sekine et al., University of Tokyo, Japan
This model, if true, would also mean that the source of Titan’s nitrogen would be different than that of other outer worlds, like Pluto, and even inner planets like our own.
Top image is a combination of a color-composite of Titan made from raw Cassini data taken on October 12, 2010 and a recolored infrared image of the comet Siding Spring, taken by NASA’s WISE observatory on January 10, 2010. The background stars were also taken by the Cassini orbiter. NASA / JPL / SSI and Caltech/UCLA. Edited by J. Major.
Note: the image at top is not scientifically accurate…the comet’s tail would be, based on the lighting of Titan, pointing more to the ten o’clock position as well as forward toward the viewer’s left shoulder. This would make it ‘look’ as if it were going the opposite direction though, away from Titan, and so I went with the more immediately decipherable version seen here. To see a more “realistic” version, click here.
Titan holds yet more secrets, far beneath its haze...
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Saturn’s moon Titan just keeps throwing surprises at us. A multi-layered atmosphere thicker than our own? Check. A hydrologic cycle that relies on methane as the operating liquid? Check. Rivers, streams and lakes filled with this same liquid? Check, check and check. And now, scientists are suspecting that Titan may have yet another surprise: a subsurface ocean.
Observations of Titan’s rotation and orbit, carried out by researchers at the Royal Observatory of Belgium using Cassini data, point at an unusual rotational inertia; that is, its resistance to changes in its motion, also known as moment of inertia or angular mass. Basically Titan moves in a way that is not indicative of a solid body of its previously assumed density and mass. Rather, its motion – both around its own axis and in its tidally-locked orbit around Saturn – are more in line with an object that isn’t uniformly solid.
Titan's thick clouds hide its surface well. NASA / JPL / SSI / J.Major
According to the math, Titan may very well be filled with liquid!
Or, at least, have a liquid layer of considerable depth beneath its surface. How far below the surface, how deep and exactly what kind of liquid are all speculative at this point…it’s suggested that it may be a subsurface ocean of yet more methane. This would help answer the question of where Titan gets all of its methane in the first place; methane, – a.k.a. natural gas – is a compound that breaks down quickly in sunlight. In fact, the high-level haze that surrounds the moon like a wispy blue shell is made up of this broken-down methane. So if this stuff is raining down onto the surface in giant, frigid drops and filling streams and lakes, but is still being broken down by ultraviolet light from the Sun to enshroud the entire moon (Titan is BIG, remember…at 5,150 km – 3,200 miles – wide, it’s over a third the size of Earth!) then there has to be somewhere that this methane is coming from.
If these calculations are right, it may be coming from underground.
We propose a new Cassini state model for Titan in which we assume the presence of a liquid water ocean beneath an ice shell… with the new model, we find a closer agreement between the moment of inertia and the rotation state than for the solid case, strengthening the possibility that Titan has a subsurface ocean.
– Rose-Marie Baland et al.
Of course in order for this hypothesis to be proven many more numbers are going to have to be crunched and more data reviewed. And more possibilities considered, too; Titan’s orbital irregularities may in fact be the result of external forces, such as a close pass by a comet or other large body. Still, there’s something to be investigated here and you can bet there’ll be no shortage of attention on a problem as intriguing as this!
Titan may soon be joining the short list of moons speculated to possess subsurface oceans, alongside Jupiter’s Europa and Ganymede and sister Saturnian satellite Enceladus…and who knows how many others?
NASA’s Cassini spacecraft chronicles the change of seasons as it captures clouds concentrated near the equator of Saturn’s largest moon, Titan, on 18 October 2010. Credit: NASA/JPL/Space Science Institute
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Titan’s skies dump methane rain on the bizarre moon a quarter of the year, which collects in northern methane lakes and maintains gullies and washes once presumed to have been sculpted in a wetter age.
Elizabeth Turtle from the Johns Hopkins University Applied Physics Laboratory (APL) is lead author on the new Science paper reporting that Cassini seems to have caught a storm in action last year: “We report the detection by Cassini’s Imaging Science Subsystem of a large low-latitude cloud system early in Titan’s northern spring and extensive surface changes,” write Turtle and her co-authors in the new paper, which appears today. “The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan’s low latitudes are carved by seasonal precipitation.”
While Saturn’s largest moon has methane lakes at high latitudes, its equatorial regions are mostly arid, with vast expanses of dunes. Researchers first observed dry, riverbed-like channels in these regions in Huygens probe images, but generally believed them to be remnants of a past wetter climate.
Turtle and her colleagues observed sudden decreases in the brightness of the surface near Titan’s equator after a cloud outburst. The authors consider several possible explanations for these changes, including wind storms and volcanism, but they conclude that rainfall from a large methane storm over the region is most likely responsible for the darkening they observed. The surface changes they noted after the storm spanned more than 500,000 square kilometers, about the size of California.
In a related Perspectives piece, Tetsuya Tokan from the Universität zu Köln in Köln, Germany wrote that Titan’s precipitation climatology “is clearly different from that of Earth, and exotic climate zones unknown in Köppen’s classification may exist.” He was referring to a widely-used climate classification system coined by Wladimir Köppen in 1884.
Tokan writes that while Earth’s global circulation patterns concentrate precipitation in rainy belts along the equatorial regions, Titan’s “convergence zone” appears migrate north and south over time, distributing precipitation more equitably across the moon.
Source: “Rapid and Extensive Surface Changes Near Titan’s Equator: Evidence of April Showers,” by Elizabeth Turtle et al. and the related Perspectives piece, “Precipitation Climatology on Titan,” by Tetsuya Tokan. Both articles appear today in the journal Science.
Saturn, its rings and moons small to large in this Cassini image. Credit: NASA/JPL/Space Science Institute
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This latest offering from the Cassini spacecraft shows a wide-angle view of Saturn, its rings, and a sampling of the planet’s moons in varying sizes. Saturn’s largest moon, Titan, is in the center of the image, with the smaller moon Enceladus on the far right, while appearing just below the rings on the far left beyond the thin F ring is teeny-tiny Pandora. Oh, to have this view out your spacecraft window as you approach the ringed-world for a flyby!
How do the moons shown here vary in size? Titan is 5,150 kilometers, or 3,200 miles, across. Enceladus is 504 kilometers, or 313 miles across, while Pandora is 81 kilometers, or 50 miles across. This view looks toward anti-Saturn side of Titan and toward the northern, sunlit side of the rings from just above the ringplane.
The image was taken with the Cassini spacecraft wide-angle camera on Jan. 15, 2011, from a distance of about 844,000 kilometers (524,000 miles) from Titan. Image scale is 50 kilometers (31 miles) per pixel.
Titan peeks from behind two of Saturn's rings. Another small moon Epimetheus, appears just above the rings. Credit: NASA/JPL/Space Science Institute
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It seems Titan is getting more Earth-like all the time. There are lakes, rainfall (never mind that any liquids on Titan are frigid hydrocarbons), dust storms, lightning and all sorts of other activity going on it the atmosphere, along with clouds. And now, not just any clouds but cirrus clouds, very similar to what we have on Earth: thin, wispy clouds of ice particles high in the atmosphere. A team of researchers at NASA’s Goddard Space Flight Center say that unlike Titan’s brownish haze, the ice clouds are pearly white.
“This is the first time we have been able to get details about these clouds,” said Robert Samuelson, an emeritus scientist at Goddard and the co-author of a new paper published in the journal Icarus. “Previously, we had a lot of information about the gases in Titan’s atmosphere but not much about the [high-altitude] clouds.”
Using the Composite Infrared Spectrometer (CIRS) on NASA’s Cassini spacecraft scientists can get a “weather report” of sorts. Previously, scientists have found that Titan’s intriguing atmosphere has a one-way cycle that delivers hydrocarbons and other organic compounds to the ground as precipitation.
Those compounds don’t evaporate to replenish the atmosphere, but somehow the supply has not run out.
Additionally, puffy methane and ethane clouds had been found before by ground-based observers and in images taken by Cassini. But these new clouds are much thinner and located higher in the atmosphere.
“They are very tenuous and very easy to miss,” said Carrie Anderson, the paper’s lead author. “The only earlier hints that they existed were faint glimpses that NASA’s Voyager 1 spacecraft caught as it flew by Titan in 1980.”
So what are these cirrus clouds made of?
This mosaic of Titan was created from the first flyby of the moon by Cassini in 2004. Credit: NASA/JPL/SS
More than a half-dozen hydrocarbons have been identified in gas form in Titan’s atmosphere, but many scientists feel there are probably many more that haven’t yet been identified.
The clouds on Titan can’t be made from water because of the planet’s extreme cold. “If Titan has any water on the surface, it would be solid as a rock,” said Goddard’s Michael Flasar, the Principal Investigator for CIRS.
Instead, the key ingredient is likely methane. High in the atmosphere, some of the methane breaks up and reforms into ethane and other hydrocarbons, or combines with nitrogen to make materials called nitriles. Any of these compounds can probably form clouds if enough accumulates in a sufficiently cold area.
To find these cloud, the team focuses on the observations made when CIRS is positioned to peer into the atmosphere at an angle, grazing the edge of Titan. This path through the atmosphere is longer than the one when the spacecraft looks straight down at the surface. Planetary scientists call this “viewing on the limb,” and it raises the odds of encountering enough molecules of interest to yield a strong signal.
So, when the researchers look at the data, they can separate the telltale signatures of ice clouds from the other aerosols in the atmosphere. “These beautiful, beautiful ice clouds are optically thin, and they’re diffuse,” said Anderson. “But we were able to pick up on them because of the long path lengths of the observations.”
A close look at Enceladus, with Saturn's rings in the background. Credit: NASA/JPL/Space Science Institute
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The Cassini spacecraft has taken a some recent images of two of Saturn’s most notorious moons, where in both images the planet’s rings serve as a backdrop. Above, Enceladus stands out with its cratered surface, but Cassini’s camera also catches a glimpse of the planet’s rings in the background. Geologically young terrain in the middle latitudes of the moon shifts to older, cratered terrain in the northern latitudes.
The image was taken during the spacecraft’s flyby of Enceladus on Nov. 30, 2010, in visible with Cassini’s spacecraft narrow-angle camera, from a distance of approximately 46,000 kilometers (29,000 miles) from Enceladus. Image scale is 276 meters (906 feet) per pixel.
Below is a ‘raw’ view of Titan, and the rings.
A closeup of Titan rings, in front of Saturn's rings. Credit: NASA/JPL/Space Science Institute.
This close-up view of Titan was taken on January 15, 2011, shows the cloudy atmosphere of the moon, with the rings in the background. Cassini was about 839,213 kilometers away from Titan.
Titan and Tethys line up for a portrait of 'sibling' moons. Credit: NASA/JPL/Space Science Institute
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This image reminds me of when I was young, my parents would line me and my siblings up for pictures, oldest and tallest in the back and youngest and smallest in the front. Here, the Cassini spacecraft sees two of Saturn’s moons lined up for a family photo, showing the hazy orb of giant Titan beyond smaller Tethys.
On Tethys, the large Ithaca Chasma can be seen running roughly north-south for more than 1,000 kilometers (620 miles). Titan’s hazy atmosphere covers up the interesting surface below.
This view looks toward the Saturn-facing sides of Titan (5,150 kilometers, or 3,200 miles across) and Tethys (1,062 kilometers, or 660 miles across).
A potential cryovolcano – or ice volcano -- region on Saturn's moon Titan is shown in this image from NASA's Cassini spacecraft. This radar swath was laid on top of an image taken by Cassini's visual and infrared mapping spectrometer. Sotra Facula is located around 15 degrees south latitude, 40 degrees west longitude. Image credit: NASA/JPL-Caltech/USGS/University of Arizona
While icy cryovolcanoes on Titan have been theorized in the past, scientists didn’t have any hard evidence for them. But now, researchers from the Cassini mission have found proof that jumped out of their data in the form of 3-D mountain peaks. Using a new three-dimensional mapping technique, the team was able to create a realistic 3-D flyover of a region on Titan, above, where volcanic-like mountains appear to be lined up in a mountain range-type formation, complete with calderas and material flows. If cryovolcanoes do exist on Titan, they would potentially answer the question of why Titan has so much methane in its atmosphere.
“A combination of features makes us think we’ve found the best evidence so far for icy volcanoes on the moon Titan,” said Randy Kirk, a geophysicist with the U.S. Geological Survey and a member of the Cassini team. “Sotra Facular is a classic volcano with a crater on it and lava flows coming out of it.
Kirk presented the team’s findings at the American Geophysical Union conference in San Francisco.
Rather than erupting hot, molten rock, it is theorized that the cryovolcanoes of Titan would erupt volatiles such as water, methane, and ammonia. “A volcano is a place where material on the inside of a planetary body has gotten warm enough that it can erupt to the surface,” Kirk said. “When a body is made of ice and not rock, you get a cryovolcano.”
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Scientists have suspected cryovolcanoes might populate Titan, and the Cassini mission has collected data on several previous passes of the moon that suggest their existence. Kirk shared radar imagery from early in the Cassini spacecraft’s mission that showed Sotra Facular as bright spot on Titan’s equatorial sand sea, as seen above.
“There were thousands of places where bright ground peeks out of the dark places,” Kirk said, “and in particular we noticed a rose-type round feature, which we called The Rose, with a flows coming from it and we wondered if it was a volcano.”
Combining new data from Cassini’s radar instrument and the visual and infrared mapping spectrometer, the team was able to create the 3-D flyover movie, which shows two peaks more than 1,000 meters (3,000 feet) tall and multiple craters as deep as 1,500 meters (5,000 feet). It also shows finger-like flows. All of these are land features that indicate cryovolcanism.
“We were excited and quite happy when we saw the video,” Kirk told Universe Today at a press briefing on Tuesday. “There was a long time lapse between seeing the image of The Rose, and everybody was wondering if it was a volcano. When we finally managed to create the three dimensional from the topographic maps, I was shocked, and I made it from our own data set! I showed the video to team and they shared that reaction.”
Kirk said the flows were quite thin – thinner than anticipated at less than 100 meters (300 feet) thick — but there were more volcanoes in the same field as Sotra Facula than what the team expected.
In the video, mountains appear, with a huge pit like a volcanic calderas –“ a big bite out of the mountain,” as Kirk described it.
The topography in the video has been vertically exaggerated by a factor of 10. The false color in the initial frames show different compositions of surface material, as detected by Cassini’s visual and infrared mapping spectrometer. In this color scheme, dunes tend to look relatively brown-blue. Blue suggests the presence of some exposed ice. Scientists think the bright areas have an organic coating that hides the ice and is different and lighter than the dunes. The finger-like flows appear bright yellowish-white, like the mountain and caldera. The second set of colors shows elevation, with blue being lowest and yellow and white being the highest. Here, the dunes appear blue because they tend to occupy low areas.
This photograph shows a fissure and a row of craters in Laki, a volcanic region in the south of Iceland. Kirk said the region on Titan where potential cryovolcanoes have been found could look very similar to this landscape. Image credit: R. M. C. Lopes
Cryovolcanism could release methane from Titan’s interior, which explains Titan’s seemingly continuous supply of fresh methane in its atmosphere. Without replenishment, scientists say, Titan’s original atmospheric methane should have been exhausted long ago.
“One of mysteries on Titan is the source of methane,” said Linda Spilker, Cassini project scientist, “so cryovoclanoes offer the perfect opportunity to get methane from interior into the atmosphere of Titan.”
Kirk and his team calculated that a Sotra-sized volcanic eruption every 1,000 years would maintain the current level of methane in Titan’s atmosphere.
Jeff Kargel from the University of Arizona, who provided an independent assessment of the potential of cryovolcanoes on Titan, said that no one yet knows what the flows are made of from these volcanoes, but — providing a tantalizing visualization — said an ammonia-water cryolava with methane and carbon dioxide would make frothy, pumice-like deposits on Titan.
Kargel also added that the strongest evidence for cryovolcanoes on Titan is the topographical data that Kirk and his team have provided. “The strong evidence here is the is juxtaposition of the high and low topography in this region on Titan. There are very few tectonic activities that can produce comparable conic mountain like this.”
For more imagery from Kirk’s presentation and other presentations about the Saturn system at AGU, see this NASA webpage.
Titan's thick haze. Image: NASA/JPL/Space Science Institute.
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Saturn’s moon Titan has long been thought to be an analog of early Earth, and a recent experiment demonstrates that amino acids and nucleotide bases — which are the basic building blocks of life on Earth – could very easily be under production in Titan’s hazy atmosphere. “Our intention was to figure out what goes on in Titan’s atmosphere using high resolution mass spectrometry,” said Sarah Horst, from the University of Arizona, a member of an international team conducting the research. “We found there could be a high number of some incredibly complex molecules being created.”
Two recent exciting discoveries led the team to try and find out more about Titan’s atmosphere: first, the discovery of high energy oxygen ions flowing into Titan’s atmosphere, and second, that there are high heavy molecular ions in the atmosphere – neither of which were expected.
“When you put two discoveries together, that leads us to possibility that oxygen can get incorporated into these large molecules and in turn, that may be incorporated into life,” Horst said in press briefing at the American Astronomical Society’s Division of Planetary Sciences meeting this week.
The intense radiation that hits the top of Titan’s thick atmosphere is capable of breaking apart even very stable molecules. The international team wanted to understand what happens as these molecules are broken apart in the atmosphere.
Working with a team in France, Horst, a graduate student, and her professor Roger Yelle, filled a reaction chamber with Titan-like atmosphere, (a cold plasma consisting of nitrogen, methane and carbon monoxide), and infused radio-frequency radiation as an energy source.
“What happens is that aerosols form in levitation — they float while forming — so this probably is very representative of Titan’s atmosphere,” Horst said. “We end up with really cool looking aerosols that have very similar sizes to aerosols that are inferred in Titan’s atmosphere.”
The molecules discovered in the aerosols include the five nucleotide bases used by life on Earth (cytosine, adenine, thymine, guanine and uracil) and the two smallest amino acids, glycine and alanine.
“The experiment showed that Titan’s atmosphere is capable of producing extremely complex molecules and has the potential for producing molecules that are important for life on Earth,” Horst said, but tempered her statement by adding, “however, this doesn’t mean there is life on Titan.”
She said if there were life on Titan, mostly likely it would use molecules that life on Earth would not use, as due to lack of liquid water, life would be completely different.
“But this tells that it is possible to make very complex molecules in the outer parts of an atmosphere,” Horst said. “We don’t need liquid water, we don’t need a surface.”
This also provides another option to how life may have started on Earth. The two main theories for how life began on Earth is that it was brought here by comets or asteroids or that it formed from a primordial soup zapped to life from lightning. But it may have formed from a primordial haze high in Earth’s atmosphere.
“This helps us to understand what processes began life on Earth and what could be happening on other exoplanets in the galaxy,” Horst said.
