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Planetary News: Cassini-Huygens (2004)

No Lakes on Titan? Too Soon to Tell

Click to enlarge > Credit: NASA / JPL / Space Science Institute

For decades humans have dreamt that Titan’s hazy atmosphere hides oceans (or at least lakes) of liquid ethane or methane. And the Huygens probe is designed to float, should it land in liquid. Recent reports have called this dream into question, but Cassini’s first close look at Titan has not ruled out the possibility that hydrocarbon lakes could exist on the surface.

Cassini-Huygens passed within 340,000 kilometers (210,000 miles) of Titan on July 2, 2004. During the flyby, investigators on Cassini’s Imaging Science Subsystem (the spacecraft’s main camera) and the Visual and Infrared Mapping Spectrometer (VIMS) were searching for a telltale sign of a liquid surface: a specular reflection. A specular reflection, or glint, occurs when a smooth, mirror-like surface is oriented so that it reflects sunlight directly at an observer. (Imagine the glinting reflection of a signaling mirror.)

For a spherical body like the Earth or Titan, there will always be one point on the sunlit part of the surface such that, if that point is covered by a smooth, mirror-like surface, spaceborne instruments should be able to see such a glint. Earth-orbiting spacecraft frequently see specular reflections from the Earth’s oceans. But throughout Cassini’s observations last week, no such glint was observed at any point on Titan.

Does this mean that there are no seas or oceans on Titan? “It’s too early to say that there is no liquid,” says Cassini imaging team member and University of Arizona researcher Elizabeth Turtle. “Titan is a completely alien surface. And understanding the surface is complicated by the fact that there’s an atmosphere in between [Cassini and the surface]. I think we need to spend more time trying to understand what we’re seeing.”

Turtle explained that since specular reflections were observed by the Earth-based Arecibo radio observatory, “It’s a big surprise that we didn’t see the specular reflection.” But there could be several reasons that Cassini didn’t see the reflection apart from a lack of seas or oceans. “It’s possible that the atmosphere is affecting how we can see the specular point. It may be that the atmosphere is decreasing the amount of signal. Or it may be that some of the features that we are seeing are low clouds. We’re just not real sure what we’re seeing on the surface yet.”

One concern that the imaging team had prior to this encounter was that the atmosphere would blur the camera’s view of the surface so much that no feature smaller than tens of kilometers, or perhaps even a hundred kilometers, across could be resolved. But this recent encounter has assuaged those fears. “We’re thrilled,” Turtle says. “We don’t know what we’re seeing, but we’re seeing things.”

The spacecraft approached close enough that one pixel in a Narrow Angle Camera image spans about 2 kilometers (1.2 miles) on the surface of Titan. In the absence of an atmosphere, the smallest objects visible would be 2 to 3 pixels across, about 5 kilometers (3 miles). Titan’s thick atmosphere does blur the images slightly so that the smallest objects visible are 10 kilometers (6 miles) across. That’s sharp enough to show plenty of fine structure.

One of the most intriguing things in the camera images is that some features appear to have very sharp boundaries, while other features have very diffuse boundaries. What this means about the makeup of Titan’s surface is unclear. The diffuse boundaries could be an inherent property of Titan’s surface or they could represent “localized atmospheric effects,” according to Turtle.

Localized atmospheric effects are certainly visible near Titan’s South Pole, where a series of images captured over 5 hours shows methane clouds forming, moving, and dissipating. Could some of the diffuse, gray regions in the middle of the image also represent clouds? “We’ve looked at some of the other images, and I haven’t seen any obvious changes,” says Turtle. “We can’t say for sure. But when we see Titan again 6 months from now, we’ll be able to look for changes. Then we’ll have to see whether we can distinguish between atmospheric changes and changes that may have taken place on the surface.”

Click to enlarge > Titan's south polar clouds Four views of Titan's south pole captured on July 2, 2004 show a small field of white clouds, which clearly change over the few hours that Cassini observed them.Source Credit: NASA / JPL / Space Science Institute

The camera is not Cassini’s only instrument to examine Titan; VIMS was also able to use the close encounter to advantage. At infrared wavelengths, different types of rock, ice, and organic materials reflect light in widely varying ways, giving them different “colors” to the VIMS. Peering through the haze at relatively long wavelengths in the near- and mid-infrared part of the spectrum, VIMS mapped out the locations of ice-rich and hydrocarbon-rich surface material. In the VIMS images, ice appears dark, while hydrocarbon-rich material appears brighter. Icy areas must be solid, as water ice is as hard as rock at Titan’s frigid temperatures; but in regions rich in hydrocarbons, complex organic reactions may be taking place The brightest areas are the methane clouds at the South Pole.

Like the cameras, VIMS did not observe any telltale signs of lakes on the surface. But Titan is such an odd place, it will likely take several flybys of the moon, and the combined efforts of all of Cassini’s science teams, to begin to answer questions about what the surface is made of.

Titan at 2.0 microns At a wavelength of 2.0 microns, VIMS reveals albedo (brightness and darkness) patterns on the surface of Titan. Darker areas probably represent water ice, while brighter regions may show simple hydrocarbons. Credit: NASA / JPL / University of Arizona

Titan at 2.8 microns At a wavelength of 2.8 microns, VIMS' view of Titan is dark everywhere, suggesting a surface composition of water ice and hydrocarbons. Credit: NASA / JPL / University of Arizona

Titan at 5.0 microns VIMS' clearest view of Titan's surface is at a wavelength of 5.0 microns, where methane is nearly transparent. But there is so little solar radiation at this wavelength that the strength of the signal from Titan's surface is weak, making the image appear noisy. Credit: NASA / JPL / University of Arizona

Color view of Titan from VIMS Combining three images taken at wavelengths of 2.0, 2.8, and 5.0 microns gives VIMS a colorful map of Titan's surface. Credit: NASA / JPL / University of Arizona

The different-composition areas visible in the VIMS images outline several features on the surface, including two linear bands and what appears to be a large crater (near the top of the images). What those linear bands could be is anybody’s guess.

“What we don’t see is circles everywhere,” Turtle says, pointing out that whatever Titan’s surface looks like, it is not saturated with craters. “So there are clearly other surface processes at work. We can’t say what they are. It could be tectonic -- and personally I would be surprised if it wasn’t. But there could also be erosional processes going on,” where windblown dust, or liquids flowing across the surface, could change the surface composition or even etch topographic features.

Only time will tell -- but there is plenty of time to go. “This mission is demonstrating what you can get out of the combined efforts of complementary instruments,” Turtle says. “By combining our and their observations, we’re going to learn a lot more about the surface. I don’t think any one instrument alone can do it. We’re all in this together.”

This was just the first of Cassini’s views of Titan, and it was a distant one at that. Cassini will get two more chances to see Titan this year on October 26 and December 13. These encounters will be close enough for Cassini to use her radar instrument to penetrate the clouds and map Titan’s surface, like the Magellan spacecraft did at Venus in the early 1990s. Then, on December 25, Cassini will release the Huygens probe for her cruise toward Titan. Huygens will parachute through Titan’s atmosphere on January 14, 2005, transmitting images and other data all the way down.

It’s going to be an exciting ride.