Titan: Where’s the Wet Stuff?

Categories: Feature Stories Titan

Carolyn Porco, Imaging Team Lead. Image Credit: NASA/JPL

With only limited data, and a lot of imagination, scientists have constructed a provocative model of Saturn’s moon Titan. The giant moon, they say, is a prebiotic laboratory of sorts, with an atmosphere rich in methane and other organic compounds. Down on the moon’s frozen surface, they suspect, are pools, lakes, perhaps even oceans of liquid hydrocarbons. One of the primary scientific goals of the Cassini-Huygens mission is to search for these bodies of liquid. Cassini’s initial results, however, are inconclusive.

Cassini flew within 1200 kilometers (746 miles of Titan on Tuesday and collected a variety of data. Those most relevant to the liquid hydrocarbon question were captured by the spacecraft’s Imaging Science Subsystem (ISS) cameras and its RADAR (Radio Detection and Ranging) instrument.

Titan, unlike other moons in the solar system, is covered by a thick layer of haze, 1.5 times as dense as Earth’s atmosphere. This haze is composed mainly of nitrogen, but it also contains methane and other organic compounds. It’s like a big smog blanket lain over the entire world. As a result, images taken of the moon in visible light (the wavelengths of light that humans can see) show little more than a big fuzz ball. At these wavelengths, light from the surface can’t penetrate the haze. But at some infrared wavelengths, it can.

Cassini’s cameras are designed to see not only the wavelengths that human eyes can see, but other wavelengths of light as well. The images sent back by Cassini from its first close flyby of Titan offer a view of the planet in near-infrared light, light with slightly longer wavelengths than what we can see. We can get a sense of what the world looks like in infrared by putting an infrared filter on a camera. The filter blocks visible light but lets infrared light through. This is precisely what the Cassini camera does: it places an infrared filter in front of its lens and then captures an image.

Slicing segmented maps from remote sensing on board Cassini. Image Credit: JPL/Space Science Institute

The filter allowed Cassini’s camera to see Titan’s surface, but that was only the first step in the hunt for surface liquid. To increase the camera’s chance of detecting liquid pools, Cassini was aimed on a course that would maximize its chances of catching the glare of the sun’s reflection off any surface liquid that might be there. Perhaps you’ve had the experience of seeing the sun dazzlingly reflected off the surface of the ocean, or a mountain lake. That same effect, known as "specular reflection," is what Cassini’s cameras were looking for.

Had Cassini found liquid on Titan’s surface, it would have been like hitting a home run its first time at bat. But, says Caroline Porco, who heads Cassini’s imaging team, "We have not seen a specular reflection yet in any of our data." That doesn’t mean there’s no liquid anywhere on the surface. Cassini was only able to look for specular reflections on a "thin little region" of the planet on its first close Titan flyby. As for the rest of the moon’s surface, "we can’t say one way or the other. I think the jury is still out whether or not there’s any fluids on the surface of Titan," Porco says.

Cassini also used a second instrument, its RADAR system, to look for evidence of liquid. Cassini’s RADAR can operate in several different modes. One of these modes produces images of the surface. RADAR images aren’t like the pictures captured by the ISS camera; they’re not images of the light radiating out from the surface. Rather, the RADAR system shines its own light on Titan, using a microwave antenna as its light source. It beams microwave pulses at the surface, and then measures the strength of the signal reflected back to the spacecraft. Where there is a very strong reflection, the image gets a white pixel; where there’s no reflection at all, the image is black; various shades of gray represent intermediate signal strengths.

Sounds simple enough. Until you realize that multiple factors can affect the strength of the reflection at a particular point. One factor is slope. If the surface that catches the microwave beam is sloped toward the spacecraft, a strong signal comes back. If it’s pointed away, nothing. Roughness also matters. Rough surfaces scatter the microwaves, reducing the reflection somewhat. But a completely smooth horizontal surface (a body of liquid, for example) reflects all the light away in the opposite direction, so it appears black in the resulting image. The third factor depends on the composition of the surface material. Rocks produce a relatively strong reflection. Organic goo – and scientists expect to find lots of organic goo on Titan – produces a relatively weak reflection.

Cassini map of Titan’s dark-light contrast shows approximately 20 percent differences in albedo. Image Credit: JPL/Space Science Institute

"It’s a combination of these three factors, the slope, the roughness or smoothness of the surface, and the property of that surface, that we need to untangle, to be able to determine what that surface is really made of," says Greg Elachi, Director of NASA’s Jet Propulsion Laboratory and team leader for Cassini’s RADAR instrument. Sorting out these factors is not a task that is done easily or quickly. The initial results are hopeful, though. In the images produced by Cassini’s RADAR system, there are some regions that are very dark. One area about the size of Lake Tahoe is particularly promising. It could be just what scientists are looking for: a hydrocarbon lake. Then again, it might just be heavily coated with goo. Or slanted in the wrong direction.

It’s also worth noting that the RADAR imaging technique won’t always reveal bodies of liquid, even if they’re there. It depends on the weather. If it’s a very windy day on Titan, the winds will kick up a lot of waves. Otherwise glassy-smooth liquid surfaces will be rough and choppy. The RADAR system will see gray instead of black. So Cassini could fly right over a lake, image it with RADAR, and never even know it’s there.

The take-home message from the initial RADAR results is what has become a mantra of sorts for the Cassini science team: "We don’t have answers yet. It’s going to take a while." For one thing, the area imaged by Cassini’s RADAR during its first close flyby of Titan is only about 1 percent of the moon’s surface. Fox News might be willing to call an election with only 1 percent of the vote counted, but planetary scientists tend to proceed with a bit more caution. In addition, Cassini can’t do RADAR imaging and take ISS visible and infrared images of the same region of the moon’s surface during the same flyby. The instruments are on different sides of the spacecraft; they can’t be operated simultaneously. So there is no overlap between the ISS and RADAR data acquired to date. When scientists have both types of data for the same area (along with other types of data that Cassini can acquire), they’ll be in a much stronger position to pull together a comprehensive interpretation.

So is the surface of Titan dotted with hydrocarbon lakes? Perhaps the question is best answered by the inscrutable message from Mattel’s Magic 8-ball: Hazy now. Try again later.