Titan: Where's the Wet Stuff?
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.
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.
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.