Recent radar observations of Saturn’s moon Titan have produced the first direct evidence that the second largest moon in the solar system may be hiding pools of liquid hydrocarbons underneath its smoggy atmosphere.
On October 2, Cornell University researcher Donald Campbell and his colleagues reported in Science Express, the online journal of Science magazine, the results of 25 observations that he and his collaborators have made of Titan using the newly upgraded Arecibo radio observatory in Puerto Rico. In about 18 of the 25 observations, radar echoes came back from a surface with physical properties indicating the presence of liquid hydrocarbons.
Titan, which is about half again as large as Earth’s moon, was thought to be the largest moon in the solar system until Voyager mission scientists discovered that its apparent size was due partially to a thick atmosphere of nitrogen, argon, and methane. But that same atmosphere has prevented scientists from learning anything about the surface of Titan through optical observations.
The atmosphere’s intriguing chemistry has invited speculation on what that surface might contain. One intriguing possibility, suggested in the 1980s, was that photochemical reactions in the atmosphere caused liquid ethane to rain out of the atmosphere, possibly creating an ethane ocean covering the whole surface as much as a kilometer deep.
Sadly, “early radar observations showed that that was not the case,” said Campbell. “They indicated that Titan had a heterogeneous surface—in other words, it wasn’t totally uniform. But that’s all they could say.” That conclusion was reinforced with Hubble Space Telescope near-infrared observations, which revealed a large, bright surface region, possibly covered with water ice. We know that there are surface features on Titan, but not what those features are.
Upgrades completed on the Arecibo observatory in 1997 increased its sensitivity for studies of the solar system by a factor of 20 and allowed Campbell and his collaborators to examine areas of the surface of Titan only 200 to 300 kilometers across. His team used the Arecibo telescope to send a radio signal at Titan, and then listened for the echo at both Arecibo and the National Radio Astronomy Observatory in Green Bank, Virginia.
They studied two characteristics of the reflected signal: the strength of the reflection and how the polarization of the signal was changed by its interaction with the Titanian surface. Surfaces that are rough or jagged at the scale of the radio waves (13 cm) would scatter the reflected radio waves, while surfaces that are smooth would reflect radio waves without scattering. Furthermore, smooth, mirrorlike surfaces will show a strong reflection only when the surface is exactly perpendicular to the Arecibo radar beam (imagine the glinting reflection of a signaling mirror). This kind of glinting behavior is known as a “specular” reflection.
Under these conditions, Campbell said, “If there are open lakes on the surface of Titan, you’d expect to see specular glints. If we had not seen any specular glints, that would pretty much have ruled out the presence of liquid surfaces on Titan, at least in the areas that we looked.” In fact, they observed specular reflections from Titan in about 18 of their 25 observations. “The properties of the echo are consistent with hydrocarbon lakes,” Campbell said.
There is a possible alternative: “These could be reflections from ultra-smooth icy surfaces, but part of that surface would have to be roughened up because the amount of signal that you would get back from an ultra-smooth icy surface would be much larger than we measured. You’d have to roughen some of the surface up to get rid of some of the signal.” But no reflections of this kind have been observed from any other surface in the solar system, not even from the icy satellites of Jupiter. Titan is showing us a new kind of surface.
Unfortunately, even Arecibo’s new acuity is not sharp enough to tell for sure what the surface of Titan really holds. “Even if there wasn’t any atmosphere or haze, the ability of Earth-based telescopes to get resolution on the surface of Titan is extremely poor,” Campbell said. We can put something like 20 or 30 pixels across the surface and that’s it. So who knows? When we get a higher resolution view from the Cassini orbiter, these spots could break up into more, smaller features.”
Campbell is looking forward to seeing his group’s conclusions from recent observations of Titan tested with observations from the Cassini orbiter. “There’s been a tremendous amount of information about Titan garnered over the last few years. We’re so close with Cassini now, and everybody’s just waiting to see what’s really there.
“The Huygens Probe will be parachuting down in January of 2005. It’ll get a very good sampling of the atmosphere as it goes down, and a very good look at one small area on the surface of Titan.” Campbell is not a member of the Cassini team, so he will have to wait with the rest of us to find out if his conclusions are true.