A week after Huygens' descent, the emerging picture of the surface of that smoggy world is of an arid, icy desert, where periodic storms of methane rain create transient rivers that wash sooty soil from icy highlands out to short-lived pools and lakes. The pools dry up -- perhaps sinking into a sandy soil of glass-like water ice -- and the Titanian desert waits for another methane storm.
The picture is strangely similar to the climate in Arizona, where Huygens' camera was built, a fact that was not lost on the camera's principal investigator, Martin Tomasko of the University of Arizona. "The region that we landed in is typical of arid regions on the Earth," he said in a press conference held in Paris this morning. "We see no evidence that there is any liquid in any of these features right now, but we see evidence of streams, rivers, and rainfall. There is lots of evidence for many familiar Earth-like processes, like rainfall, and erosion, but with very exotic materials on this new world."
Actually, the materials are quite familiar ones -- water, natural gas, and smog particles -- but under Titan's temperature conditions they behave in quite unfamiliar ways. Like the Arizona desert, the Huygens landing site has steep hills, arroyos or dry gullies choked with sediment, and dry flat valleys where liquid pools. But these features are made of very different materials on Titan. Where Arizona's hills are made of silicate rocks, Huygens' hills are made of "frozen hard water ice," Tomasko said. The hills are fairly steep, with heights varying some 100 meters (330 feet) over a span of a kilometer (0.6 mile), and they exert topographic control over the pattern of the drainage visible in the Huygens images.
The icy hills are much brighter than the flat plains, which are actually brighter than the bottoms of the drainage channels visible in the Huygens images. Tomasko proposed the following explanation for these differences in brightness. "We think it's evidence of rain. Photochemical smog falls out of the atmosphere, and coats the whole terrain." When it rains, the smog particles coating the surface "get preferentially washed off the tops of the ridges, so there is a concentration of these organic [smog] materials in the bottoms of these channels." The plains, where the channels emptied, is floored by material that was eroded from the icy highlands; the surface is "sandy" and "rocky," but the sand isn't the silicate sand we Earthlings are used to, it is "crushed dirty ice." The dirty outwash from the channels emptied into these ice-sandy plains and sank in, according to Tomasko's explanation.
How often does this scenario play out on Titan's surface? Tomasko was careful to point out that Huygens was only one mission at one time at one spot on Titan, so "we don't have a long enough time series to know what is typical." It wasn't raining when Huygens descended, and it wasn't wet when she landed, but whether this is the usual state for Titan's surface is an open question, one that may take another, longer-lived, and even mobile Titan surface mission to answer.
The Rain in the Plains is Mainly Methane
One statement that the Huygens science team was prepared to make with certainty is that the liquid responsible for all these Earth-like features is methane. There are several other hydrocarbon species that are present in Titan's atmosphere, including ethane and acetylene, and small differences in pressure and temperature conditions could have made some of these other substances more important on Titan's surface. But Cassini atmospheric scientist Toby Owen, of the University of Hawaii, announced that Huygens has positively identified methane as the culprit liquid on the surface of Titan. In fact, he said, there is liquid methane just below the surface at the Huygens landing site right now.
Owen explained that the abundance of methane in Titan's atmosphere was closely observed using Huygens' Gas Chromatograph Mass Spectrometer (GCMS). This instrument performed direct measurements of the composition and even isotopic ratios of the constituents of Titan's atmosphere from an altitude of 170 kilometers (106 miles) all the way down to the surface. Once again, the Huygens instrument observed a pattern that was strikingly similar to what would have been seen on the Earth.
"In the upper atmosphere, nitrogen is the dominant gas," Owen said. "But when we get into the lower atmosphere, things change," and methane becomes much more abundant. "This is just like what happens on the Earth with water vapor. Up in the stratosphere, there is very little water. The reason is that there is a very low temperature point, a 'cold trap,' that forces the water to stay down below." Switching back to Titan, Owen continued. "Because of the cold trap [in the stratosphere], the methane increases more rapidly than the nitrogen as you go into the lower atmosphere."
Huygens' long lifetime on the surface -- 72 minutes -- permitted the GCMS to make another astonishing observation. "When you come to the surface," Owen said, "you would expect everything to be stable, and the nitrogen is. However, [after the landing], the methane suddenly jumps up by about 30 percent, boom! In 3 minutes. That methane must be coming out of the ground. That's really exciting! It means there must be liquid methane right at the surface. This isn't Mars, where the stuff that has done the erosion is buried 200 meters underground, as a solid. This is a planet where the liquids are right there. It might've rained yesterday. This is a very active situation. Methane is really there in the liquid state. It's quite extraordinary."
A Surface Science Experiment
The GCMS wasn't the only intstrument that noticed things happening following the landing. The Surface Science Package (SSP) did as well. John Zarnecki, the principal investigator for the SSP, stated that his team had to "go back to the laboratory" to come up with a material whose mechanical properties matched what was observed by the SSP penetrometer as it impacted the surface. The penetrometer is essentially a spring-loaded stick extending out of the bottom of the Huygens probe, and was the first piece of the probe to hit the ground. The penetrometer met a little bit of resistance at the beginning, but then sank fairly freely into the surface; in fact, Zarnecki reported, the entire probe "probably nestled 10 to 15 centimeters [4 to 6 inches] into the top layer of the soil."
The best match that the SSP team could come up with for the soil at the Huygens landing site was a simulant made of tiny glass particles, with a layer of solid glassy material on the top. The actual Titanian soil would not be made of glass, but instead of water ice particles, which would behave quite a bit like glass at Titan's frigid surface temperature of - 179°C (-290°F).
The "nestling" of the probe into the Titanian surface, coupled with Huygens' surprisingly long life, created an unexpected additional experiment, Zarnecki explained. "Of course Huygens was still operating. It's generating heat, so there is a heated inlet [the port for the Aerosol Collector Pyrolyser instrument] in close proximity to the surface; Marty [Tomasko]'s camera has a spotlight, a 20-Watt bulb -- the whole probe is passing heat into the soil. The methane is evaporating and percolating up through this sandy-like soil, then a few minutes after the landing the mass spectrometer is getting a whiff. Our instrument is also getting tantalizing indications of some turbulence, some material coming up to the surface."
On the whole, the Huygens science team couldn't be more pleased with the performance of the probe and the happy chance that gave them such a great landing site. "We were extraordinarily lucky to come down on a boundary between the bright material and the dark material that was seen in the Cassini images," Tomasko said. Owen echoed the sentiment, saying "we were fortunate to land in the organic goo," where the GCMS could make valuable measurements even after the landing. And although it appears to have been fairly easy for the science team to understand the broad picture of the processes that shaped Huygens' landing site, there are many questions that will keep the team busy for months or even years to come.
One such question has to do with the composition of the atmosphere as observed by Huygens. Owen explained that Huygens successfully detected argon, a noble gas, in Titan's atmosphere, confirming a measurement made by the Ion and Neutral Mass Spectrometer aboard Cassini. But all of the argon was one isotope, argon-40, which is produced through the radioactive decay of the potassium-40 that was included in Titan's rocky core when it formed. Other isotopes of argon, along with other noble gases like krypton and xenon -- which should have been present in Titan's primordial atmosphere when the moon first formed -- are completely absent, down to the sensitivity of the GCMS instrument. "We find these noble gases in our atmosphere, on Venus, and on Jupiter, but we cannot find them on Titan," Owen said. "Surely there's an interesting clue there as to how Titan formed, which we'll be working on."
The scientists were careful to emphasize that although the Huygens data set is probably as rich as they could conceiveably have hoped for, it still has its limitations. In the end, Huygens lasted only three and a half hours, and she landed in only one place. There's no way that this one mission will be able to answer all the questions about the four-and-a-half-billion-year history of the entire world of Titan. "This is one single place on a very interesting world that is very different from what we know," Owen said. "I think it's important to remember the exploration of Mars; our first three spacecraft passed over the most boring place on Mars; no volcanoes and no channels. They were talking about canceling the Mars program because it was so boring. So it's important not to generalize too much" about what Huygens saw on Titan.
Indeed, the scientists are already looking ahead to possible future missions. The most important aspect of a future Titan mission, according to Huygens project scientist Jean-Pierre Lebreton, would be "mobility." The most effective way to do this on Titan is with "a floating machine," he said. (Titan, with its low gravity and thick but not too thick atmosphere, is the most flight-friendly body in the solar system.) But Lebreton laughed as he added that he had just had a phone call from the Mars Exploration Rover team, who "now are dreaming of sending their rovers on the surface of Titan. From what we have seen of the surface, this is now highly possible: we can dream of sending rovers on Titan." Perhaps Huygens' most lasting contribution to the study of Titan will be as a pathfinder, showing that sending a long-lived, mobile spacecradt to Titan's surface would be a worthwhile endeavor.