Huygens' Discovery of Earthlike Terrain on Titan Seen as Boost for Exploration
Aviation Week & Space Technology
01/24/2005, page 24
Frank Morring, Jr. and Michael A. Taverna
Darmstadt, Germany
Michael A. Dornheim
Pasadena, Calif.
Discovery of Earthlike terrain on Titan by Europe's Huygens probe seen as boost for exploration
Rover Territory
The next Titan landing will likely include a rover, now that Europe's Huygens probe to Saturn's largest moon has delivered enticing images and data of a landscape that looks a lot like Earth--except with hydrocarbon rain, marshy methane lakebeds and granite-like ice canyons.
When the probe touched down Jan. 14 on soft but solid ground at the edge of what scientists initially believed is at least a seasonal body of standing liquid methane, it answered a question that has persisted since Dutch astronomer Christiaan Huygens spotted Titan with his homemade telescope in 1655.
"We have always been talking about the need for mobility in the atmosphere of Titan, but we did not want to talk about mobility on the surface because we had no idea what [would move] on the surface," says Jean-Pierre Lebreton, the European Space Agency Huygens mission manager and project scientist. "But now it's clear that rovers on the surface of Titan are going to be contemplated when we discuss future missions."
Another Titan landing isn't likely anytime soon, given the complexity and expense--460 million euros, or about $600 million--of building the 318-kg. (700-lb.) probe and getting it the billion miles to the surface of the unique moon this time. David Southwood, ESA's science director, says he doesn't expect a follow-up landing there in his lifetime. But the success of the mission--sponsored by ESA, NASA and the Italian space agency, ASI--is sure to give new impetus to Europe's planetary exploration efforts, and to international cooperation in probing the outer planets as well.
ESA Director General Jean-Jacques Dordain says the agency is counting on Huygens--Europe's first foray beyond the inner planets--and its recent accomplishments at Mars and the Moon, to generate enthusiasm for other international missions, including flights to the outer planets. Dordain hopes to enlist the financial resources of the European Union in the effort.
Southwood says the agency is studying what projects might be submitted at the next ESA ministerial summit in early 2006, probably through the Aurora solar system exploration program. The agency has already held a number of informal discussions with NASA on the Prometheus space nuclear power and propulsion program, which has targeted a mission to Europa and the other icy moons of Jupiter to demonstrate its technology.
ASI Director General Sergio Vetrella indicated that Italy, too, is interested in pursuing future exploration of the outer solar system, both through ESA and on a bilateral basis with NASA. Alfonzo Diaz, NASA associate administrator for science, and U.S. scientists in Darmstadt to receive the Huygens data also made clear their enthusiasm for further international missions, with Neptune's moon Triton and Europa high on the list of candidates.
Meanwhile, the images and data returned from Titan in the Huygens probe's nail-biting parachute descent and landing are likely to keep scientists and their computers busy for years. At the European Space Operations Center in Darmstadt last week, science teams gave up sleep to take a first look at the long-awaited data.
By Jan. 18, just four days after the landing, they had put together a "reference reconstructed trajectory" for the probe's entry and parachute descent through the atmosphere, against which the "rich data set" will be analyzed, Lebreton said.
"The landing site has been pinpointed down to probably less than a kilometer in the DISR [descent imager and spectral radiometer], but we still need to do a lot of work to put in the overall Titan surface coordinates," he said.
The picture that was emerging from the data by week's end was a combination of confirmation and surprise, with Titan still withholding some of its secrets. The imagery collected by the DISR clearly showed the results of weather beneath the clouds, but weather unlike any ever seen before.
SUPERFICIALLY, it seemed Huygens had dropped into the Italian lake country. Deep canyons cut through hilly terrain to a sea, with low-lying clouds floating over barrier islands and sandbars. But according to Peter H. Smith, a DISR co-investigator from the University of Arizona's Lunar and Planetary Laboratory, the chemistry was very different from Northern Italy.
"It's definitely water ice on the ground, and it's not pure water ice, but it's mixed with some other material," said Smith, who is also principal investigator on the Phoenix lander set to dig for ice on the arctic plains of Mars in 2008. "We can see methane also, and then we see a third component but haven't been able to identify it. It has a very broad signature. Probably, most likely, it's the haze material that's raining down out of the sky, what we call tholins."
Tholins--organic polymers produced by the action of ultraviolet light or other energy sources on the methane in Titan's atmosphere--are of interest because in warmer temperatures they can dissolve in water to form amino acids that on Earth are the building blocks of life. But on Titan, the surface temperature hovers at about -290F, so cold that water ice takes on the consistency and durability of granite. Smith said, at least on the first look, the terrain appears to be water ice--consistent with the lower density of Titan--eroded by at least periodic downpours of methane, other hydrocarbons and the soot-like tholins.
"We're suspecting that while this is definitely a continuous, ongoing process, there may be rainy seasons somewhere within the 29-year cycle of Saturn going around the Sun," he says. "We just happened to get there with kind of good weather. It's not raining. At least we didn't notice it."
As a result, it wasn't immediately clear whether the features that look like seas in the images are standing bodies of methane or dry, flat beds that fill periodically. Still, it seems the ground was at least marshy where Huygens fell, in a 15g impact on a feature that looks like an island in the dark sea just offshore of the eroded ice hills. John Zarnecki of the U.K.'s Open University, principal investigator on the Huygens surface science package, said the ground had the consistency of wet sand or clay, with a crust suggestive of creme brulee.
The relatively soft impact may explain why Huygens survived to keep broadcasting from the surface. According to Claudio Sallazzo, ESA Huygens mission operations manager, the probe continued to function for the 1 hr. 12 min. that remained after touchdown until it lost its relay when the Cassini orbiter set below the horizon. Huygens could be heard via radio telescopes on Earth for more than 2 hr. after that.
ALTHOUGH THE SURFACE terrain, while startling, at least fit with hypotheses developed on the basis of what can be seen and detected above Titan's clouds, there were some immediate surprises as well. The probe's gas chromatograph and mass spectrometer, designed to analyze the atmosphere's chemical composition, did not detect some of the inert gases that scientists had expected to find.
"They built the instrument to measure the concentrations of these things, but they don't see them," says Smith. "Why not? Somehow all these inert gases, which are part of the constitution of our solar system, got driven off of this planet; so it must have gone through a hot phase, hot enough to drive off the gas. Maybe the original atmosphere was completely washed away, and a new atmosphere bubbled out of the ground. There are a number of possibilities, but it's brand new, and it isn't what was expected."
Technically, however, the mission was less than perfect. Disappointment in the control room at Europe's main spacecraft control center was palpable, even through closed-circuit video as the first data returns started coming in. Soon it was clear that an apparent human error had cost scientists some of the data they had waited so long to receive.
Southwood, in his capacity as ESA's chief scientist, says he will ask for a formal inquiry into how it happened that a critical command was left out of the command sequences loaded into Huygens through Cassini, which kept one of two redundant data channels from functioning as planned during the descent. He stressed that the error was made within ESA, and he's dismayed at suggestions that NASA might have been to blame for the loss of the European data.
"We were responsible for everything going through Cassini" to the Huygens probe, Southwood said.
Early thinking blamed the loss of data on a failure to include the command to switch on the receiver ultra-stable oscillator (RUSO), which was needed to supply a steady frequency for the Doppler Wind Experiment (DWE) on Cassini, and on the apparent failure to double-check the command sequences. Because of the complexity of the RUSO, it was installed only on data channel A; when it didn't switch on, the channel was unable to close the link between Huygens and Cassini.
AS A RESULT, the DWE couldn't generate wind profiles by measuring the Doppler shift in the radio signal from Huygens to Cassini, and half of the images collected by the Descent Imager/Spectral Radiometer never were received by Cassini.
The failure was mitigated somewhat by a backup to the DWE that will use tracking data from the probe's carrier wave received by a network of 18 radio telescopes on Earth. With very large baseline interferometry (VLBI) and a lot of computer processing, researchers hope to be able to plot the probe's position with accuracy comparable to what would have been generated by the DWE.
Leonid Gurvits, senior scientist at the Joint Institute for VLBI in Europe, predicted accuracy of 1 meter/sec. or better from the process, which will involve matching the signals received by all pairs of the 18 radio telescopes in the network to fix Huygens' positions as it descended, and building a wind profile from that. Beginning with the Green Bank telescope in West Virginia, telescopes in the U.S., Australia, China, Japan and Europe were able to receive the carrier wave directly. A test pair between two telescopes in Australia demonstrated that the approach will work, Gurvits says.
An industrial team led by France's Alcatel Space built Huygens, with EADS/ Astrium providing the silica-fiber tile heat shield and Lockheed Martin the DISR. Mission operations manager Sallazzo says the entry and descent was well within margins all the way down, including a 65.2-deg. entry angle that was only 0.2 deg. off nominal, a condition he attributes to the highly accurate delivery provided by the probe's Cassini mothership at separation on Dec. 25.
THE THREE parachutes performed perfectly after the 3-min. entry into the dense atmosphere cut the probe's speed from 21,000 kph. (13,000 mph.) to 1,800 kph. in about 3 min. A 2.5-meter pilot chute pulled out the 8.3-meter main chute, which carried Huygens for 15 min. until a 3-meter chute was deployed so that Huygens could reach the surface before loss of the data relay link through Cassini.
In the lower atmosphere, the probe descended at about 5.4 meters/sec., drifting to the east at about 1.5 meters/sec. First swinging as much as 20 deg. off level, it gradually settled to a 3-deg. tilt. The methane haze layer proved thicker than expected, clearing at only 30 km. above the surface instead of the 50-70 km. scientists had predicted. The thick haze also blocked a clear view of the Sun's position in the sky, complicating the job of orienting the images on the surface of Titan.
At 700 meters above the surface, a downward-facing 20-watt lamp switched on to give the DISR instruments a view of surface colors filtered from the natural sunlight by the methane in the atmosphere. Landing came after 2 hr. 48 min. on the chutes, which were not visible in the images collected after touchdown. That suggests the side-mounted camera is not facing east.
The failure of data channel A left some blanks in the panoramas the DISR team had hoped to generate, because they decided early on to split the images between the two channels. But DISR principal investigator Marty Tomasko, also of the University of Arizona's Lunar and Planetary Laboratory, says the haul still equaled the roughly 350 images that would have reached Earth had the team chosen to send each image redundantly, even though two functioning channels would have doubled that number.
Even as the team prepared to present more complete results from Huygens at a full-dress press conference at ESA headquarters in Paris, scientists were already contemplating ways to answer the queries that will remain unanswered after the Huygens data are thoroughly analyzed.
Smith stressed that the Huygens science team felt very fortunate that the probe descended over such varied terrain, instead of one of the featureless dark areas, but added "it would sure be nice to see the rest of the planet."
Tobias Owen, a Cassini interdisciplinary scientist from the Institute for Astronomy in Honolulu, says researchers are studying a family of low-cost generic probes and carriers within NASA's Prometheus project that could enable exploration of several outer planets and moons. As many as eight probes could be dropped off at multiple destinations from a single carrier, just as Cassini dropped off Huygens. The idea will be discussed with ESA in the coming months.
However, a number of obstacles beyond obvious funding difficulties stand in the way of a future international cooperative effort. One is the fact that ESA's second largest contributor, Germany, has yet to commit to the Aurora planetary exploration program. German Research Minister Edelgard Bulmahn, who's responsible for space, says Germany will discuss participating in a future outer planet exploration initiative.
Sigmar Wittig, director general of the German aerospace center DLR, says his country would like to contribute its robotics skills to such an effort. Details of a German contribution, and whether it should be conducted through Aurora or another initiative, remain to be worked out.
ANOTHER STUMBLING block to future international exploration initiatives, according to ESA science chief Southwood, would be the use of radioisotope thermoelectric generators (RTGs) or future nuclear propulsion hardware sources, which is opposed by several European nations.
[why the reluctance to name names ??] This has forced ESA to rely exclusively on solar arrays; but the Rosetta comet rendezvous mission launched in January 2003 has shown the limits of solar technology, and major missions to the outer planets are inconceivable without RTGs or other nuclear power sources.
To get around this problem, ESA planners envision proposing an RTG development project through agency exploration or technology programs, which don't require unanimous consent of ESA members. ASI's Vetrella says Italy would support use of RTGs, but not through a European development effort.
Still another hurdle, says Roger-Maurice Bonnet, is what he termed "a growing list of hassles" posed by the current U.S. administration, including ITAR technology transfer rules. Bonnet, Southwood's predecessor as ESA science chief, is now executive director of the International Space Science Institute in Bern, Switzerland.
