A MACHINE from Earth is now resting on an ice-strewn plain a billion kilometres away. The Huygens probe landed on Titan last Friday. It started transmitting during its descent and kept on going far longer than anyone expected after it reached the surface. The images and other data it sent back are extraordinary.
Although analysis has barely begun, educated guesses by the Huygens team are giving us a first impression of this new world. There appear to be canyons cutting through hills of ice, surrounded by tarry plains strewn with ice cobbles and frosted with methane. There are many signs that exotic hydrocarbon liquids have shaped the landscape, and may still be present a few centimetres under Titan's surface.
After the failure of the European Space Agency's Beagle 2 trip to Mars in 2003, mission scientists were understandably tense as Huygens approached Titan on 14 January. No probe had ever landed on a world nearly as remote or poorly understood as Titan, making this an extraordinarily ambitious mission. So when radio telescopes on Earth reported that Huygens was transmitting successfully, there was a huge release of tension. "I've seen people with tears in their eyes," said ESA scientist Mike McKay.
There was one heart-stopping moment when scientists learned that one of the two data channels from Huygens was not working. But it now appears that relatively little science will be lost (see "The lost pictures"). Overall, Huygens worked almost perfectly. It survived the fierce heat of entry and the harsh cold of the atmosphere, and kept transmitting data for more than an hour after landing.
One of the most startling images shows a network of sinuous lines crossing a bright region, and a sharp boundary between that area and a featureless dark expanse. "It seems to me we are seeing evidence of drainage channels, and perhaps a shoreline," says Marty Tomasko of the University of Arizona, who heads the Huygens imaging and spectroscopy team.
The channels are shorter and stubbier than river valleys on Earth, and may be more akin to canyons, possibly carved by fluid oozing out of their sides. As the surface temperature is a chilly 94 kelvin (-179 °C), this fluid can't be water. It is probably liquid ethane, methane or some other hydrocarbon.
”The dark stuff that fills the channels is probably tarry gunk, perhaps swept down by methane or ethane rain
The dark stuff that fills the channels is probably tarry gunk, perhaps swept down by methane or ethane rain falling on the icy highlands. "Clean rain may wash off the higher parts, so dirty stuff gets washed down to the bottom," says Tomasko.
That "dirt" is thought to be constantly accumulating from organic molecules formed in the upper atmosphere, which coagulate into aerosol particles and gradually settle out. If undisturbed, this accumulation would give every surface a thick tarry coating within about 10,000 years, according to Ralph Lorenz of the University of Arizona. So the clean highlands have probably been rinsed off within that timescale, perhaps by violent methane rain storms that may occasionally scour Titan's surface.
The channels appear to drain into the dark regions of Titan. "The darker areas seem very flat and don't have a lot of features on them," says Tomasko. "It's hard to make something so flat. Probably the dark areas are evidence of flooding. We don't know whether they still have liquid on them or whether the liquid has drained into the surface."
To the disappointment of some scientists, Huygens didn't land on liquid. Instead, its cameras looked out on a plain covered in rounded, fist-sized lumps of ice. "The amazing thing to me is how familiar this looks," says Tomasko. "You can imagine yourself standing in this scene." He thinks there is also a hint of a depression in the view, which could be a flow channel, and the ice "rocks" are quite rounded, like pebbles in a river - although rain alone might eroded these shapes.
Huygens will also help experts interpret data from NASA's Cassini probe, which carried Huygens to Saturn and relayed its data back to Earth. Cassini has compiled detailed images of Titan from space using its cameras and radar, but they have been tantalisingly ambiguous. No one could tell what the complex white and dark patterns meant. Now, assuming Huygens landed in a reasonably representative area, scientists can tentatively interpret bright areas on images from Cassini's maps as icy highlands, and dark areas as tarry flood plains.
Lorenz also points out that Cassini's radar images show triangular features that connect to narrow sinuous lines, and says it is now tempting to interpret them as pebbly alluvial plains fed by drainage channels.
Like a human explorer, Huygens gathered its impressions of Titan through a range of senses: not only sight but also sound, smell and touch. Even as it landed it tested the texture of the ground using a metal rod poking out of the bottom of the craft, attached to a device to measure the force of impact. Scientists hoped this would determine whether the landing site was solid, liquid or slush - in other words, whether Huygens had landed with a crash, a splash or a squelch - but it turns out to have been none of the above.
”Even as it landed, Huygens tested the texture of the ground using a metal rod poking out of the bottom of the craft
At first the penetrometer met strong resistance, but then it slid in more easily. That may just mean it glanced off a pebble on the surface, but the more popular interpretation is that the ground has a thin, hard crust covering some uniform softer stuff. "If you look at the drop tests we did in the lab, the closest analogue in consistency is wet sand or clay," said John Zarnecki of the Open University in Milton Keynes, UK, leader of the surface science team. Another member of the team compared the overall structure to crème brûlée.
"You get the feeling that maybe this area was wet not so long ago and maybe liquid hasn't penetrated too far into the surface," says Tomasko. The soft stuff may be wet with ethane, while the crust is a harder, dried-out layer. This week, Zarnecki's group will be mixing up different kinds of crusty stuff and dropping a replica of the Huygens penetrometer onto it, to see what matches most closely. Will they actually try crème brûlée? "We're going to have to - we can't keep using that phrase without knowing quantitatively what it's like," says Andrew Ball, a member of Zarnecki's team.
”You get the feeling that maybe this area was wet not so long ago, and maybe liquid isn't too far beneath the surface
The probe's cameras also recorded the spectrum of light reflected from the surface, which will be compared with spectra from a host of complex organic materials to look for a match. And Huygens's chemical analyser appears to have sniffed out some of the surface chemicals. Its inlet was heated up to vaporise some ground material, so it could study the gases given off. It appears to have worked: the instrument detected a puff of methane just after impact. "It's probably methane frost or dew on the surface," says Toby Owen of the University of Hawaii, an expert on the solar system.
During the descent, the same instrument detected a jump in methane levels between 18 and 20 kilometres up, probably from a cloud or unforeseen layer of haze. If it is a widespread haze layer, it may be what confounded Cassini's attempts to discover lakes on Titan, as haze could have blurred any telltale glints of sunlight. There may be fog, too: as Huygens approached the ground, its cameras got an oblique view of the canyon-riddled terrain and saw a whiteness that was not visible from above. Tomasko thinks this could be a patch of methane fog viewed edge-on.
”As Huygens approached the ground, its cameras got an oblique view of the canyon-riddled terrain
As for the organic chemistry in Titan's atmosphere, which is thought to hold clues to the origin of life on Earth, we will have to wait. The chemical analysis team has only released methane data so far. Some of the most interesting and complex molecules will take the longest to sniff out, because they were probably locked in aerosols. Another of Huygens's instruments collected and baked these particles at 600 °C to release their constituents, but it could take months to interpret the results.
All the conclusions so far are extremely tentative. As the scientists piece the data together and combine results from different instruments, there may be much bigger surprises to come. Toby Owen remembers his own work on the Galileo probe, which pierced Jupiter's clouds in 1995: "One of our biggest discoveries came five years later."
The composite images opposite were prepared not by ESA or NASA scientists, but by a growing number of amateur enthusiasts who are analysing the data from space missions themselves.
Anthony Liekens, whose website carries the images (
They started with the raw images from Huygens, which are available from the University of Arizona at www.lpl.arizona.edu/~kholso/, and built up these striking mosaics using standard image-processing software. Some of these images were posted at 7 am GMT on Saturday, only hours after the raw data was made available, and hours before the first mosaic released by ESA.
These images are not as accurate as those NASA and ESA will assemble. For example, the orange cast of one of the images above gives only a rough idea of the light on Titan ESA and NASA, though, should be able to use the cameras' spectrum-analysing capabilities to produce pictures with true colours.
There is one blemish on ESA's astonishing technical achievement. One of the two data streams broadcast by Huygens was not picked up by its support module on Cassini. It means that precious images of Titan have been lost.
The immediate cause was a missing command that should have been sent from Earth. It was part of a sequence of commands to turn on the Doppler wind experiment, designed to measure winds on Titan by monitoring slight fluctuations in the 2040-megahertz "chain A" radio signal from Huygens. The receiving circuit tuned to that frequency simply didn't switch on.
David Southwood, ESA's head of science, squashed early rumours that the error was NASA's. "There will be an ESA inquiry into how that command came to be missing. This is our responsibility."
But all was not lost. Huygens sent the second data stream, chain B, on a separate frequency of 2098 megahertz, and it was picked up flawlessly. Most of the results from Huygens were duplicated on both chains, so most of the instrument teams have lost nothing. The camera team, however, decided to try and get as many pictures home as possible, so they sent 350 different images in each stream. With half now lost, some of the panoramic mosaics are going to have holes in them.
Southwood is not entirely sympathetic. "That's scientists trying to screw the system. We don't have redundant systems to get more data down, we have redundant systems for redundancy."
Although the Doppler wind experiment initially appeared to be a write-off, remarkably, radio astronomers on Earth picked up enough of Huygens' signal to do the job. A network of 18 radio dishes not only managed to detect a transmitter with the power of an ordinary mobile phone more than a billion kilometres away, but should eventually pin down its frequency to within less than 1 part in a trillion.
That's precise enough to detect Doppler shifts from even gentle breezes pushing Huygens around, so we should learn something about the winds of Titan after all.
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18:53 05 September 2008
