The Huygens Chronicles

Unveiling Titan

Clutching three black-and-white photographs, Marty Tomasko beamed as he strode into a late-night press briefing on Jan. 14 in Darmstadt, Germany. With a crowd gathering around him at the European Space Agency’s operation center, he showed off images taken by the Huygens probe, which just hours before had landed on Saturn’s moon Titan. No other humanmade object had ever touched such distant ground.

MOON SLIVERS. Fish-eye view of the Huygens landing site on Saturn’s moon Titan as seen from 3,000 meters above the central black region, which may be a lake bed where methane recently flowed. Assembled from 45 images taken by the probe’s three cameras, the mosaic shows a bright area (upper left) that’s about 100 m higher than the rest of the terrain. Fine, dark lines (one indicated with arrow) there appear to be drainage channels, leading to what may be a shoreline with river deltas and sandbars. Tomasko, M. Bushroe, et al./UNIV. Ariz., ESA, NASA

TOUCHDOWN. Huygens lands on Titan in this illustration. An orbiting craft received Huygens’ signals for 3 hours and 37 minutes. D. Ducros/ESA

FLOWING PANORAMA. Titan as seen by the Huygens probe 800 meters above the surface. In this mosaic of 17 images, a ridge of ice boulders stretches from lower left to upper right, projecting through dark lake beds where organic goo might have concentrated. Seepage of fluid, most likely methane, between the boulders cuts the dark sediment into channels (arrow), which are 40 m wide. Tomasko, M. Bushroe, et al./Univ. Ariz., ESA, NASA

“Everybody was saying ‘Congratulations! The images are great!'” recalls Tomasko, a planetary scientist at the University of Arizona in Tucson. But even as he sipped champagne, Tomasko feared that he might have precious little more to reveal after these few moments of glory.

In the best of all possible worlds, he and his camera team might have already begun to determine the composition of Titan’s surface. But because of several malfunctions as the probe descended through Titan’s atmosphere and a communications problem with its mother craft, the Cassini orbiter, the data didn’t seem to make sense.

The spectra taken by Tomasko’s cameras looked OK, but the scientists couldn’t figure out exactly where the spectrometers had been pointing when the recordings were made. That’s because the probe was swinging so violently and the atmosphere was so thick that Huygens’ sun sensor couldn’t function properly. It had been intended to schedule when spectra were taken and how to orient the spectrometer, with respect to the sun.

Furthermore, the parachuting probe had unaccountably reversed its direction of rotation, confounding the task of assembling panoramas of Titan and its atmosphere from the probe’s three cameras, each of which pointed in a different direction.

Beyond that, half of the 1,200 images relayed by Huygens had been lost because of a major snafu—only one of the two receivers on Cassini had been switched on. Assembling the images into panoramas was “like putting together a jigsaw puzzle when half the pieces are missing,” Tomasko recalls.

But now, after more than 3 months of painstaking work, scientists are lifting the veil on Titan. Shrouded in hydrocarbon haze and slightly bigger than Mercury, the moon has the largest unexplored surface in the solar system. The Huygens mission team is finding that Titan in part resembles Earth yet is also strikingly alien. Tomasko and other scientists presented some of the new findings in March at the annual Lunar and Planetary Science Conference in Houston.

Singin’ in the methane

Riveting images of Titan’s surface show a network of drainage channels, highlands, and perhaps shorelines. The drainage channels, Tomasko notes, are evidence for some kind of precipitation and flowing liquid, most likely methane. Although most of the surface is made of water ice, the pattern of light and dark features indicates the deposition of hydrocarbon goo.

Hydrocarbon aerosols fall onto the surface and solidify into a uniform coat of dark, organic gunk, Tomasko says. “But then there’s methane rain, which is relatively clear, that washes it off [the highlands], concentrating it in the bottom of the drainage channels and in the flat areas.”

The Huygens lander discovered the channels, which are about 40 m wide. “To see these drainage channels for the first time—to have [the images] shout ‘rain, precipitation, erosion’—it was thrilling,” says Tomasko.

“We’ve got the Titan equivalent of what happens on Earth, the erosion of underlying rocky material by streams of liquid,” says John Zarnecki of the Open University in Milton Keynes, England. “But the big difference is that the fluid is liquid methane, not water, and instead of silicate rocks or granite, we have [pebbles made of] water ice.”

Measurements made by Huygens’ gas chromatograph-mass spectrometer are corroborating the abundance of methane at and just beneath Titan’s surface. During the first 3 minutes after landfall, a heated inlet on the instrument made contact with the surface and vaporized material just below the ground.

This chromatograph recorded a 30 percent jump in methane over that measured in the atmosphere. The sudden increase suggests that Titan has substantial reserves of methane only a few centimeters beneath its surface.

What’s more, the force of Huygens’ impact recorded by a tiny sensor protruding from the bottom of the probe, suggests that the craft encountered a hard crust and a mushier interior, notes Zarnecki. The finding is also consistent with methane lurking just beneath an icy carapace.

An accelerometer on the probe suggests that the landing was not only gentle but that the probe slid or skipped along the surface for a few seconds before coming to a halt. Other sensors indicate that Huygens nestled into the ground at a 10° tilt and that the probe shifted position slightly during the 70 minutes that Cassini could receive its signals, Zarnecki told Science News.

These preliminary findings also point to a subsurface made of a mushy material, perhaps methane mixed with grainy ice, he says.

There’s even a hint that methane was raining down when Huygens descended. The ultraviolet spectra taken as the probe parachuted through the atmosphere show a dramatic change 25 kilometers above Titan, the altitude at which methane becomes a major component of the lower atmosphere. The size of aerosol particles increased while their density decreased.

The scientists speculate that liquid methane coats the aerosol particles, increasing their girth, and also washes out some of the particles, decreasing the aerosol’s density.

Tomasko notes that his team has yet to analyze spectra using 300 other wavelengths. These data should help clarify what was going on in the atmosphere when Huygens showed up, he says.

Rain or no rain, methane on Titan poses a mystery. Although methane is constantly recycled between Titan’s atmosphere and its surface, much of it is broken down by sunlight into other hydrocarbons that coalesce into smog. In just a few million years, these sunlight-driven conversions should have depleted Titan of all its methane, says Toby Owen of the University of Hawaii in Honolulu.

“Why is the methane still there?” asks Owen. “Why hasn’t it disappeared after making all this smog material? Either we’re very lucky that we came along [with Huygens] at a time before all the methane was gone or something is replenishing it.”

Huygens’ detection of methane just beneath Titan’s surface favors the replenishment scenario. There may be larger liquid reserves even farther down, Owen says.

If Titan has enough internal heat, such underground deposits may rise to the surface. The heat could come from gravitational flexing of Titan by Saturn as the moon orbits its mother planet.

Absence of nobility

Frigid Titan is the Peter Pan of the solar system. Because Titan is so cold, complex chemistry hasn’t modified the moon’s components over billions of years. “It’s the little world that never grows up,” says Owen.

Titan is much too cold to support living things as we know them, yet it has two prerequisites for life’s formation: organic matter and a thick atmosphere that filters out harmful ultraviolet radiation.

“There’s a basic notion that by exploring Titan, we can travel back in time to Earth nearly 4 billion years ago,” just before life arose, says Owen. “We can look at a very primitive environment that has been preserved here for us today, where some of the kinds of processes that happened on the early Earth are still taking place,” he says.

Using the Huygens data to characterize those processes on Titan has been slow going, in part because signals from the gas chromatograph have been difficult to interpret. Even so, Owen and his colleagues have eked out several new findings.

Huygens failed to detect an abundance of some materials that scientists had predicted would be present. In one case, hypotheses from more than a decade ago suggested that when sunlight breaks down methane gas in Titan’s atmosphere, most of the fragments combine to make ethane, which is one chemical notch up in size and complexity from methane. Ethane aerosol particles would settle on the surface, where they could easily be identified by the Huygens’ mass spectrometer.

But when the probe landed, it recorded only tiny amounts of ethane. This indicates that “the early photochemical models suggesting that ethane was the end point in the atmospheric photochemistry were wrong,” says Owen. “There must be further processing to more-complex hydrocarbons like benzene.”

Yet so far, the surface compounds identified by the gas chromatograph are the same as those detected in the upper atmosphere, which had been characterized by an infrared spectrometer on the orbiting Cassini spacecraft. “We are still looking for evidence of more-complex [chemical] species” on the surface, notes Owen.

In another case of missing materials, Huygens found no trace of any noble gases, such as neon and xenon, on Titan, despite their prevalence on Mars, Venus, and Earth and in comets. That’s a puzzle because half of Titan is made of ice, and ice at low temperatures acts as an efficient trap for the noble gases.

“Not finding those noble gases is a clue to Titan’s formation,” Owen says. Indeed, he now proposes that the moon formed under warmer conditions, much closer to its mother planet Saturn, than planetary scientists had suggested.

He notes that below 80 kelvins, water ice has structural features that trap noble gases. At temperatures above 80 K, however, laboratory experiments show that ice doesn’t hold these gases nearly as well.

Researchers had previously proposed that Titan coalesced from bits of ice and rock in a cold region, far from Saturn, and migrated toward the planet. But formation in a more temperate zone nearer to Saturn may be the only way that ice could have remained at temperatures too warm to trap the noble gases, notes Owen.

The absence of noble gases may also shed light on the origin of Titan’s atmosphere. Molecular nitrogen now accounts for 95 percent of that atmosphere, but several lines of indirect evidence indicate that the nitrogen didn’t come to Titan in its molecular form.

If Titan’s nitrogen didn’t start out as N2, the most likely source for nitrogen is ammonia (NH3), says Owen. Simulations of the early solar system indicate that ammonia was a common ingredient in the particles of ice and rock that collected close to Saturn during its formation. If Titan coalesced near Saturn, as Owen proposes, the fledgling moon could have easily incorporated ammonia.

No one has yet found signs of ammonia in the Huygens data. But a storehouse of the material deeper within Titan could account for features seen in radar images recently taken by the orbiting Cassini spacecraft, says Jonathan Lunine of the University of Arizona.

The radar images show surface features reminiscent of lava flows on Earth. Titan was never warm enough for molten rock to erupt, but volcanic eruptions of water-ice and ammonia could be common, says Lunine.

The presence of ammonia could spawn icy volcanism in several ways, he notes. For starters, ammonia acts as an antifreeze, lowering the melting point of water by 100°C. As a result, less energy is required for an eruption to occur. Adding ammonia to liquid water also makes it more buoyant than water-ice, enabling the mixture to rise to the surface more easily. Finally, the addition of ammonia to liquid water creates a more viscous fluid, similar to lava.

Floating on Titan

Even before Huygens landed, planetary scientists were thinking about future visits to Titan’s surface. The mission’s success has fueled those ambitions. The next lander on Titan would stay much longer than just a few hours and would probably have the capability of traveling from place to place.

With Titan’s thick atmosphere and low gravity, “the place just cries out” for an airship, says Torrence Johnson of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. The craft could float along, landing on a variety of intriguing sites. “There’s no question that we have to go back to Titan,” says Zarnecki.

For now, he notes, the Huygens data occupy most of his time. But when he has a moment to relax, he looks back to the night of Jan. 14. At the operations center in Darmstadt, “there was a terrible time waiting for science data that was supposed to be coming,” he recalls. “For 4, 5, 6 minutes, we were essentially looking at a blank screen. Then, those green figures lit up.” From there, he says, “everything just took off, and it was fabulous.

“It made everything in the past 15 years seem worthwhile,” says Zarnecki.

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