October 9, 2010 | 9
PASADENA—They say that null results never get published, either in science or in journalism. Well, I’m about to break that rule. Some of the most interesting results to come out of the Division for Planetary Sciences meeting this week concern non-discoveries.
In recent years, planetary scientists have gotten excited by the prospect that Mars and Saturn’s moon Titan might still be geologically active. Volcanic eruptions would account for fresh-looking terrain, the detection of methane in the atmospheres of both worlds, and the sheer bulk of Titan’s atmosphere. Besides, ongoing geologic activity would carve out habitats for living things, which would be undeniably cool.
But almost every scientist I know is a contrarian by temperament and likes nothing better than to question conventional wisdom, and the case of geologic activity is no exception. No sooner did scientists propose that Mars and Titan were alive than others figured it was time to do a takedown. The result has been a fascinating debate. Here I’ll summarize some talks at the meeting that captured the contrarian arguments.
On Wednesday morning, Ken Edgett of Malin Space Science Systems described how he and his colleagues have scoured high-res images of the Red Planet, turned over every rock, revisited every claim for recent volcanic activity, and yet found nothing. Terrain that seemed smooth, hence young, proved to be heavily pocked; what looked like geysers turned out to be dust devils; thermal scans failed to reveal a single hotspot. The team concluded that Mars has not seen volcanism or geothermal activity for at least 10 million years. Veteran planetary science journalist Kelly Beatty has more discussion over at Sky & Telescope.
What about the whiffs of methane detected in the Martian atmosphere? In a poster paper, Kevin Zahnle of NASA Ames Research Center and his colleagues cast doubt on that finding, too. To look for methane, scientists have used spectroscopic observations, both from Martian orbit and from telescopes on Earth. The putative concentration of the gas, just parts per billion, is so slight that the spectral hints are very subtle. Worse, ground-based observations have to subtract out the effects of Earth’s atmosphere, a notoriously error-prone process. "They are fundamentally hard measurements," Zahnle says.
Zahnle is not a spectroscopist—far from it. He comes at the whole problem from the standpoint of modeling Mars’s atmosphere. "The introduction of methane would f- it up," he says. This gas, especially if its concentration varies with time and position as some of the observations imply, would entail a vigorous cycle of chemical activity—contradicting other atmospheric measurements.
That concern led him to conduct some simple reality checks on the methane detections. In many of those observations, the methane signal is deeply buried in the noise. Even the most reliable are troubled. The relative motion of Mars and Earth shifted the Martian spectrum so that the methane signal overlapped with the signal of an isotopic variant of methane in Earth’s own atmosphere. Simply put, Zahnle worries that observers may have measured methane in the atmosphere not of Mars, but of Earth. "As an outsider, I’m not convinced at all," he says.
Titan has an even more convoluted tale. The satellite is shrouded in haze, so most of the images you may have seen of the surface are in fact radar images made by the Cassini space probe. At first glance, evidence for volcanism (or, rather, cryovolcanism, in which ices play the role of magma) abounds. Round features sure look like domes, pits like volcanic calderas, smooth areas like flows. But Jeff Moore of Ames and Bob Pappalardo of the Jet Propulsion Lab began to second-guess these interpretations in 2008. Radar images are notoriously tricky to interpret, and the ones of Titan are fairly low-res. "We were lobbing grenades," Pappalardo recalls. "We were stirring things up."
Both gave talks this week that piled on more doubts. Pappalardo pointed to stereo radar images, which show that many of the supposed domes are actually flat plains and that the supposed flows go uphill. Moore scrutinized the two leading candidates for cryovolcanic flows, Hotei Regio and Tui Regio, and suggested that they look more like dry lakebeds: they sit in the middle of broad depressions and are surrounded with what look like riverbeds.
At the same time, the new thinking is that Titan’s interior is too cold to power cryovolcanoes. Earlier this year the Cassini team used spacecraft tracking data to measure Titan’s gravitational field and therefore its moment of inertia—a measure of its interior structure. A uniform ball would have a value of 0.4. Earth weighs in at 0.33, indicating that its interior is layered into a core and mantle. Titan, however, has a value of 0.34, suggesting that its interior is not fully stratified. Evidently the satellite never got toasty and gooey enough for heavy materials to sink and light ones to rise.
That jibes with the latest theories for how Titan and the other satellites of the outer planets formed. Amy Barr of the Southwest Research Institute in Boulder proposed that these bodies coagulated so slowly, over millions of years, that the heat released in the process was able to dissipate into space—leaving them with frigid interiors. Subsequent bombardment by asteroids and comets heated them from the top down. Titan got off fairly lightly. But Jupiter’s largest moon, Ganymede, might as well have had a big target painted on its side, by virtue of being so close to the giant planet, whose mighty gravity pulls in all manners of space rocks. So Ganymede probably did get pretty warm. And indeed it has a moment of inertia of 0.31, indicating its interior is even more stratified than Earth’s, let alone Titan’s. "We’re rewriting the first billion years of the formation of these moons," Barr says.
To be sure, not everyone agrees. Bruce Bills of JPL cautioned that Titan’s interior structure remains uncertain; the satellite’s spin axis doesn’t quite line up with what the models predict. And others point out that isotopic measurements indicate that gas from the interior has been vented into the atmosphere. "There’s been a fair amount of back and forth," Karl Mitchell of JPL told his colleagues. He cautioned against getting carried away with the case for either an active Titan or a dead one. And indeed it seemed at the meeting that the Titan-is-dead point of view, once contrarian, has gone mainstream—which should surely arouse scientists’ counter-contrarian instrincts.
Image: Putative geyser on Mars was seen by the Viking mission in August 1977 (left), but similar features spotted by the Mars Reconaissance Orbiter this past March (right) were just dust devils. Courtesy of Malin Space Science Systems