Martian Methane: Rocky Birth, Then Gone with the Wind?
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Meeting Division for Planetary Sciences CAMBRIDGE,U.K.—In the medieval city where Isaac Newton worked on the gravitational laws, about 850 scientists gathered from 4 to 9 September Martian Methane: Rocky Birth, for the 37th meeting of the American Astro- Then Gone With the Wind? nomical Society’s Division for Planetary Sciences. Last year, a spectrometer on board the Euro- which would then be quickly destroyed by the concentrations of some 10 parts per bil- pean Space Agency’s Mars Express space- oxidation, ultraviolet sunlight, and possibly lion seen in the atmosphere. craft detected methane above areas of the also by electrical activity of atmospheric dust. So where does all the methane go? Given martian surface where there also appears to Atreya says basalt reacts with liquid water that methane concentrations vary widely over be subsurface ice (Science, the martian surface, it must be 1 October 2004, p. 29). Many destroyed too quickly for the gas to researchers hailed the find as pos- Methane muddle. Who’s found the right spread out evenly. The explanation sible evidence that bacteria are liv- concentration, Mars Express or Gemini may lie in the electrostatic charg- ing in the ice and producing the South (inset)? ing of dust particles, says Atreya. gas. After all, almost all the In small dust devils and larger dust methane in Earth’s atmosphere is storms, electric fields as strong as produced by living organisms. 25 kilovolts per meter could be Indeed, says planetary scientist produced. Such voltages would Sushil Atreya of the Uni- break up water molecules, and the versity of Michigan, hydroxyl molecules created would Ann Arbor, many then oxidize methane. If this alternative explana- removal mechanism is indeed tions for the existence operating on Mars, it could mean of the methane don’t that the production rate of methane work. Volcanic activity is actually much higher than has would also produce been assumed until now. sulfur dioxide, which is Indeed, Michael Mumma of not observed. A freak cometary NASA’s Goddard Space Flight impact in the past few thousand Center in Greenbelt, Maryland, years could have delivered methane to the to produce minerals known as serpentines, observed Mars with telescopes on Earth martian surface, but then the gas wouldn’t be releasing hydrogen in the process. The hydro- and found much higher methane concentra- concentrated in specific regions. gen then reacts with carbon dioxide to pro- tions (up to 250 parts per billion) in some But, Atreya announced at the meeting, it’s duce methane. The process operates at tem- equatorial regions. However, Atreya says too soon to invoke martian microbes as the peratures of about 40° to 90° Celsius and is “something is weird” about these observa- source. Instead, a little-known geochemical distinct from the high-temperature hydro- tions. Such high concentrations would process known as low-temperature serpen- thermal activity also seen on Earth’s ocean almost blind Mars Express’s sensitive spec- tinization could be the culprit. In this process, floors. At a few kilometers beneath the mar- trometer, a problem that does not occur. which has been observed on Earth’s ocean tian surface, low-temperature serpentiniza- Mumma is currently reanalyzing the data floor, liquid water chemically alters basalt to tion in reservoirs of liquid water could pro- using new and better calibrations, but so far produce the gas. Atreya thinks it might pro- duce up to 200,000 tons of methane per year, there’s no indication that the high values duce huge amounts of martian methane, Atreya says—more than enough to explain will go away, he says. Snapshots From the Meeting Irregular satellites explained? No one really knows how to explain the large number of “irregular” satellites that swing around the giant A rapidly rotating rugby ball. A recently discovered miniplanet in planets in slow, eccentric, tilted orbits. Most likely they’re asteroids, the outer solar system is almost twice as long as it is wide, long ago slowed by gas drag and captured by the rotating disks of gas says David Rabinowitz of Yale University. The object, known and dust from which each of the planets formed. But computer sim- as 2003 EL61, has the shape of a squashed rugby ball, ulations show that such captured objects quickly spiral into the nas- measuring 1960 × 1520 × 1000 kilometers. The elongated cent planet unless something boosts their orbits well outside the shape results from the object’s rapid rotation; its period of cluttered inner parts of the planet-spawning disk. 3.9 hours is the fastest ever measured for a large solar system Now, Brett Gladman and Matija Cuk´ of the University of British body. Using the 10-meter Keck Telescope at Mauna Kea, Columbia in Vancouver think they’ve found such a mechanism. Hawaii, Rabinowitz and his colleagues have also detected a According to their numerical simulations, an orbital resonance small satellite orbiting the miniplanet at a surprisingly large between Jupiter and Saturn that occurred in the distant past would distance of almost 50,000 kilometers. It’s unclear how the have “pumped up” the orbits of Saturn’s irregular satellites to a safe system could have formed or whether the rapid rotation and the distance from the planet. A similar past resonance between Saturn strange satellite are somehow related. Says Rabinowitz: and Uranus may have preserved the latter planet’s irregular satel- ´ “2003 EL61 may not quite be as big as Pluto, but it’s much more lites, says Cuk. However, he admits that Jupiter’s troop of irregulars interesting dynamically.” is not so easily explained. –G.S. CREDIT: D. DUCROS/ESA; INSET: GEMINI OBSERVATORY/AURA 1984 23 SEPTEMBER 2005 VOL 309 SCIENCE www.sciencemag.org Published by AAAS N EWS FOCUS As for the origin of the gas, Mumma says Weirdest of all is Saturn’s thin, braided, out by “methane monsoons,” as Lunine’s he’s not sure that Atreya’s low-temperature kinky F ring, which lies just outside the main colleague Ralph Lorenz calls them after a serpentinization scenario applies to Mars. ring system. Cassini’s images show that vari- description in Arthur C. Clarke’s 1975 novel “I’d keep the biological option open,” he ous strands of the F ring are actually one and Imperial Earth. Taking into account Titan’s says. A definitive check on the origin of the same narrow dust ring, tightly wound into seasons, atmospheric properties, and solar methane will likely have to wait for NASA’s a spiral. This unique Mars Science Laboratory, scheduled for structure—unrelated launch in 2009. Says Mumma: “This is going to the spiral density to be a long tale.” waves that have been seen in other parts of the ring system Several New Twists (Science, 9 July 2004, p. 165)—may be for Saturn’s Rings caused by a small moonlet discovered Rough terrain. Stereoscopic images of Titan’s surface from the Huygens probe. They may appear serene and eternal, but by Cassini in an Saturn’s rings are changing, and changing eccentric orbit that appears to cross the F ring. radiation, Lorenz estimates that the “mon- fast. Over the past 25 years—the mere blink That orbit is a mystery in itself: The F ring is soons” happen every few centuries and last of an eye in planetary evolution—one partic- believed to contain many large boulders and for months. They’re like the episodic rain- ular ringlet in the innermost, tenuous part of moonlets, which would make it hard for a storms in the Arizona desert, but on a differ- the ring system moved 200 kilometers inward small satellite to survive multiple crossings. ent time scale, he says. and became one-tenth as bright. “That’s radi- Even so, the tiny object (denoted S/2004 S6) The methane in Titan’s atmosphere must cal,” says Carolyn Porco of the Space Science has been observed for almost a year. be continuously replenished because ultra- Institute in Boulder, Colorado, head of the Cassini has also spotted more bright knots violet sunlight is constantly breaking down imaging team for NASA’s Cassini spacecraft. close to the F ring, some of which are very the gas. Researchers do not yet know whether Porco’s team discovered the rapid change by elongated. “We have a hard time deciding methane has been stored in the mantle since comparing Cassini ring photos with images which of these objects are real moons and Titan’s formation or whether it is being pro- the Voyager spacecraft sent to Earth in 1980. which of them are clumps of dust,” says duced by geochemical processes beneath the “This is one of the reasons why we wanted to Porco. Even S/2004 S6 may turn out to be a surface. According to planetologist Gabriel come back,” she says. The dramatic change loose clump rather than a solid object. Future Tobie of the University of Nantes, France, suggests that this part of the ring system observations of Saturn will surely reveal new various forms of outgassing—such as cryo- could be young and rapidly developing, small satellites. Says Cassini’s project scien- volcanism, which brings water-ammonia ice although no one yet knows how to interpret tist Dennis Matson of JPL: “The complexity containing trapped methane to the surface— the observations. in the rings is just dumbfounding. We will would then release the gas into the atmos- Other ring results presented at the meeting continue to bring you excitement.” phere episodically. Indeed, radar images of are equally baffling. For instance, Cassini’s Titan’s surface obtained by NASA’s Cassini temperature measurements of the rings indicate spacecraft—Huygens’s mother ship—show that ring particles are 15° cooler on their night evidence of volcanic domes, craters, and side than on their day side.