Not Life on Titan Again
Something weird is going on at the large moon of Saturn. “What is Consuming Hydrogen and Acetylene on Titan?” teased a press release from Jet Propulsion Laboratory”s Cassini mission:
Two new papers based on data from NASA’s Cassini spacecraft scrutinize the complex chemical activity on the surface of Saturn’s moon Titan. While non-biological chemistry offers one possible explanation, some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan’s surface. According to one theory put forth by astrobiologists, the signatures fulfill two important conditions necessary for a hypothesized ‘methane-based life.’
What are these signatures? One is that hydrogen molecules are apparently flowing down through the atmosphere and disappearing. Another is that complex hydrocarbons are accumulating, but not acetylene as expected. What do these details imply? Chris McKay of NASA’s Ames Research Center thinks it may imply that unobserved organisms down there are eating the acetylene as food. “We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth,” he said. “If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth.” No one has ever observed such a life form anywhere, of course. “To date, methane-based life forms are only hypothetical,” the next sentence hastened to add. “Scientists have not yet detected this form of life anywhere, though there are liquid-water-based microbes on Earth that thrive on methane or produce it as a waste product.”
Most astrobiologists consider liquid water as an essential ingredient for life. On Titan, however, H2O is hard as rock. Methane and ethane, which are liquid at its -290°F surface temperature, are the only candidates left: “The list of liquid candidates is very short: liquid methane and related molecules like ethane.” Astrobiologists believe that liquid water is not a requirement for life – that one of these other liquids might work, although water is the best liquid when you can get it.
Back to Titan’s disappearing hydrogen. Darrel Strobel of Johns Hopkins University, a Cassini interdisciplinary scientist, was surprised that the molecular hydrogen produced in the atmosphere by UV photolysis of methane, some of which escapes to space, and some of which falls to the surface, was not accumulating at the surface. It is apparently not being converted to acetylene, either. At this point, the reader, salivating for the exciting climax of the story – for scientific evidence that the best possible explanation is that exotic life-forms are consuming the hydrogen and acetylene – encounters nothing of the sort. Instead, there is only evidence that unknown “organic chemistry” is “happening” and coating the ice: “Titan’s atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again,” said Roger Clark of the U.S. Geological Survey in Denver.“All that implies Titan is a dynamic place where organic chemistry is happening now.” Presumably that leaves room for alien seekers; after all, one could describe Earth as a dynamic place where organic chemistry is happening.
As for his view on the life hypothesis, “Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,” Allen said. “We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results – for example, reactions involving mineral catalysts.” Project scientist Linda Spilker was noncommittal but appropriately diplomatic: “These new results are surprising and exciting,” she said. “Cassini has many more flybys of Titan that might help us sort out just what is happening at the surface.”
Mere mention of the L-word life is enough to make some reporters go bonkers. Immediately, the Daily Mail in the UK, as expected, threw all caution to the wind: “Scientists find a ‘hint of life’ on Saturn’s moon Titan” blared the headline. Right away, the aliens were breathing and eating: “They have discovered clues that primitive aliens are breathing in Titan’s atmosphere and feeding on fuel at the surface.” They found a quote mine in John Zarnecki, one of the Huygens Probe science czars: “We believe the chemistry is there for life to form. It just needs heat and warmth to kick-start the process. In four billion years’ time, when the Sun swells into a red giant, it could be paradise on Titan.”
Astrobiologists have enough difficulty on their hands explaining life’s origin here on the paradise planet (01/26/2008), let alone the poison pit of Titan. Instead of being distracted by the life hypothesis, may we direct your attention to this paragraph in the JPL press release:
Strobel found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10,000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escape out of the upper atmosphere.
“It’s as if you have a hose and you’re squirting hydrogen onto the ground, but it’s disappearing,” Strobel said. “I didn’t expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should ‘float’ to the top of the atmosphere and escape.”
Why didn’t Dr. Strobel expect this result? And why were the makers of the Huygens Probe, who designed it to float on a global ocean of liquid ethane (02/15/2008), surprised when it landed with a thud on a moist lakebed? Maybe we need to question some old assumptions, and boldly ask new questions (05/16/2010).