Yet this hypothesis has a shortcoming. Huygens data rule out an underground source of acetylene; this compound must ultimately come from methane in the atmosphere. Thus, it seems like a circular argument: to produce methane (by microbes), one needs methane. Moreover, the sheer abundance of methane on Titan is so immense that methanogens would have to work in overdrive to produce it, severely depleting the available nutrients.
In view of these obstacles, a biological explanation for methane is much less attractive on Titan than on Mars. Nevertheless, the hypothesis of habitability bears investigating. Some scientists argue that this moon might have been or still be habitable. It receives enough sunlight to turn nitrogen and methane into molecules that are the precursors to biology. An underground water-ammonia brine, with some methane and other hydrocarbons thrown in, could be a friendly environment for complex molecules or even living organisms. In the distant past, when the young Titan was still cooling off, liquid water may even have flowed on the surface.
One crucial measurement that could help determine the sources of methane on Mars and Titan is the carbon isotope ratio. Life on Earth has evolved to prefer carbon 12, which requires less energy for bonding than carbon 13 does. When amino acids combine, the resulting proteins show a marked deficiency in the heavier isotope. Living organisms on Earth contain 92 to 97 times as much carbon 12 as carbon 13; for inorganic matter, the standard ratio is 89.4.
On Titan, however, the Huygens probe measured a ratio of 82.3 in methane, which is smaller, not larger, than the terrestrial inorganic standard value. This finding argues strongly against the presence of life as we know it. To be sure, some scientists suggest that life could have evolved differently on Titan than on Earth or that the inorganic isotope ratio may be different there.
No one has yet determined the carbon isotope ratio for Mars. This measurement is challenging when the concentration of the gas is so low (one billionth of that on Titan). NASA’s Mars Science Laboratory (MSL) rover, scheduled to arrive at Mars in 2010, should be able to carry out precise measurements of the carbon isotopes in methane and possibly other organic materials. It will also study solid and gaseous samples for other chemical signs of past or present life, such as a very high abundance ratio of methane to heavier hydrocarbons (ethane, propane, butane) and chirality (a preference for either left-handed or right-handed organic molecules).
Tied up with these issues is the question of why organics seem to be missing from the surface of Mars. Even in the absence of life, meteorites, comets and interplanetary dust particles should have delivered organics over the past four and a half billion years. Perhaps the answer lies in Martian dust devils and storms and ordinary saltation (the hopscotching of windblown dust grains). These processes generate strong static electric fields, which can trigger the chemical synthesis of hydrogen peroxide. Being a potent antiseptic, hydrogen peroxide would quickly sterilize the surface and scrub out the organics. The oxidant would also accelerate the loss of methane locally from the atmosphere, thus requiring a larger source to explain the abundances observed in the Martian atmosphere.
In summary, methane serves as the glue that holds Titan together in some mysterious ways. The presence of methane on Mars is equally intriguing, not the least because it evokes visions of life on that planet. Future exploration of both bodies will seek to determine whether they were ever habitable. Although life as we know it can produce methane, the presence of methane does not necessarily signify the existence of life. So planetary scientists must investigate thoroughly the sources, sinks and isotopic composition of this gas, along with other organic molecules and trace constituents in both gaseous and solid samples. Even if methane is found to have no connection to life, studying it will reveal some of the most fundamental aspects of the formation, climate histories, geology and evolution of Mars and Titan.