The Methane Mystery
|Looking back toward the sun brings out the thin haze that hovers 500 kilometers (310 miles) above Saturn’s moon Titan. |
The methane giving an orange hue to Saturn’s giant moon Titan likely comes from geologic processes in its interior according to measurements from the Gas Chromatograph Mass Spectrometer (GCMS), a Goddard Space Flight Center instrument aboard the European Space Agency’s Huygens Probe. The GCMS, which descended with five other instruments on the probe through the moon’s thick atmosphere on Jan. 14, 2005, also found evidence of liquid methane in the surface material.
Methane, a molecule consisting of four hydrogen atoms bound to a carbon atom, is the primary component of natural gas on Earth. It can be produced by life, by degradation of organic debris or by geologic processes like volcanoes.
The origin of methane in Titan’s atmosphere is a mystery because it gets broken down by sunlight and particle radiation from space in the upper atmosphere. If surface lakes and pools were the only source, all of Titan’s methane would be lost by this mechanism in less than a hundred million years, a short time for a moon that’s been around since the formation of the solar system 4.5 billion years ago. Components of the methane molecules react with each other and atmospheric nitrogen. As they descend, they form larger and heavier molecules that comprise the orange haze that blankets the moon. Because Titan is very cold (292 degrees below zero F, or minus 180 degrees Celsius) these heavy compounds condense and rain out on the surface.
"We have determined that Titan’s methane is not of biological origin, so it must be replenished by geologic processes on Titan, perhaps venting from a supply in the interior that could have been trapped there as the moon formed," said Dr. Hasso Niemann of Goddard, principal investigator for the GCMS and lead author of a paper on this research to appear in Nature on Dec. 8. An advance online publication will be available on Nov. 30 at
|The descent of the Huygens probe has allowed the first detailed study of the atmosphere of Saturn’s moon Titan, revealing startling parallels and stark contrasts with that of Earth. Both atmospheres are nitrogen-dominated, but the low temperature of Titan means that the carbon-carrying gas in its atmosphere is methane (1.6% of the total) rather than carbon dioxide (present at only 345 parts per million). Photochemical reactions involving this methane produce a smog at middle altitudes, and an organic rain of methane and nitrogen-containing aerosols falls steadily onto the satellite’s surface, creating an Earth-like terrain of extended river networks. Radiogenic argon (40Ar), which makes up 1% of Earth’s atmosphere, is in short supply on Titan (just 43 parts per million). The still smaller amount of primordial argon (36Ar) suggests that the nitrogen in the atmosphere must have arrived in the form of compounds such as ammonia, rather than as molecular nitrogen. Credit: Nature|
Titan is believed to be too cold for life. Nevertheless, Niemann’s team of scientists used Goddard’s GCMS instrument to rule out a biological source for Titan’s methane. The GCMS instrument identifies different atmospheric constituents by their mass. Molecules and atoms are given an electric charge (ionized) and are separated by their mass as they traverse an electric field in a quadruple mass spectrometer.
The carbon in methane molecules comes in different varieties, or isotopes carbon-12 (12C) and carbon-13 (13C). Each 13C atom has an extra neutron in its nucleus, making them slightly heavier than 12C atoms, so the GCMS can distinguish between methane with 12C and methane with 13C.
Living organisms have a preference for carbon-12. As a result, carbon-containing molecules, such as methane, that are associated with life on Earth get enriched in 12C. The ratio of 12C to 13C is a marker or signature of life. However, the team did not see 12C enrichment in the methane on Titan.
Also, when the heated inlet of the GCMS came in contact with the surface, it vaporized some of the surface material. After impact, the GCMS detected a 40 percent increase in the number of methane molecules measured, and this level remained for about 50 minutes after impact. This long-lived burst is best explained by the presence of liquid methane mixed with the surface material, according to the team.
Other Huygens observations, such as pictures from the Descent Imager and Spectral Radiometer instrument, show features that look remarkably like dry riverbeds. Scientists have theorized that at Titan’s cold temperatures, liquid methane plays the role of water on Earth, while deeply frozen water substitutes for rock. On Titan, liquid methane could cut channels in water ice the same way water carves canyons through rock. The presence of liquid methane on Titan’s surface supports this scenario.