Ten years ago, the Huygens probe landed on the surface of Saturn’s giant moon, Titan. Huygens, named after Christiaan Huygens who discovered Titan, was built by the European Space Agency as part of the joint NASA/ESA Cassini-Huygens mission to the Saturn system. The mission’s major objective was to collect chemical and isotopic data from Titan’s atmosphere with a view to understanding the origin of Titan’s atmosphere. Titan is unique among the moons of our solar system in having a thick atmosphere, composed primarily of molecular nitrogen and methane. Titan’s atmosphere contains unique clues to the origin and evolution of carbon, nitrogen, and other volatile elements in the outer solar system.
Prior to the Huygens landing, scientists hoped the gas chromatograph-mass spectrometer (GC-MS) onboard Huygens would measure noble gases in Titan’s atmosphere and provide clues to the origin of volatiles on Titan. Surprisingly, the GC-MS found primordial noble gases in very low abundances, and the heaviest noble gases krypton and xenon were not detected down to 10 parts-per-billion. These measurements suggested Titan formed in a warm environment around Saturn called a subnebula. The relatively high temperatures would make it difficult for noble gases to be trapped inside the icy building blocks of Titan. An important corollary is that other chemical species of similar volatility, such as molecular nitrogen and methane, would also remain as free gases in the subnebula. This finding leads to the startling conclusion that the gases in Titan’s atmosphere are not primordial, but must have been produced on Titan instead.
But where and how? Analysis of the gravitational field of Titan combined with theoretical geophysical modeling suggests that the moon may have a core made up of hot rocks. Detection of the radiogenic isotope of argon in Titan’s atmosphere by the Huygens GC-MS seems to imply that volatile species can be transported from Titan’s core all the way up to its atmosphere by a process known as cryovolcanism. In a paper accepted for publication in Icarus, Christopher Glein (University of Toronto, Canada, and Carnegie Institution of Washington, USA) builds on these ideas. In the paper, he describes a new geochemical model for the origin of Titan’s atmosphere based on hydrothermal production of molecular nitrogen and methane by cooking ammonia and carbon dioxide, respectively, in Titan’s core . Hydrothermal activity is unavoidable on an icy body like Titan, where water is in contact with hot rocks. By performing mass balance and thermodynamic calculations, Glein shows that this type of (endogenic) model explains the pattern of noble gases in Titan’s atmosphere, and the abundances and isotopic compositions of molecular nitrogen and methane in Titan’s atmosphere. Ten years after Huygens, we have a self-consistent model of the data that helps lead us into the future of Titan exploration.
An exciting implication of this model is that Titan’s atmosphere represents a window to understanding deep geochemical processes, such as the Sabatier synthesis of methane, elsewhere in the solar system. These insights provide opportunities for expanding our knowledge of the behavior of carbon and other volatile elements in planetary interiors. The model also draws attention to the emerging theme of a “hydrothermal solar system”, where processes driven by heat and water can bring life to the cold and otherwise dead outer solar system.
Images: Above left: Titan's atmosphere makes Saturn's largest moon look like a fuzzy orange ball in this natural color view from the Cassini spacecraft. Titan's north polar hood is visible at the top of the image, and a faint blue haze also can be detected above the south pole at the bottom of this view. NASA/JPL-Caltech/Space Science Institute. Above right: Hydrothermal vent. NOAA.