On January 14 the Huygens Probe, built by the European Space Agency, made a soft landing on the surface of Saturn's moon Titan. The first data from the atmosphere and surface reveal a remarkable place indeed, as described in a science press conference held in Paris on January 21.
Titan is one of the three large icy moons in the solar system, together with Callisto and Ganymede in the Jupiter system. All three have diameters of about 5000 km and compositions that are a mixture of water ice and stony material. All three are also so cold that the water ice is hard-frozen at the surface. What dramatically differentiates Titan from the other two, however, is its thick atmosphere. The primary constituent of this atmosphere is nitrogen, with methane making up a few percent. The break-up of methane molecules in the upper atmosphere also produces small quantities of many complex organic molecules, such as ethane, acetylene, and hydrogen cyanide. The density of the atmosphere at the surface is 4 times greater than that of Earth's atmosphere.
The Huygens probe descended for more than 2 hours by parachute, analyzing the atmosphere and clouds and photographing the landscape beneath. It landed on a soft or muddy surface, where the probe continued to operate for another two hours.
The most remarkable initial findings concerned the role of methane in the lower atmosphere and on the surface. This gas is a liquid at the temperature of the surface (90 Kelvins), and evidently it also forms condensation clouds or fog in the lower atmosphere. Photos during the probe descent show dramatic evidence of liquid erosion in the form of complex dendritic drainage channels. The characteristics of the landing site suggested a thin hard crust above a soft layer; when this layer was analyzed it was found to consist of liquid methane "mud". Fist-size "stones" photographed on the surface might be made of water ice, floating on the mushy methane swamp. From this initial evidence it seems that Titan is like the Earth in having a sort of hydrological cycle, but based on methane not water. This is, apparently, a world of cyclic processes, not a frozen fossil of the geologic past.
The latest results from Huygens, including the photos of the surface and a report on the science news conference, can be found on the ESA website http://www.esa.int/SPECIALS/Cassini-Huygens/. For more information, planetary scientist and astrobiologist Jonathan Lunine of the University of Arizona provided a fascinating overview of the initial results on at a NAI Director's Seminar on January 24, which was attended by more than 200 people (mostly by video link). Lunine is an Interdisciplinary Scientist on the Cassini-Huygens mission and is Chair of the NAI Titan Focus Group. His presentation can be accessed on the NAI webpage.