Main Astrobiological Aspects ofTitan

Titan's dense atmosphere, which extends up to about 1500 km, is mainly composed of N2 like the atmosphere of the Earth. The other main constituents are CH4 (~1.6-2% in the stratosphere as measured by the CIRS on Cassini and the GC-MS on Huygens) and H2 (~0.1%). Although Titan is much colder, with a troposphere (~94—70 K), a tropopause (70.4 K) and a stratosphere (~70-175 K), its atmosphere as recently evidenced by Cassini-Huygens presents a similar complex structure and includes, like the Earth, a mesosphere and a thermosphere. These similarities are linked to the presence of greenhouse gases and anti-greenhouse elements in both atmospheres. On Titan CH4 and H2 are equivalent to terrestrial condensable H2O and noncondensable CO2. In addition, the haze particles and clouds in Titan's atmosphere play an anti-greenhouse effect similar to that of the terrestrial atmospheric aerosols and clouds.

Methane on Titan seems to play the role of water on Earth with a complex cycle not yet fully understood. Cassini ISS has detected surface features near the South Pole that might be a lake; radar has discovered the presence of lakes and seas in the north polar region, likely to be of liquid methane and ethane (Fig. 3.9). Moreover, the DISR instrument on Huygens has provided pictures of Titan's surface that clearly show dentritic structures resembling terrestrial fluvial net (Fig. 3.8). These features, free of crater impacts, are in a relatively young terrain where liquid flowed recently. As shown by the Huygens GC-MS data, the CH4 abundance in the low troposphere above the landing site reaches the saturation level at an altitude of approximately 8 km, allowing the possible formation of clouds. The potential resulting rain may be the cause ofthe observed fluvial-like net. In addition, GC-MS data also suggest the presence of condensed methane on the landing site. Other observations from the Cassini instruments clearly show the presence of various surface features of different origins indicative of volcanic, tectonic, sedimentological and meteorological processes similar to those that are found on Earth (Fig. 3.10).

INMS and GC-MS have detected the presence of argon (Ar) in Titan's atmosphere. Like in the Earth atmosphere, the most abundant isotope is 40Ar, which should come from the radioactive decay of40K. Its stratospheric mole fraction, measured by GC-MS, is about 4 x 10-5. The abundance of primordial argon (36Ar) is about 200 times smaller. The other primordial noble gases have a mixing ratio ofless than 10 ppb. This strongly suggests that Titan's atmosphere is ofa secondary origin formed by the degassing oftrapped gases. Now, N2 could not have been efficiently trapped in the icy planetesimals from which Titan formed, but NH3 could. Thus it is believed that Titan's primordial atmosphere was initially made of NH3 which was then progressively transformed into N2 by photolysis and/or impact driven chemical processes. The 14N/15N ratio measured by GC-MS in the stratosphere ( = 183) is 1.5 times smaller than that of primordial nitrogen. This indicates that the atmosphere was probably lost several times since its formation with the simultaneous

Figure 3.7. Experimental device for laboratory simulation of Titan's atmosphere chemistry at LISA (Laboratoire Interuniversitaire des Systèmes Atmosphériques, Universités Paris 12 et Paris 7). A gas mixture of N2 (98%) and CH4 (2%) flows continuously flows through an open reactor and is maintained at low pressure (~1 hPa) and low Temperature (-150 K) (Credit : P. Coll).

Figure 3.7. Experimental device for laboratory simulation of Titan's atmosphere chemistry at LISA (Laboratoire Interuniversitaire des Systèmes Atmosphériques, Universités Paris 12 et Paris 7). A gas mixture of N2 (98%) and CH4 (2%) flows continuously flows through an open reactor and is maintained at low pressure (~1 hPa) and low Temperature (-150 K) (Credit : P. Coll).

Figure 3.8. Image of Titan's surface taken by the DISR instrument on Huygens at 6.5 km altitude and showing channel networks, highlands and dark-bright interface. Credit: ESA/NASA/J PL/University of Arizona.

transformation of methane into organics, suggesting the presence of large deposits of organics on Titan's surface.

0 0

Post a comment