Titan as an Exobiology Laboratory
Basic Properties of Titan (Ref: "Saturn" by Arizona Press) Surface radius = 2575 km Mass = 0.022 Earth mass Surface gravity = 135 cm/s2 Mean density = 1.881 g/cm3 Rock/Ice ratio = 52:48 (by mass) Distance from Saturn = 20 Saturn radii Distance from Sun = 9.546 AU Orbital period = 15.95 day Heliocentric period = 30 yr Surface temperature = 92 K Effective temperature = 86 K Tropopause temperature = 71.4 K ( 42 km) Statopause temperature = 170 K (200 km) Bond albedo = 0.20 Solar flux = 1.1% of flux at Earth Surface pressure = 1496 mbar
Titan may serve as a model of a highly reduced early Earth atmosphere in which the first stages of organic chemical evolution could take place in the atmosphere. Laboratory simulations show that a Titan-like atmosphere, primarily a N2/CH4 mixture, under various energetic excitations forms not only HCN, (CN)2, and HC3N (observed on Titan as a result of photodissociation reactions), but also CH3CN, (HCN)4, and finally, adenine, a component of DNA. When simple organic compounds such as methane or ethane are irradiated by solar UV radiation, a heteropolymer (or Tholin) forms. Such Tholins do not form on present-day Earth, but are found in great abundance on the surface of Titan. It is important to note that these Tholins are distinct from the so-called "ice Tholins", which form by irradiation of clathrates of water and organic compounds such as methane or ethane. Titan is not the only body in the outer solar system that produces Tholins. The surfaces of comets, centaurs, and other ice covered moons may also be rich in deposits of Titan and ice Tholins. There has been much speculation that comets rich in Tholin compounds may have delivered organic compounds to Earth early in its history. These organic compounds may have lead to the development of life on Earth. This makes Titan an object of exceptional interest for exobiology studies.
The following diagram depicts the various cycles and processes that occur in Titan's atmosphere.
How do the properties of Titan compare with those of Earth's?
The following diagram shows how complex molecules might be formed on Titan.
The diagram below shows the various haze layers in Titan's atmosphere.
Constituent Formula Equator North Pole Nitrogen N2 0.9-0.99 0.9-0.99 Argon Ar <0.06 ? <0.06 ? Methane CH4 0.017-0.045 0.017-0.045 Hydrogen H2 0.001 0.001 Ethane C2H6 1.3E-5 1.65E-5 Acetylene C2H2 3E-6 6.5E-6 Propane C3H8 5E-7 1.2E-6 Hydrogen Cyanide HCN 1.7E-7 1.5E-6 Ethylene C2H4 1.5E-7 1.5E-5 Cyanoacetylene HC3N ? 4.5E-8 Propyne C3H4 5E-9 3.7E-8 Diacetylene C4H2 1.4E-9 2.7E-8 Cyanogen C2N2 - 2.2E-8 Carbon Monoxide CO 5E-5 6E-5 Carbon Dioxide CO2 1.4E-8 1.3E-8 Water Vapour H2O 8E-9 Dicyanoacetylene C4N2 - solid phase Note: Shown are the stratospheric mole fractions after Gautier and Raulin (1997) and Coustenis et al. (1998). The presence of Argon-36, which is cosmogonically abundant and not condensable in Titan's atmospheric condition, is predicted in order to explain the observed mean molecular weight in excess of 28 amu, but it has not been discovered yet. The estimated upper limit of Argon is 10%.
Life on Titan ? Surface is too cold (about 92 K) and not energetic enough to provide the right conditions. However, the (hypothetical) subsurface ocean, may be suitable to life. Fortes (Icarus, 2000) shows that there are no insurmountable obstacles: With a possible temperature of a subsurface ocean as high as about 260 K and the occurrence of cryovolcanic hotspots allowing 300 K Even at a depth of 200 km, the expected pressure of about 5 kbar is not incompatible with life In regards to pH values: 15% wt NH3 is equivalent to a pH of 11.5. Some bacteria can grow on Earth at pH 12. Energy comes mainly from radiogenic heat flow (about 5E11 W). If 1% of that is used for volcanic activity and if 10% of the later is available for living systems metabolism: this gives about 5E8 W. 5E8 W corresponds to the production of about 4E11 mol of ATP per year and about 2E13 g of biomass per year. If one assumes a turn over of the order of a year: the biomass density would be 1 gram per square meter: 1000 times lower than Earth, but not negligible. A good candidate for life on Titan would be Methanogenic microorganisms. Could these be the source of CH4 in Titan's atmosphere, or even of the atmospheric N2 ? However, it is important to note that there are several other sources for CH4, including cometary impact chemistry!
