Analogies between Titan and the Earth

With a diameter of more than 5100 km, Titan is the largest moon of Saturn and the second largest moon of the Solar System. It is also the only one to have a dense atmosphere. This atmosphere, as is clearly evidenced by the presence of haze layers (Figures 14.1, 14.2) extends up to approximately 1500 km (Fulchignoni et al., 2005). Like Earth, Titan's atmosphere is mainly composed of dinitrogen (= molecular nitrogen), N2. The other main constituents are methane, CH4, with a mole fraction of about 0.016-0.02 in the stratosphere, as measured by the composite infrared spectrometer (CIRS) instrument on Cassini (Flasar et al., 2005) and the gas chromatograph-mass spectrometer (GC-MS) on Huygens (Niemann et al., 2005) and dihydrogen (= molecular hydrogen,) H2, with a mole faction of the order of 0.001. With a surface temperature of approximately 94 K, and a surface pressure of 1.5 bar, Titan's atmosphere is nearly five times denser than the Earth's. Despite

Table 14.1. Cassini-Huygens science instruments and interdisciplinary scientists (IDS) and the potential astrobiological return of their investigation

Cassini instruments and interdisciplinary programs

PI, team leader, or IDS

Country Astrobiological return

Optical remote sensing instruments composite infrared spectrometer (CIRS) imaging science subsystem ultraviolet imaging spectrograph (UVIS) visual and IR mapping spectrometer Fields particles and waves instruments Cassini plasma spectrometer D. Young cosmic dust analysis ion and neutral mass spectrometer magnetometer

C. Porco L. Esposito

R. Brown

E. Grün H. Waite magnetospheric imaging instrument radio and plasma wave spectrometer Microwave remote sensing Cassini radar radio science subsystem Interdisciplinary science magnetosphere and plasma rings and dust magnetosphere and plasma atmospheres satellites and asteroids aeronomy and solar wind interaction

D. Southwood/ M. Dougherty S. Krimigis

D. Gurnett

C. Elachi A. Kliore

M. Blanc J.N. Cuzzi T.I. Gombosi T. Owen L. A. Soderblom

USA USA

Germany

USA USA

France

Huygens instruments and PI or IDS interdisciplinary programs

Country Exobiological return gas chromatograph-mass spectrometer aerosol collector and pyrolyser Huygens atmospheric structure instrument descent imager/spectral radiometer Doppler wind experiment

H. Niemann G. Israël M. Fulchignoni M. Tomasko M. Bird

Germany +

266 Titan: a new astrobiological vision from the Cassini-Huygens data Table 14.1. (cont.)

surface science package J. Zarnecki

Interdisciplinary science aeronomy D. Gautier atmosphere/surface J. I. Lunine interactions chemistry and exobiology F. Raulin

France USA

France

Fig. 14.1. This image taken with the Cassini spacecraft narrow-angle camera shows the complex structure of Titan's atmospheric haze layers. It was taken on 3 July 2004, at a distance of about 789 000 kilometres from Titan. Image courtesy of NASA/JPL/Space Science Institute.

these differences between Titan and the Earth, there are several analogies that can be drawn between the two planetary bodies.

The first resemblance concerns the vertical structure of its atmosphere (see Table 14.2). Although Titan is much colder, with a troposphere (~94-70K), a tropopause (70.4 K), and a stratosphere (~70-175 K), its atmosphere presents a similar complex structure to that of the Earth and also includes, as evidenced by

Fig. 14.2. This ultraviolet image of Titan's night side limb, also taken by the narrow-angle camera, shows many fine haze layers extending several hundred kilometres above the surface. Image courtesy of NASA/JPL/Space Science Institute.

Table 14.2. Main characteristics of Titan (including the HASI-Huygens data)

surface radius 2.575 km surface gravity 1.35 m s-2 (0.14 Earth's value)

mean volumic mass 1.88 kg dm-3 (0.34 Earth's value)

distance from Saturn 20 Saturn radii (~1.2 x 106 km)

orbit period around Saturn ~ 16 days orbit period around Sun —30 years

Atmospheric data surface tropopause stratopause mesopause

Altitude (km)

Temperature (K)

Pressure (mbar)

1470 135

Fig. 14.3. A picture of Titan taken near the south pole by the ISS narrow-angle camera on Cassini shows, in addition to bright clouds, the presence of a dark feature on the upper left which could be a liquid hydrocarbon lake. The pole is in the middle of the picture. Image courtesy of NASA/JPL/Space Science Institute.

Fig. 14.3. A picture of Titan taken near the south pole by the ISS narrow-angle camera on Cassini shows, in addition to bright clouds, the presence of a dark feature on the upper left which could be a liquid hydrocarbon lake. The pole is in the middle of the picture. Image courtesy of NASA/JPL/Space Science Institute.

Cassini-Huygens, a mesosphere and a thermosphere. Because of a much higher density in the case of Titan, the mesosphere extends to altitudes higher than 400 km (instead of only 100 km for the Earth), but the shape looks very much the same.

These analogies are linked to the presence in both atmospheres of greenhouse gases and antigreenhouse elements. Methane has strong absorption bands in the medium and far infrared regions corresponding to the maximum of the infrared emission spectrum of Titan and is transparent in the near ultraviolet and visible spectral regions. It can thus be a very efficient greenhouse gas in Titan's atmosphere. H2, which also absorbs in the far infrared (through bimolecular interaction - its dimers) plays a similar role. In the pressure-temperature conditions of Titan's atmosphere, CH4 can condense but H2 cannot. Thus, on Titan, CH4 and H2 are equivalent respectively to terrestrial condensable H2O and non-condensable CO2. In addition, the haze particles and clouds in Titan's atmosphere play an antigreenhouse effect similar to that of the terrestrial atmospheric aerosols and clouds (McKay et al., 2005).

Indeed, CH4 on Titan seems to play the role of water on the Earth, with a complex cycle which still has to be understood. Although the possibility that Titan is covered with hydrocarbon oceans (Lunine, 1993) is now ruled out (West et al., 2005), Cassini data show that Titan's surface includes lakes of methane and ethane, as detected by the radar instrument in the north polar regions of Titan (Stofan et al., 2007; Sotin, 2007). In addition, the imaging science subsystem (ISS) camera on Cassini has detected dark surface features near the south pole (Figure 14.3) which could be such liquid bodies. Moreover, the descent imager/spectral radiometer (DISR) instrument on Huygens has provided pictures of Titan's surface which clearly show dentritic structures (Figure 14.4) which look like a fluvial net in a relatively young terrain (fresh crater impacts), strongly suggesting recent liquid flow on the surface of Titan (Tomasko et al., 2005). In addition, GC-MS data show that the methane mole fraction increases in the low troposphere (up to 0.05) and reaches the saturation level at approximately 8 km altitude, allowing the possible formation of clouds and rain (Niemann et al., 2005). Furthermore, GC-MS analyses recorded an ~50% increase in the methane mole fraction at Titan's surface, suggesting the presence of condensed methane on the surface near the lander.

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, as we find on Earth (Figure 14.5).

Analogies between Titan and the Earth have even been pushed further by comparing Titan's winter polar atmosphere and the terrestrial Antarctic ozone hole (Flasar et al., 2005), although these implied different chemistry.

Another important comparison concerns the noble gas composition and the origin of the atmosphere. The ion neutral mass spectrometer (INMS) on Cassini and GC-MS on Huygens have detected argon in the atmosphere. Similarly to the Earth's atmosphere, the most abundant argon isotope is 40Ar, which comes from the radioactive decay of 40K. Its stratospheric mole fraction is about 4 x 10-5, as measured by GC-MS (Niemann et al., 2005). The abundance of primordial argon (36Ar) is about 200 times smaller. Moreover, the other primordial noble gases have a mixing ratio smaller than 10 ppb. This strongly suggests that Titan's atmosphere, like that of the Earth, is a secondary atmosphere produced by the degassing of trapped gases. Since N2 cannot be efficiently trapped in the icy planetesimals which accreted and formed Titan, unlike NH3, this also indicates that its primordial atmosphere was initially made of NH3. Ammonia was then transformed into N2 by photolysis and/or impact driven chemical processes (Owen, 2000; Gautier and Owen, 2002). The 14N/15N ratio measured in the atmosphere by INMS and GC-MS (183 in the stratosphere) is less than the primordial N and indicates that several times the present mass of the atmosphere has probably been lost during the history of the satellite (Niemann et al., 2005). Since such evolution may also imply methane transformation into organics, this may also be an indication of large deposits of organics on Titan's surface.

Fig. 14.4. Channel networks, highlands, and dark-bright interface seen by the DISR instrument on Huygens at 6.5 km altitude. Image courtesy of ESA/NASA/JPL/University of Arizona.
Fig. 14.5. Titan, seen by the Cassini spacecraft narrow-angle camera, shows a very diverse surface, with bright (like the so-called 'Xanadu' region in the middle of the picture) and darker areas. Image courtesy of NASA/JPL/Space Science Institute.

Analogies can also be made between the organic chemistry which is now very active on Titan and the prebiotic chemistry which was active on the primitive Earth. In spite of the absence of permanent bodies of liquid water on Titan's surface, both chemistries are similar. Several of the organic processes which are occurring today on Titan imply the presence of the organic compounds which are considered as key molecules in the terrestrial prebiotic chemistry, such as hydrogen cyanide (HCN), cyanoacetylene (HC3N) and cyanogen (C2N2). In fact, the atmosphere of Titan (dinitrogen with a small percentage of methane) is one of the most favourable atmospheres for prebiotic synthesis, as shown by Miller's experiments. Until recently, such atmosphere composition was supposed to be far from that of the primitive Earth. However, a new model of the hydrogen escape in the primitive atmosphere of the Earth suggests that it may have been much richer in hydrogen and methane than previously thought (Tian et al., 2005). This suggests that Titan may be even more similar to the primitive Earth than we thought.

Indeed, a complex organic chemistry seems to be present in the three components of what one can call, always by analogy with our planet, the 'geofluids' of Titan: air (gas atmosphere), aerosols (solid atmosphere) and surface (lakes).

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