An Active Organic Chemistry

Analogies can also be made between the active organic chemistry which is currently going on on Titan and the prebiotic chemistry on primitive Earth. Several organics have already been detected in Titan's atmosphere, including compounds usually considered as key molecules in terrestrial prebiotic chemistry, such as hydrogen cyanide (HCN), cyanoacetylene (HC3N) and cyanogen (C2N2). The detected organics in the stratosphere21 are hydrocarbons (both

Figure 3.9. Cassini Radar image of Titan's surface at high latitude regions obtained in February 2007, showing the presence of a large lake (several 100 km, with a ~100 km size island. Credit: NASA/JPL.
Figure 3.10. Top: The see sand dunes seen by Cassini on Titan surface (top) look like Namibian sand dunes on Earth (bottom). Credit: upper photo NASA/JPL—lower photo; NASA/JSC.

with saturated and unsaturated chains) and N-organic compounds, mainly nitriles, as expected from laboratory simulation experiments. Since the arrival of Cassini in the Saturn system, the presence of water and benzene has been unambiguously confirmed by the CIRS instrument.

Direct analysis of the ionosphere by the INMS instrument during the low altitude Cassini fly-bys ofTitan shows the presence ofmany organic species at detectable levels at very high altitudes (1100-1300 km). The mass range of this instrument is limited to a maximum mass of 100 Daltons. However, extrapolation of the data strongly suggests that high molecular weight species up to several thousand Daltons may be present in the ionosphere.22 These new data open a fully new vision of the organic processes occurring in Titan's atmosphere, with a strong implication ofthe ionospheric chemistry in the formation of complex organic compounds in Titan's environment, which was not envisaged before (Fig. 3.11).

On the contrary, the GC-MS instrument on board Huygens19 did not detect many organics in the low atmosphere. This is probably due to their condensation on the aerosols. These particles, for which no direct data on chemical composition were available before, have been analyzed by the ACP instrument.20 The data show that the aerosols are made of refractory organics that release HCN and NH3 during pyrolysis. Moreover, the nature ofthe pyrolysates indicates the potential presence of nitrile, amino and/or imino groups.

Table 3.2A. Instruments on the Cassini spacecraft, InterDisciplinary Programs (IDP), the leading scientists and the potential for astrobiological return of their investigation

Instrument or IDP

P.I., T.L. or IDS*

Country

Astrobiological Return

Optical Remote Sensing Instruments

Composite infrared spectrometer (CIRS)

V. Kunde,

USA

+++

M. Flasar

Imaging science subsystem (ISS)

C. Porco

USA

+++

Ultraviolet imaging spectrograph (UVIS)

L. Esposito

USA

++

VIS/IR mapping spectrometer (VIMS)

R. Brown

USA

++

Fields Particles and Waves Instruments

Cassini plasma spectrometer

D. Young

USA

+

Cosmic dust analysis

E. Grün

Germany

+

Ion and neutral mass spectrometer

H. Waite

USA

+++

Magnetometer

D. Southwood,

USA

NA'

M. Dougherty

Magnetospheric imaging instrument

S. Krimigis

USA

NA

Radio and plasma wave spectrometer

D. Gurnett

USA

NA

Microwave Remote Sensing

Cassini radar

C. Elachi

USA

+++

Radio science subsystem

A. Kliore

USA

++

Interdisciplinary Program

Magnetosphere and plasma

M. Blanc

France

+

Rings and dust

J.N. Cuzzi

USA

+

Magnetosphere and plasma

T.I. Gombosi

USA

+

Atmospheres

T. Owen

USA

+++

Satellites and asteroids

L.A. Soderblom

USA

+

Aeronomy and solar wind interaction

D.F. Strobel

USA

++

*P.I. = Principal Investigator; T.L. = Team Leader; IDS = InterDisciplinary Scientist. *NA = not applicable.

*P.I. = Principal Investigator; T.L. = Team Leader; IDS = InterDisciplinary Scientist. *NA = not applicable.

This result strongly supports the hypothesis that the aerosols have a molecular composition very close to that of the laboratory tholins. These particles are probably made of a refractory organic nucleus, covered with condensed volatile compounds, with a mean diameter of the order of1 ^m (Fig. 3.12).

After sedimentation the aerosols should accumulate on the surface to form a deposit of complex refractory organics and frozen volatiles. The DISR data show the presence ofwater ice but no clear evidence—so far—of tholins. On the other hand, analysis of the atmosphere near the surface by GC-MS subsequent to Huygens'

Table 3.2B. Instruments on the Huygens entry probe, InterDisciplinary Programs (IDP), the leading scientists and the potential for astrobiological return of their investigation

Instrument or IDP

P.I. or IDS*

Country

Astrobiological Return

Gas chromatograph-mass spectrometer (GC-MS)

H. Niemann

USA

+++

Aerosol collector and pyrolyser

G. Israël

France

+++

Huygens atmospheric structure instrument

M. Fulchignoni

Italy

++

Descent imager/spectral radiometer (DISR)

M. Tomasko

USA

+++

Doppler wind experiment

M. Bird

Germany

+

Surface science package (SSP)

J. Zarnecki

UK

+++

Interdisciplinary Program

Aeronomy

D. Gautier

France

++

Atmosphere/surface interactions

J.I. Lunine

USA

++

Chemistry and exobiology

F. Raulin

France

+++

Figure 3.11. Averaged mass spectrum of Titan's ionosphere taken by the Cassini-INMS instrument near 1,200 km altitude. The spectrum shows signature of organic compounds including up to 7 (carbon/nitrogen) atoms. Image Credit: NASA/JPL/University of Michigan.

landing indicates the likely presence of many organics, including N-organics, C3 and C4 hydrocarbons and benzene, vaporized by the heating of the probe. This is in agreement with the hypothesis that the surface is rich in condensed volatile organics and that most of the organic compounds are in the condensed phase in the low atmosphere.

With this picture of Titan's organic chemistry, the chemical evolution of the main atmospheric constituents—dinitrogen and methane—produces mainly ethane which accumulates on the surface or the near sub-surface, eventually dissolving in methane-ethane lakes and seas and complex refractory organics which accumulate on the surface together with condensed volatile organic compounds such as HCN and benzene. In spite ofthe low temperature, contrary to what was often said, Titan is not a congealed Earth: the chemical system is not frozen. Titan is an evolving planetary body and so is its chemistry. Once sedimented on Titan's surface, the aerosols and their complex organic content may follow a chemical evolution of astrobiological interest. Laboratory experiments show that Titan tholins can release many compounds ofbiological interest, such as amino acids, once in contact with liquid water. This seems possible even with water ice. Those processes could be particularly favourable in zones of Titan's surface where cryovolcanism is occurring. Thus one can envision the possible presence of such compounds on Titan's surface or near subsurface. In situ measurement of Titan's surface would thus offer a unique opportunity to study by a ground truth approach some of the many processes that could be involved in prebiotic chemistry, including isotopic and enantiomeric fractionations.

In fact, even the presence of liquid water on Titan's surface is a possibility in spite of the low surface temperature. Cometary impacts can melt the surface water ice and provide, during periods as long as about 1000 years, conditions of terrestrial-like prebiotic syntheses with localized liquid water bodies.23 The hypothetical internal water-ammonia ocean mentioned above is also a place where an active prebiotic chemistry can occur, like in the case of Europa. These prebiotic processes are less likely now, since Titan's ocean is probably not in contact any more with the silicate layer, but they might have occurred in the early history of Titan, with the possible presence of hydrothermal activity allowing a CHON (Carbon, Hydrogen, Oxygen and Nitrogen) prebiotic chemistry over long periods of time (several million years). Thus even the presence of life in this hidden ocean today cannot be excluded.

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