Origin(s) of Life, Extraterrestrial Environments and Astrobiology

Since life emerged on Earth, about four billion years ago, our planet has drastically changed. Fluvial and aeolian erosions, plate tectonics and volcanism have erased most of the traces of the environment of the primitive Earth and fully remodelled the planet. It is thus extremely difficult to replace all the prebiotic processes, which allowed the first living system to arise, in a proper environmental context. One possible approach is to search for extraterrestrial environments were at least some of the conditions – if not all – which were prevailing on the primitive Earth four billion years ago are present today. Such planetary bodies may thus offer us the possibility of studying now, in a real planetary environment, by remote sensing and maybe also by in situ observation, some of the prebiotic processes which were occurring on Earth when life was emerging.

By carrying out experiments on prebiotic chemistry, we are currently trying to reproduce in the laboratory some of the steps – in particular the chemical ones – involved in the origin of life on Earth. But in such laboratory experiments, there is one parameter that we cannot reproduce experimentally: this is time duration! Again, another approach is to find environments were similar processes of prebiotic chemistry are still occurring today, on long duration timeframe.

Life on Earth is so far the only example we know. But it went through many steps still very badly known. What was the molecular structure of the pre-RNA first living systems? of the RNA ones? What were the properties of LUCA? Furthermore, if life arose after a long chemical evolution, there is no reason to believe that this occurred only on our planet. But would chemical evolution through prebiotic chemistry yield exactly the same result elsewhere? Most of these fundamental questions could be answered by the discovery of a second genesis. This requires searching for life in extraterrestrial environments: this is a third approach. The discovery of a second example would also allow us to define in a more easy way what is Life!

Now, the three approaches briefly described above are part of astrobiology. By definition, astrobiology is the study of the origin, distribution evolution and destiny of life in the universe. It includes the study of organic and prebiotic-like chemistry in extraterrestrial environments, as well as the study of the origin of life on Earth. Thus it is indeed essential for answering many of the basic questions about the origin of life.

I thus do not agree with the statement that “what we learn from cosmic products is of limited interest regarding prebiotic molecular evolution” for several reasons:

  • “Cosmic products” often include macromolecular matter of interest for prebiotic chemistry, and in particular macromolecular prebiotic chemistry. Its molecular structure is still very poorly known, because it is difficult to determine, but its study is a way to look at the emergence of chemical complexity in a real cosmic environment.

  • Such macromolecular cosmic products are issued from processes which often involve kinetic control and thus may have many analogies with the terrestrial prebiotic processes.

  • They involve very long duration time, not reachable in the (human) laboratory: thus it is a way to test our prebiotic chemical scheme in more realistic conditions.

It is also a way to approach the problem of the origin of chirality: with the current technology we can analyse by sample return mission as well as in situ the eventual enantiomeric excess of chiral molecules in extraterrestrial samples.

There are several “cosmic“ environments of interest, in that frame:

  • Meteorites and comets, and specially the cometary nucleus with the likely presence (to be confirmed) of complex macromolecular organics,

  • Titan, with its many analogies with the Earth, and again, a complex organic chemistry in its atmospheric haze particles (recently demonstrated thanks to Cassini-Huygens), in spite of its low temperatures.

The Case of Titan

Analogies with the Earth

With a diameter of more than 5,100 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 with the presence of haze layers masking the surface in the visible spectral domain. Like the Earth, Titan’s atmosphere is mainly composed of N2. The other main constituents are CH4, about 2.0% in the stratosphere and H2 about 0.1%. With a surface temperature of approximately 94 K, and an average surface pressure of 1.5 bar, Titan’s atmosphere is nearly five times denser than the Earth’s. Despite of these differences between Titan and the Earth, there are several analogies that can be drawn between the two planetary bodies (Raulin 2007).

The first resemblances concern the vertical atmospheric structure (Tables 1 and 2). Although much colder, Titan’s atmosphere presents a similar complex structure to that of the Earth. These analogies are linked to the presence in both atmospheres of greenhouse gases and antigreenhouse elements. 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. Indeed, methane on Titan seems to play the role of water on the Earth, with a complex cycle, which still has to be understood. The DISR instrument on Huygens has provided pictures of Titan’s surface which clearly show dentritic structures (Fig. 1) suggesting recent liquid flow on the surface of Titan. In addition, the Huygens GC-MS data indicates the presence of condensed methane on the surface near the landed probe (Nature 2005). Moreover, recent surface observations of Titan by the Cassini Radar strongly suggest the presence of lakes of methane and ethane in the polar regions.

Table 1 Main characteristics of Titan (including the HASI-Huygens data)
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Table 2 Atmospheric data
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Fig. 1
figure 1

Channel networks, highlands and dark-bright interface seen by the DISR instrument on Huygens at 6.5 km altitude. Credit: ESA/NASA/JPL/University of Arizona

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Similarly to the Earth atmosphere, 40Ar is present in Titan’s atmosphere and comes from the radioactive decay of 40K. This indicates that it is a secondary atmosphere, produced by the degassing of trapped gases. The 14N/15N ratio measured in the atmosphere by Cassini indicates that the present mass of the atmosphere was probably lost several times during the history of the satellite (Nature 2005). This may be also the indication of large deposits of organics on Titan’s surface.

An Active Extraterrestrial Organic Chemistry

With a N2–CH4 atmosphere, Titan has one of the most favourable atmospheres for prebiotic syntheses. Recent modelling of hydrogen escape in the primitive atmosphere of the Earth (Tian et al. 2005) suggests that Titan atmosphere maybe even more similar to that of the primitive Earth than we previously thought.

Analogies can thus also be made between the organic chemistry which is very active now 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 some 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). The most complex organics seem present in the aerosols particles the analysis of which by the ACP instrument on Huygens shows the presence of refractory organics, made of C, H and N atoms (Nature 2005). This complex organic matter seems close to the laboratory Titan’s refractory organic products, usually named Titan’s tholins, which release amino acids in the presence of water. These particles play a role in the radiative properties of Titan’s atmosphere; similar particles may have been present in the atmosphere of the primitive Earth, and may have played a similar role, of crucial importance when considering the problem of faint early sun.

Life on Titan?

Even the possibility of Life on Titan cannot be excluded. The models of internal structure of the satellite strongly suggest the presence of an internal ocean of liquid water (with a few % of ammonia), with conditions which are not incompatible with life, as we know it (Fortes 2000).

This gives us three main scientific reasons to astrobiologically explore Titan. The Cassini–Huygens mission will continue its observations of Titan within a probable extended mission until 2011. But it will not solve the question of life in Titan nor the question of the nature and evolution of the organics on its surface. A future mission of exploration of Titan with horizontal mobility is already under study. Its scientific return, should clearly concern the question of the origin of life.