The surprinsingly rapid start of life on Earth - in geologic terms - has raised the question about whether life originated from purely endogenic processes, or whether it was favored to a greater or lesser degree by exogenous agents. The most extreme of the latter type of theories is panspermia that assumes that life was created elsewhere in the universe and brought to Earth by some vector, as for instance comets.
Comets are very rich in organic matter (see Table 3.2). Therefore, besides water comets supplied the organic ingredients to form the broth in which more complex polymers and finally microorganisms emerged. Could microorganisms have formed within comet nuclei themselves and seeded the primitive Earth, thus answering the question about why life started so early? Could comets offer a friendly environment for the development of lifeforms? Two distinguished scientists as Fred Hoyle and Chandra Wickramasinghe have argued that bacteria, algae and viruses populate the interstellar medium and comets, based on the similarities between the spectrum of interstellar dust with those produced by micro-organisms (Hoyle and Wickramasinghe 1979). However, the interpretation of the broad spectral features in terms of algae and bacteria is not unique, and other more conventional non-biological sources are as well possible, so we should take their claim with extreme caution.
The origin and early evolution of life on Earth argues in favor of liquid water as a necessary ingredient. There are no known organisms on Earth that can thrive on pure ice or that can extract liquid water from ice using metabolic energy (see, e.g. McKay 1997). Therefore, the search for life in comets - or in other celestial bodies - should follow the search for present or past existence of liquid water. Several authors have considered the possibility that the heat released by short-lived radioactive isotopes, in particular 26Al, would have maintained a liquid water core for a time long enough to allow the development of micro organisms (e.g. Wallis 1980, Prialnik et al. 1987). 26Al is a very powerful heat source of very short lifetime (7.4 x 105 yr). The product of its decay is 26Mg, and an excess of this isotope (that is stable) was found in Ca-Al inclusions in the Allende meteorite. 26Al was probably extant in the protoplanetary disk, probably produced by nearby ordinary novae and/or massive stars present in the early dense galactic environment of the Sun (cf. Section 10.8).
For a ratio 26Al/27Al~ 5 x 10"5 for the cometary dust, Podolak and Prialnik (1997) estimate an average heating rate of Q ~ 2 x 10"3 erg g"1 s"1 from 26Al decay. Let us consider a spherical cometary nucleus of radius RN, density p, and thermal conductivity K. The energy produced within a volume of radius r(< RN) will be transferred outwards by thermal conduction, so if we neglect other energy sinks, the energy balance equation gives
3 dr which upon integration between the limits 0 < r < RN leads to
For loose snow Wallis (1980) adopts: K = 1.67 x 104 erg cm"1 s"1 K"1 and p = 0.25 g cm"3, whereas for solid ice + dust (at ~ 200 K): K = 2.93 x 105 erg cm"1 s"1 K"1 and p = 1 g cm"3. By introducing the appropriate numerical values in eq. (11.1) for loose snow, we can see that interior temperatures in the comet nucleus raise above the melting point of ice in a 10 km-nucleus. We must also check that the interior pressure be above the triple point (,>6x 103 dyn cm"2). From eq. (10.40) we find that a 10-km radius nucleus has a central pressure of ~ 104 dyn cm"2, which is just above that required for keeping liquid water. Wallis (1980) thus concluded that comet nuclei greater than ~ 10 km radius could have had melted cores that lasted for about one Myr, i.e. until the 26Al source was exhausted (Fig. 11.5). These melted cores might have been surrounded by a 1-km thick ice shell as the water vapor generated in the central core leaked through cracks and interstices in the overlying layers and then recondensed into ice. He argues that the central fluid core would have provided a very suitable and protected environment for colonies of bacteria that could have survived the refreezing of the fluid core for several Gyr, so such frozen bacteria might still be apt for triggering life if they meet a suitable environment. Wallis then considers comets as a vector for panspermia.
More recently, Podolak and Prialnik (1997) have re-evaluated the melting of ice in the cometary interior and its maintainance, reaching a more pessimistic conclusion. They argue that efficient cooling mechanisms, such a heat conduction through the ice and heat carried by the flow of gas through the porous nucleus would have made difficult to reach the above conditions for core melting. Thus, if we allow for phase transition from amorphous to crystalline ice when the core reaches a temperature T ~ 137 K, this will result in the release of a latent heat of 9 x 108 erg g_1. This heat is sufficient to transform the adjacent layer. This layer too releases latent heat and crystallizes, and the process continues moving outwards until the subsurface layers. The heating and crystallization process stops in the outer layers because they are efficiently cooled by radiation. Because the thermal conduction of crystalline ice is about one order of magnitude greater than that of amorphous ice, the rate of cooling is much more rapid, so the temperature through the nucleus drops quickly.
There is an additional problem so far overlooked: comets are far from being considered safe heavens that have gone through very peaceful existences. As we analyzed before (cf. Section 10.11), in the early solar system comets were subject to mutual collisions that led to successive fragmentations and re-accummulations of fragments (Stern and Weiss-man 2001), thus making virtually impossible that liquid cores could have been preserved undisturbed. Giant comets ( > 100 km) might have been spared from catastrophic collisions, and they may be the best candidates for searching liquid environments in their interiors.
Another possibility, analyzed by McKay (1997), is that the solar nebula could have contained dormant lifeforms that were accreted together with the solid material into the planetesimals. This enters into the panspermia scheme in which life pre-dates the Earth and is ubicuitous in the universe, present in interstellar matter where the Sun and other stars form. In this scenario comets would have been the suitable vectors to carry such lifeforms from the nebular material to the Earth's surface. We do not know enough yet about the survival of dormant microorganisms under hostile environments, with high dose of radiation and in absence of liquid water, to assess the plausibility of this scenario.
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