Читать книгу Planet Formation and Panspermia - Группа авторов - Страница 16

The possibility that panspermia could act over interstellar distances has been debated for at least two decades. Initial estimates of the probability that a rock ejected from Earth could be captured by another terrestrial planet in a different stellar system in the solar neighborhood were deemed too small to be relevant as a life-spreading mechanism [2.29]. Thus, interstellar panspermia was also dismissed as implausible. However, subsequent studies argued that such a conclusion was probably too pessimistic [2.45, 2.13, 2.46]. In fact, it was shown that the capture probability could increase in crowded environments, such as in star-forming clusters [2.2, 2.7], and can be significantly enhanced by interactions with binary systems [2.24].

Because the survivability of microorganisms in deep space depends on the shielding mechanism provided by the rocks, there is probably a minimal mass to life-carrying fragments for panspermia to work at interstellar distances, of order ~1–10 kg. If the typical survival time of microorganisms trapped in the rocks is τ_s, the fraction of surviving microorganisms after a travel time t can be modeled as P ∝ e^-t/τs [2.13]. No exact estimate for τ_s exists, although values of order ~10⁵ years or higher seem possible given favourable conditions. In this regard, we note that the assumption that microorganisms can only survive when shielded within rocks is rather conservative: more speculative scenarios can be envisioned, where microorganisms endure the vacuum of space without insolation and atmosphere (powered, for example, by slow chemical reactions or even long-lived radionuclides), leading to much larger values of τ_s. Whichever the case, by adopting this simple survival model and assuming a dynamical mechanism for the transfer of material, one can estimate the rate of life-bearing rocks impacting a terrestrial planet at any location in the Galaxy.

The possibility that interstellar panspermia played a role over the entire disk of the Milky Way is certainly not established conclusively, but it cannot be entirely dismissed either. Because of the different stellar densities at various locations, the effectiveness of the mechanism is not homogeneous over the whole Galaxy, and it might have been more important within specific subvolumes. In [2.5], we argued that the eventuality of lithopanspermia should be given special consideration for planets residing in the galactic bulge, where the high density of stellar systems might make the transfer more likely than in the disk (see also [2.8]). We made an initial estimate of the efficiency of panspermia in the bulge by adopting the model outlined in [2.29, 2.2] for the rate of life-bearing rocks impacting a terrestrial planet in another stellar system

[2.1]

where v is the relative velocity of rocks with respect to the stars, nL is the number density of life-bearing rocks, and σ is the impact cross-section. The latter can be computed as the product of the capture cross-section from a stellar system, σ_c, and the probability that a rock impacts a terrestrial planet in the system once is captured, P_impact. Plausible values for σ_c are in the range 0.01–0.05 AU2 [2.29] and are expected to vary based on the average stellar velocity dispersion, orbital configurations and multiplicity of the stellar and planetary systems, ejection velocity, rock size distribution, and so on. In our analysis, we adopted the values σ_c = 0.025 AU² and P_impact = 10–⁴ from [2.29]: these are probably conservative in general, and in particular with respect to the conditions in the bulge. In fact, the value adopted for σ_c applies to planetary systems with a Jupiter-type planet in a Jupiter-like orbit. However, only ~10% of all systems meet this criterion. As already mentioned, binary star systems (that make up roughly 40% of all stars) have a much higher cross-section [2.24, 2.13]. Similarly, the capture rate can be enhanced in systems that contain massive hot Jupiters or brown dwarfs, both of which could have habitable exomoons. As an illustration, using the fit for σ_c from [2.2] and assuming a velocity dispersion ~120 km/s for stars in the bulge [2.44] would result in a value σ_c = 0.045 AU².

The number density of life-bearing rocks per year can be assumed to be proportional to the star density, nL = γnt, with γ~15/yr [2.2]. Then, the typical diffusion timescale for life between stellar systems in the galactic bulge can be found by τ = 1/г and is

[2.2]

If indeed life “colonizes” a suitable planet after transport, t_D represents the typical timescale for the evolution of the fraction of inhabited planetary systems in the bulge. Adopting a realistic model for the stellar density n leads one to conclude that, all over the bulge, even a single inhabited planet might in principle spread life to all other suitable stellar systems in a time ~1 Gyr, much smaller than the age of the Galaxy [2.5].

While this is not a full-fledged examination of the problem, it gives some support to the idea that the galactic bulge could be seeded with biological material much more efficiently than the solar neighborhood. A possible hindrance, in this respect, is the effect of the radiation environment, which is certainly harsher near the galactic center than in the disk [2.4, 2.5], as well as the higher risk of potentially sterilizing events such as supernovae or tidal disruption events [2.35]. Even if life is not completely wiped out, ionizing radiation can still influence planetary habitability by enhancing the rate of atmospheric mass loss (see, e.g., [2.34]). Furthermore, it has been argued that a high level of ultraviolet radiation could suppress the formation of terrestrial planets via protoplanetary grain evaporation [2.1]. However, strong ultraviolet doses could also have beneficial effects, for example, by increasing the rate of prebiotic synthesis of biomolecular building blocks [2.25].

It should also be noted that there might be safer routes for microorganisms if the transfer happens indirectly, for example, through cometary bodies that are subsequently captured into a protoplanetary disc: this mechanism could in principle spread life through the Galaxy at a rate of ~5 kpc Gyr^–1, covering the entire Galaxy in just a few Gyr [2.42], a short time compared to the age of the oldest stars in the Milky Way, 13.41 ± 0.54 Gyr [2.37].