Panspermia: |
Directed Panspermia
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Contents Introduction
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Although aimed at specific targets, the microbial payloads may carry life further in space and time. First, much of the microbial swarm will miss or transit the target. Secondly, of the initial 1E13 comets that capture capsules in the accreting system, up to 99% will be ejected into interstellar space [11], carrying the microbial content. These embedded capsules, shielded from radiation and preserved at 3 K, may survive in the comets for many Gyr, until eventually captured in accreting systems in other regions of the galaxy. Of the 1E11 comets remaining in the accreting system, most will remain in the cold <10 K Oort cloud which will be eventually ejected into interstellar space. Therefore the majority of the launched biomass will eventually carry the microbial payload further into the galaxy. The spread of microbial life by comets is similar to the proposals of Hoyle and Wickramasinghe [17], but we postulate here a directed origin. Future programs may aim intentionally to seed the entire galaxy. It is interesting to assess the feasibility of such a program. Once launched randomly into the galactic plane at v = 0.01 c, the microbial packets will traverse the galaxy (r = 7E4 ly [25]) in 7E6 yr. The packets are gravitationally bound to the galaxy and will eventually perform random paths. At these speeds, mm size capsules will transit all thin regions and will be captured only in protostellar condensations or denser accretion zones. The mass ratios above showed that 1E-13 of the captured biomass in these areas will be delivered to planets. With 100 capsules of 1E-10 kg, ie., a biomass of 1E-8 kg required to seed a planet, and with star-formation rate of 1 yr-1 in the galaxy, biomass needs to be launched at the rate of 1E5 kg/yr for 5E9 yr to seed all new stars during the lifetime of the solar system. For example, the biomass can be dispersed in pulses of 1E12 kg to seed the population of star-forming clouds as it is renewed every 1E7 yr. The total required biomass is 5E14 kg, compared for example with the 1E19 kg organic carbon (1%) in the 1E21 kg total asteroid mass. This resource allows increasing the launched biomass up to a factor of 2E6 to account for undoubtedly substantial losses. As a more conservative estimate, assume a 5 au capture zone, with a volume of 2E36 m3, with the total capture volume of 2E47 m3 about 1E11 stars. With a capture probability of 1E-5 and for delivering 100 captured capsules of 1E-10 kg each, 1E-3 kg needs to be placed about each star. This corresponds to a density of 5E-40 kg biomass m-3 in these circumstellar volumes. Assuming that this is achieved by establishing a similar biomass density through the 5E61 m3 volume of the galaxy, then the total biomass needed in the galaxy is 2.5E22 kg. Renewing this density each 1E9 yr for the 5E9 yr lifetime of the solar system, to seed every new planetary system during the first Gyr after its formation, gives a material requirement of about 1E23 kg, about 10% of the 1% C content in 1E26 kg of the total cometary mass. The material requirements can be reduced by many orders of magnitude if the missions are directed to star-forming regions rather than distributing biomass through the galaxy at random. Of course, the microbial population may be subject to substantial losses, but may be enhanced in the target zones by gravitational attraction. The fate of biological objects traversing the galaxy requires detailed analysis. It may be possible to grow the necessary large amounts of microorganisms directly in carbonaceous asteroids or comets. Carbonaceous C1 meteorites, and presumably asteroids, contain water in about the biological ratio of 5:1 H2O/C, and N in the biological ratio of 10:1 C/N, as well as biologically usable forms of the other macronutrients S, P, Ca, Mg, Na and K in at least the biological C/X elemental ratios [19]. Once the nutrient components are extracted, the residual inorganic components may be used for shielding materials for the microbial capsules. As a possible method for converting comets to biomass, the loose icy, cometary matrix may be fragmented and enclosed in membranes in 1 kg spheres. Warming and melting such a unit, from 10 to 300 K, requires 5.1E9 J, which can be provided by the solar energy flux of 325 W m-2 at 2 au, incident on the 3.1 m2 cross-section of a 1 m radius object during a two-months perihelion transit about 2 au. The microbial experiments show that in 6 - 8 days after inoculation, this organic solution will yield microbial densities of >1E8 CFU/ml which can survive for several months [18, 19]. Subsequently, the microbial solution can be converted to 1 mm "hailstones". These microbial ice capsules can be accelerated out of the solar system, for example, by first accelerating the comets sunward into parabolic orbits, and in this manner dispersing the Oort cloud at the rate of 20 comets yr-1 during 5E9 yr. This rate is comparable to the natural rate of 3 new comets/yr plus up to 1E9 new comets per/year during cometary showers [16], and the task may be accomplished at the required rate by processing every new comet that arives naturally from the Oort cloud.An interesting experiment in this direction would be to inoculate the sub-crust zone of an inbound comet, and of enclosed samples of the cometary material embedded in the comet, the latter to allow melting near the perihelion without evaporation. Embedded sensors could monitor microbial growth during the perihelion passage and, for a short-period comets, during further passages, to verify microbial growth in cometary materials and environments. Laboratory microbiology experiments with returned cometary materials would be also of interest. The above considerations suggest that a single technological civilisation can seed the galaxy. Similarly, one past panbiotic civilisation could have seeded the galaxy, accounting for the rapid emergence of life on Earth and possibly on Mars [2, 3, 26]. However, if ours is the first technological civilisation, the potential to seed the galaxy demonstrates the significance of directed panspermia that we can accomplish. Furthermore, by extrapolation, the material resources of 1E11 solar systems in one galaxy may be sufficient to seed all the 1E11 galaxies. Of course these are speculative long-term prospects. However, even a few comet-based missions in the nearer future, using a small fraction of the comet’s material, is sufficient to target one star-forming cloud for a major biological expansion. |
