Contents
Technical Sections

Directed Panspermia
- Technical Considerations -

9. Biological Considerations

The biological requirements were considered in relation to missions to nearby solar systems [4,5]. Some key points are as follows. 

The microbial design must allow survival during transit, and subsequently in diverse planetary and possibly cometary environments, and facilitate evolutionary pressures that will lead to higher evolution.

These criteria suggest a diverse microbial assembly. The anaerobic environment will require at least facultative anaerobes. Blue-green algae, and possibly eukaryotic algae may be the best colonising organism, the latter may lead to higher plant evolution. The photosynthetic organisms may survive first and establish an oxygen-containing atmosphere. Higher aerobes, including predatoryheterotrophs can grow from the capsules that are meanwhile stored in comets and asteroids, and are delivered to the planet later. The ensuing predator/pray selection pressures will lead to higher evolution. This may require aerobic conditions, although coceivably, higher, including intelligent anaerobes may be possible. 

The inclusion of simple multicellular eukaryotes is crucial, as this development may be a major evolutionary bottleneck. This development required billions of years on Earth, but then led rapidly to higher life-forms. Such a low probability event may not occur at all in other evolving ecosystems. 

Even the most primitive single-cell organism must include the complex DNA and protein structures for replication, as well as complex energy mechanisms and membrane transport systems. The origin of such a complex system would seem to have a low probability. Panspermia helps to overcome this probability barrier. However, possible finding of Martian micro-organisms [26] may suggest that the origin of primitive life is more probable. Even in this case, overcoming the second probability barrier to the emergence of multicellular eukaryotes may in itself justify the panspermia program.

For interstellar transit, the microbial payload may be freeze-dried, as is the current practice for preserving microbial cultures. For UV survival, the capsules must be shielded appropriately, at least with UV resistant films. It may be also desirable to include a nutrient medium in the capsule, and to enclose it in a selective membrane that will allow the supplied nutrient to slowly absorb and mix with the local planetary nutrients, so that the microorganisms can gradually adjust to the planetary chemistry (pH, redox potential, toxic components, specific local nutrients). For aerobic eukaryotes, it may be desirable to enclose them in separate capsules with shells that will dissolve only in oxygen-containing environments. This will preserve the aerobic eukaryotes until photosynthetic organisms create a suitable oxygen-containing atmosphere.

It may be possible to provide some of this shielding and nutrient using the solar sail that launches the capsule. The sail must constitute about 90% of the total mass of the small vehicles. The sail could be possibly made of proteinaceous or other biodegradable organics. It may be designed to fold over the microbial packets after propelling them from the solar system, and provide shielding during transit and capture, and eventually to provide nutrient materials on the host planet.

For succesful missions, the microorganisms must find adequate nutrients, which may be carbonaceous materials accumulated from dust particles, comets and asteroids, with organic content resembling carbonaceous chondrites. As a model, soil nutrient analysis of the Murchison C2 meteorite showed biologically available nutrient content (in mg/g) of: C and N in hydrothermally (121 ^oC, 15 minutes) extractable organics, 1.8 and 0.1; S as soluble SO₄^-2, 4.5; P as PO₄^-3, 6.4E-3; and extractable cations by 1 M CH3COONH4 solution at pH 7, Ca, 4.0; Mg, 1.7; Na, 0.57; K, 0.65 mg/g; and cation exchange capacity of 5.8 milliequivalents/100 g. All of these are values are comparable or higher than in average terrestrial agricultural soil. Use of the organic meteorite nutrients as sole carbon source was demonstrated by light emission from Pseudomonas fluorescence modified with a lux gene when challenged with the meteorite extract, and preliminary observations of growth of the thermophile eubacteria Thermus and Thermotoga in the extract. The soil microorganisms Flavobacterium oryzihabitans and Nocardia asteroides grew in materials extracted from 100 mg meteorite powder into 1 ml water, as illustrated in figure 3, to populations up to 5E7 colony forming units/ml in 4-8 days, similar to extracts from agricultural soils, and retained stable populations in the meteorite extract for several months. Biological effect on higher plants was demonstrated by Asparagus officinalis and Solanum tuberosis (potato) tissue cultures. When the above meteorite extract was added to partial 10 mM NH₄H₂PO₄ nutrient solution, the average fresh weight of asparagus plants grew from 1.5± 0.3 to 2.1± 0.8 g, and of potato from 3.0± 1.2 to 3.9± 1.2 g, and both showed enhanced green coloration. Correspondingly, the elemental S content of asparagus dry mass increased from 0.07 to 0.49%, of Ca from 0.02 to 0.26, of Mg from 0.03 to 0.41, of K from 0.18 to 0.32, of Fe from 0.02 to 0.03% [18,19].

These observations suggest that microorganisms entering young planetary environments, and even higher organisms, can grow on the large amounts of accreted interplanetary dust, meteorite and cometary [23] materials. Implanted microorganisms may multiply as well in carbonaceous asteroid parent bodies during the warm hydrothermal alteration phase, and in dust-sealed comets if they contain sub-surface water when warmed to 280-380 K during perihelion transits [27]. After landing, microorganisms can use the meteorite matrix materials. In fact, water in fissures in carbonaceous meteorites can create concentrated organic and mineral nutrient solutions conducive to prebiotic synthesis, and provide early nutrients after life arosein these meteorite microenvironments [19].