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During the past few decades, the delivery of water, organics, and prebiotic chemicals to the Biosphere of Earth
during the Hadean (4.5-3.8 Ga) period of heavy bombardment by comets and asteroids has become more widely
accepted. Comets are still largely regarded as frigid, pristine bodies of protosolar nebula material that are devoid of
liquid water and therefore unsuitable for life. Complex organic compounds have been observed in comets and on the
water-rich asteroid 1998 KY26 and near IR observations have indicated the presence of crystalline water ice and
ammonia hydrate on the large Kuiper Belt object (50000) Quaoar that has resurfacing suggesting cryovolcanic
outgassing. Spacecraft observations of the chemical compositions and characteristics of the nuclei of several comets
(Halley, Borrelly, Wild 2, and Tempel 1) have shown that comets contain complex organic chemicals; that water is
the predominant volatile; and that extremely high temperatures (~350-400 K) can be reached on the surface of the
very black (albedo~0.03) nuclei of comets when they approach the Sun. Impact craters and pinnacles observed on
comet Wild 2 suggest a thick crust. Episodic outbursts and jets from the nuclei of several comets indicate that
localized regimes of liquid water and water vapor can periodically exist beneath the comet crust.
The Deep Impact mission found the temperature of the nucleus of comet Tempel 1 at 1.5 AU varied from a
minimum of 280 ±8 K the 330K (57 °C) on the sunlit side. In this paper it is argued that that pools and films of
liquid water exist (within a wide range of temperatures) in cavities and voids just beneath the hot, black crust. The
possibility of liquid water existing over a wide range of temperatures significantly enhances the possibility that
comets might contain niches suitable for the growth of microbial communities and ecosystems. These regimes
would be ideal for the growth of psychrophilic, mesophilic, and thermophilic photoautotrophs and chemolithotrophs
such as the motile filamentous cyanobacteria (e.g., Calothrix, Oscillatoria, Phormidium, and Spirulina) that grow in
geothermal springs and geysers of Earth at temperatures ranging from 320K to 345K and are also found growing in
cold polar desert soils. The mineralized remains of morphotypes of all of these cyanobacteria have also been found
in the Orgueil CI1 and the Murchison CN2 carbonaceous meteorites that may derive from cometary parent bodies.
Observational results that support the hypothesis that liquid water can in active regions just beneath the surface of
comets and that comets, carbonaceous meteorites, and asteroids may have played a significant role in the origin and
evolution of the Biosphere and in the distribution of microbial life throughout the Solar System.s
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