A crucial issue for prebiotic chemistry and exobiology is the maximum temperature reached by cometary matter during its history. Prialnik and Podolak (1999) consider that comets form rapidly, which leads to a key role of 26 Al decay in heating the interior of that comet. The maximal temperature reached somewhere in the nucleus ranges from low (35-90K for a small 1-km radius comet with low/ high (0.1/0.9) porosity) to 270-210K for a 100-km radius object. A very different situation is found in McKinnon (2002), who modelled the thermal history of large icy bodies in the outer solar nebula (Kuiper Belt). The accretion there is slow enough for 26 Al to have mostly decayed, and collisional accretion is the main heat source. The temperature remains below 100K in most places for radii below 100km, but reaches more than 250K in the case of KBO Varuna (R ~ 400km).
The prospect for high temperature inside the nucleus raises the question of possible liquid water inside comets or Kuiper Belt objects. Liquid water is a key parameter for chemical evolution toward prebiotic molecules, such as amino acids more complex than glycine. Therefore liquid water events on comets might allow the synthesis of more molecules than those one would expect in "dry" comets. From the model quoted above, liquid water seems excluded at present times in the bodies of modest size (< 100km diameter) reaching the zone of terrestrial planets. Conditions were more favourable in the past but only a very limited parameter range would allow liquid water to be present for some time (Podolak and Prialnik 1997); in the case of KBOs, the body should be among the largest (Pluto?).
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