Europan biospheres (Galactic classification type T-EU) occur when the entire surface of a rocky world is completely covered in water, the top portion of which is frozen, and there is no significant atmosphere above the ice. (The pressure of a significant atmosphere would melt the ice producing either type T-L or type H-N biospheres). Such biospheres can exist outside the habitable zone for a standard terrestrial (type T-L) biosphere, furthermore the presence of the ice cap means that minimum mass of the world for a viable biosphere is lower than for type T-L. However such biospheres are never suitable locations for the evolution of potential and hence of little Galactic interest. Under 1 per_128 of Europan biospheres occur on planets near stars. Proto-planets with sufficient water experience a greenhouse effect sufficient to establish type T-L biospheres, further away the temperature is low enough for gas giant formation. However there should be an interval where neither of these processes occur. Unfortunately, the aforesaid interval typically lies within the zone rendered non-viable by virtue of the resonant interaction with the system's nascent gas giants, that disrupts planetary accretion and produces an asteroid belt.
Planetary Europan biospheres, due to the high albedo and low conductivity of the ice, do not receive a significant energy flux from their parent stars, but are driven solely by that received from geothermal sources. Such geothermal sources typically provide under 1 per_1024 of the Galactic standard incident solar energy flux for a T-L planet, and are thus incapable of supporting the evolution of the multiple trophic levels which are a precondition for multicellularity.
The majority of Europan biospheres occur inside gas giant moons. Investigation is discouraged by the Hydrogen order, but enough is known to indicate that they do not form viable hosts for the development of potential. These moons are subject to tidal heating that can produce an energy flux greatly in excess of that available to planetary based type T-EU biospheres. However, Europan planets have a frozen surface by definition, so the net energy flux cannot be high enough to raise the surface temperature above the melting point of water at the surface. Therefore, the mean Europan biosphere receives a net energy flux of no more than 4 per_128 of the Galactic standard incident solar energy flux for a terrestrial class C planet. Europan biospheres get enough energy to allow the natural evolution of multicellularity, and of simple organisms, but not to support a biomass sufficient for the evolution of potential after allowances are made for thermodynamic constraints on the maximal efficiency of such organisms due to the low temperature of the energy source. Indeed, it is insufficient for the evolution of carnivorous behavior. These constraints ensure that the maximal complexity of natives in a normal T-EU biosphere is low (comparable with that of Terran algal mats, or at best sponges).
Some Retired races have been known to study these biospheres and are thought to have introduced designed species suitable for those environments. However, it would be inappropriate for any Main Sequence species to interfere in the studies of our ancestors, whose wisdom surpasses our comprehension.
Tg.R.S.: We believe that the odds of finding promising life in Europan environments has been significantly under-estimated in Galactic Library sources. First the conclusions are based on very old data --often in the range of 2 Giga hab-years. As usual there has been little incentive to resurvey to confirm the Library data is still valid. Even if promise develops very slowly in Europan ecologies, 2 GY is a very long time. Second, it is likely that many Europan planets have served as refugium for "gray" species with the result that biotic complexity has been artificially increased or that redeemed species may be present. Also, many Europan planets will host active gray colonies that Earth wants to contact. Third, the argument that Europan planets are sacrosanct because Retired species have very occasionally taken an interest in one or two biospheres is spurious. A tiny fraction of Europan planets have been subject to manipulation by the Retired. Furthermore, the Retired tend to clearly exclude Main Sequence species when they want "privacy." So, if the Retired have not explicitly quarantined an area, they probably have no objection to Main Sequence activity in the region. There are also no GIM or GUI directives against activity in "unreserved" zones of Retired activity or previous activity. (Unfortunately, the same is NOT true of more zealous species and clans who would be incensed by what they would regard as trespassing on Retired territory.)
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by Robert Shaw, contributing editor