**4. Considerations for astrobiology**

Regarding the phenomena of silicate and cryovolcanism, all of them may be powered by the energy sources discussed, but conditions for and evolution of these power sources are differing. Considering constraints for life as we know it, new aspects arise. Water in liquid form would be assumed as a requirement, in some alternative chemistry also ammonia or methane are discussed as possible solvents, liquid silicate/rock is less considered as being favorable for life. Also, a longer period of stability of these solvents is seen as favorable.

As accretion/radioactivity powered volcanism is high after formation and presumably gives rise to liquid silicates, it is a narrow gap of parameters depending on the size of the object and seeding of elements, which would allow a long and stable period of solvents as water. Bigger objects (starting already with radii just below 1000 km) might keep the heat over Gys too high, for example, water to rain down on the surface. Objects with sizes of several hundred kilometers and below may cool down very fast, allowing liquid water on the surface or in layers deeper in the crust for short periods of some 10 or 100 Mys [96]. Volcanism by tidal heating seems to be, if special conditions are met, more stable, as may be seen from all moons in our system with known active volcanism or tectonics, for example, Europa, Ganymede, or Enceladus. Even if becoming presumably unstable as Triton, it is after many Gys.

Considering the distribution of stable (e.g., considered from formation until now) volcanism powered by accretion/radioactivity or by tidal heating in our system, only Earth may be considered as accretion/radioactivity powered and many tens of objects powered by tidal heating confirmed or strongly assumed. If not for the power of the sun, habitable biotopes on Earth would be pretty much the same as the assumed ones on the moons discussed, that is, around vents deep in the liquid oceans below an ice crust covering (nearly) the whole surface. If we postulate such black smokers as life forging and maintaining harbors, in general, all over the universe, tidal heating may stably sustain such sources over many Gys, independent of a central stellar object even (and especially) on tiny objects. The requirements for tidal heating to power the cryovolcanism and rendering solvents liquid maybe not easily met, but considering the vast number of tiny objects (in contrast with bigger ones), the overall abundance of the self-powered systems may be seen as relatively high.
