The paradox of Fermi and the habitable worlds of our solar system

The paradox of Fermi and the habitable worlds of our solar system https://danielmarin.naukas.com/files/2018/11/888.png

The paradox of Fermi and the habitable worlds of our solar system







            
We all know that the definition of planet-potentially-habitable in astronomy is as restrictive as that of "metals" (astronomers are people who tend to simplify things a lot). To say that a planet is suitable for life to develop in it only if it is located in the habitable zone of its star, leaves out all those worlds where there may be liquid water but not on the surface, hence it would be better call this area "water zone" or something similar. In our solar system there are many worlds with liquid water that are not located in the habitable zone, all of them located in the outer solar system. But, how much water are we talking about?



Worlds with oceans in the solar system (Bob Pappalardo, personal communication).

It is difficult to know because we have not studied them in detail, so we will have to wait for the Europa Clipper and JUICE probes to better know the volume of the internal oceans of Europe, Ganymede and Callisto (in reality they are "mantos" of water instead of rock as in the rocky worlds). Obviously, not all of these aquatic worlds of the outer solar system are the same. There are several informal classifications, but we can keep three types. The first type includes those worlds in which the ocean is at the same time in contact with outer space through fissures in the ice crust and with a rocky bottom rich in minerals. This is the case of Enceladus and, probably, Europe.



The second type are worlds with oceans in contact with a rocky bottom, but not with the outside due to the high thickness of the ice crust, while the third includes oceans totally isolated between layers of ice. The astrobiological potential of the first type is obviously the greatest, while in the case of the second and third, it will depend on the particular conditions. Of course, we can only study from a distance the worlds of the first type, since in the rest we will have to dig tens or hundreds of kilometers through the ice crust to reach the internal ocean. Or oceans. In the case of Ganymede, there could be several water-based layers, most lying between different types of ice. Yes, types of ice, in plural. In addition to ice I, the "our", there are other variants that occur at high pressure, such as ice V or ice VI.



Amount of water in the ocean worlds of the solar system (David L. Clements).

As we said, the amount of liquid water in these worlds is still under discussion, but as a first approximation we can measure the amount of ice. In this sense, the worlds of the outer solar system with internal oceans - or that could harbor them - have a quantity of water that exceeds several times that of the Earth. In some cases, tens or hundreds of times higher. Only the small Enceladus and Dione - with an internal ocean yet to be confirmed - have less liquid water than Earth. And it is that, in reality, despite its extensive oceans, our planet is quite dry. Of course, another debate facing the appearance of life in these bodies is the available energy. The circumstance occurs that the worlds with the most energy available to living beings are those that can be of type two or type three, that is, with internal oceans beyond the reach of our instruments.

Obviously, the implications with regard to Fermi's paradox is that most potentially habitable worlds in the Galaxy would not be like Earth, but would have oceans isolated from the outside by ice crusts. Although it is perfectly possible to imagine intelligent life in these worlds, it is almost impossible for technological civilizations to appear. Perhaps most of the intelligent beings of the Universe are locked in oceans separated from the rest of the Universe.

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