Life on the surface of the Earth is very abundant. Both on the continents and in the oceans, biological organisms have developed and flourished. Even in the atmosphere, microorganisms have been identified at higher stratospheric levels. Another biological area that has been recognized during the last decades is the underground. At the bottom of wells, wells, caves and galleries of mines you can find a large number of microorganisms, particularly bacteria, confirming the extraordinary adaptability of living beings.
This must be taken into account when we examine the possibility of life on the surface of other planetary bodies, both in our Solar System and in other star systems.
In the case of the Solar System it can be verified that the surface environments are very different from those of the Earth, either because the temperatures can be too high (Venus) or too cold (eg Mars, most of the asteroids and satellites of giant gas planets). In some cases the atmospheres may be absent and in others they may have a lethal composition for life as we know it.
However, the subsurface conditions of many planetary bodies can be similar to ours with greater possibilities for the existence of active biological processes.
The relationship between pressure and temperature with depth is, of course, different, but the chances that life has developed at a certain depth may not be very different from Earth's.
Hydrocarbons (methane and others) have been detected spectroscopically on the surfaces and atmospheres of many of these bodies, and it can be considered that there is (liquid) water underground in most of them.
The water seems to have been abundant in the cloud of gas and dust that formed the planets, and ice has been identified in several planetary bodies and comets that are colder than the earth. In these bodies, rock formations, such as those on earth, contain some oxidized components that will serve as oxygen donors. The hydrocarbons would provide the necessary energy when oxidized. Under these conditions there could be an active biology in the planetary crusts.
Mars would be the least expensive planet to investigate for evidence of subsurface extraterrestrial life. Perhaps there is no need to launch any spacecraft to begin such an effort. Some meteorites that occasionally fall to earth carry the chemical signature of Mars. This is the case of several meteorites collected from the ice fields of Antarctica.
The proportions of trace elements, such as the sequence of noble gases from neon to xenon, as well as the unusual proportion of nitrogen isotopes in the Martian atmosphere, were already very similar measurements and values appear in these meteorites. It seems very unlikely that the remains of any other body can match these compositions so exactly.
Millions of years after an impact on Mars caused the ejection of Martian material, the orbits of some of these rocks collided with Earth. In 1996, one of those meteorites (called ALH84001) yielded strong evidence that the rock had been altered by microbial life while it was still on the original planet.
We can extend this reasoning even more. Probably the subsurface conditions of several planetary satellites (such as the Moon, Io, Ganymede, Titan, Europa, Enceladus, etc.) as well as the larger asteroids (Ceres, Pallas), can also be similar and, therefore, can have active organisms inside.
Even smaller bodies, such as comets, can have these kinds of conditions, especially when they approach the sun.
One final conclusion is that these deep underground environments probably exist on many planets, perhaps on most of the planets in the galaxy.
With respect to these thoughts, we would like to end by quoting the famous astronomer Fred Hoyle when he said "Life is a property of matter".
Partially adapted from "The Deep Hot Biosphere" by Thomas Gold.
