Without an atmosphere, life on the planet is impossible - it is the atmosphere that protects it from destructive rays from space.
Every now and then very fast and rather heavy particles, and even whole nuclei of atoms - mostly protons - fly through the empty space of space. The energy of these tiny projectiles is colossal in the world of elementary physics - hundreds of millions of times higher than anything our most powerful accelerators are capable of.
Some of these particles were ejected by the Sun, while others appeared in other parts of the galaxy or beyond. The earth is constantly bombarded by them: getting into the earth's atmosphere, they generate a "rain" of secondary cosmic rays, consisting of new particles. These cosmic rays are part of our natural background radiation, but on other planets they can put a limit to the development of life.
Today, to assess the potential habitability of the planet, astronomers most often turn to those of them that are most similar to ours. Such a planet should, first of all, have approximately terrestrial dimensions and a stable orbit located within the "habitable zone" of its star - that is, it should be constantly at such a distance from it that water could remain on the planet in liquid form. However, radiation should also be an important factor: too strong fluxes will literally sterilize the planet.
A recent study by astronomers from the Blue Marble International Space Institute is dedicated to this issue. Physicist Dimitra Atri and colleagues decided to evaluate two key factors that affect the amount of cosmic radiation that hits the planet: the strength of its magnetic field and the density of the atmosphere. The magnetic field deflects a large number of charged particles, the atmosphere absorbs and scatters them.
“For the first time I thought about it, mentally comparing the Earth and Mars, these are two very close planets, and meanwhile take a look at our blooming biosphere - and the lifelessness of our neighbor. Why is this so? - says Atri. "The main factor is the high level of radiation on Mars: its atmosphere is very rarefied, there is no global magnetic field - and there is no protection from cosmic rays."
The authors simulated a number of possible scenarios on a computer: planets with a magnetic field, like that of the Earth, and without it at all, with an atmosphere with a density from 1 to 0.1 Earth. Unexpectedly for them, the most important factor of protection was not the magnetic field, but the atmosphere. They calculated that if the Earth did not have a magnetic field at all, the level of radiation generated by cosmic rays would only double - this is not a threatening value for life. But with a decrease in the density of the atmosphere to 10%, the radiation would jump by two orders of magnitude at once. This is already fatal.