The ice shell does not at all condemn the heavenly body to uninhabitability.
Scientists from the University of Toronto (Canada), using simulations, have found that completely ice-covered planets, today considered unsuitable for life, in fact, must have regions that consistently maintain positive summer temperatures. To do this, they need only an atmosphere, close in density to that of the Earth, and a moderate amount of liquid water. The text of the corresponding article can be found on the Cornell University preprint server.
At the moment, it is believed that for sustainable habitability, the planet must have a working carbon cycle. This is the name of the carbon cycle in nature, when carbon dioxide of the atmosphere forms carbonates due to chemical interaction with rocks. The latter, due to plate tectonics, sink into the mantle, from where they are eventually raised by mantle flows, due to which, during volcanic eruptions, carbon dioxide periodically breaks back into the atmosphere.
If any link in this chain is damaged, then there will be no climate stable and acceptable for complex life on the planet near the yellow dwarf. For example, on Venus, the mechanism for removing carbon dioxide from the atmosphere "broke", and as a result, it is too hot there. On Mars, there is a mechanism for re-entering the same gas into the atmosphere, and therefore it is too cold there.
The problem with such a scheme is that it is really prone to "breakdowns", and it may not come out of such "breakdowns" by itself. For example, if the temperature on Earth is now set noticeably below -100 degrees Celsius (in theory, in some cases, this is possible), almost all carbon dioxide will simply fall out in the form of snow, which will end the carbon cycle. And it will not be possible to raise the temperature again, because without this key greenhouse gas, the planet will never get warmer again. Because of this, many exoplanets, which, according to calculations, lie in the habitable zone, in fact, may turn out to be snowball planets. They will receive from the luminary as much energy as the Earth, but solid ice will reflect its main part into space, and the planet will remain a lifeless snowy desert.
The authors of the new work, using a specialized model, calculated what would be the effect of general glaciation of the Earth (when the entire planet is covered with ice) for a long-term climate. They found that, contrary to earlier ideas, in fact, even on a once icy planet, a continuous ice cover in the equatorial region can break open.
A number of factors can help. For example, warm ocean currents can locally overheat the ice sheet, even if the planet as a whole remains fairly cold. High steep mountains in the equatorial region can create rocky patches where the sun's rays are actively absorbed by dark stones, and therefore the ice cover cannot be fixed there.
Moreover, it turned out that even with a very limited opening of the ice sheet in this area, a real carbon cycle will begin to work. On a snowball planet in a place of local heating, dry ice (solid carbon dioxide) will undergo sublimation and begin to react with rocks. As a result, carbonates are formed, and when plate tectonics is working, they will begin to descend into the mantle, then rise upward, with mantle updrafts.
Promotional video:
Moreover, modeling has shown that summer temperatures at the equator of a snowball planet, which are close to Earth in parameters, will stably exceed 10 degrees Celsius. As a result, seasonal vegetation will become possible there.
Interestingly, the authors offer reliable remote indicators that will distinguish such a snowball planet from an ordinary earth-like one. Snowball atmospheres will have an increased ratio of carbon dioxide to water vapor. The fact is that water evaporation is very low on "snowballs", because the seas and oceans are covered with ice - there is nowhere for water to evaporate. But carbon dioxide, on the contrary, has nowhere to go, because rocks will be able to bind it only in equatorial zones, where warm oases can exist. Therefore, the spectra of such planets will contain more of the usual traces of carbon dioxide and less water vapor.
Such a set of indicators will soon make it possible to determine in practice whether the authors' hypotheses about the habitability of snowball planets are correct. The new James Webb Space Telescope, which the United States plans to launch into space in the 2020s, will be sensitive enough to analyze the composition of the atmospheres of nearby terrestrial exoplanets.