Most recently, we said goodbye to the Cassini spacecraft, which, after 13 years of faithful service to the orbit of Saturn and its satellites, descended directly into the depths of the giant planet's atmosphere. The reason for this grand finale was the warning of the possibility that the Cassini would crash into one of Saturn's moons - in particular, Enceladus.
Enceladus is unique for its curtain of geysers and inland ocean. Today, this tiny icy moon is considered a potential habitat for life, so we couldn't let it be contaminated by the Cassini spacecraft. New research, published in Nature Astronomy, suggests that this ocean has existed under the surface of Enceladus for a very long time - long enough to provide all the conditions for life to develop.
Enceladus Geysers are emissions of salty water-ice mixtures with traces of carbon dioxide, ammonia, methane and other hydrocarbons that erupt from cracks in the south polar region of Enceladus. It was because of these geysers that scientists decided that Enceladus must have a subsurface ocean and that this ocean is active (convective). Subsequent observation showed that hydrogen was present in the emissions, which led to an additional conclusion about hydrothermal activity - chemical reactions caused by the interaction of water and rock. But scientists have not been able to explain what kind of heat source could lead to this activity.
With additional observations, the mystery of the missing heat source only intensified. Geysers are associated with the so-called "tiger stripes" - a set of four parallel faults on the surface, 100 kilometers long and 500 meters deep. These bands are hotter than the rest of the ice crust, so they were supposed to be cracks in the ice. There are almost no impact craters in the area of the tiger stripes, so they must be very young, on the order of a million years. Any model that could explain the heat source would also have to take into account its focused nature - the world's ocean, but why is only the south polar region active?
For several years in a row, scientists have preferred the explanation of "tidal heating" - the result of the interaction of bodies of planetary sizes. For example, tidal interaction with our own moon is responsible for the ebb and flow of water on Earth. Enceladus is in orbital resonance with the moon Dione, which affects the shape of Enceladus's orbit around Saturn. But this influence is not enough to explain the power required to keep the geysers active - about 5 GW. It would be enough for a city the size of St. Petersburg.
Porous core
Promotional video:
Scientists came close to solving the puzzle when they looked at the internal structure of Enceladus. This moon has a low enough density that it is composed primarily of ice with a small solid core. This was considered for many years, since Voyager 2 took the first pictures of Enceladus and determined its radius, and then its volume. Enceladus's gravitational pull on the Cassini made it possible to estimate the mass of the moon and derive the value of the body's density. Cassini's measurements also showed that the core has a low density, which suggests that the core is porous, with ice-filled pores.
The new series of calculations fills the core pores with water rather than ice, because the tidal forces associated with the water in the pores are more than enough to explain how Enceladus' heat is generated. The model is excellent in that it explains not only the porosity of the core, but also its permeability (how easily the liquid passes through it) and strength (will it crack when the liquid passes?).
Combining all these parameters in one equation allows it to be resolved with the creation of an elegant model of heat flow inside Enceladus.
The authors create a three-dimensional image of where and when the heat of tidal movements within porous spaces is transferred to the subsurface ocean. The distribution of heat in the core is not uniform, but rather in the process of connected narrow upwellings, mainly at the South Pole. And because the heat sources (whose temperature reaches 85 degrees Celsius) are so concentrated, there must be increased hydrothermal activity near them, explaining the hydrogen in the eruptions.
Finally, the striking observation that can be made in analyzing this model is that the amount of heat generated by the internal tide is sufficient to sustain the Enceladus underground ocean for billions of years. Another question arises: what does all this mean for life on Enceladus? The warm world ocean, which has existed for several billion years, would become a wonderful cradle for life - on Earth it took only 640 million years for it to transition from the form of microbes to mammals. Unfortunately, Enceladus itself can be quite young - it may have formed just 100 million years ago. Is this enough time to live?
Maybe. Most likely, life on Earth was formed over several hundred million years in much more severe conditions of heavy bombardment. But then it took another 3500 million years to expand its areas of influence. Maybe this will be the future of Enceladus. Maybe this satellite will become not the planet of the apes, but the planet of mermaids?
Ilya Khel