MOSCOW, December 22, Tatyana Pichugina.In the subglacial ocean of Enceladus, Saturn's moon, many organic compounds involved in the synthesis of proteins and DNA were discovered. This is very similar to the “primordial soup” that existed on Earth billions of years ago. There may be life on a distant, icy world at the edge of the solar system.
Water in the Saturn system
Saturn has 146 moons, the most in the solar system. The largest one, Titan, has long been considered the most interesting. The Huygens lander was sent to it as part of the Cassini mission in 2005. Behind the dense atmosphere, the probe saw a lifeless, cold world, shrouded in smog, with seas and lakes of methane. Some evidence suggests that there is an ocean below the surface. But there are no direct observations.
Little was known about the sixth largest moon, Enceladus. Like other moons of Saturn, it reflects sunlight well, which means it is covered with ice. Its diameter is about five hundred kilometers. It was believed that the subsoil had cooled long ago — there were no volcanoes or movement of tectonic plates. However, the estimates of the age of the surface were confusing — less than a million years, too young.
While approaching the satellite, the Cassini spacecraft recorded powerful geysers in the area of the south pole, shooting tens of kilometers in height. This created a sensation. Water vapor, ice grains and hydrocarbons were found in the emissions. From which they concluded: below there is a liquid ocean. And the depths have not cooled down.
As it turned out, Enceladus is the largest source of water in the Saturn system. Geysers formed its outer ring E. The belt of ice crystals around the planet was studied in the submillimeter range by the Herschel space observatory.
Recent observations with the James Webb Telescope's Near-Infrared Spectrometer have helped map the plume of gaseous water from Enceladus. The ejection speed is 300 kilograms of matter per second.
Since the satellite rotates very quickly around the planet (one revolution per 1.37 Earth days), a plume of water vapor remains in orbit, closing in a ring. It is estimated to account for up to 32 percent of H2O emissions. The rest, the researchers believe, is distributed throughout the Saturn system.
The Mystery of the Oceans
On the outside, Enceladus is covered by an icy shell 30 kilometers thick at the equator, ten at the south pole. The ice is thicker in the northern hemisphere. This indicates its flow from the poles to the equator.
Underneath the ice shell is a global ocean 40 kilometers deep, washing a solid silicate core. It is assumed that it is porous, which explains the low density of Enceladus.
Where the ocean contacts the core, there is hydrothermal activity, the rocks reacting with salt water. The same thing happens on Earth, in the deep-sea parts of the World Ocean, along the line of mid-ocean ridges.
In order for an ocean of liquid water to form in such a cold place, a constant source of heat is needed. On Enceladus these are gravitational tidal forces: Saturn's rotation transfers some of the energy to its satellites.
Heat is also generated by libration — the wobble of the satellite due to orbital resonance. Perhaps the core itself, its core, also contributes.
How thermal energy is redistributed between the shells of Enceladus is unclear. Among the main factors is the salinity of the water. It is believed to be small, from two to forty grams per kilogram of water. For comparison, in the Earth's oceans there are about 33. According to modeling, heating is determined by glacier topography and salinity.
«Primal broth» for life
Cassini's direct samples and other methods, including observations during eclipses and occultations, showed that geysers at Enceladus's south pole erupt at altitudes of 15 to 40 kilometers. At high flow rates, complex organic matter is destroyed. But recently, scientists from the United States experimentally demonstrated that amino acids are preserved even at 4.2 kilometers per second.
In geyser samples and spectra, several organic compounds and simple molecules, such as nitrogen, but necessary for the synthesis of life, were identified , carbon, nitrogen.
An international team of scientists analyzed data on ice grains collected by Cassini during its flight through the E ring. In ice particles of the third type, with a higher content of sodium and potassium salts, they found phosphates — necessary for the synthesis of biological macromolecules such as DNA, RNA, proteins . This is unexpected; this has not been discovered before in alien oceans.
The amazing discoveries did not end there. Recently an article was published in Nature Astronomy, the authors of which tried to determine the substances contained in the ocean.
Based on data from Cassini's low-altitude flyby in 2011-2012 and recently published observations from the INMS instrument — an ion and neutral mass spectrometer — a library of spectra of organic compounds was compiled. We added the results of laboratory experiments to simulate icy worlds with oceans. And they scanned tens of billions of possible models of geyser composition.
It turned out that the composition of 10-15 compounds is optimal. In addition to the previously known H2O, CO2, CH4 and NH3, it reliably contains HCN, C2H2, C3H6, CO, and most likely C2H6 (ethane). Alcohols and molecular oxygen, argon-40 are possible. It was not possible to identify 43 fragments.
This indicates the rich chemical diversity of the ocean on Enceladus, and the fact that conditions for the synthesis of complex organic matter, including the building blocks of living matter, were formed there. For the chemical evolution leading to the emergence of life, the composition of the ocean floor is extremely important, information about which is not yet available. But the already established facts are enough for Enceladus to become a priority target for astrobiologists.
Due to its great distance, a mission to Enceladus requires lengthy preparation. NASA's Enceladus Life Finder (ELF) and Enceladus Life Signatures and Habitability (ELSAH) projects are currently at the concept stage.