The subterranean ocean of the Saturn moon Enceladus contains abundant phosphorus, one of the six essential elements for life as we know it on Earth, according to observations made by the Cassini spacecraft. The discovery, which is presented today in the journal Nature, reinforces the hypothesis that Enceladus has the necessary conditions and ingredients for life forms to have emerged under its surface.
Saturn's small moon, just 500 kilometers in diameter, has a surface of icy water, making it an exceptionally bright star. Interest in it skyrocketed in 2005 when NASA's Cassini spacecraft discovered a huge geyser of gases and icy particles emerging from a breach in its surface. Subsequent studies using the Cassini data concluded that Enceladus has a global ocean of liquid water under its icy crust and a rocky core below the ocean.
The analysis of the composition of the geyser, as well as of one of Saturn's rings that is made up of particles from Enceladus, revealed that the moon contains abundant carbon and nitrogen, two of the essential elements for life. It also has abundant oxygen and hydrogen, which form its surface and ocean water. Sulfur, also necessary for the chemistry of life, was detected in 2009 in smaller amounts in a geyser.
But it remained to be clear whether Enceladus has enough phosphorus to be habitable. On Earth, phosphorus is part of the structure of RNA - the primordial nucleic acid from which life is believed to have arisen - and its descendant, DNA. It is also an irreplaceable component of ATP, the fundamental molecule that provides cells with energy. This element had not been detected so far in any of the stars with oceans discovered outside of Earth.
New analyzes of Saturn's E ring, the one formed with dust from Enceladus, have detected grains of sodium phosphate, made up of phosphorus and sodium atoms. Data from the Cosmic Dust Analyzer, one of the instruments Cassini carried on board before completing its mission in 2017, indicates that Enceladus' ocean also contains potassium, chlorine, bicarbonate, and carbon trioxide, among other atoms and molecules.
From these observations, complemented by experiments carried out in the laboratory and with computer modeling studies, the authors of the research deduce that the ocean of Enceladus contains phosphorus in a concentration more than one hundred times higher than that of Earth.
A previous study by Harvard University (USA), published in 2018, had indicated that the availability of phosphorus would be the bottleneck that would limit the habitability of subterranean oceans on icy worlds such as Enceladus or Jupiter's moon Europa.
With the new results, "there is no longer any doubt that there is a large amount of dissolved phosphorus in the ocean of Enceladus," says Frank Postberg, an astronomer at the Free University of Berlin (Germany) and director of the research, in an email. teams from the United States, Finland and Japan have participated.
This finding, coupled with the large number of organic molecules present in Enceladus' ocean and a source of energy sufficient to power geysers, makes Saturn's moon one of the most likely places in the solar system for life to have arisen. extraterrestrial, notes Postberg.
"With the discovery of phosphates, the ocean of Enceladus satisfies what is considered the strictest habitability requirement," the authors of the research conclude in Nature.
“The next step is clear; we have to go back to Enceladus to see if its habitable ocean is actually inhabited,” said Nozair Khawaja, co-author of the research, from the Free University of Berlin, in a statement. NASA is currently studying a project to send a mission to Enceladus in the next decade, consisting of a satellite that would be placed in orbit around the moon and a lander that would land on its surface.
