The subsurface ocean of Saturn's 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, presented yesterday in the journal Nature, reinforces the hypothesis that Enceladus gathers 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 water ice, which makes it an exceptionally bright star. Interest in this satellite soared in 2005 when NASA's Cassini spacecraft discovered a huge geyser of gases and icy particles emerging from the surface. Later studies based on Cassini data concluded that Enceladus has a global ocean of liquid water beneath its icy crust and a rocky core beneath the ocean.
The analysis of the composition of the geyser, as well as of one of Saturn's rings which is made up of particles from Enceladus, revealed that the moon contains carbon and nitrogen in abundance, two of the essential elements for life. It also has abundant oxygen and hydrogen, which make up the surface water and its ocean. Sulfur, also necessary for the chemistry of life, was detected in 2009 in small amounts in a geyser.
But it needed to be clarified 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 thought to have arisen – and its descendant, DNA. It is also an irreplaceable component of ATP, the fundamental molecule that provides cells with energy.
New analyzes of Saturn's E ring, which is made up of dust from Enceladus, have detected grains of sodium phosphate, composed of phosphorus and sodium atoms. Cassini data indicate that Enceladus' ocean also contains potassium, chlorine, bicarbonate and carbon trioxide, among other compounds.
Based on these observations, supplemented by laboratory experiments and computer modeling studies, the authors of the research deduce that the ocean of Enceladus contains phosphorus in a concentration more than a hundred times higher than that of Earth.
A previous study from Harvard University (USA) had pointed out that the availability of phosphorus is the bottleneck that can limit the habitability of subsurface 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 phosphorus dissolved in the ocean of Enceladus," Frank Postberg, an astronomer at the Free University of Berlin (Germany) and director of the research, says by email. in which teams from the United States, Finland and Japan have participated. This finding, coupled with the large amount of organic molecules in Enceladus' ocean and a source of energy intense enough to activate geysers, makes Saturn's moon one of the places in the solar system most likely to extraterrestrial life has arisen, Postberg points out.
"The next step is clear; we have to go back to Enceladus to see if its habitable ocean is really inhabited", says 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.
