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Research focused on Enceladus, one of Saturn’s natural satellites, has gradually ranked it among the top candidates to potentially harbor life, its surface and its ice ocean containing key elements. However, the final piece of the puzzle – phosphorus – was not directly detected in the Enceladus dust samples collected by the Cassini probe as part of an initial analysis. But an international study has recently confirmed its presence (and in large quantities), completing the list of six essential elements for all forms of life and placing the satellite among the favorites for habitability (within the solar system).
On our planet, every form of life is made up of six main elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. Without these key elements, life would not have appeared on Earth. Other parameters must also be present, including liquid water. Over the past 25 years, astrophysicists have focused on icy stars, some of which have oceanic layers beneath their surface. This parameter would even be quite common in our solar system: Europa, Titan, Enceladus as well as some planetary bodies located outside the orbit of Pluto.
Earth maintains its liquid oceans thanks to its ideal distance from the Sun. But in the case of planetary bodies like Enceladus, the Sun is unfortunately too far away to heat the surface to maintain a liquid ocean. But beneath a thick icy surface, Enceladus’ oceans are kept afloat by its hot and active core. This feature, which would not be so rare in space, thus significantly increases the number of potentially habitable planets in our galaxy.
Since the presence of liquid water is not as rare as previously thought, ” the search for extraterrestrial habitability in the solar system has shifted focus as we now search for the building blocks of life, including organic molecules, ammonia, sulfur compounds and the chemical energy needed to sustain life “, states Christopher Glein, co-author of the new study published in a press release PNAS and PhD researcher at the Southwest Research Institute.
Phosphorus in particular is a very active area of research, as scientists previously thought it would be scarce in Enceladus’ oceans. However, phosphorus molecules are essential metabolic and structural components for all forms of life, being present in DNA synthesis, in the structure of cell membranes and even in the marine microbiome.
In 2015, when the Cassini spacecraft passed about 49 km from the surface of Enceladus, its mass spectrometer was able to analyze samples of dust projected from the geysers on its surface, feeding the E ring of Saturn, its parent planet. The spectrometer then detected various compounds such as organic and inorganic carbon, molecular hydrogen, various nitrogenous and oxygenated and aromatic compounds, ammonia, etc.
Phosphorus could not yet be detected, however, as initial analyzes could only be carried out in the intermediate spectra. The new study reanalyzed the samples collected by the probe and compared the results with those (at high resolution) obtained previously by other researchers. ” While the bioessential element phosphorus has yet to be directly identified, our team has uncovered evidence of its availability in the ocean beneath the satellite’s icy crust. says Glen.
A higher concentration of phosphorus than on Earth
Indirect detection of phosphorus in dust samples from Enceladus was carried out by researchers from the Chinese Academy of Sciences, Space Science and Engineering Division of the Southwest Research Institute, CSIRO (Australia), from the University of Berlin and the University of Washington. In particular, the team performed experiments based on thermodynamic and kinetic models to simulate the geochemistry of the satellite’s oceans. This simulation aims to determine whether the satellite’s conditions allow phosphorus to be present, as well as to assess whether it can occur in large quantities, in the form of orthophosphate.
The researchers then discovered that the underlying geochemistry of Enceladus made the presence of phosphorus inevitable, and this in amounts equal to or greater than those in our oceans. In addition, 9 of the 1000 dust particles from the Cassini samples contained traces of phosphorus. This could indicate that the concentration of this element in the oceans of Enceladus would be 100 to 1000 times higher than the concentration of the terrestrial oceans.
However, phosphorus has not yet been detected in molecules containing organic carbon. But the researchers hope that further research will uncover them. ” What this means for astrobiology is that we can be more confident than before that the ocean of Enceladus is habitable. Glen concludes.