Geyser basin in the southern hemisphere of Saturn's moon Enceladus (image from 2010)
Photo: NASA / JPL / Space Science Institute / dpa
The chemical building block phosphorus is central to the emergence of life. Now it is said to have been found in the ocean of Saturn's moon Enceladus, according to archival data from the Cassini spacecraft. An international research team has re-analysed the data and compared it with laboratory experiments on Earth.
"With this discovery, it is now known that the ocean of Enceladus fulfills the strictest requirement for life," said co-author Christopher Glein, according to a statement. The next step is clear: "We have to return to Enceladus to see if the habitable ocean is actually inhabited." The results were published in the journal Nature.
In addition to hydrogen, oxygen and carbon, sodium, sulfur and phosphorus are considered by researchers to be essential for the existence of life. Phosphorus is not only an important component of the genetic material DNA, but also the central element for energy transport in cells. But "Of these six elements, phosphorus is the one that is rarest in the cosmos," the research team writes.
Fountains shoot ice particles into space
In the search for life in our solar system, scientists have increasingly focused on the icy moons of the planets Jupiter and Saturn in recent years. This is because many moons have deep oceans of water under kilometre-thick ice sheets.
Even though there was no evidence of phosphorus in these waters for a long time, teams did not stop looking for it. Saturn's moon Enceladus was of particular interest to many researchers because it is tectonically active at its south pole and shoots ice particles and water vapor several thousand kilometers into space from several fountains.
The US Cassini spacecraft orbited Saturn from 2004 to 2017 and flew through these fountains several times. In the ice particles captured by a special instrument, researchers first detected organic – i.e. carbon-based – molecules, but no phosphorus. Experts therefore remained controversial as to whether the ocean of Enceladus contains sufficient amounts of this element to theoretically enable the existence of life.
Postberg and his colleagues have now re-analyzed data from a total of 345 ice particles captured by Cassini using new, more precise methods and came across nine particles that show an unusual composition that differs from the other particles: they contain molecules whose mass corresponds to sodium phosphates. In order to estimate how much phosphate the ice particles and thus the ocean of Enceladus contain, the team also conducted a series of laboratory experiments.
"Our observations and our laboratory experiments indicate that phosphorus is available in the ocean of Enceladus in the form of phosphates," the researchers conclude. "And in concentrations that are a hundred times higher than in the Earth's oceans."
The discovery suggests that the liquid oceans beneath the icy crusts of this and other icy moons in the solar system contain all the necessary ingredients for life. Mikhail Zolotov of Arizona State University Tempe writes this in an accompanying commentary in "Nature". At the same time, doubts remain, as Cassini has found only nine phosphorus-carrying particles. Further measurements on site could help to learn more about the components of the ocean and the processes in the water.