A terrestrial microorganism has managed to grow in laboratory water designed to reproduce the conditions of an extraterrestrial ocean: Enceladus's.
This small moon of Saturn is covered by a thick layer of ice. Beneath it lies what may be a vast ocean of liquid water in contact with a rocky seafloor. Jets regularly expel vapor and ice grains into space. The Cassini probe passed through these plumes and detected salts as well as carbon-containing molecules.
Representation of hydrothermal activity on Enceladus based on data from the Cassini-Huygens mission
Credit: ESA
The researchers wanted to find out whether a terrestrial organism could live in water with characteristics similar to this ocean. They chose Methanothermococcus okinawensis, a microorganism that naturally lives near underwater hot springs. It belongs to the archaea, microscopic organisms distinct from bacteria. It can live without oxygen and produces methane.
The laboratory therefore recreated highly alkaline water, with a pH reaching as high as 11. This water also contained a great deal of carbonate and very little available carbon dioxide. The researchers reproduced the reactions that could occur between the water and hot rocks on the ocean floor.
In a standard nutrient medium adjusted to the same pH, the microorganism did not grow because of the lack of dissolved carbon dioxide. In water reproducing that of Enceladus, by contrast, it continued to grow and produce methane. Hydrogen released by reactions between the water and rocks provided it with some of the resources it needed.
Illustration of NASA's Cassini orbiter passing through a gigantic vapor jet above the moon Enceladus.
Credit: NASA/JPL-Caltech
In practical terms, the organism managed to adapt its functioning to the very low quantities of available carbon dioxide. This indicates that chemistry comparable to that envisioned on Enceladus does not necessarily prevent this type of life from existing.
Water droplets expelled from Enceladus's ocean could contain organic substances. A future probe would not necessarily need to land or drill through several kilometers of ice. It could pass through the plumes and analyze them, as Cassini already did with instruments less suited to the search for life.
The European Space Agency is currently studying a future mission to Enceladus devoted in particular to this question.