April 10, 2014
By creating a map of Enceladus gravitational field, researchers have discovered strong evidence of a water ocean beneath moon's south pole.
Artist's impression of Enceladus with its icy surface, water vapours at the south pole and the ocean underneath. Credit: Nasa/JPL-Caltech
When we talk about water on other planets or moons in the solar system, Mars or Jupiter's moon Europa come to mind instantly, but Saturn's moon Enceladus was also suspected to have water under the surface.
A new study published in journal Science and based on NASA's Cassini space probe has found strong evidence that there is an ocean on Enceladus that could be the best place to search for extraterrestrial life. First indications of water on the moon came in 2005 when Cassini imaged jets of water vapour coming from Enceladus' south pole, but now there is an evidence it originates from the large body of water 10 kilometres under the surface.
The discovery was made by observing how the Cassini spacecraft first sped up and then slowed down just a few millimetres per second as it flew past Enceladus. This change of speed was due to variations in the gravitational field of the moon. These variations suggest that under the icy surface of Enceladus, there are materials of different densities.
After creating a map of Enceladus' gravitational field, its shape revealed that there is something more dense than ice and less dense than rock beneath the south pole.
"Given the kinds of materials we know are used to make bodies like this, the natural thing to look for is water, because water is more dense than ice, and because it's a natural thing to have in that environment," said David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, USA.
At the 1.4 billion kilometres from the Sun, water on the surface of Enceladus is frozen, but thanks to Saturn's gravitational pull and resulting tidal force, the ice under the surface of the moon might be melted to create an underground water ocean. If the ocean was salty, the freezing temperature would be lower than zero degrees Celsius and thus more likely to stay liquid.
What makes Enceladus so interesting, perhaps even more likely place to support life than Europa, is that its water contains elements necessary for life, such as phosphorus, potassium and sulfur. By previously analyzing the vapour plumes mentioned above, scientists already know they contain these basic elements as well as organic molecules.
"The question is what conditions do you need to form life and, of course, we don't know what temperature the ocean is today, nor do we know what it was back in the geological past," said one of the researchers, Jonathan Lunine, from Cornell University in New York, USA. "But it's conceivable that it was warm enough, with circulation of water coming from the silicate core as well, to allow life to form even if today that ocean is maintained by antifreeze and is slightly below the freezing point."
A future mission could sample the vapour plumes in search for organic biomarkers and if the results turn out to be positive, the answer to one of the most important questions in science could be within our grasp.