For years, the motto among astrobiologists – people who look for life on distant worlds and try to understand what life is, exactly – has been follow the water.
You have to start the search somewhere, and scientists have started with liquid water because it’s the essential agent for all biochemistry on Earth.
Now they’ve followed the water to a small, icy moon orbiting Saturn. Scientists reported Thursday that Enceladus, a shiny world about 300 miles in diameter, has a subsurface regional sea with a rocky bottom.
This cryptic body of water is centered around the south pole and is upward of 5 miles deep. It has a volume similar to that of Lake Superior, according to the new research.
The moon’s liquid reservoir had already been inferred from the presence of plumes of water vapor emerging from the south pole. The plumes stunned scientists when they were detected by NASA’s Cassini spacecraft in 2005. This latest report adds the detail of the rocky sea floor, which is significant because the contact between liquid water and rock creates the potential for the kind of interesting chemistry that gets astrobiologists excited.
This bulletin from the outer solar system could boost Enceladus as a possible target of a future robotic space mission. A spacecraft could fly through the plumes and study whatever’s coming out of the moon – something Cassini has done, but with instruments from the previous century that were not designed to look for signs of life.
To become a target for a new mission, however, Enceladus would likely need to outshine Jupiter’s moon Europa, which also appears to have subsurface ocean.
NASA is putting together preliminary designs for a possible Europa mission, though budgetary pressures for now make any new major, costly venture problematic.
The description of the subsurface sea on Enceladus is based entirely on indirect evidence. The body of water, if it actually exists, is covered with at least 20 miles of ice, according to the new report. But there are several lines of evidence that point to its presence.
The first is gravitational: The Cassini spacecraft, which has been exploring the Saturn system for nearly a decade, has made multiple fly-bys of Enceladus. Faint changes in the wavelengths of radio signals sent back to Earth have enabled scientists to calculate how the moon’s gravity tugs on the spacecraft.
But ultimately the scientists created a model for the moon’s interior and what appears to be a striking gravitational asymmetry. Around the moon’s south pole, there’s something that’s slightly off, and the calculations seem to be begging for the model of the interior to include some material denser than water ice. Liquid water – about 7 percent denser than ice in those conditions – seems to be the answer.
Another line of evidence is the moon’s shape: It has a shallow dimple, a depression, at the south pole. There’s missing mass. This fits with the hypothesis that there’s denser water down below, deforming the planet’s shape.
Finally, there are those plumes, which spew water vapor into space. It’s possible to generate such a phenomenon without geysers; instead, you could make plumes by rubbing blocks of ice together. But the plumes could be created by a deep ocean sending water up through cracks and into space.