Move over, Mars. In the search for extraterrestrial life, moons are now in the limelight.
Enceladus—one of the Ringed Planet’s icy and austere orbiters has been on astronomers’ shortlist of potential hosts of alien life, especially since they discovered geysers of ice crystals shooting out of its south pole in 2005. They hypothesized that a deep ocean the size of Lake Superior sits underneath its highly tectonic and veiny surface, potentially feeding those gushers.
Normally, liquid water couldn’t exist that far out in the solar system, but the gravitational pull of Enceladus’s neighbor, Dione, bends the icy moon’s outer layer, creating heat through friction. NASA’s Cassini spacecraft has repeatedly flown by the surface of Enceladus to better understand the temperature dynamics that create these explosive geysers.
Kenneth Chang, writing for The New York Times:
Cassini has no instruments that can directly detect water beneath the surface, but three flybys in the years 2010-12 were devoted to producing a map of the gravity field, noting where the pull was stronger or weaker. During the flybys, lasting just a few minutes, radio telescopes that are part of NASA’s Deep Space Network broadcast a signal to the spacecraft, which echoed it back to Earth. As the pull of Enceladus’s gravity sped and then slowed the spacecraft, the frequency of the radio signal shifted, just as the pitch of a train whistle rises and falls as it passes by a listener.
Using atomic clocks on Earth, the scientists measured the radio frequency with enough precision that they could discern changes in the velocity of Cassini, hundreds of millions of miles away, as minuscule as 14 inches an hour.
What they detected was a gravitational asymmetry—the pull of gravity was weaker near the south pole, which made sense because there’s a depression in the planet’s landscape there. But the depression is so large that it should have made the gravity weaker than what they measured, so they knew that something more dense must be making up the difference—something like liquid water, which is denser than ice by 8%. Using that information, they calculated that a sea up to six miles thick probably exists 20 to 25 miles beneath the surface.
Perhaps more exciting is that astronomers now have proof of this water coming into contact with Enceladus’ rocky core. Water touching rock may seem prosaic here on Earth, but the union of the two can make for some compelling chemistry, creating conditions that could foster life. Here’s Alexandre Witze, writing for Nature:
On Earth, for instance, hot water from sea-floor hydrothermal vents interacts with rocks to generate chemical reactions and nutrients that life can feed off. Enceladus may well have similar molecules at the bottom of its ocean, McKay says. Cassini has spotted tiny silica particles in the Enceladus plume, which may be fragments of the rocky core that could carry nutrients.
What remains to be seen is how long this ocean has been around and whether or not it’s going to keep cooling or stay in its current condition for some time. Regardless, the chemical conditions on Enceladus may be our best hope for finding life in solar system, which has some astronomers proposing sending robots to investigate. Other candidates for such a mission would be Jupiter’s moon Europa or Saturn’s other moon, Titan.
Astrobiologists have found icy plumes on Europa, too, but only on occasion. And Titan has liquid methane, which working with other compounds could concoct life, but in a very different way from what we know on Earth. Only Enceladus is known to have the four essential ingredients for Earth-like life: liquid water, energy, carbon, and nitrogen. It’s still a dark, largely unknown territory, but a future mission manages to capture particles from Enceladus’ plumes, it could confirm the existence of life beyond Earth.