LIFE ON ONE OF SATURN’S MOONS?
(HINT: THEY COULD HARBOUR LIFE)
In what is likely to be its final big discovery before it plunges into the gas giant planet Saturn later this year, the NASA spacecraft Cassini has discovered what could be a habitable ocean environment on Enceladus, one of Saturn’s dozens of moons.
The discovery, reported in the journal Science and announced by NASA Thursday, shows there is hydrogen in the massive plumes of gases and water that explode like geysers from Enceladus’s south pole, out of geological features known as “tiger stripes.”
The National Post’s Joseph Brean breaks down the reasons this has generated excitement among scientists looking for signatures of extraterrestrial life.
Q What are these plumes? A These plumes are massive geysers that burst forth from the ocean of liquid water that lies under 30 kilometres of ice on Enceladus, surrounding its rocky core. As it orbits the ringed giant Saturn, it is distorted by gravity, which creates heat, and drives hydrothermal processes in the deep ocean, just like in Earth’s oceans. Eventually, the heated water bursts through to the surface, where it almost instantly freezes in the minus-200 C weather, but not before offering the orbiting Cassini spacecraft a glimpse of what lies beneath.
Q Why does it matter that they contain hydrogen gas?
A Hydrogen is a potential source of energy for microscopic life on Earth, and likewise could be on this moon of Saturn. This is the big excitement. At 500 km wide, Enceladus is among the larger of Saturn’s moons, but it is too small for hydrogen to be left over from its formation, so it has to be coming from inside. In the Earth’s oceans, water reacts with ironbearing minerals in rocks to create hydrogen gas, which is then exploited by tiny organisms to make methane as an energy source, in a process called methanogenesis. So if that can happen on Earth, around the deep sea vents that teem with strange life forms, it could also be happening on Enceladus. “The presence of H2 (hydrogen gas) in the plume of Enceladus could therefore suggest the occurrence of temperatures and chemical energy sources necessary for habitable conditions in the moon’s interior,” the paper reads. But it also points out “the favourable thermodynamics alone are agnostic as to whether methanogenesis is actually occurring.”
Q How can NASA detect hydrogen without landing?
A The research involved the chemical analysis of those plumes by a device on Cassini called the Ion and Neutral Mass Spectrometer, which can measure the chemical content of the upper atmosphere of Saturn and its moons as it passes through. The spacecraft Cassini has been in Saturn’s orbit since 2004, and this research is almost its swansong. In two weeks, on April 26, Cassini will begin its descent, in ever tightening orbit through Saturn’s rings, until September, when it will plunge headlong into the planet, never to be heard from again.
Q Is Enceladus unique for having an ocean under an ice crust?
A Not at all. Many moons and other objects like comets are made of water, although most are frozen solid. Another discovery announced Thursday was based on images from the Hubble Space Telescope, which has been looking at Europa, one of Jupiter’s four moons, about the same size as Earth’s moon. Astronomer William Sparks said it is thought to have an ocean under an icy crust, with almost twice as much water as Earth. A few years ago, Hubble detected evidence of water vapour plumes from the surface of Europa, just like on Enceladus. On Thursday, Sparks announced the discovery of a similar plume at the same location, increasing confidence that it is a real observation, and it means science could study sub-surface water without having to drill through miles of ice. That is a key goal of the Europa Clipper mission, set to launch in about five years to study Europa.