For those who are hoping to find evidence of life somewhere else in the Solar System, there was some exciting news this week. Two moons, Europa and Enceladus, were already thought to be among the best places to search, since both have liquid water oceans beneath their outer icy shells. And now, new data from the Cassini spacecraft and the Hubble Space Telescope has increased the potential for some form of living organisms to be found.
Europa and Enceladus are known as "ocean moons," among at least several others in the Solar System such as Ganymede, Titan, Mimas and a few others, and maybe even the dwarf planet Pluto. The difference with Europa and Enceladus, however, is that their oceans are closer to the surface compared to the others, and are in contact with the rocky interior below them, just like on Earth. This means that chemical nutrients should be available. Other than that and the liquid water, a source of heat/energy is also required, at least for ocean life as we are familiar with on Earth, even if just microorganisms. Both moons appear to possess all three requirements, which makes them prime candidates in the search for life.
Now, the new results are showing that both worlds should indeed be habitable; this is not proof of life itself, but evidence for habitable conditions.
“This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment,” said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at Headquarters in Washington. ”These results demonstrate the interconnected nature of NASA's science missions that are getting us closer to answering whether we are indeed alone or not.”
First, Enceladus. Data from the Cassini spacecraft indicates that the ocean contains hydrogen, which is a key finding. On Earth, microscopic marine life can use hydrogen as a food source. In a chemical reaction called methanogenesis, the hydrogen is combined with carbon dioxide in the water to produce methane. Cassini found all three gases in the water vapour geysers which erupt through the surface ice from the ocean below, as well as oxygen, ammonia, salts and organics. Cassini's deepest dive through one of the plumes was on Oct. 18, 2015, which is the most recent time the data was obtained. The paper detailing these new findings was published in the journal Science.
"Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth," said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
Cassini doesn't have the instruments to detect life itself, but the new findings are important clues that at least the right conditions exist.
"Although we can't detect life, we've found that there's a food source there for it. It would be like a candy store for microbes," said Hunter Waite, lead author of the Cassini study.
The large amount of hydrogen found, about 1 percent in the plumes, is additional evidence for active hydrothermal vents on the ocean bottom of Enceladus. On Earth, the heat and nutrients from similar vents have helped create oases of a wide variety of life forms. The hydrogen would be produced by chemical reactions between the rock and hot water. It has been suggested by some people that the large amount of hydrogen means there's no life, since it should be consumed otherwise. But other sources of nutrients are possible as well, or it may be that some hydrogen is consumed, but the huge amounts being produced overwhelm that and help maintain the overall high levels seen.
On Europa meanwhile, there is additional evidence from the Hubble Space Telescope for active plumes on that moon as well. Possible plumes had been seen before in 2014, but were not able to be confirmed. The new observations are almost a confirmation - not quite, but very close. In the new images, the likely plume can be seen to reach about 62 miles (100 kilometres) above Europa’s surface. What also makes this more interesting now is that both this plume and the one seen in 2014 are in the same location on the surface where an unusually warm region is also present. Enceladus' plumes are erupt from "hot spots" in cracks on the icy surface. Cracks were also seen in this region on Europa by the Galileo spacecraft in the late 1990s.
Origin of Europa's plumes
“The plumes on Enceladus are associated with hotter regions, so after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map. We discovered that Europa’s plume candidate is sitting right on the thermal anomaly," said William Sparks of the Space Telescope Science Institute in Baltimore, Maryland. Sparks led the Hubble plume studies in both 2014 and 2016.
It would seem then, that the processes behind the plumes on both moons are similar. This is good news, since it means that water from the oceans below can reach the surface, where it could be analyzed by spacecraft. Cassini has already done this at Enceladus, but subsequent probes now being planned could analyze the water vapour plumes directly for signs of life. This would be much preferable to having to drill through the ice, as in some other future mission concepts. The Europa Clipper mission, now being designed for launch in the 2020s, would be able to do just that. Of course though, who wouldn't want to be able to actually send a probe into these oceans themselves, to "swim around" in alien waters?
The search for life has so far focused on Mars, and justifiably so, but it would seem that if you want to find current life, not just ancient fossils of microbes, then Enceladus and Europa might well be the better bet. For the first time, we know of oceans somewhere other than Earth. There's even Saturn's moon Titan, where some scientists even think that the surface lakes and seas of liquid methane/ethane might be able to support some type of exotic biology.
These discoveries are not proof of alien life, not yet anyway, but they are a fascinating indication that such habitable environments seem to be common, not only in our Solar System, but perhaps on moons or planets orbiting other stars as well. Being habitable is not the same as actually being inhabited, but to find out if either moon really is home to alien microbes or even something like jellyfish, we must go back. Only with additional robotic missions can we determine whether these potentially habitable worlds really are a home for life or not.