Possible food source for life detected in Enceladus' plumes
Thursday, April 13, 2017, 5:30 PM - Could there be alien life in the oceans of Saturn's Enceladus or Jupiter's Europa? New evidence boosts the potential habitability of these icy moons in our solar system!
Of all the places in our solar system that we search for signs of life beyond Earth, few - if any - rank as highly as Europa and Enceladus.
These two moons - Europa around Jupiter and Enceladus around Saturn - have both given up evidence that they have subsurface oceans of liquid water under their icy shells, and both have demonstrated that these oceans can "communicate" with the surface, by squeezing of plumes of water vapour out into space through cracks in their shells, which then rain (and snow) back down onto the surface.
Over the past few years, NASA's Cassini spacecraft has been gathering even more from Enceladus, by passing close to the moon and even flying through the plumes of water streaming from near its south pole. The results of these flybys have revealed that Enceladus' ocean is very likely a global one, rather than only being a small pocket of liquid water near the pole, and that some of the components in the plumes point to the likely presence of hydrothermal vents on the floor of this ocean.
This artist's rendition of Enceladus' south polar region features the water vapour plumes and a cutaway of the icy crust to reveal the subsurface ocean and hydrothermal vents. Credit: NASA/JPL-Caltech
Here on Earth, water seeping into cracks in the ocean floor interacts with hot magma deep down, dissolving minerals and chemicals, which are then transported back to the ocean as the hot water boils out from the cracks.
This process tends to form tall spires of mineral deposits above the vent, and the area around these vents supports an abundance of life - microbes that use the chemicals in the water as a food source, bigger organisms that feed off the microbes, which in turn are used as food for even larger organisms, and so on.
Cassini's latest plunge through Enceladus' plumes has now turned up evidence that this same process is going on along the sea floor of Enceladus' subsurface ocean.
Examining the data from that pass through the plumes, a team of scientists led by Hunter Waite, from the Southwest Research Institute, found molecular hydrogen - H2.
"Hydrogen is a source of chemical energy for microbes that live in the Earth’s oceans near hydrothermal vents," Dr. Hunter Waite, principal investigator of Cassini’s Ion Neutral Mass Spectrometer (INMS) at the Southwest Research Institute (SwRI) said in a statement. "Our results indicate the same chemical energy source is present in the ocean of Enceladus. We have not found evidence of the presence of microbial life in the ocean of Enceladus, but the discovery of hydrogen gas and the evidence for ongoing hydrothermal activity offer a tantalizing suggestion that habitable conditions could exist beneath the moon’s icy crust."
There's one important thing to note here, though. While this makes it clear that Enceladus very likely has hydrothermal vents supplying an abundant source of food for microbial life along the floor of its ocean, there's no way to know if anything living is actually taking advantage of that food source. Since NASA didn't even know about Enceladus' plumes, its subsurface ocean, and the potential for hydrothermal activity there until after Cassini had arrived at Saturn, the spacecraft isn't equipped with the right instruments to take the investigation any further.
This NASA infographic details the processes that could be forming molecular hydrogen in Enceladus' oceans. Credit: NASA/JPL-Caltech
Meanwhile, one gas-giant planet over from Saturn, another investigation - this time using the Hubble Space Telescope - has revealed water plumes from cracks in the surface ice of Europa.
This is now the third detection of plumes near the southern polar region of Europa. The first was in December 2012, the second was in January 2014, and the latest was in February of 2016. Each time, these plumes were revealed by Hubble images taken in the ultraviolet spectrum, while Europa was transiting in front of Jupiter. While the first two sightings were tentative, at best, this third occurrence makes it highly likely that we are, indeed, seeing water vapour plumes from Europa!
Europa's plumes of water vapour, imaged roughly two years apart, by the Hubble Space Telescope. The blue background is the UV imagery from Hubble. A composite image of Europa has been added, from the Galileo and Voyager missions, to show the location of the plumes relative to the moon's surface features. Credit: NASA/ESA/STScI/USGS
According to NASA:
The newly imaged plume rises about 62 miles (100 kilometers) above Europa’s surface, while the one observed in 2014 was estimated to be about 30 miles (50 kilometers) high. Both correspond to the location of an unusually warm region that contains features that appear to be cracks in the moon’s icy crust, seen in the late 1990s by NASA's Galileo spacecraft. Researchers speculate that, like Enceladus, this could be evidence of water erupting from the moon’s interior.
"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," said William Sparks, an astronomer with the Space Telescope Science Institute who led the 2014 and 2016 efforts to image Europa's plumes with Hubble. "We discovered that Europa’s plume candidate is sitting right on the thermal anomaly."
While there is no spacecraft at Jupiter that is capable of investigating these plumes closer (Juno's orbit and instruments are all wrong for that purpose), NASA is currently planning the Europa Clipper mission, which will launch sometime in the 2020s. Also, while Cassini's time at Saturn ends on September 15 of this year, the question of whether Enceladus supports life could be the driver for a followup mission (perhaps ELF, the Enceladus Life Finder, which was proposed by Linda Spilker and Hunter Waite at the 46th Lunar and Planetary Science Conference, in 2015.
While we wait for those opportunities, though, these are some incredible discoveries, and herald even more to come.
"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment," Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate, said during the NASA teleconference. "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."