The prospects of life in our solar system beyond Earth and finding it within a decade or two have improved with two scientific findings announced by NASA on Thursday. The agency confirmed the presence of hydrogen in plumes from Saturn’s small moon Enceladus, and also reported that plumes are highly likely on Jupiter’s moon Europa.
Both findings are significant. It means not only that most of the ingredients needed for life must exist in Enceladus’ oceans, but also that a few probes planned to explore Europa have a much higher chance of finding some life there. NASA’s Jim Green, who oversees the agency’s planetary exploration plans, said somewhat understatement: “This is a very exciting time to explore the solar system.”
The findings support a recent focus by NASA on expanding a program to explore these outer solar system ocean worlds, including Enceladus, Europa and Saturn’s methane-covered moon Titan. This has been a particularly important goal for Texan Republican John Culberson, who serves as chairman of the House subcommittee on NASA’s budget.
“This is really exciting news,” he told Ars on Thursday afternoon. “The findings on Enceladus and Europa reaffirm that the best place to look for existing life in the solar system is on these ocean worlds. Almost certainly life could have evolved there, as we see in hydrothermal vents here on Earth.” Culberson added that he would continue to push not only for an orbiter and lander to be sent to Europe, but also for a special mission to Enceladus to follow.
Planetary scientists weren’t particularly excited about Enceladus’ small, icy moon before the Cassini spacecraft arrived in the Saturn system about 13 years ago. However, when Cassini began making observations throughout the Saturn system, scientists soon found evidence of plumes emanating from cracks in the moon’s surface. Later observations confirmed the existence of those plumes, plus a large ocean beneath Enceladus’s surface.
So as the Cassini mission neared its end, scientists planned some more close observations of Enceladus, including flybys through the plumes themselves. In 2015, the spacecraft made its closest approach to within 30 miles (49 km) of the moon’s surface. During that flyby, the spacecraft detected a significant amount of molecular hydrogen in the plume.
In their subsequent analysis, scientists ruled out a number of explanations for the hydrogen and concluded that it most likely originated from the interaction of warm water near a rocky core of the moon, similar to the hydrothermal vents in Earth’s oceans. On Earth, large communities of microbes thrive near these vents, sustaining them through a process known as methanogenesis. These organisms use carbon dioxide and hydrogen to create methane, a chemical reaction that gives the microbe a boost of energy. This way they can survive without any solar energy.
Could such a process unfold in the oceans of Enceladus? Absolutely, scientists said Thursday at a NASA briefing. Astrobiologists believe that life as we know it requires water, chemical elements to make the building blocks of cells, and chemical energy. Saturn’s small moon, which is only about 500 km across, has all three. “Now we just need to know if Enceladus has had enough time to develop life and make an imprint,” said Mary Voytek, a senior astrobiology scientist at NASA Headquarters.
One of four Jovian moons observed by Galileo, Europa is quite a bit larger, about the size of Earth’s moon. Scientists are convinced that the ice-covered Europa is also home to a large ocean, probably much larger than that of Enceladus. And while planetary scientists are eager to explore Europa’s ocean and its potential for life, they’re not sure how to get through the moon’s icy crust, which is likely at least a few miles thick.
Using the Hubble Space Telescope, astronomers have found some evidence in recent years that, like on Enceladus, plumes may also be emanating from Europa’s internal ocean. But they weren’t sure: spying on water vapor at such a great distance was lame at best, even for the large space telescope. But on Thursday, William Sparks, an astronomer at the Space Telescope Science Institute in Baltimore, said new evidence from Hubble provides very strong evidence for plumes.
Over the course of 12 different observations, Hubble twice found evidence of water vapor emanating from the surface of Europa from the same location on the moon, near the equator, in 2014 and 2016. a 99.99 percent chance that the observations were not due to random chance. In addition, the location of the plume corresponds to where the Galileo probe, which flew through the Jovian system in the 1990s, observed thermal “hot” spots on the surface of Europa. “It’s really intriguing,” Sparks said. “It’s actually pretty amazing.”
The confluence of plumes at a thermal hotspot may be due to liquid water, below the ice but not too far from the surface, Sparks said. Alternatively, the plumes themselves, blowing as much as 100 km into space, could rain a fine fog vapor back onto Europa’s surface that alters the moon’s thermal character in a local area. Either way, as NASA plans to send both an orbiter and a lander to Europe in the 2020s, the existence of plumes would make the task of sampling the vast ocean below much easier for scientists.
All this talk about oceans is exciting, but what kind of life could exist there? Certainly, no one expects to find whales, sharks or other large marine animals in the oceans of Europa or Enceladus. Chemosynthetic microbes seem much more likely. But actually we have no idea. If life existed, it would be exotic.
As for which ocean holds the best prospects for life, NASA astrobiologist Voytek said she’d still bet on Europa. That Cassini found such high concentrations of hydrogen in the plumes of Enceladus could well mean that there is nothing in the moon’s oceans to consume it. There is also evidence that Enceladus could be significantly younger than Jupiter’s moons, giving life there less time to form. “My money is currently still on Europa, but it could be on any of these moons,” she said. “It would be great if life belonged to all of them.