Sub-surface seas on distant moons could be habitable

NASA’s Cassini spacecraft has detected hydrogen in plumes of icy material spewing from a sub-surface sea on Saturn’s moon Enceladus, a source of chemical energy and the latest indicator that the unseen ocean could be a habitable abode for microbial life, scientists said Thursday.

“The hydrogen is coming from a hydrothermal vent on the sea floor of Enceladus, going out into space in the plume,” said Linda Spilker, Cassini project scientist at the Jet Propulsion Laboratory in Pasadena, California. “This is a very significant finding because the hydrogen could be a potential source of chemical energy for any microbes that might be in Enceladus’s ocean.”

Jupiter’s moon Europa is another so-called “ocean world” in Earth’s outer solar system that is believed to harbor a sea of tidally heated water beneath its frozen crust.

In a double-barrel announcement, a second team of researchers reported Thursday that the Hubble Space Telescope has spotted, for the second time, what may be towering plumes of water ice erupting from cracks in Europa’s crust.

A similar plume was spotted two years earlier and both originate in the same area where surface temperatures are slightly higher.

NASA plans to send a spacecraft -- the Europa Clipper -- to Jupiter in the early 2020s to repeatedly fly by Europa, using a suite of powerful instruments to map the moon and to possibly sample the plumes, measuring the constituents of the sub-surface ocean and helping determine its habitability.

“These observations are really informing us of major things happening in these ocean worlds right now,” Jim Green, director of planetary science at NASA Headquarters in Washington, told reporters. “We’re pushing the frontiers, we’re finding new environments, we’re looking in a way we never thought possible before for environments in our solar system which may harbor life today.”

Cassini, nearing the end of its extended 13-year mission at Saturn, discovered plumes jetting away from Enceladus after the spacecraft arrived in 2004. It has photographed the jets, determined they are made up of 98 percent water ice and has even flown through them, sampling the material with a spectrometer and detecting ammonia, carbon dioxide, methane and organic compounds.

The latest findings are based on data collected during Cassini’s deepest dive through the plumes in October 2015. The data show about 1 percent of the plume material is hydrogen. And hydrogen, believed to be entering the ocean through hydrothermal activity on the sea bed, could provide a source of energy -- food -- for any microbes that might be present.

Hunter Waite, lead author of a paper published Thursday in the journal Science, said the calories represented by the hydrogen seen in the plume is roughly equivalent to 300 pizzas per hour.

The hydrogen is important because life, at least as it’s known on Earth, requires energy for metabolism. It also requires liquid water and a variety of key elements, primarily carbon, hydrogen, nitrogen, oxygen, phosphorous and sulfur.

Cassini has not yet detected phosphorous or sulfur, NASA said in a release, but scientists believe they likely are present because the moon’s rocky core is believed to be chemically similar to other solar system bodies that contain both elements.

Detecting hydrogen “really represents a capstone finding for the mission,” Spilker said. “Because we now know that Enceladus has almost all of the ingredients you would need to support life as we know it on Earth.”

Waite said the hydrogen is likely produced “by chemical reactions between warm water and rocks. We think that hydrothermal fluids are circulating below the ocean floor on Enceladus.”

“This exposes rocks to warm water, which drives geochemical transformations,” he said. “Certain minerals that are rich in iron react with water to form new minerals, and hydrogen is produced.”

He said Enceladus could feature buildups of precipitates on the ocean floor similar to so-called “white smokers” on Earth, which build up around warm-water vents and provide habitats for animal life beyond the reach of the sun.

Waite said a process known as methanogenesis could provide the spark for life on Enceladus.

“This is where microbes combine hydrogen with carbon dioxide to make methane, and they get a jolt of energy out of the process,” he said. “They use this energy to synthesize some of their complex biomolecules. ... We haven’t discovered evidence of organisms on Enceladus, but I’m encouraged by the geochemical data, which could allow for this possibility.”

Cassini is virtually out of fuel after an enormously successful mission. To prevent any chance that the spacecraft could eventually crash into Enceladus or any other moon, possibly contaminating a pristine environment, NASA has decided to crash the spacecraft into Saturn’s atmosphere in September, vaporizing the probe in a kamikaze-like plunge.

“Enceladus has got a warm, saltwater undersea ocean. We cannot risk an inadvertent contact with that pristine body,” Cassini project manager Earl Maize said at a recent briefing. “Cassini has got to be put safely away, and ... the only choice was to destroy it in some controlled fashion.”

In what NASA calls Cassini’s “Grand Finale,” the spacecraft will fly past the moon Titan on April 24, using the moon’s gravity to bend the probe’s trajectory enough to set up repeated, somewhat risky, passes between Saturn’s cloud tops and the inner edge of its ring system. The Titan flyby will set up the first of 22 passes on April 26.

NASA used a similar end-of-mission strategy for the Galileo spacecraft that explored Jupiter and its moons in the 1990s.

During Galileo’s mission, researchers noted what appeared to be cracks in the icy surface of Europa. Using the Hubble Space Telescope, researchers spotted what appeared to be an icy plume erupting from that same area in 2014, climbing to an altitude of about 30 miles.

In a paper published Thursday by The Astrophysical Journal Letters, researchers announced spotting a second plume, this one rising some 62 miles, in the same area in 2016. The measurements are at the limit of Hubble’s abilities, but researchers say they are confident the plumes actually exist.

Both presumed plume sightings “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,” NASA said in a statement.

If the warm spot and the plumes are, in fact, related, “it could mean water being vented from beneath the moon’s icy crust is warming the surrounding surface,” the statement said.

The Europa Clipper will be equipped with a powerful ultraviolet imaging system that will be able to make close-up, high-resolution observations that should reveal the nature of the presumed plumes and whether they represent water escaping through the surface cracks.