Most of the moons of our Solar System are intriguing, frigid, and dimly lit ice-worlds in orbit around the quartet of outer, majestic, gaseous giant planets that circle our Star, the Sun, from a great distance. In our quest for the Holy Grail of discovering life beyond our Earth, some of these icy moons are considered to be the most likely worlds, within our own Solar System, to host life. This is because they are thought to hide oceans of life-sustaining liquid water beneath their alien shells of ice--and life as we know it requires liquid water to emerge, evolve, and flourish. In April 2017, a team of planetary scientists announced that they have discovered the presence of hydrogen gas in a plume of material erupting from Enceladus, a mid-sized moon of the ringed, gas-giant planet Saturn, indicating that microbes may exist within the global ocean swirling beneath the cracked icy shell of this distant small world. Currently, two veteran NASA missions are providing new and intriguing details about the icy, ocean-bearing moons of the gas-giant planets, Jupiter and Saturn, further heightening scientific fascination with these and other "ocean worlds" in our Solar System--and beyond.
The findings of the two missions are presented in papers published on April 13, 2017, by planetary scientists with NASA's Cassini mission to Saturn and the venerable Hubble Space Telescope (HST). In one of the papers, Cassini scientists announced their discovery that a form of chemical energy life can feed on appears to exist on Enceladus. In the second paper, HST researchers report additional evidence of plumes erupting from Jupiter's moon, Europa, whose fascinating frozen crust of ice resembles a cracked eggshell. It has long been recognized by planetary scientists that beneath Europa's bizarre cracked shell of ice, there is a sloshing global ocean of liquid water.
"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment. 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," commented Dr. Thomas Zurbuchen in an April 13, 2017 NASA Press Release. Dr. Zurbuchen is associate administrator for NASA's Science Mission Directorate at Headquarters in Washington D.C.
The paper from planetary scientists with the Cassini mission, published in the journal Science, suggests hydrogen gas, which could potentially provide a chemical energy source for living tidbits, is gushing into the subsurface global ocean of Enceladus from hydrothermal vents on the seafloor of this distant ice-world.
Cassini is an unmanned spacecraft ourocean2018 to the Saturn system. It is the fourth space probe to visit the ringed planet, as well as the first to enter orbit. It has been studying Saturn and its many moons since arriving there in 2004.
There is a bizarre rocky landscape, well hidden from our prying eyes, in the secretive shadows under the oceans of our Earth. Here, in this strange and alien domain, it is always as dark as midnight. Thin, tall towers of craggy rock emit billows of black smoke from their peaks, while all around the towers stand a weird, wavy multitude of red-and-white, tube-like organisms--that have no eyes, no intestines, and no mouth. These 3-foot-long tubeworms derive their energy from Earth itself, and not from the light of our nearby Sun--a feat that most biologists did not believe possible until these wormish creatures were discovered back in 2001. The extremely hot, superheated black water, billowing out from the hydrothermal vents erupting on Earth's seafloor, provides high-energy chemicals that sustain the tubeworms, as well as other weird organisms that apparently thrive in this very improbable habitat.
"Hydrogen is a source of chemical energy for microbes that live in the Earth's oceans near hydrothermal vents. 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," explained Dr. Hunter Waite in an April 13, 2017 Southwest Research Institute (SwRI) Press Release. Dr. Waite is principal investigator of Cassini's Ion Neutral Mass Spectrometer (INMS), and lead author of the paper titled Cassini Finds Molecular Hydrogen in the Enceladus Plume: Evidence for Hydrothermal Processes. The SwRI is in San Antonio, Texas.
The hydrothermal vents on Earth's seafloor shoot out mineral-laden, hot fluid. This sustains some very unusual and unique forms of life--such as the wavy, wormish tubeworms--and other creatures that are able to thrive in this strange environment. Microbes can convert mineral-laden fluid into metabolic energy, making these ecosystems possible--both on Earth's seafloor and elsewhere.
The HST findings, published in The Astrophysical Journal Letters, report on recent observations of Europa, dating from 2016, in which a probable plume of material was observed shooting out from the moon's cracked icy surface. This rising plume occurred at the same location that HST had previously observed signs of a plume in 2014. The HST images provide strong evidence that the plumes observed shooting out from the surface of Europa, could be real eruptions. The observed plumes could be seen flaring up intermittently in the same region on the moon's surface.
The more recently imaged plume erupts to a height of 62 miles above Europa's surface, while the one seen in 2014 was estimated to rise almost half as high at 30 miles above its surface. Both erupting plumes are located in an unusually warm region of this icy small world. This relatively toasty area shows some strange features that appear to be cracks in the moon's shell of ice, that were first observed back in the late 1990s by NASA's Galileo spacecraft. Planetary scientists propose that, like Enceladus, this could be a sign of water erupting from a sloshing global ocean of liquid water, swirling around in the moon's interior, that is hidden beneath its crust of surface ice.