Ocean Moon – Worlds Of Ice
By Judith E Braffman-Miller
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 sent 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.
The Ocean Worlds Of Our Solar System
There are more than 100 moons in our Solar System that do their mysterious gravitational dance around the eight major planets belonging to our Sun’s family. Most of them are icy and small, containing only tiny quantities of rocky material, and they circle around the quartet of giant gaseous planets that dwell in the outer regions of our Solar System. The four majestic, giant denizens of the outer limits–Jupiter, Saturn, Uranus, and Neptune–are cloaked in blankets of gas, and they are orbited by sparkling, icy moons and moonlets. Of the quartet of relatively small, rocky terrestrial planets–Mercury, Venus, Earth, and Mars–Mercury and Venus are moonless, and Mars is circled by a pathetic duo of tiny and somewhat deformed moons (Phobos and Deimos). The two little moons of Mars are interesting objects, frequently considered to be asteroids that escaped from the Main Asteroid Belt between Mars and Jupiter, only to be snared by the Red Planet’s gravitational pull when our Solar System was young. Earth’s own beautiful, beguiling, bewitching Moon is the only large one inhabiting the inner kingdom of our Solar System.
A moon is defined as a natural satellite in orbit around another body that, in turn, is in orbit around its Star. The moon is kept in its position by both its own gravity, as well as its host’s gravitational grip. Some planets have many moons, some have only a small number, and still others have none at all. Several asteroids inhabiting our Solar System are circled by very small moons, and some dwarf planets–such as Pluto–also host moons.
Water in its life-sustaining liquid phase exists beyond our own planet, both in our Solar System–and elsewhere. With oceans of water sloshing around on 71% of our own planet’s surface, Earth still remains the only planet known to have stable bodies of liquid water. Liquid water is essential for all known life forms on Earth. The existence of water on the surface of Earth is the outcome of its atmospheric pressure and a stable orbit in our Sun;s circumstellar habitable zone. The habitable zone is that Goldilocks region, surrounding a star, where the temperature is not too hot, not too cold, but just right for life sustaining water to exist in its liquid phase. However, the origin of Earth’s water still remains unknown.
Only recently have space missions begun to solve this beguiling Solar System mystery–that a small number of distant moons have been successfully hiding, from the curious eyes of astronomers, life-sustaining liquid water beneath secretive shells of ice.
These icy moon-worlds are the next important step in the scientific quest for the Holy Grail of life beyond our own planet. It is a strange era in human history. Astronomers have collected large amounts of data revealing bewitching clues that habitable ocean moon-worlds may be out there, within the family of our very own Star. Humanity is poised at the beginning of a new era. Sophisticated new technology might very soon answer the profound, and very ancient question, “Are we alone?”
As of December 2015, confirmed liquid water in our own Solar System–excluding Earth–accounts for about 25 to 50 times the volume of our planet’s water. The moons of our Sun’s family thought to possess liquid water are:
Europa: Planetary scientists generally think that a layer of liquid water swirls around beneath Europa’s surface, and that heat from tidal flexing causes the subsurface ocean to remain liquid. It is estimated that the outer crust of solid ice is about 6 to 19 miles thick, including a ductile “warm ice” layer that hints that the liquid ocean underneath may be 60 miles deep. This means that Europa’s oceans would amount to slightly more than two times the volume of Earth’s oceans.
Enceladus: Enceladus has shown geysers of water that were confirmed by the Cassini spacecraft in 2005. Gravimetric data obtained from 2010 to 2011 confirmed the existence of a subsurface ocean. Even though originally it was thought to be localized, most likely in a region of the icy moon’s southern hemisphere, evidence collected in 2015 indicates the subsurface ocean is actually global. Furthermore, in additon to water, these geysers from vents located near the south pole of Enceladus contain tiny quantities of salt, nitrogen, carbon dioxide, and volatile hydrocarbons. Tidal flux from Saturn is apparently responsible for the melting of the ocean water, as well as the geysers.
Ganymede: Ganymede is both the largest moon of Jupiter, our Solar System’s planetary behemoth, as well as the largest moon in our entire Solar system. Observations of Ganymede by the HST in 2015 suggested the existence of a subsurface saline ocean. This is because patterns in auroral belts and rocking of the magnetic field hinted at the presence of an ocean. It is estimated to be approximately 100 kilometers deep with a surface situated below a crust of 150 kilometers.
Titan: Titan, the tormented, hydrocarbon-slashed largest moon of Saturn–and the second largest moon in our Solar System, after Ganymede–could possess a subsurface, salty ocean that may well be as salty as the Dead Sea on Earth. The salty water could begin approximately 31 to 62 miles beneath Titan’s icy shell, according to recent estimates. Meanwhile, on Titan’s smog enshrouded surface, “life as we do not know it” could swim in alien lakes and rivers that flow with liquid methane and ethane hydrocarbons–instead of water.
NASA’s future exploration of ocean worlds is enhanced by HST’s monitoring of Europa’s possible plume activity and Cassini’s long-term observations of the plume of Enceladus. In particular, the investigations of both ocean worlds are providing the groundwork for NASA’s Europa Clipper mission, which is planned for launch in the 2020s.
“If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper we will be ready for them,” commented Dr. Jim Green in the April 13, 2017 NASA Press Release. Dr. Green is Director of Planetary Science at NASA Headquarters.
HST’s detection of a site, which appears to show persistent, intermittent plume activity on Europa, provides a promising target for the Europa mission to investigate. Equipped with its new and sophisticated suite of science instruments, the mission can detect whatever may potentially be swimming around in the hidden global ocean sloshing around beneath its secretive crust of ice.
Beneath The Icy Crust Of Enceladus
The research paper written by scientists with the Cassini mission, published in the journal Science, suggests the presence of hydrogen gas. Hydrogen gas, that could potentially provide a chemical energy source for life, is pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor of this bewitching, distant moon-world.
The existence of ample amounts of hydrogen in the subsurface ocean of Enceladus indicates that microbes–if any exist there–could use it to obtain energy by mixing with carbon dioxide dissolved in water. This particular chemical reaction, termed methanogenesis, because it manufactures methane as a byproduct, may have been of critical importance in the emergence of life on our planet.
Life as we know it depends on the presence of three ingredients: liquid water; a source of energy for metabolism; and the right chemical ingredients, mainly carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. With this new discovery of the existence of hydrogen, in the tattle-tale plume shooting out from the surface of Enceladus, Cassini has revealed to the prying eyes of curious astronomers, that this small, icy moon has almost all of these ingredients important for habitability. At this point, Cassini has not detected the presence of phosphorus and sulfur in the hidden subsurface ocean of this distant small world, but many planetary scientists suspect that they will eventually be detected because the rocky core of Enceladus is believed to be similar to certain meteorities that contain these two critical elements.
“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,” commented Dr. Linda Spilker in the April 13, 2017 NASA Press Release. Dr. Spilker is Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
During Cassini’s close flyby of Enceladus on October 28, 2015, it detected molecular hydrogen as the spacecraft zipped through the plume of ice grains and gas spraying out from cracks slashing though the icy crust of the moon-world. Earlier flybys provided hints that a global subsurface ocean did, indeed, exist, sloshing around above a rocky core. Molecular hydrogen in the plumes could indicate hydrothermal processes, which could play the important role of providing the chemical energy so necessary to support life as we know it. In order to hunt for hydrogen specifically originating on Enceladus, the spacecraft dived particularly close to the strange slashed surface.
From these observations planetary scientists were able to determine that almost 98% of the gas in the plume is water, about 1% is hydrogen, and the rest is a combination of other molecules that include methane, ammonia, and carbon dioxide.
This important measurement was made using Cassini’s INMS instrument, which detects gases with the goal of determining their composition. INMS was designed to sample the upper atmosphere of Saturn’s large, smoggy moon Titan. However, after Cassini’s surprising discovery of a tall plume if icy spray erupting from cracks on Enceladus in 2005, planetary scientists turned its detectors to that small moon.
“We developed new operations methods for INMS for Cassini’s final flight through Enceladus’ plume. We conducted extensive simulations, data analyses, and laboratory tests to identify background sources of hydrogen, allowing us to quantify just how much molecular hydrogen was truly originating from Enceladus itself,” explained Dr. Rebecca Perryman in the April 13, 2017 SwRI Press Release. Dr. Perryman is INMS operations technical lead.
Cassini wasn’t originally designed to spot signs of life in the Enceladus plume. In fact, planetary scientists didn’t even know that the plume existed until after the spacecraft reached Saturn.
The scientists also considered other possible sources of hydrogen from the little moon itself, such as a preexisting reservoir in the icy crustal shell or a global ocean. Subsequent analysis indicated that it was unlikely that the observed hydrogen was obtained during the formation of Enceladus or from other processes on the moon-world’s surface or in the interior.
“Everything indicates that the hydrogen originates in the moon’s rocky core. We considered various ways hydrogen could leach from the rock and found that the most plausible source is ongoing hydrothermal reactions of rock containing minerals and organic materials,” Dr. Waite noted in the April 13, 2017 SwRI Press Release.
The new findings are an independent line of evidence that hydrothermal activity is taking place in the subsurface ocean of Enceladus. Earlier results, published in March 2015, indicated hot water is interacting with rock beneath the sea of this distant moon. The new discoveries support that conclusion and add that the rock appears to be reacting chemically to produce the hydrogen.
Dr. Waite commented that “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.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.