Europa: Frozen Ocean in Motion
The Jovian moon, Europa, is the smallest of the four satellites first discovered by Galileo in 1610. Slightly smaller than the Earth’s moon, Europa’s two-thousand mile diameter however reflects about five times as much light as our Moon. This brightness hints at what makes Europa such an intriguing place for astrobiologists: Europa’s cracked surface is not as smooth as a mirror, but has an icy-sheet that covers the entire satellite. Tens of miles underneath this ice-sheet may slosh a liquid and murky ocean.
|High resolution close-up of Europa’s cracked surface, with the veiny, vermillion capillaries that intrigue geologists and astrobiologists. Sir Arthur C. Clarke wrote the following famous and enigmatic passage in his novel, 2010: Odyssey 2: "All these worlds are yours – except Europa. Attempt no landing there."
Named for a Phoenician princess, Europa was captured by the mythological Jupiter. When Jupiter disguised himself as a white bull (Taurus), she was picking coastal flowers, but took stride on his back underneath the surf on her way to the island of Crete. When so named, the moon Europa was not known to have a subterranean sea and vast floating rafts of ice.
Shown in false color, the images of Europa highlight the coarse ice in blue and the mineral-rich cracks in red and brown. The blue-ice is the older layer of nearly pure frozen water. The northern hemisphere in particular is tinted brown with mineral deposits like magnesium sulfate (Epsom salts) contaminating the ice.
Unlike the other moons of the solar system, where meteor strikes riddle their pockmarked faces, Europa is relatively smooth and self-healing after impacts. The moon is not sheltered from such strikes, but shows few signs of catastrophic hits. The places where a few craters are preserved, their depths are shallow and ripple out like a soft surface has swallowed the rocky debris. Gaps are filled with slush from underneath and frozen in place. What is virgin surface today may be 10 to 250 million years old, a mere blink of an eye in geological timelines. In closer view, many of these young ridges and criss-crossing patterns look as though plastered over with a rough knife. The zig-zag pieces of the moving puzzle on Europa are shifted by daily tides as Europa orbits the massive parent, Jupiter. Jovian gravity shapes the icy to fit together and cap the sea with an frozen pack.
|Double-ridge pattern characteristic of close views of the Europan ice pack. "I’m increasingly aware that some of the most interesting things in astronomy and astrophysics, for instance, can change the way people understand the universe, how it got started and where it’s going. I found those Voyager pictures of the moons of Jupiter incredibly exciting, these beautiful color pictures .." -Robert C. Richardson, Nobel Laureate, Physics, Cornell, (1996)
Image Credit: NASA/ Galileo
The close view (left) of the ice pack shows a curious double-ridge pattern that marks much of the cracking. Whether these ridges spring geyser-like to heal stress fractures or alternatively more closely resemble a slush spreading through a crack, the pairing of edges makes Europa a surprisingly ordered puzzle of bonded rafts. In many places, the height of a ridge may rival the terrestrial cliffs of Mount Rushmore.
But to witness anything comparable on Earth, one might have to drain the oceans to see the scarred marks of continental drift, as it rips the terrerstrial crust and drives together tectonic plates. Europa, when viewed in this way, is like a terrestrial ocean inverted: the water shifts underneath a ceiling of land. Europa’s shell is sometimes compared to a candy, with a hard-coating, a liquid layer, and a molten iron and nickel core.
The probable sixty-mile thick ocean layer, if truly liquid, is so deep that its volume equals all of Earth’s oceans combined.
Even farther out, a wispy gas layer of oxygen shrouds Europa. Like only four other bodies in our solar system–Earth, Mars, Venus, and Ganymede–Europa has trace evidence of molecular oxygen in its highly rarefied atmosphere. But unlike Earth, where living things contribute the bulk of oxygen, ultraviolet radiation from the Sun and Jupiter’s powerful magnetic cloud strip the water ice to make whatever oxygen has been found haloing Europa to a distance of 125 miles above the frozen pack.
|"I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times… there are four erratic sidereal bodies performing their revolutions around Jupiter." –Galileo, 1610, discovering the four Galilean moons.
Most critical to astrobiologists studying Jupiter’s moons, the eccentricity or oval shaped orbits of Jupiter’s moons are pumped or oscillated by tidal forces as they orbit. This input of Jupiter’s gravitational energy heats up the inner moons particularly like Io without relying only on the Sun’s radiant heat, and thus gives an interesting way to provide one of the three ingredients for life–an energy source–even if far from the Sun.
What remains to be found among the Giant Planets like Jupiter and Saturn are some candidates that combine all three ingredients for primitive life: energy, liquid water and some atmosphere. Only Saturn’s moon, Titan, has an appreciable atmosphere, and only Jupiter’s Europa or Ganymede have any indications of water ice. But uniquely powerful tidal forces around the Giant Planets do offer some promising, non-radiant and non-volcanic heat sources.
A lander on Europa would greet a relatively windless surface world. The tropical conditions near the equator only reach a surface temperature of minus 260 degrees Fahrenheit (-130 C). The acidity of its briny ocean may compare to battery acid, rich in the sulfurous deposits characteristic of the other Jovian satellites. As inhospitable as this world might seem, the presence of sulfur and sulfuric acid may not differ from some views of the early Earth, and are not incompatible with many extreme environments where bacteria have evolved to master radically different life-cycles based on extracting energy without sunlight. Indeed, like the electrochemistry of a battery, terrestrial life has found ways to extract energy from the charged reactions of sulfuric acid.
A mystery of where Europan acid comes from has perplexed researchers, since the discovery that the side of Europa that permanently faces another of Jupiter’s moon, Io, is rich in sulfur. Because Io spews volcanic sulfur into space, the pairing of these two moons may provide evidence for transport between two satellites unlike what was imagined between more isolated worlds. Alternatives to explain the oxidizing acids on Europa include that sulfuric minerals rise up from the depths of the ocean, or are ionized by the intense radiation of Jupiter itself. Whether Europa mines sulfur from Jupiter, Io, or its own ocean has contributed to more curiousity about the cracked moon.
Setting the clock on this imagined Europan lander would challenge terrestrial expectations. The Europan day, its time to complete a full rotation, is equivalent to its year, the time to orbit Jupiter (or about 3.55 Earth days). To stand at the longitude that always faces Jupiter, would present an overhead specter of the parent Jupiter always shining above filling the sky. Like most moons that are phase-locked with their planet, there is a far-side to Europa that is relatively more dark. But a global ocean separating the core from its surface may offer a slippery version of this traditional view. Jupiter’s gravity would spin the ice pack faster than the core, and the floating world drifts unlike any other known moon or planet. This drift may be slow, however, since no appreciable surface features differed when the Voyager and Galileo probes first photographed Europa during flybys separated by twenty years.
Europa challenges many assumptions about what the outer planets might offer as hospitable conditions for primitive life to develop. So far from the Sun, Europa draws tidal energy from its parent planet. The murky ocean that may separate its core from a floating surface world is salty enough that it conducts electrically as it orbits. If some microbial life could survive this far from the Sun, it would need to generate its chemical energy from the briny liquid and depend on the shelter of a frozen cap to weather the intense radiation of its Jovian host.
Where water, energy and some mineral nutrient has been found on Earth, life has flourished in what often offers some of the more exotic biochemistry hardly imagined even a few decades ago. If life exists on Europa is highly speculative, but would depend on sulfur for its fuel. Whether Europa’s primitive conditions ever sparked life or not, the recent end of the Galileo probe’s mission took precautions against contaminating whatever may lurk underneath the tidally-flexed, Europan surface.