Human beings have long looked to the stars and hoped that alien life might look back at us. Yet the truth is that the first extraterrestrial life we discover is far more likely to be microbial — a prospect less romantic perhaps than the idea of bipedal aliens shaking hands with humans after landing on Earth.
Such microbial life has been theorized to have existed in the early days of Mars, before its water dried up, though we still don't know for certain. Now, astrobiologists are turning their gaze towards another nearby neighbor, Europa — an icy gray moon of Jupiter — as a suddenly much more alluring candidate for simple life.
"Early microbial life on Earth evolved in the liquid salt water environment of our oceans — which is what makes the hint of salt water on Europa so tantalizing.
Renewed interest in Europa's potential to harbor life stems from a new study about the peculiar moon. The subject of curiosity is the giant ridges that criss-cross the planet's surface like scratches on a cue ball. Underneath those ridges, explain the authors of a new paper in the journal Nature Communications, there may be pools of salty, liquid water. And since those ridges are ubiquitous, that means the pools could also be commonplace.
Of course, early microbial life on Earth evolved in the liquid salt water environment of our oceans — which is what makes the hint of salt water on Europa so tantalizing. The unique geography of Europa also happens to very much resemble Northwest Greenland, which is the other half of what the study concerns.
"Here we present the discovery and analysis of a double ridge in Northwest Greenland with the same gravity-scaled geometry as those found on Europa," the authors explained. "Using surface elevation and radar sounding data, we show that this double ridge was formed by successive refreezing, pressurization, and fracture of a shallow water sill within the ice sheet. If the same process is responsible for Europa's double ridges, our results suggest that shallow liquid water is [ubiquitous] across Europa's ice shell."
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Europa is not a particularly large world; a mere 2,000 miles in diameter, it is not even as large as Earth's own moon. Yet Europa's surface is unique, festooned in giant double ridges that can tower as high up as 1,000 feet into the air.
When a team of scientists at Stanford University learned about Europa's double ridges, they decided to study smaller geological structures in Greenland's northwest. More specifically, they studied the little double ridge feature in Greenland and learned how it was formed. It turned out that they came into existence because shallow pools of water beneath the surface first froze and then wound up breaking through on multiple occasions. This repeatedly pushed up the twin ridges.If the analogous ridges on Europa were formed the same way, as seems probable, the constant churning could have helped bring about the chemical reactions necessary to create life. It is an intriguing premise, to say the least, and is part of a long history of astrobiological interest in Europa.
"Gravity measurements also tell us that below this ice/water layer is a layer of rock and then a metallic core at the center," Phillips added. If you want there to be life in the universe, these are all good signs, as they suggest the basic ingredients could exist on the enigmatic moon.
"Scientists know from a combination of observations by Earth-based telescopes and spacecraft such as Galileo that the surface of Europa is covered primarily with water ice," Dr. Cynthia B. Phillips, Europa Project Staff Scientist and Science Communications Lead from the NASA Jet Propulsion Laboratory, told Salon by email. Astronomers estimate that Europa's surface has the same density as water ice and is roughly 100 kilometers thick, but the gravity measurements used to obtain that estimate do not answer questions about exact composition. How much of this is solid ice and how much of this is liquid water?
"Gravity measurements also tell us that below this ice/water layer is a layer of rock and then a metallic core at the center," Phillips, who was not involved in the most recent study, added. If you want there to be life in the universe, these are all good signs, as they suggest the basic ingredients could exist on the enigmatic moon.
"There are three things needed for life as we know it," explained Dr. Christopher Chyba, Professor of Astrophysical Sciences and International Affairs at Princeton University, in an email to Salon. In addition to liquid water and a source of useable energy, you need "the so-called biogenic elements" — like carbon — "that our kind of life is based on," plus a source of useable energy. "NASA's strategy for searching for life has long been 'follow the water,' and Europa and Enceladus in our Solar System are the two places, besides Mars, where we have a lot of evidence for liquid water that is probably accessible to exploration," Chyba, who was not involved in the study explained.
Chyba said it would be "bizarre" if Europa did not form with the "usual complement" of biogenic elements one finds on celestial bodies, "but even if Europa somehow formed without them, the late Betty Pierazzo showed that Europa would have accumulated a significant inventory of them over Solar System history from comet impacts." Pierazzo was a researcher at the Planetary Science Institute who specialized in impact craters.
Dreamers of Europa-pean life can also take heart in the magnetic field results, which gave strong evidence of an "induced field" while Europa orbits around Jupiter, which has a very strong magnetic field of its own. What accounts for Europa's magnetic field?
"The best explanation for the source of this induced field is a global salty water ocean," Phillips told Salon. "We think that Europa has the ingredients for life as we know it — more liquid water than all of Earth's oceans combined, plus the right other chemical elements and an energy source. On Earth, we find life wherever we have these three ingredients, so we think that Europa is one of the best places to look for life in our solar system beyond the Earth."
Chyba echoed this view when he wrote that it is "possible, based on what we know so far, to imagine types of microorganisms that could live in Europa's ocean." There was the important caveat, though, that we are dealing with the truly alien — we cannot know for sure that life can only develop as it has on Earth because we do not fully understand what causes "life" to exist in the first place.
"We don't know if there is life there or not, because we don't have enough of an understanding of the origin of life (on Earth or anywhere else) to say whether Europa's conditions would have favored the origin of life," Chyba observed.
For more Salon articles on extraterrestrial life: