“Since ’tis certain that Earth and Jupiter have their Water and Clouds, there is no reason why the other Planets should be without them. I can’t say that they are exactly of the same nature with our Water; but that they should be liquid their use requires, as their beauty does that they be clear. This Water of ours, in Jupiter or Saturn, would be frozen up instantly by reason of the vast distance of the Sun. Every Planet therefore must have its own Waters of such a temper not liable to Frost.”
— Christiaan Huygens, “Cosmotheoros (Book I),” 1698
In his seminal book, “Cosmotheoros,” 17th-century Dutch mathematician Christiaan Huygens speculated about the possible habitability of other planets in the Cosmos, which he viewed as being not less capable than Earth in hosting their own complex forms of life. Yet even though the scientific exploration of the Universe hasn’t discovered the existence of any extraterrestrial life to date, it has helped to reveal a Universe that is much more dynamic, complex, and fascinating than what was ever previously thought. A series of spectacular discoveries in the last 50 years of entire terrestrial ecosystems, thriving on places that were previously thought to be too hostile to life, have similarly transformed our concept of habitability and the possible environments where life could take hold. The latest findings of microbial life flourishing in the extreme environment of subglacial Lake Whillans in Antarctica, which were detailed in the first part of this article, are further hinting at the possibility of life existing in a similar fashion as well in the mysterious, underground alien waters of Europa.
The range of habitable environments on which life was traditionally thought to arise were determined by the presence of three key ingredients: liquid water, organic compounds, and sunlight—the same ingredients that allowed for the emergence of life on Earth approximately 3.9 billion years ago. The revolutionary discovery in the 1970s of a thriving complex marine ecosystem around the hydrothermal vents of the Galapagos Rift on the ocean floor of the eastern Pacific forever changed our understanding of habitability showing that life could also arise and flourish in the complete absence of sunlight in conditions that were utterly toxic to any other life forms on Earth. Biologists were surprised to discover that the ecosystems found on the ocean floor along these mid-ocean ridges, consisting of complex organisms like tube worms, clams, and crabs, were dependent for their food on thermophilic chemosynthetic bacteria, which produced organic compounds using the oxidation of inorganic molecules as an energy source, instead of sunlight. These bacteria were found to live under extreme conditions of temperature and pressure, while utilising the minerals that were dissolved in the hot water that was emanating from these hydrothermal vents from deep within the Earth, at temperatures as high as 400 degrees Celsius. In the following decades, similar extremophilic microorganisms have also discovered around the globe in some of the harshest terrestrial environments imaginable, from the boiling geysers of the Yellowstone National Park in the U.S. to the highly alkaline, acidic, and toxic waters of the Rio Tinto river in southwestern Spain to the Mono Lake in California, just to name a few. If life was able to showcase such a tenacity on Earth, could it mean that life could have arisen on other parts of the Solar System, in equally challenging environments?
The water world of Europa
The most obvious candidate for the search of alien life has traditionally been Mars, a world that has captivated the imagination of scientists and the general public alike for centuries like none other. It is no exaggeration to state that largely because of its proximity and similarity to Earth, the Red Planet has been deeply ingrained through the years in the public’s conscience as the archetypal place of origin of extraterrestrial life, fueling a plethora of unsettling visions of ominous intelligent aliens that were hell-bent on conquering and destroying humanity.
Even though the series of robotic space missions that were sent to the Red Planet in the second half of the 20th century finally put to rest these earlier notions of Mars as a present-day habitat for complex and intelligent alien life, they nevertheless revealed a planet that could have been habitable at an earlier stage in its history, billions of years ago. The latest such findings come from NASA’s Curiosity rover, which has been exploring the plains of Gale Crater on Mars for past signs of habitability ever since it landed there in August 2012. The intrepid robotic explorer successfully completed its primary scientific objective when it discovered strong evidence early on in its mission that a place called Yellowknife Bay near its landing site, constitutes an ancient dried-out lake bed which was once filled with drinkable liquid water billions of years ago and could have sustained life—if any were present. Yet, as exciting as these findings were, for many members of the scientific community Mars does not represent the ideal place to look for life in the Solar System today, arguing that our searches should focus instead on the icy moons of the outer Solar System, like Jupiter’s Ganymede, Callisto, and Europa and Saturn’s Titan and Enceladus. “Europa is the most likely place in our Solar System beyond Earth to possess life,” says Dr. Robert Pappalardo, a Senior Research Scientist at NASA’s Jet Propulsion Laboratory in California. “Europa is the most promising in terms of habitability because of its relatively thin ice shelf and [the presence of] an [underground] ocean.”
Prior to the advent of the space age, the outer Solar System was largely seen as a vast, icy planetary graveyard. Conventional wisdom held that so far from the Sun the moons of the outer planets would just be dead and frozen chunks of rock and ice. NASA’s twin Voyager spacecraft, which flew by those outer planets during the 1970s and ’80s, revealed a very different picture: an assortment of dynamic, active, and ever-changing moons orbiting their equally fascinating planets, in set-ups that could best be described as mini solar systems in their own right. One of these worlds, Europa, a 3.120-km-wide moon orbiting Jupiter, exhibited a surface full of chaotic geologic formations that were unlike anything else seen in the Solar System, one that was crisscrossed by a vast network of red-colored ridges, spanning the entire globe. The Galileo mission that studied the Jovian system during the 1990s and early 2000s confirmed the scientists’ speculations that these ridges were fractures, or cracks, on Europa’s icy surface, caused be the intense tidal forces of the massive, nearby Jupiter and the orbital resonances with the other nearby moons of the Jovian system. “Just like the Earth’s oceans have tides, because they are pulled by the Moon’s gravity, Europa should have a tide, because it’s pulled by Jupiter’s gravity,” explains Pappalardo. “And Europa’s orbit gets a little closer and a little further from Jupiter. So, when it’s closer to Jupiter, it’s stretched out more, when it’s further from Jupiter it will contract more.” This constant flexing of Europa by Jupiter’s immense gravity melts its interior in the same way it melts that of neighboring Io, causing the latter to be the most volcanically active moon in the entire Solar System. But unlike Io, Europa’s interior does not feature a global magma ocean but a liquid water one, which lays beneath its few-dozen-kilometers-thick surface ice layer. In addition, the data that were collected by the Galileo spacecraft helped scientists to calculate that Europa’s underground ocean not only spans the entire moon, but it holds approximately twice as much water as Earth’s oceans.
The single best evidence for the existence of these alien waters came from Galileo’s onboard magnetometer. During its eleven flybys of Europa, the spacecraft found that the moon didn’t possess any internal magnetic field of its own. But its findings were equally interesting. It detected an induced magnetic field close to the surface, with an electric current running through the moon’s interior. Induction occurs when a moving magnetic field generates an electric current inside a conducting material. In the case of Europa, because of its orbit, which lays inside Jupiter’s massive magnetosphere, the latter’s magnetic field lines were found to be passing right through the moon itself. That could only mean that a highly conductive material was present near the moon’s surface. “Ice isn’t conductive enough to create such a field,” explains Dr. Kevin Hand, Deputy Chief Scientist for Solar System exploration at JPL. “So the best explanation is a region of salty, liquid water below the frozen surface, which supplies a conductive layer. When you go through a metal detector at the airport with a conductor such as keys in your pocket, the alarm goes off. Likewise, when Galileo flew by, Europa set off the alarm.” “It’s very hard to get that pattern, without having an ocean underneath the ice,” adds Dr. John Spencer, an astronomer at the Southwest Research Institute in Boulder, Colo.
Perhaps one of the most fascinating and important discoveries in recent years, which further strengthened the case for the existence of Europa’s underground ocean, has been the detection by the Hubble Space Telescope in 2013 of water vapor above the moon’s south pole. The most possible cause, according to researchers, for the observed water vapor traces around Europa is the presence of erupting water plumes on the moon’s surface, not unlike those that have been directly observed on Saturn’s moon Enceladus by the Cassini spacecraft. “By far the simplest explanation for this water vapor is that it erupted from plumes on the surface of Europa,” says Lorenz Roth, a Postdoctoral Researcher at the Southwest Research Institute. “If those plumes are connected with the subsurface water ocean we are confident exists under Europa’s crust, then this means that future investigations can directly investigate the chemical makeup of Europa’s potentially habitable environment without drilling through layers of ice. And that is tremendously exciting.”
Panspermia on Europa?
Located inside Jupiter’s immense magnetic field, Europa is constantly irradiated by streams of charged particles that are trapped in the massive planet’s radiation belts. When these particles collide with the moon’s surface, they break down the water molecules of the upper ice layer into oxygen and hydrogen, which in turn quickly recombine to form chemical compounds like hydrogen peroxide, which researchers believe could be playing an important role in the possible habitability of the underground ocean. “The availability of oxidants like peroxide on Earth was a critical part of the rise of complex, multicellular life,” says Hand. “At Europa, abundant compounds like peroxide could help to satisfy the chemical energy requirement needed for life within the ocean, if the peroxide is mixed into the ocean.”
Yet the biggest concern regarding the possible habitability of Europa has been the apparent isolation of the ocean from the rest of the moon. If biologically useful surface chemical compounds can’t reach the depths of the ocean below, the latter could end up hosting an environment that would be less conducive for life. Researchers have long speculated that water could find its way to the surface through openings along the cracks and ridges of the moon’s chaotic geologic terrain. A study by a research team in 2011 came to support this notion, by providing evidence for the existence of lakes several kilometers below the moon’s icy surface. These lakes, that would be located deep in Europa’s icy crust, could be communicating with the liquid water ocean below, while providing it with chemical elements from the surface that would be a valuable energy source to any potential life forms. “One opinion in the scientific community has been, ‘If the ice shell is thick, that’s bad for biology – that it might mean the surface isn’t communicating with the underlying ocean’,” says Dr. Britney Schmidt, a research scientist at the University of Texas and lead author of the study. “Now we see evidence that even though the ice shell is thick, it can mix vigorously. That could make Europa and its ocean more habitable.”
Another mechanism by which these biologically valuable chemical elements could be delivered to Europa’s ocean is through impacts by comets and asteroids. A study that appeared in the 2013 Fall meeting of the American Geophysical Union in San Francisco, Calif., by a team led by Jim Shirley, a research scientist at NASA’s Jet Propulsion Laboratory, presented evidence for the existence of a narrow ring of phyllosilicates on the moon’s surface, which were uncovered through an analysis of photographs taken with Galileo’s Near-Infrared Mapping Spectrometer. Phyllosilicates are clay-like minerals, formed in the presence of a neutral-pH water, and are indicative of a habitable environment. These minerals have also been found on chondritic meteorites on Earth, as well as on the surface of Mars by the Opportunity and Curiosity rovers. But Europa’s only habitable environment is believed to exist in its underground ocean, rather than on its surface. What Shirley and his colleagues noticed by examining Galileo’s images was that the phyllosilicates on Europa laid 120 km away from a 20-km-wide crater on the moon’s surface. Study of the images indicated that the crater was probably the result of a shallow impact with a comet or asteroid, at an angle of approximately 45 degrees. This shallow angle would have permitted some of the ejected material from the collision to fall back on the surface, spraying it with the observed phyllosilicate traces. “Organic materials, which are important building blocks for life, are often found in comets and primitive asteroids,” says Shirley. “Finding the rocky residues of this comet crash on Europa’s surface, may open up a new chapter in the story of the search for life on Europa.”
These cosmic impacts are also the key mechanism with which life, or at least its chemical building blocks, are delivered between worlds, according to the panspermia hypothesis which posits that the seeds of life are common throughout the Universe and can be spread from planet to planet by cometary and asteroid impacts. A study that was published in the Astrobiology magazine in late 2013 by Rachel Worth, an astronomy graduate student at Penn State University, bodes well with this hypothesis, as well as with the observations by Shirley’s team. Worth and her team conducted a series of simulations of 100,000 collisions throughout the Solar System on a timescale of 10-30 million years, with material originating from Earth and Mars. What the results showed was that the majority of these ejecta would fall toward the Sun and back to their parent planets in a short amount of time. According to the study, that would ensure that Earth would be re-seeded with life, following big, cataclysmic events like the Late Heavy Bombardment which happened approximately 4 billion years ago when the space environment around the Earth was dominated by untold numbers of big asteroids in the aftermath of its formation. Yet Worth’s simulations showed that several thousands of collisions on the gas giants would also take place and as much as 10 collisions for each of the moons of Jupiter and Saturn as well. Worth and her colleagues are quick to point out that the results of the simulations do not mean that life actually did arise on the outer moons, but rather that it is possible that it could have. “We find that transfer of rock capable of carrying life has likely occurred from both Earth and Mars to all the terrestrial planets in the Solar System and Jupiter, and transfer from Earth to Saturn is also probable,” conclude the researchers. “Additionally, we find smaller but significant probabilities of transfer to the moons of Jupiter and Saturn from Earth, and to the moons of Jupiter from Mars. The probability of life surviving such a journey or finding a tenable environment on arrival, is beyond the scope of our research. Ultimately, we conclude that the possibility of transfer of life from the inner Solar System to the outer moons, cannot be ruled out based on current knowledge.”
Prospects for the future
As fascinating as the results of these studies may be, they only provide tantalising evidence regarding Europa’s potential habitability. More definitive conclusions can only be drawn by future space missions to that fascinating Jovian moon, which would allow scientists to fill the gaps in our understanding of Europa’s potential for life. During the last 20 years, there hasn’t been a shortage of proposed mission concepts by NASA that called for a more detailed exploration of this distant world and, sadly, didn’t advance beyond their conceptual phase, mostly due to budgetary constrains. Despite a series of further proposed cuts to the space agency’s planetary science budget in recent years by the White House, Congress has nevertheless been much more supportive of a Europa mission, appropriating more than $100 million during the same time period for the further development of a mission concept called the Europa Clipper, which is currently under study by NASA. The concept envisions a spacecraft that would conduct approximately 40 close flybys of the Jovian moon, in order to study its surface geology and internal ocean in detail.
“The Galileo spacecraft left us with just enough information to feel confident about the pressence of [Europa’s] global liquid water ocean, but not enough information to know whether or not that ocean is in fact habitable,” says Hand. There are many pressing, fundamental questions about whether this immensely fascinating moon is indeed a place where life has arisen for the second time in our Solar System, and they can only be answered by future missions like the Europa Clipper.
The sooner we send the latter on its way, the sooner we will have the answers.
You can read Part 1 here.
Video Credit: NASA
- ‘Europa: Exploring a cold, distant world’, the von Karman Lecture Series, NASA/JPL, 19 June 2014.
- ‘Ices and Oceans in the outer Solar System’, Dr. Robert Pappalardo, public talk at the University of Nebraska–Lincoln, 14 April 2014.
- ‘The Lure of Europa’, public discussion by the Planetary Society in Congress, Washington, DC, 15 July 2014.
- ‘Finding Life Beyond Earth’, PBS, 2012