(L.Billings – Scientific American, 17/02/17) – It’s not something NASA likes to advertise, but ever since its creation in 1958, the space agency has only conducted one direct, focused hunt for extraterrestrial life—and that was more than 40 years ago.
It happened in 1976, when the twin Viking landers touched down at separate sites on Mars to look for any signs of life lurking on the planet’s desolate, freeze-dried surface. The Viking mission was—and still is—the most expensive planetary science mission ever launched as well as a technical tour de force that laid the foundations for all future interplanetary exploration. Both landers came up empty in their search for life, however, and ever since NASA has favored a series of missions—most of them to Mars—that transformed our understanding of our neighboring worlds as they tiptoed around the central question of whether any of them harbor life.
Now, after decades of wandering in Martian deserts, NASA’s astrobiologists are at last preparing to rekindle a direct search for a “second genesis” of life in our solar system—but not where one might think. This time, they will look well beyond Mars, the most Earthlike of our planetary neighbors, to the dark reaches of the outer solar system, where probes and space telescopes have revealed ever-more tantalizing signs of oceans hidden inside icy moons and dwarf planets. Warmed by tidal forces rather than sunlight, those environments could harbor life, scientists say. “These oceans may be close to the surface, or may be deeper, with thicker ice crusts, but there must be water in liquid or slush form there—even all the way out to Pluto,” says James Green, director of NASA’s Planetary Science Division and an architect of the agency’s embryonic Ocean Worlds exploration program.
The program’s central focus is Europa, a moon of Jupiter that despite being slightly smaller than Earth’s lunar companion is thought to contain an ocean twice as voluminous as all our planet’s seas combined. Data from previous spacecraft flybys hint Europa’s ocean is billions of years old and in direct contact with the moon’s hot, rocky core, offering life sufficient time and energy to get started somewhere within. Locked below a crust with an average thickness of at least dozens of kilometers, any Europan biosphere might have remained forever out of reach. Occasionally seawater wells up through fissures in the crust to freeze at or near the surface, however, and recent observations by the Hubble Space Telescope suggest the ocean may even be venting vast amounts of water vapor into space through geyserlike plumes erupting from beneath the surface. If astronomers could collect the frozen material or the vapor they might learn what, if anything, lurks within Europa.
ALL EYES ON EUROPA
NASA is already developing a spacecraft set to launch in the 2020s called the Europa Multiple Flyby Mission. The EMFM will orbit Jupiter, swooping by Europa 45 times to study the reputed plumes, measure the thickness of the moon’s icy crust and map the surface at high resolution. Yet the EMFM is only a prelude. Heeding a directive handed down by Congress in 2015, the agency is studying concepts for a lander to touch down on Europa’s surface with the explicit purpose of gathering and studying samples in search of alien biology. A new study produced by a 21-member panel of biologists, geologists, space scientists and flight engineers describes the potential lander in detail, and projects it could land on Europa as early as 2031. “That’s what we really want to know,” Green says. “What’s in that ocean, and is it alive? The lander is really all about that next step… I would like to see the lander sitting under a plume—the plume sloshing on its deck, fresh material coming out of the crack. Now, are we there yet? Not quite.”
Despite Congress’s clamor for NASA to explore Europa, there is no guarantee it will actually provide the agency with funding necessary for the mission, which is conspicuously bereft of a price tag. Cost estimates would come later, NASA officials say, after careful consultation with the scientific community, not to mention sympathetic and powerful members of Congress.
Bob Pappalardo, a senior research scientist at NASA’s Jet Propulsion Laboratory (JPL) and EMFM’s project scientist, believes there is a compelling case to launch both the orbital and lander missions in rapid succession. “It’s like peanut butter and jelly—neither one makes a great sandwich by itself but they are wonderful together,” he says. “The EMFM will get at the key questions of Europa’s global habitability—its geology, chemistry, surface variation and the locations of its liquid water. If you want to really go for the brass ring and search for signs of life, then you’ll need to follow up on those findings by zooming in and going down to the surface…. I thought this was so far off that we would not see it in our lifetimes. Now, I’m not so sure.”
Jonathan Lunine remains skeptical. The Cornell University planetary scientist has seen too many mission proposals crash and burn long before they reach the launch pad to be overly optimistic about near-term prospects for a lander. “I want to see this happen in my scientific career, but we are at an early stage and so it is hard to predict when this will happen,” he says. “I’ve always found that the political process—getting approval and funding—represents the most hazardous environment a planetary mission can be exposed to.”
LESSONS FROM VIKING
Whenever—if ever—NASA’s mission planners green-light a life-seeking Europa lander, the specter of Viking’s pitfalls will loom over every challenge. How can it safely land? Where should it go? And above all, how should it search for alien life?
Low-resolution orbital imagery and simplistic retrorockets forced the Viking landers to touch down on drab, boulder-strewn plains that proved unfavorable to the search for life. The landers carried three relatively crude life-detecting experiments conceived when genetics and microbial ecology were still in their infancy and when knowledge of the Martian environment was much more limited. Each experiment investigated soil samples for signs of organic metabolism, the chemical reactions organisms rely on to produce and use energy. The samples, though, were scraped directly from the surface where intense ultraviolet radiation and cosmic rays would have killed almost any conceivable microbe, eliminating potential metabolic signatures. These and other troubles ensured that instead of making a robust case for life on Mars, Viking’s experiments delivered confusing, conflicting results.
In contrast, a Europa lander would have to rely on profoundly different technologies for landing, operating and looking for life, largely based on lessons learned from Viking.
STICKING THE LANDING, GATHERING SAMPLES
Before the lander even approached Europa, the EMFM’s high-resolution reconnaissance would help locate a compelling landing site—ideally a region of young ice enriched with fresh material from the ocean beneath, perhaps pushed up through cracks or falling like snow from a nearby plume. It would touch down using a “skycrane” like the one that gently placed NASA’s Curiosity rover on Mars in 2012, improving the odds of achieving a pinpoint landing in obstacle-filled terrain. “The greatest technical hurdle is designing a spacecraft that can safely land on a surface that is largely unknown,” says Curt Niebur, program scientist for NASA’s outer solar system missions. “But if we can meet the challenge of landing at Europa, then we can land anywhere.”
Although mission planners have yet to map Europa at very high resolution, the lower-resolution images they have already seen show a topography rugged enough to give them nightmares, says Britney Schmidt, a planetary scientist at Georgia Tech and study co-author. “Icy surfaces on Earth are incredibly complex, and Europa is rough on every scale we’ve ever observed it, so finding a flat spot might be impossible,” she says. “It’s hard not to be worried about that. Mars has been difficult for us—and it’s way flatter than Europa.”
Many of the most tantalizing landing spots may in fact also be the most dangerous—so called “chaos regions” defined by the jumbles of ridges, pits and fissures that sprawl haphazardly across them. Such regions may be sites where liquid water has come close to the surface through relatively thin crust, causing the ground above to collapse and shift due to cycles of melting and refreezing. Schmidt’s personal favorite site for a lander—and one of the leading candidates in the study—is Thera Macula, a chaos region near a possible plume source that also resides in a relatively radiation-free area of the Europan surface.
If it makes it to the surface successfully, a Europa lander would deploy a sophisticated instrument package to characterize its surroundings and perform a much broader search for life than anything possible during the Viking era. Stereoscopic cameras would find targets for sample collection and seismometers would map the subsurface using the echoes from icequakes. Instead of focusing on metabolism, spectrometers and microscopes would look for the biochemical building blocks of life—organic molecules, perhaps even individual cells—in pristine samples carved or drilled by a robotic arm that could penetrate as much as ten centimeters beneath the moon’s surface.
Despite benefiting from 40 years of technological and scientific progress, there is one key area in which the Europa lander will be at a distinct disadvantage in comparison to Viking. The Jovian moon is a far more alien place with fewer obvious similarities to Earth or to Mars to guide the design of experiments. “Europa is the right next place to ask the always-tough question about how life might be detected beyond Earth,” says Jim Garvin, chief scientist at NASA Goddard Space Flight Center and co-chair of the lander study team. “What makes this both exciting and daunting is engineering the necessary analytical measurements to occur in an environment that is outright ‘nasty’ in comparison to Mars.”
At Europa’s equator, the average surface temperature hovers around a chilly –160 degrees Celsius, and the entire surface is continuously pummeled by deadly radiation from particles trapped in Jupiter’s immense magnetic field, not to mention the occasional incoming space rock. Most of the lander’s delicate instruments would be kept relatively warm and protected within a radiation-shielded vault, leaving little more than the robotic arm and cameras exposed. The lander would operate for perhaps a month before expiring on that cold, hostile surface.
A BIOSPHERE IN THE SHADOWS
The mysterious aquatic world within Europa pushes standard concepts of habitability to extremes and demands an entirely new approach to searching for life. “The influence of abundant photosynthetic productivity permeates our atmosphere, oceans and upper crust, so our intuition about what an inhabited world looks like is very much couched in this context,” says Tori Hoehler, an astrobiologist at NASA Ames Research Center and co-author of the lander study. “A Europan biosphere, if one exists, is constrained by a very different set of environmental factors.”
There can be no life-giving sunlight in Europa’s ocean, so organisms there, scientists believe, would probably be chemosynthetic rather than photosynthetic, much like the creatures that live at hydrothermal vents on Earth’s seafloor. Life in that cold, dark, briny abyss would likely be quite languorous, with biochemistry throttled by a relative paucity of usable energy and nutrients, analogous to the minimalist aquatic ecosystems found in Antarctica such as the subglacial Lake Vostok and the hypersaline Lake Vida. Unable to probe these undersea environments directly, the Europa lander would instead have to look for biological by-products that might suffuse the sea and become incorporated in surface ice.
In a similar fashion, scientists can estimate deep-sea biological activity on Earth by measuring concentrations of cells and amino acids diluted in huge volumes of seawater. Based on such terrestrial measurements, the Europa study team set high standards for a lander’s life-seeking experiments, which must be able to discern organic material diluted to roughly one part per 50 billion and as few as 100 cells in a cubic centimeter of ice. “We basically wanted to have a very strong approach to understanding any ambiguous results,” explains Kevin Hand, a planetary scientist at JPL and co-chair of the lander study team. “If the lander finds no evidence of complex organics or cells of microbes in the ice, we’ll know that if there is life on Europa, it leaves only a faint bio-signature that is below the organic and cell count levels found in places like Antarctica’s Lake Vostok.”
Such a result would be disappointing, but according to Hoehler and his co-authors the greater disappointment would be if it was perceived as a failure that stifled momentum for further missions to Europa and other icy moons. It would be unlikely for Europa to give up all its secrets to the very first lander that sets down there, and such a mission could be just the beginning for NASA’s Ocean Worlds program. Missions could someday explore subsurface seas in Saturn’s Enceladus and Titan, Neptune’s Triton or even deep down in Pluto. Sensing a coming sea change, optimistic researchers are already sketching out wild ideas like interplanetary submarines built to bore or melt through kilometers of ice.
“Even if we somehow convinced ourselves that Europa wasn’t inhabited, and I don’t really think it’s possible to do so,” Hoehler says, “it would remain an extraordinarily interesting place to understand.”