Centauri Dreams

Our recent discussions of the Jovian moons Ganymede and Europa highlight a fact that not so long ago would have seemed absurd. Three of the four bright dots that Galileo saw through his primitive telescope around Jupiter are potential habitats for life. Even battered Callisto gives evidence of an internal ocean, as do, of course, both Ganymede and Europa. But why stop there? Further out, Titan is worth exploring both on the surface and under it, and tiny Enceladus may be both the easiest to study and the most bizarre astrobiological possibility we’ve yet found.

The ‘easy to study’ part comes from the fact that Enceladus conveniently spews vapor from its own internal reservoirs into space, making it possible for a space probe to analyze the contents without ever touching down on the surface. The ‘bizarre’ part comes from the fact that those fissures exist, surely a sign of Saturn’s gravitational grip upon the flexing moon, but also a reminder that these outer moons have leaped into our consciousness as liquid water-bearing places. Remember, it wasn’t that long ago that we assumed Europa itself would be just another crater-scarred, inert ball of rock and ice. The Voyager missions changed everything.

Lee Billings is sufficiently encouraged by facts like these to put Europa at the very top of his list of destinations, speculating in his new essay in Aeon Magazine that our enthusiasm for Mars may be misplaced. We’re always ‘following the water,’ knowing that water helps in the transmission of biochemical energy, nutrients and waste, not to mention its shielding effects against cosmic radiation and its ability to retain warmth. But Billings, the author of Five Billion Years of Solitude (Current, 2013) sees our current Mars efforts as ‘cautious and procedural, perhaps to a fault, as a result of past overreaches in the search for Martian life.’

Indeed, scientists who specialise in Mars have been forced to dial down their dreams, hypothesising ever-smaller windows of opportunity for past life on the red planet, and ever more inaccessible refuges for anything now living there. Native Martians, if they exist at all, are most probably microbes clinging to life almost unreachably deep beneath the surface. This does not diminish the importance of exploring our neighbouring planet, but it must be admitted that there might well be more promising places to seek alien life. Indeed, if following the water is the prime directive in the search for extraterrestrial life, it increasingly appears that we should look beyond Mars to an icy moon of Jupiter called Europa.

Image: This artist’s concept shows a simulated view from the surface of Jupiter’s moon Europa. Europa’s potentially rough, icy surface, tinged with reddish areas that scientists hope to learn more about, can be seen in the foreground. The giant planet Jupiter looms over the horizon. Credit: NASA/JPL-Caltech

The russet upwellings of mineral salts that mark Europa’s cracks and fissures helped us see that a warm ocean sustained by Jupiter’s tidal forces and the flexing of the interior could exist, and that that ocean could have existed for billions of years. Imagine finding evidence that life of some kind existed under that ice. The discovery would implicate the other moons I’ve mentioned, and could, as Billings reminds us, take us out as far as Pluto in the hunt for subsurface water, looking for tidal heating or radioactive decay as sources of a comfy astrobiological warmth.

Europa, of course, is a very tough nut to crack. For one thing, you’re dealing with magnetic fields around Jupiter that produce extreme radiation hazards not just for manned missions but robotic orbiters, which adds greatly to the cost of any contemplated mission. Then there’s the matter of Europa’s crust, which might be a few kilometers deep or a hundred. Here the Enceladus model may come to our rescue, for just as Enceladus vents subsurface materials into space, so too may Europa. It was just last year that the Hubble Space Telescope was used to detect water vapor here — an estimated 7000 kg of water per second — blown 200 kilometers into space.

Fly a mission through these plumes and it should be possible to learn a great deal about what’s going on by way of chemical and physical processes beneath the ice, perhaps even evidence of biological activity or, as Billings adds with a touch of whimsy, ‘you might even catch a flash-frozen fish.’ For that matter, a robotic lander near a Europan fissure might snare highly interesting results. Sure, Mars is a much easier target, but look at the sheer number of orbiters and landers both in place and planned and contrast that commitment to the less than $1 billion NASA is now targeting as the pricetag for the Europa mission it’s gathering concepts for.

Image: Reddish spots and shallow pits pepper the enigmatic ridged surface of Europa in this view combining information from images taken by NASA’s Galileo spacecraft during two different orbits around Jupiter. The spots and pits visible in this region of Europa’s northern hemisphere are each about 10 kilometers across. The dark spots are called “lenticulae,” the Latin term for freckles. Their similar sizes and spacing suggest that Europa’s icy shell may be churning away like a lava lamp, with warmer ice moving upward from the bottom of the ice shell while colder ice near the surface sinks downward. Other evidence has shown that Europa likely has a deep melted ocean under its icy shell. Ruddy ice erupting onto the surface to form the lenticulae may hold clues to the composition of the ocean and to whether it could support life. Credit: NASA/JPL/University of Arizona/University of Colorado.

Billings ends his essay with characteristic eloquence:

Even if Mars proves totally, irrevocably dead, one can still squint up at its ruddy disk in the night sky, and envision a better future for it. Someday, humans might walk there, perhaps even live. No one has such dreams for Europa. If Mars is a warped mirror we stare into, while imagining ourselves as explorers in some pleasantly familiar frontier future, then Europa must be a locked door, or maybe a matte-black monolith, cold and indifferent, an abyss that might, some day, gaze back at us, if only we could first convince ourselves to look.

I like that, especially the nod to Clarke’s monoliths, and I thought about the distance between Bradbury and Clarke as I absorbed Lee’s essay. Clarke went to Mars as well, in The Sands of Mars (1951), his first published novel, while Bradbury’s Mars was a splendid, visionary dream sequence. Both writers depicted a Mars that could be tamed by humans, but it’s the mysterious Europa of Clarke’s Space Odyssey series that draws me more.

Clarke had planned to delay 2061: Odyssey Three until the Galileo mission to Jupiter was operational, but he went ahead after that mission’s launch delay, so that when the book was published in 1987, it couldn’t have drawn on any of the Galileo findings. But even with Galileo’s imagery, Europa’s hidden depths are still enigmatic, their exciting promise hidden by their layered ice. Clarke’s Europa would be all about transformation as an ignited Jupiter (‘Lucifer’) heated up all the Jovian moons to bring life to a once desolate system.

What might come out of a thawed Europan ocean in a scenario like that? Life is all about transfiguration — it emerges out of an environment, changes and is changed by that environment — and we are left to wonder how complex it might become given the right mix of oxidized mineral salts filtering back down through the fractured Europan ice and the chemical reactions near deep water hydrothermal vents. The prospect is so compelling, the possibilities so alien to our earlier conceptions of the Galilean moons, that surely we can come up with a mission following up ESA’s Jupiter Icy Moons Explorer (JUICE) to make the 2030s the decade of Europa.

Remember as you ponder NASA’s Request for Information about a Europa mission that the agency is contributing three instruments to the European Space Agency’s JUpiter ICy moons Explorer (JUICE) mission, to be operational in Jupiter space in the 2030s. The goal here is to explore Europa, Callisto and Ganymede through numerous flybys, with the craft finally settling into orbit around Ganymede. This would be the first serious look at multiple Jupiter moons by a visiting spacecraft since the Galileo mission, which explored the system from 1995 to 2003.

The large Jovian moons have always been of interest, with not just Europa but Callisto and Ganymede also thought to have deep oceans beneath their icy crusts. Galileo, in fact, found evidence for salty seas within Ganymede, probably containing magnesium sulfate. At the Jet Propulsion Laboratory, a team led by Steve Vance is offering new research showing that what we may have on Ganymede is more than a simple sea between two layers of ice. Using computer modeling of the processes involved, the scientists are talking about regions of ice and oceans stacked in layers. “Ganymede’s ocean might be organized like a Dagwood sandwich,” says Vance.

Image: This artist’s concept of Jupiter’s moon Ganymede, the largest moon in the solar system, illustrates the “club sandwich” model of its interior oceans. Scientists suspect Ganymede has a massive ocean under an icy crust. Previous models of the moon showed the moon’s ocean sandwiched between a top and bottom layer of ice. A new model, based on experiments in the laboratory that simulate salty seas, shows that the ocean and ice may be stacked up in multiple layers, more like a club sandwich. Credit: NASA/JPL.

Larger than Mercury, Ganymede may have 25 times the volume of water found on Earth. The JPL work, reported in a new paper in Planetary and Space Science, suggests that the sea bottom in the various layers may not be coated with ice but with salty water. Earlier models had shown the probability of dense ice at the bottom of a huge ocean with enormous pressures, but Vance’s team added salt to its models and found that liquid dense enough to sink to the sea floor was the result. Salt, as it turns out, makes the ocean denser, particularly under the extreme pressure conditions found within Ganymede and other moons.

Oceanic pressure on Ganymede would be high, for the moon’s oceans may be up to 800 kilometers deep. The deepest, densest form of ice thought to exist on Ganymede is known as Ice VI. If ordinary refrigerator ice (Ice I) floats in a glass of water, heavier forms of ice produced by extreme pressures show much more compact crystalline structure, with the molecules packed more tightly together, which is why some forms of ice can fall to the bottom of the ocean.

That would seem to produce an icy ocean floor, making potentially life-producing chemical interactions between water and rock unlikely. But the team’s models show up to three ice layers and a rocky seafloor, with the lightest ice on top and salty, dense liquid at the bottom. Moreover, the layers in the diagram above account for odd phenomena, with Ice III in the uppermost ocean layer being formed in the seawater and salts precipitating out. As the heavier salts begin to settle to the bottom, the lighter ice would float upward, perhaps melting again before it reaches the top of the ocean or leaving slush layers in the Ganymede ocean. When it’s snowing on (or within) Ganymede, in other words, the snow floats up, not down.

This JPL news release offers more, including the notion that the ‘club sandwich’ structure the researchers describe varies over time, sometimes returning to a single ocean found below a layer of Ice I and existing on top of regions of different high-pressure ices. In any case, the idea of chemical interactions where water and rock meet is intriguing from an astrobiological viewpoint, suggesting that a wet seafloor could produce the necessary conditions for life. The salts the Vance team added to its model can produce a sea bottom with the needed dense liquids.

The paper is Vance et al., “Ganymede?s internal structure including thermodynamics of magnesium sulfate oceans in contact with ice,” Planetary and Space Science published online 12 April 2014 (abstract). Also of interest: Vance et al., “A Passive Probe for Subsurface Oceans and Liquid Water in Jupiter’s Icy Moons,” submitted to Icarus (preprint).

Stanley G. Weinbaum is best known for the 1934 short story “A Martian Odyssey,” lionized by readers and critics alike after it appeared in the July issue of Wonder Stories. Isaac Asimov would later opine that “A Martian Odyssey” was one of a handful of stories that changed the way all later science fiction was written. But Weinbaum’s depiction of a genuinely alien being called Tweel sometimes obscures his other work, which you can find collected in The Best of Stanley G. Weinbaum (1974), a worthwhile addition to the library of any SF fan, and a reminder of the loss the genre suffered when the author died at age 33.

This morning I’ve been thinking back to a little known Weinbaum story called “Redemption Cairn,” which ran in the March, 1936 Astounding Stories and which, because I have a good run of Astounding issues from that era, sits not ten feet away from me on my shelf. I don’t know if this is the first appearance of Europa in science fiction, but “Redemption Cairn,” with its exotic biosphere in a valley on the moon, shows us a time when Jupiter was thought to produce enough heat to make the Galilean moons habitable. Arthur C. Clarke would imagine a warm Europa as well, but his, in 2061 Odyssey Three (1988) was the result of Jupiter’s transformation into a small star and the birth of a biosphere.

One thing we’re not going to find when we get a dedicated probe to Europa is a tropical habitat, but the musings of science fiction writers remind us that we shape our aspirations around our dreams, and the encounter with the unknown becomes just as meaningful in real life whether the ocean we’re probing lies under balmy skies or a kilometers-thick layer of ice. Want to see an icy, science fictional Europa? Try Kim Stanley Robinson’s Galileo’s Dream, in which the astronomer is transported from Padua into the depths of Europa’s deep ocean.

Image: Science fiction pioneer Stanley G. Weinbaum.

A NASA Request for Information

Galileo had plenty of Europan connections, from being the person who discovered the moon to having his name attached to the spacecraft that sent us our best images of the surface. Like all of us, NASA would like more information about Europa than the Galileo mission could provide, and while it takes science fiction to get us into the Europan ocean for now, down the road we may have more concrete options. The agency’s recent issuance of a Request for Information (RFI) asks the scientific and engineering communities to come up with ideas that can help us answer some of our longest-standing questions. The ultimate goal: A $1 billion mission (excluding launch) that can achieve the following goals, or at least as many of them as possible:

• Characterize the extent of the ocean and its relation to the deeper interior
• Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface-ice-ocean exchange
• Determine global surface, compositions and chemistry, especially as related to habitability
• Understand the formation of surface features, including sites of recent or current activity, identify and characterize candidate sites for future detailed exploration
• Understand Europa’s space environment and interaction with the magnetosphere.

These requirements come from the National Research Council’s 2011 Planetary Science Decadal Survey. Why we need a mission like this is clear enough. For all its achievements, Voyager could give us nothing more than a quick flyby, and while the Galileo spacecraft was able to make repeated flybys (fewer than a dozen), it labored under serious communications problems with the failure of its high-gain antenna. We’ve seen what Cassini can do in the Saturn system with repeated observations of high-value targets like Titan and Enceladus, but Europa is a tough nut to crack, particularly given the radiation environment that surrounds Jupiter.

For more on the RFI, whose deadline is May 30, visit the NSPIRES site. NASA has been funding work into mission concepts and in particular the science instruments that will be needed for Europa, including possible ways to penetrate surface ice. The Decadal Survey considers a Europa mission among the highest priority scientific pursuits for the agency, and the recent findings from Hubble of possible water vapor ejections from the moon’s surface add punch to the statement. The RFI, says John Grunsfeld, associate administrator for the NASA Science Mission Directorate at the agency’s headquarters, “is an opportunity to hear from those creative teams that have ideas on how we can achieve the most science at minimum cost.”

Image: Two views of the trailing hemisphere of Jupiter’s ice-covered satellite, Europa, returned by the Galileo spacecraft. The left image shows the approximate natural color appearance of Europa. The image on the right is a false-color composite version combining violet, green and infrared images to enhance color differences in the predominantly water-ice crust of Europa. Dark brown areas represent rocky material derived from the interior, implanted by impact, or from a combination of interior and exterior sources. Bright plains in the polar areas (top and bottom) are shown in tones of blue to distinguish possibly coarse-grained ice (dark blue) from fine-grained ice (light blue). Long, dark lines are fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 kilometers (31 miles) in diameter. This crater has been provisionally named “Pwyll” for the Celtic god of the underworld. Credit: NASA/JPL.

Through Galileo’s Lens

But back to science fiction, and Kim Stanley Robinson, a fine science fiction author indeed. In Galileo’s Dream, before the celestial voyaging that will give Galileo a much closer look at what he sees in his telescope, Robinson depicts the discovery of the Galilean moons:

On the night of January 12, Galileo trained the glass on Jupiter in the last moments of twilight. At first he could see again only two of the little bright stars, but an hour later, when it was fully dark, he checked again, and one more had become visible, very close to Jupiter’s eastern side.

He drew arrows trying to clarify to himself how they were moving, shifting his attention between the view through the glass and his sketches on the page. Suddenly it became clear, there in the reiterated sketches: the four stars were moving around Jupiter, orbiting it in the same way the moon orbited the Earth. He was seeing circular orbits edge-on; they lay nearly in a single plane, which was also very close to the plane of the ecliptic, in which the planets themselves moved.

He straightened up, blinking away the tears in his eyes that always came from looking too long, and that this time came also from the sudden urge of an emotion he couldn’t give a name to, a kind of joy that was also shot with fear. “Ah,” he said. A touch of the sacred, right on the back of his neck: God had tapped him. He was ringing.

Image: Portrait of Galileo by Ottavio Leoni (1578-1630).

The pace of space exploration is sometimes frustrating, particularly when we gauge it against the optimism of the Apollo era and the dreams of von Braun. But when I think about Europa and our opportunities there, I always think back to this passage in Robinson, and ultimately back to Galileo himself. We have come so far since the days when he identified those bright objects around Jupiter as moons. Surely the drive for discovery — the zest, the enchantment of it — that drove Galileo is something hard-wired into our species, a sort of ‘ringing,’ as Robinson describes it, or perhaps a kind of inner fire that won’t allow us to turn away from these explorations.