by Paul Gilster | May 2, 2014 | Outer Solar System |
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).
by Paul Gilster | May 1, 2014 | Outer Solar System |
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.
