Emily LakdawallaNov 17, 2011

Is Europa's ice thin or thick? At chaos terrain, it's both!

Yesterday there was a press briefing about a paper published in Nature* about lakes inside Jupiter's moon Europa. I didn't have time to watch the briefing but I've just read the paper and it's a really important one. It's an academic paper, but in my opinion it also represents a detente in a long-running conflict.

Among Europa scientists there are two warring factions: the thick-icers and the thin-icers. (I know science isn't really supposed to work that way, but all too often, it does.) The question is how thick is the ice shell that overlies Europa's subsurface ocean (the existence of which pretty much everyone agrees on). The thin-icers claim that the water comes very, very close to the surface, sometimes even melting through. How else, they said, could you get a landscape that looks like icebergs floating in a frozen ocean:

High-Resolution Mosaic of Conamara Chaos, Europa (20 February 1997)
High-Resolution Mosaic of Conamara Chaos, Europa (20 February 1997) High-resolution mosaic across Conamara Chaos captured by Galileo, colorized with lower-resolution data taken in September and December 1996.Image: NASA / JPL / UA / color mosaic by Daniel Machacek

Not so fast, say the thick-icers. No matter how much Europa's "chaos terrain" looks like icebergs floating in an ocean, physics makes that pretty much impossible. Europa's surface is directly exposed to space. Space is cold. Very cold. So cold that ice at Europa's distance from the Sun behaves, thermodynamically speaking, like rock does on Earth. On Earth, some lava gets through to the surface sometimes, but you never have hundreds of square kilometers of rock suddenly liquefying into lava at the surface, letting blocks of crust bob around like icebergs in a red-hot liquid rock lake. (Thankfully.) That's equivalent to what the thin-icers were saying happened from time to time on Europa.

In the decade since the ending of the Galileo mission, the thick-icers' mathematically rigorous arguments have pretty much carried the day. But the thin-icers haven't gone away, and the geophysicists have had a tough time trying to come up with an explanation for how Europa's chaos terrain forms that does make sense thermodynamically.

That's why this paper is important. Titled "Active formation of 'chaos terrain' over shallow subsurface water on Europa," it's a mathematically rigorous paper that describes a mechanism in which a thick-iced Europa can produce chaos terrain, without liquid water ever melting through to the surface. Europa's ocean remains at great depth, but there are perched "lakes" close to the surface, whose formation drives the creation of chaos. In a way, it's a bit of thin ice on top of a lot of thick ice.

The authors are Britney Schmidt, Don Blankenship, Wes Patterson, and Paul Schenk. Wes and Paul have done a lot of work mapping the icy moons and can describe in detail what chaos terrain looks like -- where it's high, where it's low, and so on. Paul has even made movie flyovers of Europa's chaos.

Don Blankenship is a geophysicist who's done lots of fieldwork in Antarctica, studying how water melts and moves at the base of thick ice sheets, and who has applied that knowledge to Europa. And the first author, Britney Schmidt? She's a young scientist, a freshly minted Ph.D. in geophysics, who has, in this publication, fulfilled an ambition she'd formed as a 20-year-old undergrad: to become a scientist studying Europa when she grew up. And here she is, first author on a paper about Europa in the extremely prestigious journal Nature. Awesome. Here's a lengthy interview of Britney, in which she explains how she got there, and offers advice to young people who want to follow a path like hers.

Back to Europa. Here is the story that Britney and coauthors tell to explain how chaos forms (remember, geology is all about storytelling). I did my best to translate the story, but it can be kind of hard to explain why ice can move around while thoroughly solid and why it melts at some times and not at others. If you don't follow me, that's OK, just skip past the image caption, and I'll explain why their story is a good one.

Diagram explaining formation of Europa chaos
Diagram explaining formation of Europa chaos How Europa's chaos terrain is made, in sketch form. In (a), a plume of warm ice (not liquid water, but a solid ice plume, like the rocky plumes in Earth's mantle) rises upward. Above the upwelling plume, the surface might (but might not) warp upward. At great depth in Europa's crust, the pressure of the overlying ice overcomes the relatively warm temperatures to keep ice in solid form. On to (b): when the upwelling plume of warm ice pushes the nearer-surface ice upward, this balance is disturbed, and ice within the crust at a few kilometers below the surface begins to "sweat," partially melting. Liquid water is slightly denser than frozen ice, so takes up less volume. The reduced volume means that over the area of the melting, the surface sinks downward. The thicker crust at the edge of the downwarped area produces higher pressure on the liquid melt than the thinner crust at the center, so as more of the crust melts, the water flows from high pressure toward low pressure, producing a "lens" of water, thickest in the center and thinnest at the edges, and the melting water is pinned in a confined area above the uprising plume. Then comes (c): As the "lid" over the plume sinks downward, it also bends, and cracks open in the bottom of the lid to accommodate that bending. Briny liquid from the lake, under pressure from above, squirts into these cracks and percolates into the porous granular ice in the crust. In this way the crust never melts through but it is saturated with water, and large chunks of of crust can "calve" off as the fissures crack. If the blocks are narrow, they may tilt sideways. Finally, in (d), as the geologic activity subsides and the lens of liquid water refreezes, so does the water saturating the crust. Freezing water expands, so the brine-wetted matrix material in between the calved blocks domes upward.Image: Nature Magazine

Why is this story better than any other I've heard? Because it explains a lot of the descriptive evidence that geologists like Wes and Paul have assembled over years of work. Among other things, the story explains:

  • Europan terrains like Conamara Chaos and Thrace Macula are approximately circular.
  • They contain what appear to be floating blocks calved off of the adjacent crust, stuck in a "matrix" of disrupted, darkened material.
  • But Conamara Chaos stands higher than the surrounding crust and also has matrix domes between its blocks, while Thera is sunken below the adjacent crust.

But this isn't just a story. It is, in fact, a scientific theory that makes testable predictions. It suggests that Conamara is a relatively old chaos, where there may once have been a lake but now it's all frozen. By contrast, at low-lying Thera, we're seeing chaos in the middle of this formation process. Britney and her coauthors wrote:

At Thera Macula, we are probably witnessing active chaos formation....[the evidence] indicates that the lens below Thera Macula was liquid at the time of the Galileo encounter. Today, a melt lens of 20,000-60,000 cubic kilometers of liquid water probably lies below Thera Macula; this equates to at least the estimated combined volume of the Great Lakes....Thera Macula may have noticeable changes between the Galileo encounter and the present day.

So for all of you people who were secretly hoping the thin-icers would win the argument because you are hoping to see humans send a probe onto Europa's surface and maybe even drill through the ice to its ocean, you have a consolation prize. The ocean's still deep below the surface, 10 to 20 kilometers, but if Britney and her coworkers are right, there very likely are liquid water lakes at only maybe 3 kilometers' depth. And water from those lakes has squirted upward, helping wet and break up Europa's crust all the way to the surface. The dark stains associated with chaos could well be the salts and other stuff that are dissolved in that lake water. So, land at Thera, and you might be able to taste Europa's ocean!

Thera Macula, Europa
Thera Macula, Europa A view of Thera Macula on Europa from Paul Schenk's Atlas of the Galilean Satellites.Image: NASA / JPL / UA / Paul Schenk

*I forgot to do this earlier when I posted, but I wanted to make sure that people were aware of the "womanspace" controversy going on over a "humor" piece in Nature. Lots of good thoughtful blogging happening.

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