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

Posted By Emily Lakdawalla

2011/11/17 04:32 CST

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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:

NASA / JPL / UA / color mosaic by Daniel Macháček

Conamara Chaos, Europa

High-resolution mosaic across Conamara Chaos captured by Galileo, colorized with lower-resolution data.

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.

Nature Magazine

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.

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!

NASA / JPL / UA / Paul Schenk

Thera Macula, Europa

A view of Thera Macula on Europa from Paul Schenk's Atlas of the Galilean Satellites.

*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.

Comments:

Franz: 01/29/2013 04:51 CST

Dear Emily, It is interesting that the main point of your article rotate around a particular explanation for the formation of the chaotic terrain presented as a final proof of the thick ice theory. I would like to direct you to the Book of Richard Greenberg: “Unmasking Europa” that basically formed my opinion on this subject. This is an amazing book! After reading this book my opinion is that the existence of “floating icebergs” is a very strong evidence in favor of the thin ice theory. I feel that this particular explanation of the formation of the chaotic terrain via the existence of subsurface lakes is a far-fetched, twisted explanation that try to rescue the thick ice theory who is melting under the scrutiny of more careful observations and analysis. Did you know that the initial explanation for the chaotic terrain was relying on convection that requires a thick ice of at least 20km deep? Once the thick ice advocate realized that the presence of floating iceberg is incompatible with convection (and therefore thick ice) they tried to show up with this subsurface lake explanation. Rigorous mathematics does not mean much if you build your theory with the wrong assumptions. Ptolemy for exemple managed to describe mathematically and with relative accuracy the motion of the planets by assuming that earth is the center of the universe. No, earth is not flat. Sorry. This theory of the subsurface lake has many problems: How can we explain the melting of the warm plume of ice once it comes near the surface where the temperature is lower? Increased pressure? How the ice could melt a kilometer or less near the surface and not also deeper where the pressure is higher? Also the temperature will rise substantially only if the pressure increase rapidly. But how can this happen since the ice is solid and move upward very slowly? Moreover the warm plume will force the ice to bulge up before the chaotic terrain start to form and there is no such bulge at the surface of Europa. Finally how will you explain the irregular shape of the chaotic terrain? The thin ice theory is also supported by rigorous mathematical models. It assumes a warmer ocean due to tidal heating. It is simpler and also explains most if not all the others Europa surface features. The thin ice theory accommodates very elegantly the famous double ridges as cracks that crisscross the entire moon that form under tidal stress. Also the presence of ice tectonic, the aspect of the craters and the overall smoothness of the terrain are hard to explain if the ice is thick. The thick ice theory was born out of superficial observations of early low resolution images by people with an earth geological background bias. They were expecting ice to behave like rocks. The problem is that obviously ice behave very differently than rocks even when it is very cold and molten lava is physically very different from slushy ice or liquid water. It is fair to say that no one know for sure if the ice is thin (few kilometers) or thick (tens of kilometers) at this point. Personally I am a Richard Greenberg advocate and I am suspecting that the ice is thin. But since we are all dying to know let petition NASA for more probes to study Europa!

Emily Lakdawalla: 01/31/2013 12:16 CST

Greenberg's book is not widely respected by other scientists, partially because of some disagreements in details with his science, but mostly because he accompanied his arguments with ridiculous and unseemly ad hominem attacks. I've been attending Europa sessions at scientific conferences for a long time and there is lively debate about how its ice works; probably not very many people are totally won over by Britney's model, either. There is definitely not a Thick Ice Cabal made up of some all-powerful group meeting in a smoke-filled-room. There are lots of good scientists doing independent work and agreeing in some places and disagreeing in others. And most of that work, these days, is yielding estimates for the ice that are on the thick side -- except for Britney's, which offers this intriguing notion that there may be thick ice operating on a regional scale but thin ice locally.

Franz: 02/05/2013 04:54 CST

Dear Emily, The fact that Richard Greenberg has some criticisms about the way research is conducted around NASA does not invalidate his science and of course he has the right of his own opinion. This does not remove the merit of NASA, scientists and Engineer involved in this tremendous space exploration achievement. But let forget about cabal and politic for a moment. Of course there is not enough information yet about Europa to be certain about the thickness of the ice or about the actual mechanisms of formation of surface features. However, let face it, and let say that the current available data do not support a thick ice model. A solid state ice convection model that require an ice crust of at least 20km would lead to the formation of diapirs and could form some “pits” and “domes” structures. Unfortunately even if these “pits” and “domes really existed it would not explain the constant formation of the lineas and it will not explain how the water could melt and vaporize as indicated by the formation of brown deposits around nearly every single geologic feature. It can not explain the formation of large ice segment strike-slip, convergence and divergence. Most of all if the ice was even only 15km thick convergence of ice segment would in some area raise mountain of at least a kilometer or two in height. There is no such mountain. On the other hand a thin ice model explain everything: the lineas as crack through the ice down to the ocean surface due to tidal stress that could not happen if the ice was too thick, the big and small young and old chaotic terrains as melt through, the obvious break through the ice at even small craters. Even the pit and domes if any could be explain as an early phase of melt through. So of course I am very surprised when you claim that “most of that work, these days, is yielding estimates for the ice that are on the thick side” May be you should take the time to read Richard Greenberg book “Unmasking Europa” or “Europa The Ocean moon” by yourself and form your own opinion and not just listen to these academic scientists who clearly got pissed off by Richard Greenberg who committed the cardinal sin of questioning their integrity. If you like space exploration and NASA stuff as much as I do and if you can ignore the polemic you are going to enjoy the reading of this otherwise fascinating book! (By the way I enjoyed the polemic too!)

john delano: 04/18/2013 10:44 CDT

The answer to the thick/thin ice of Europa is the same as the incoming comets that came into its gravitational field over 4.6 billion years. Its agreed then that a lessor amount of ice/water hit Europa than those comets that hit the larger gravitational field of our planet Earth. The thin ice people loose the argument because it is not important. What is important is how the ice movement or non-movement has a thickness, but how it reflects how Earth 's earths life of ice over 4.6 billion years moved. Our ice on Earth moved, enough ice to fill the "7 seas" , accumulated in one half of planet Earth. The Moons excavation ,removing enough moon forming material to excavate the Pacific Ocean "hollow" , is where all of Earths water//methane/ice had a profound effect on the entire ocean/land layout of our planet Earth.

john delano: 04/18/2013 10:54 CDT

Europa does not revolve around its axis, like planet Earth. "THE CRUST BELOW AN ASSEMETRIC MASS ON A ROTATING PLANET WILL MOVE TOWARD THE EQUATOR." [Albert Einstein] Europa does not rotate=no coreoulus effect ,no force to move anything on the surface anywhere. see theory-Book on Amazon-Delano-The Origin of Mountains.

john delano: 04/18/2013 10:59 CDT

Send comments to sunnyday1@optonline.net My noteworthy book on Amazon- is un/noteworthy. It is on the top 5,400,000 books sold. Am I that unsuccessful?