Today’s ‘Galaxy’ Insight: “The Whales of Europa”

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"I know, I know," writes Mike Brown, Caltech's Richard and Barbara Rosenberg Professor of Planetary Astronomy and author of the mikebrownsplanets blog, who specializes in the discovery and study of bodies at the edge of the solar system. "We have all been instructed by Arthur C. Clarke to attempt no landings on Europa. But if you did land on Europa, wouldn’t you like to know where to go?

If, on a whim, you climbed down a crack on the surface of Europa and made your way down into the ocean (which, interestingly, might be something you actually could do; though it is more likely you would get stuck and squeezed to death; hard to tell) and then you figured out how to swim down to the rocky bottom something like 100 km below the base of the ice (a depth 10 times greater than the Marianas Trench, by the way) you would instantly be able to answer what to me is one of the most interesting mysteries about Europa. What is happening at the boundary of the rocky core and the ocean? The answer has profound effects on the type of world that Europa ultimately is.

The most interesting possibility – at least the most interesting possibility that I can think of – is that the rocky bottom of the ocean is almost like a miniature Earth, with plate tectonics, continents, deep trenches, and active spreading centers. Think about mid-ocean ridges on Earth, with their black smokers belching scalding nutrient-rich waters into a sea floor teaming with life that is surviving on these chemicals. It doesn’t take much of an imagination to picture the same sort of rich chemical soup in Europa’s ocean leading to the evolution of some sort of life, living off of the internal energy generated inside of Europa’s core. If you’re looking for Europa’s whales – which many of my friends and I often joke that we are – this is the world you want to look for them on Europa.

 

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The announcement a couple of years ago, that plumes of water jetting from Europa’s south pole had been discovered by the Hubble Space Telescope, seemed to have answered the question: land at the pole, and wait for plumes to rain down upon you (or, perhaps even more easily, fly through the plumes and collect samples without even landing!). The bad news, however, is that the plumes now appear to be elusive at best and non-existent at worst. Since their initial detection no one has been able to see them again. Are they (very) sporadic? Was the initial detection an unfortunate spurious signal that was misinterpreted? No one yet knows, but no one today is going to count on plumes for measuring the chemical composition of the ocean.

Luckily, our new paper shows that we don’t need plumes to sample the interior, and we even conveniently point out a potential landing area that is large enough to easily target with your favorite lander.

Mapping the composition of the surface of Europa has shown that a few large areas have large concentrations of what are thought to be salts. These salts are systematically located in the recently resurfaced "chaos regions." One such region, named Western Powys Regio, has the highest concentration of these materials presumably derived from the internal ocean, and would make an ideal landing location for a Europa surface probe.

 

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On Europa, "chaos terrains" are regions where the icy surface appears to have been broken apart (NASA image above) , moved around, and frozen back together. Observations by Caltech graduate student Patrick Fischer and colleagues show that these regions have a composition distinct from the rest of the surface which seems to reflect the composition of the vast ocean under the crust of Europa.

Chaos terrain was noticed early on in the Galileo mission as regions which look like the surface of Europa has become cracked and jumbled and – intriguingly – perhaps even melted in recent times. If you had to vote for a location on Europa where ocean water had recently melted through and dumped its chemicals on the surface, you would vote for chaos terrain. And now Patrick had found that on large regional scales chaos terrain has a different composition than the rest of the surface of Europa!

And what do the spectra tell us that the unique composition of this chaos terrain is? Sadly, we can’t yet tell. To date, we have not found unique compositional indicators in the spectra of this region, though our search is ongoing. Our best bet, though is that we are looking at salts left over after a large amount of ocean water flowed out on to the surface and then evaporated away. The best analogy would be to large salt flats in desert regions of the world. Just like these salt flats, the chemical composition of the salt reflects whatever materials were dissolved in the water before it evaporated. On the Earth, salt flats can contain any number of exotic salts, depending on the surrounding rock chemistry. On Europa, the salts will tell about the rock chemistry, too, though the rock is the material far below at the base of the ocean.

We think, then, that we have found a giant salty patch on the surface of Europa, and very likely the region of most recent resurfacing and undisturbed chemistry. I have tried very hard to get Patrick to call this salty patch Margaritaville, but he does not think that graduate students are quite established enough to make jokes like that. I’ll make it for him, though. And I will tell you: attempt a landing there!

Margaritaville will not only have salts that tell you about the rock-ocean interaction, but it will also have samples of everything else that the ocean has to offer. Is there organic chemistry taking place in the oceans? Look in Margaritaville. Carbonates? Margaritaville. Microbes? Definitely Margaritaville. All of these are best searched for with the types of instruments currently roving around on Mars, where you grab a sample, put it into a machine, and read back out the chemical composition.

The Daily Galaxy via mikebrownsplanets.com

Image credits: top of page, with thanks to Bryant Austin's "Dwarf Minke Whale Portrait 1186" is a life-size work from the Great Barrier Reef; geysers of Europa, courtesy of K. Retherford, Southwest Research Institute and NASA

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