Sea salt (part 3)

[You should probably start with Part 1]

The first thing that you notice when you look at a spectrum of Europa -- from the Earth, from a spacecraft, it doesn’t really matter – is the ice. Ice is everywhere. The spectrum of ice is a very distinctive looking thing, with a quickly recognizable pattern of regions where the sunlight reflects strongly from the surface and regions where there is less reflectance (and remember the regions here means spectral regions, which means, essentially, we stare at one small spot on the surface, put the light through a prism to spread it all out, and see which colors of the rainbow are present and which are absent. In our case our rainbow is in infrared light that your eye can’t see, but the idea is still the same).

The second thing that you notice – and this is so noticeable that it was recognized by astronomers at telescopes on the ground long before Galileo arrived and long before adaptive optics gave pristine views -- is that on the reddish trailing side of Europa the water ice looks funny.  As you remember from Part 1, the water ice looks funny because is because we are not really water ice, but water that is adsorbed on to something else. It looks similar to water ice, but different.

The first thing we did with the global spectral maps of the surface of Europa was to figure out where there was water ice and where there was something else.  And we made a map that looks like this:

This map of the non-water ice stuff on Europa was taken by measuring spectral ratios across the disk. Where the ratio is low (the red and black regions) there is mainly non-water ice material. Where the ratio is high (white, yellow) there is mostly water ice. In the blueish regions there is a mixture. The image also shows the limit of spatial resolution at the Keck telescope. From the Voyager image on top pick out the crater just south of the equator at about 270 degrees longitude. Now find that spot in the Keck image. You can see it. But just barely. Am I looking forward to the next generation of bigger telescopes on the ground that will have ~3 times better spatial resolution and be able to isolate features such as that crater? Why yes, I am.

The darkest regions on the map show the places with the least water and the most of whatever-else-it-is. The blue to yellow shows some ice to nearly pure ice regions (the white parts show the regions we didn’t quite cover). The image above it is the composite map from the Voyager flyby.  The thing you see most clearly is what I told you earlier. The non-water-ice stuff is strongly concentrated on the “trailing” hemisphere of Europa, which is the one that Io’s sulfur slams into and the one where the reddish material is. Interestingly – and this will play an important role when we try to explain what is going on – there is non-water-ice stuff on the leading hemisphere, too, just not as much.

But we can do more. Much much more. Instead of just figuring out where the non-water-ice stuff is, we can also try to figure out what it is. And we do that by looking at the spectrum.  If we isolate all of the most non-water-icy material – the stuff that is black in the map above – and look at the spectrum of it, we see nearly exactly the same thing that the Galileo spacecraft saw 15 years ago. There is one tiny, almost unnoticeable exception. Right next to one of the spectral regions that is dark because of water ice there is a slight decrease in the amount of light reflected. It’s a small enough spectral region that the Galileo spectrograph didn’t have the chance to see it, but, looking at 40 times greater spectral detail you can’t miss it.

Though that little blip (“spectral feature” we call it) is tiny, and we didn’t immediately know what caused it, we did know one thing immediately. It was not caused by sulfuric acid. Just this little bit of information is critical: something is on the surface of Europa that is not an obvious by-product of sulfur ions hitting water ice. This result already answers one of the questions we set out to address: is the reddish non-water-ice stuff on the trailing side of Europa all sulfuric acid? No. No it’s not. While much of the reddish material is likely still sulfuric acid (remember: it has to be there), there is definitive evidence now that there is something else there.

But we can do more. Much much more. We can try to figure out what chemicals on Europa’s surface cause that new spectral feature.  The first thing we did was to carefully map out where we see that new spectral feature. We had found it by looking at the reddish material, but where else might it be? The answer, which surprised us greatly, is: nowhere else. This new spectral feature, which doesn’t look like any expected chemical species that we would expect from sulfur ions bombarding water ice, appears on the parts of the surface where sulfur ions are bombarding water ice, and nowhere else. Weird. Very weird.

We went to the laboratory to try to reproduce that new spectral feature. (I should mention, at this point, that “we” is me and my friend and colleague Kevin Hand, who is a scientist at JPL specializing in Europa, among other things). We tried normal things (dissolve salts in water, freeze them, look at the spectrum), we tried slightly unusual things (freeze Epsom salt, crush it, sieve out big particles, medium particles, tiny tiny partucles, take spectra), we tried truly strange things (freeze Drano, grind it up, take a spectrum). In all of these experiments we only found one material with the new spectral feature. And that material is one that had been suspected all along: epsomite, a magnesium sulfate salt with water bound to it. If you remember back to part 1, epsomite was one of the favorite salts that people thought had been detected on the surface of Europa. But, if you remember back to part 1, you also remember that the actual evidence was quite thin and that it was more of an inference based on what people thought the ocean composition might be. The argument was, essentially: we seen non-water-ice stuff on Europa which might be salt, we expect the oceans to have magnesium and sulfate, thus those salts must be magnesium sulfates, and, with the not-very-precise data that we currently have, they could well be magnesium sulfates. I will admit that I always found this line of reasoning unconvincing, so magnesium sulfates were the last thing that we expected to find. But there they were, just as had been predicted for 15 years.

Two parts of this story don’t add up. If magnesium sulfates are coming from the internal ocean and making it to the surface of Europa, why do they only do it on one side? And why does that one side happen to be the side with sulfur raining down on top of it? Suspicious, no?

The other part of the story that doesn’t add up is that, as you will recall from the map above, the leading hemisphere of Europa does have non-water-ice stuff on it, but that non-water-ice stuff does not appear to have a large concentration of magnesium sulfate. Looking at the spectra of the leading hemisphere, even with the better view form Keck, we still don’t really know what it is.

And, finally, there is one more clue.  More than 15 years ago, when I was a freshly-minted Ph.D. looking around the solar system, I discovered anatmosphere of sodium atoms surrounding Europa. A few years later (when I was at Caltech and finally could use the mighty Keck telescope on my own!) I found potassium alongside the sodium.  All of these years, we have assumed that the sodium and potassium come from the salts on the surface of Europa and that these salts get knocked off the surface of Europa (by the radiation, among other things) where we can see them in the atmosphere.  So the question you might want to ask is: hey, what about magnesium? If there are magnesium salts shouldn’t magnesium get kicked up into the atmosphere? More than ten year ago, my then summer undergraduate research fellow, Sarah Horst, looked for magnesium using data from the Hubble Space Telescope. She didn’t see any. “Huh” we both said, and put the data in a drawer and didn’t think about it for most of the last decade. When we detected the magnesium sulfates a few months ago, I realized it was time to dust off those results. I called Sarah, who had gone on to graduate school, finished, and is now a postdoctoral fellow at the University of Colorado, and said “let’s write that paper.” We did, and it came out recentlyin the Astrophysical Journal Letters. In that paper we showed that magnesium is undetectable in Europa’s atmosphere, and, for this to be true, magnesium can’t be dominant.

All of the pieces of the puzzle are now in place. From everything that I just told you, Kevin Hand and I constructed a hypothesis. I’ll call it a hypothesis here, because much of it is a story that we constructed to fit all of the available evidence, but it is a story that we do not yet have the data to verify. But here it goes:

It seems unlikely that ocean-derived magnesium sulfates would only cover one side of Europa. It seems even more unlikely that that one side would also happen to have sulfur raining down. Sulfates. Sulfur. Hmmmm. So what if, instead, the sulfate (that’s SO₄) is formed by sulfur slamming into ice. And the irradiation breaks about the magnesium salt and created magnesium sulfates. That is exactly what happens to make sulfuric acid on Europa’s surface, but now magnesium is involved instead. In what form is magnesium originally? We don’t know. We’ll call it MgX.

The leading side of Europa, which appears to have no magnesium sulfates, is, presumably, instead covered by MgX. But not too much of it, because there is more sodium and potassium than magnesium, it appears. So, most likely, the surface has NaX and KX, too.

But what is the mysterious chemical X?

Here, we have no direct evidence at all, so we have to resort to inference. This inference is not much better than the one about sulfates that I complained about earlier, but it is the best we can do, so we are going to do it. Examining the chemistry of the solar system and asking ourselves about likely oceanic components and ruling out sulfates really leaves one major suspect: chlorine.  Which would make the important salts on the surface NaCl, KCl, and MgCl₂. You’ve heard of NaCl before. You call it salt. The others are in our oceans here on earth, too.

This chlorine hypothesis – the “sea salt hypothesis” we like to call it – if true, implies major difference in the ocean chemistry with chlorine being an important component of a chemically reduced ocean. But we’ll talk about chemical implications later.  

We made this cool picture for the press release. How exactly it is supposed to demonstrate our results I am not certain, but it's cool looking, no?

For now these results have a few key conclusions:

• The surface of Europa has magnesium in some salt form. Salt from the ocean gets on the surface. If salts get on the surface, other stuff gets on the surface.

• While people have speculated for a while about drilling down to the ocean to find out its composition, it doesn’t sound like that is important. If you want to know the composition of the Europa ocean, go lick the surface.

• When you do, we suspect that it may taste surprisingly familiar to those who have recently ingested an accidentally mouthful of sea water.

• While no one is going to be licking the surface of Europa anytime soon, the great power of modern giant telescopes at Earth will be used increasingly to take spectral fingerprints of increasing detail to finally understand the mysterious details of the salty ocean beneath the ice shell of Europa.

What happens next? We look for chlorine, I think. The existence of chlorine as one of the main components of the non-water-ice surface of Europa is the strongest prediction that this hypothesis makes. We have some ideas on how we might look; we’re working on them now. Stay tuned.

While these posts have been extra long (longer than the scientific paper, I think, since there was a lot of background to get through), you now have enough background and inside knowledge that you might even consider, for fun, reading the scientific paper on  which all of this is based. You can find it right here.