Europa (one of the moons of Jupiter) is said to have the smoothest, youngest-looking surface of any planet or moon in the whole Solar System. But Europa’s surface, as astonishingly smooth as it is, still isn’t perfectly smooth. As you can see in the totally legit Hubble image below, there are dark-colored cracks and rough patches, and there are also blob-shaped discolorations that kind of look like the birthmark on Mikhail Gorbachev’s head.
Fifteen to twenty years ago, when I started teaching myself about space, the things I read about Europa made it sound like scientists had no idea what caused the discolorations on Europa. The blue-grey regions were frozen water, obviously; but the reddish-brown stuff… that could be anything! Tholin? Sulfur? Amino acids? Alien poo? Anything. Those reddish-brown areas may as well have been marked “here be dragons,” chemically speaking.
Today, though, it seems like scientists have seriously narrowed down the range of possibilities.
Sulfuric Acid: Io, one of Jupiter’s other moons, happens to be the most volcanically active object in the Solar System. Io is so volcanically active that sulfur from Io shoots up into space and spreads to the neighboring Jovian moons. On Europa, Io’s sulfur can react with Europa’s frozen water to create sulfuric acid (H2SO4). This could explain some of the discoloration we see on Europa.
Epsom Salts: The discoloration could also be explained by a different sulfur compound: magnesium sulfate (MgSO4). Also known as Epsom salts, magnesium sulfate is found in Earth’s oceans, and it’s reasonable to guess that it might be found in Europa’s subsurface ocean as well. If so, magnesium sulfate could be spilling onto Europa’s surface through cracks and fissures in the surface ice.
Table Salt: In a previous post, I told you about the intense radiation environment on Europa’s surface. Recent laboratory experiments have shown that sodium chloride (NaCl) can change color when exposed to that much radiation. Just like magnesium sulfate, sodium chloride could be welling up to the surface through cracks and fissures in the ice. And after a bit of radiation exposure, sodium chloride could cause the kind of discoloration we see on Europa.
So which of these three chemicals causes the discoloration on Europa? Or is it some combination of all three? From what I’ve read, I don’t think the scientific community has reached a consensus on that. Much of the discoloration we see is in the vicinity of cracks, fissures, or other breaches in Europa’s surface. That seems to favor sodium chloride and/or magnesium sulfate as the explanation. However, one hemisphere of Europa is more exposed to the sulfur cloud coming from Io than the other. And guess what! The hemisphere that’s more exposed to Io is also more discolored! That evidence seems to favor sulfuric acid as the explanation.
But again, I don’t think there’s a consensus about this yet. This is still a topic of some debate among the scientific community. However, the fact that we’ve gone from “it could be anything, here be dragons (chemically speaking)” to “it’s one or more of these three chemical substances” seems like real progress to me.
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I relied on these three research papers for this post. Together, I think they show the evolving conversation about Europa’s discolored regions over the last few years.
- “A New Energy Source for Organic Synthesis in Europa’s Surface Ice,” published in 2002. Please note that this research is very out of date. I’m including it as an example of the “it could be anything” wild speculation that was more common fifteen to twenty years ago.
- “Salts and Radiation Products on the Surface of Europa,” published in 2013.
- “Sodium Chloride on the Surface of Europa,” published in 2019.
I wish I could recommend some easier and more accessible articles on this topic, but the ones I read all made claims like “scientists prove Europa’s covered in Epsom salts!” Those sorts of articles do not reflect what the actual research papers are saying.