This answer on Movies.SE collects some fascinating facts about the sulfur-dominated surface and atmosphere of Jupiter's moon Io. But it made me wonder where all the sulfur comes from in the first place. Volcanoes powered by tidal heating of the interior, it says -- very well, but volcanoes don't make sulfur.

Is Io simply unusually rich in sulfur, right back from when it accreted? Or do the other Galilean moons contain similar amounts of sulfur but just don't wear it on their outsides? Because they lack volcanism? Earth does have volcanism, but that hasn't led its surface being sulfur-dominated. Because we had less sulfur in the first place, or just because our atmosphere retains lighter elements that dominate over the same amount of sulfur that Io also has?

  • $\begingroup$ All gold on Earth originates from one single meteorite strike. So "specialized" composition of bodies much smaller than Earth doesn't seem all that odd. $\endgroup$ – SF. May 14 '16 at 23:11
  • $\begingroup$ @SF. I don't think that's true. I looked it up, and it seemed to be a period of meteor showers 200 million years long that brought the gold. $\endgroup$ – Rikki-Tikki-Tavi May 15 '16 at 1:35
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    $\begingroup$ @Rikky-Ticky-Tavi What on Earth are you talking about? Gold falling from heaven in a meteor shower billions of years after Earth formed? How then did the gold (or sulfur) concentrate in a single asteroid to begin with? $\endgroup$ – LocalFluff May 15 '16 at 3:18
  • $\begingroup$ @SF and LocalFluff -- Rikki-Tikki-Tavi is correct. Most geologists are of the opinion that all (or almost all) of the gold in the Earth's crust is not primordial, having instead been deposited during the Late Heavy Bombardment (also the cause of most of the Moon's craters). This was a 200 to 300 million year long period of intense meteoric activity starting about 4.1 billion years ago. However, most of the Earth's gold presumably is primordial, but that primordial gold is down in the Earth's core rather than near the surface. $\endgroup$ – David Hammen May 15 '16 at 13:08

It's erroneous to think of those volcanos on Io as sulphur volcanos. This is an old idea. Io's volcanos are instead somewhat similar to volcanos on the Earth, where the primary product is mafic rock (possibly ultramafic rock in the case of Io), but with lots of volatiles as secondary products.

In the case of volcanos on Earth, those volatiles include water, carbon dioxide, and various gaseous sulfur compounds. Lots of sulfur compounds. The 1783-1784 eruption of Laki in Iceland and the 1815 eruption of Mount Tambora in the Philipines dumped massive quantities of sulfur dioxide into the atmosphere. Both of these eruptions had very significant (but short-lived) impacts on the Earth's climate. Much more severe volcanic eruptions are now believed to be responsible for many of the extinction events, including the biggest of all, the Permian–Triassic extinction event.

In the case of Io, there's no water, and very little carbon dioxide. The volatiles released by Io's volcanos are mostly sulfuric compounds. If Io ever did have water, it lost it long ago. The same is true for carbon dioxide. Water and carbon dioxide are both light and highly volatile. Sulfur dioxide is 45% denser than is CO2, and is significantly less volatile that are either water or CO2. While Io can't quite hold onto its thin atmosphere of sulfur dioxide, the decreased volatility means that a good portion of that vented sulfur dioxide gas condenses and falls to Io's surface, eventually to be recycled into Io's interior.

With regard to where all that sulfur came from, sulphur 32 is the tenth most abundant isotope in the Solar System and in the Galaxy. The reason for this abundance is that 32S is on the alpha ladder, a fusion process that eventually results in the production of 56Ni (which quickly decays into 56Co and then 56Fe) in supernovae. The alpha ladder is why we see so much carbon, oxygen, neon, magnesium, silicon, sulfur, and iron in the universe.

  • $\begingroup$ Very nice! This or this link might be helpful for some to think about the alpha ladder. $\endgroup$ – uhoh May 15 '16 at 15:16
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    $\begingroup$ @uhoh -- I thought of doing that when I wrote my answer, and then I had second thoughts. I've seen too much that is obviously wrong in wikipedia in place where I am a subject matter expert. I've seen too much that appears to be wrong in wikipedia (and has taken too much research to find that it is wrong) in places where I am but a casual expert. Wikipedia is no longer a good source. I'll link to it when I know that it's right, but even then, I should get into the habit of linking to a specific revision. $\endgroup$ – David Hammen May 15 '16 at 15:43
  • $\begingroup$ Ya, I understand exactly what you mean. Wikipedia is great, but it's not a reference document. I've probably used it that way too often myself - thanks for the reminder! $\endgroup$ – uhoh May 15 '16 at 17:13

We might also consider that the sulfur is simply easier to see on Io than on Earth. Volcanoes on both Earth and Io produce sulfurous compounds, but on Earth both atmospheric and biological processes intermingle the with other materials in the litho- and biospheres, making the sulfur less visible on Earth than on Io.

The atmospheric processes on Earth arise from the presence of plentiful oxygen and moisture, which react with the sulfur compounds to ultimately produce dilute sulfuric acid [1]. The sulfuric acid, and any salts it forms with rocks (such as gypsum) are then distributed into soils and bodies of water. The sulfates, in turn, may seem inert, but they actually enter the biosphere and become incorporated into a complex cycle, which is described by Warneck's review [2].


1. Patricia Shapely, "Sulfur in the Atmosphere" (2010).

2. Peter Warneck, "Sulfur Compounds in the Atmosphere", International Geophysics: Chemistry of the Natural Atmosphere_, Vol. 41, Ch. 10, pp. 484-542 (1988).


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