A whiff of blue sulfur

A lot of observations, especially those relating to the deep history of a planet, tend to become received wisdom after a time. This is not to say that they are wrong, but it's certainly true that the light of new investigations can modify what we thought to be right.

A good example of something that might just cause some hiccups is the recent discovery that the molecular forms taken up by most terrestrial sulfur may be very different in the planetary interior than closer to the surface.  A new work by Pokrovski and Dubrovinsky together with a discussion by Manning in last week's Science indicates that the forms of sulfur in our planetary innards should include a significant amount of a triple-sulfur molecular ion known as the trisulfur anion (S3-).

The part of the planet they probed is usually about 100 kilometers below us. Given its inaccessible nature they used a diamond anvil and some clever techniques to both re-create the pressure-temperature environment (about 10,000 atmospheres and a few hundred degrees) and to examine the molecular structures that form. The trisulfur anion reared its head. Previously it was generally considered that sulfate and hydrogen sulphide would be the dominant high-temperature sulfur compounds popping out on the surface of a young, geophysically active planet. It's the reactions of these compounds with atmospheric oxygen that has provided one of the key tools by which we've measured its geological abundance - since oxygen alters the isotopic ratios of sulfur atoms in mineral deposits. Throwing more trisulfur into the mix may require a revision of what we think oxygen levels were 2-4 billion years ago. This could have significant ramifications for phenomena that we relate to oxygenation, including the rise of multi-cellular life.

There is another, more poetic side to this result. Not only do the very hot geofluids under scrutiny help transport and deposit gold on our planet, the presence of trisulfur suggests a similarity to another precious resource. Lapis lazuli gets its vivid blue coloration from the shuffling of electrons related to the trisulfur anion. As Manning notes, if trisulfur is abundant at depth then much of the world beneath us could be a rather hot, but appealing ultramarine.