Sulfur was not just a trace ingredient — it was a structural player.
In a high-pressure laboratory at Rice University, a researcher named Yishen Zhang has been cooking rocks — not Earth rocks, but synthetic stand-ins for the interior of Mercury, assembled from the chemical recipe of a meteorite that fell to Earth more than a century ago. The results are reshaping how planetary scientists think about the way worlds are built.
Mercury has always been something of an outlier. Two spacecraft missions that swept past the planet returned data showing its crust is strikingly iron-poor and unusually rich in sulfur. It is also, in the language of chemistry, deeply reduced — meaning its minerals have gained electrons rather than lost them. Of all the rocky planets in the solar system, Mercury sits at the most extreme end of this spectrum. That combination of traits makes it nearly impossible to study using the frameworks scientists developed for understanding Earth.
The breakthrough came through a meteorite. Indarch fell in Azerbaijan in 1891 and has sat in collections ever since, a curiosity among researchers who noted that its chemical fingerprint closely resembles what the spacecraft data suggests about Mercury's composition. Rajdeep Dasgupta, director of the Rice Space Institute Center for Planetary Origins to Habitability, and his team recognized that Indarch might serve as a proxy — a way to bring Mercury into the lab without waiting for a sample-return mission that does not yet exist.
Zhang, a postdoctoral researcher in Dasgupta's lab and the paper's lead author, ground up a model melt based on Indarch's chemistry, loaded it into a small glass vial, and subjected it to the temperatures and pressures that spacecraft observations and theoretical models suggest exist inside Mercury. Then he watched what happened when those synthetic magmas cooled.
What he found was striking. Sulfur, it turns out, dramatically lowers the temperature at which these reduced melts begin to crystallize. On Earth and Mars, sulfur is mostly a passenger — it binds readily to iron, which both planets have in abundance, and gets locked away. Mercury's iron scarcity left its sulfur chemically unattached, free to bond instead with the major rock-forming elements: magnesium and calcium. On Earth, those elements bind to oxygen, forming the stable silicon-oxygen lattice known as a silicate network. When sulfur steps into oxygen's role, that network weakens, and the whole structure solidifies at a lower temperature.
The implications run deep. If Mercury's magmas stayed molten longer and at lower temperatures than comparable magmas on Earth, the entire history of how its mantle solidified — and therefore how its crust formed and what its surface looks like today — would have unfolded differently. Sulfur was not just a trace ingredient; it was a structural player, occupying a position in Mercury's geology that oxygen holds on every other rocky planet we know well.
Dasgupta framed the finding in terms that reach beyond Mercury itself. The tendency in planetary science has been to use Earth as the baseline — to ask how other planets differ from us. This research suggests that approach has limits. What water and carbon do to drive magmatic evolution on Earth, sulfur does on Mercury. Each planet, the work implies, needs to be understood on its own chemical terms.
The study was funded by two NASA grants and conducted through the Rice Space Institute. No spacecraft has yet returned a physical sample from Mercury's surface, and none is currently scheduled to do so. Until that changes, a meteorite that landed in a field more than 130 years ago may remain the closest thing scientists have to holding a piece of the innermost planet in their hands.
Notable Quotes
What water or carbon does to magmatic evolution on Earth, sulfur does on Mercury.— Rajdeep Dasgupta, Maurice Ewing Professor in Earth Systems Science, Rice University
These experiments show that Mercury likely formed with sulfur occupying a structural position that on Earth belongs to oxygen — and that fundamentally changes how the planet's mantle solidified.— Yishen Zhang, postdoctoral researcher and lead author, Rice University
The Hearth Conversation Another angle on the story
Why does it matter that Mercury is "reduced"? That sounds like a technical detail.
It means Mercury's minerals have held onto their electrons rather than giving them up to oxygen. That single chemical fact cascades through everything — what minerals form, how magmas behave, what the surface ends up looking like.
And sulfur is the key to understanding that?
On Mercury, yes. Sulfur is normally a follower — it latches onto iron wherever iron is available. Mercury barely has any iron, so sulfur had to find other partners. That's where things get interesting.
What partners did it find?
Magnesium and calcium — the backbone elements of rock. On Earth those bind to oxygen and form a tight, stable silicate network. When sulfur takes oxygen's place, that network is looser, weaker, and it solidifies at a lower temperature.
So Mercury's magmas stayed liquid longer?
Potentially, yes. And that changes the whole story of how the planet's interior cooled and hardened over billions of years. The crust we see today is a record of that process.
How confident can researchers be, given they're working from a meteorite and not actual Mercury samples?
Indarch is as chemically reduced as Mercury's surface rocks, and it's thought to be a possible building block of the planet. It's the best proxy available. The lab results are internally consistent with what the spacecraft data shows.
What's the broader lesson here — beyond Mercury specifically?
That Earth-centric models have a ceiling. Every planet has its own chemistry driving its own magmatic history. Sulfur does on Mercury what water and carbon do on Earth. You have to meet each world on its own terms.