Mercury Rewritten: The Sulfur Planet

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I want you to just take a second and picture volcanic magma.

Oh yeah, the classic mental image right.

Like you probably immediately think of this thick, glowing red, slowly creeping lava.

Like something you'd see in Hawaii or Iceland exactly.

And that image, that whole process is totally driven by the familiar elements we have right here on Earth, you know, mainly oxygen and iron.

Yeah, those are the big players for us.

But we need to completely shatter that mental image right out of the gate today. Oh, absolutely, throw it right out the window, because we are heading to Mercury. It's the smallest, innermost planet in our Solar system, but it is hiding this massive, fundamentally alien secret.

It really is. It's just a totally different world.

Yeah, on Mercury, the familiar rules of geology, all that oxygen and iron stuff, it's just it's gone. It's replaced by a chemical reality that is so bizarre it forces us to basically rethink how planets are even born.

It's wild. It completely flips our understanding of planetary evolution.

So that's our mission for you today. We are going to journey far away from this comfortable Earth centric view of planetary science, and we're dropping into a world where a totally different element, sulfur just reigns supreme.

Lfer is the king on Mercury, right, and.

Our goal is to understand how this one single element completely rewrites Mercury's history. It creates a crust and a geologic timeline that looks honestly nothing like.

Our own, not even close to our own.

And it's so easy, right, It's so easy to just assume all rocky planets are basically variations of Earth, like Mars is a cold Earth, Venus is a hotter Earth.

Yeah, it's the single biggest trap in planetary science, honestly.

I mean, it makes sense why we do it, of course it does.

We live here. We have naturally built our entire understanding of you know, planetary accretion, mantle convection, crust formation, all of it. The chemistry of Earth, Mars, and well Venus, because that's what we can see and measure easily exactly. And in those systems, the oxygen and fugacity, which is basically just the availability and the chemical activity of oxygen. When the planet was forming, that was relatively high.

So there's plenty of oxygen to go around.

Tons of it. Oxygen totally dominates the chemical structure on Earth. It dictates which elements bond together, it decides at what temperature rocks melt, and really importantly, how a planet loses its internal heat over bions of years.

But mercury is playing a different game.

A completely different game. It formed under entirely different initial conditions in the Solar nebula. The rules that govern how magma evolves on Earth, they just fail, They literally fail when you apply them to mercury.

Because oxygen isn't the boss there, right, We're.

Dealing with environment governed by a completely different primary driver, which is, like you said, sulfur.

Okay, So let's look at the actual data that broke this Earth centric model, because I think that's fascinating.

The NASA Messenger mission.

Yes, Messenger because before Messenger, our understanding of mercury surface was pretty much just you know, educated guessing.

Yeah, we had some fly by data from Mariner ten back in the seventies and some telescope observations from the ground, but it was really speculative.

But then Messenger finally gets into orbit around Mercury and it starts using this X ray spectrometer.

Which is such a cool instrument by the way it is.

It basically waits for solar flares from the Sun to blast the planet's surface and then measures the secondary X rays that bounce off the rocks.

Exactly. It uses the Sun as its own giant radiation source to light up the planet's chemistry.

And the chemical signature at back was just shocking. It was entirely incompatible with an.

Earth like mantle, totally incompatible. It threw everyone for a loop.

It showed that Mercury's crust was incredibly depleted in iron, like missing so much iron, but it was extraordinarily enriched in sulfur.

We are talking about sulfur levels on the surface that are orders of magnitude higher than anything we ever see on.

Earth, which is crazy. And this immediately tells us there's a huge difference in the planet's bulk chemistry. It points to this thing called an extreme reducing environment, right.

Yeah, a highly reduced environment. That's the absolute key to interpreting all this messenger data.

Okay, So help us out here break down what it actually means for a planetary body to be highly reduced because that sounds like a very dense chemistry term.

Oh sure, So when planetary scientists talk about the redock state of a planet, we're really just talking about the balance between reduction and oxidation when the planet was first clumping together.

Okay.

In an oxidized in environment like Earth, oxygen is superabundant, and oxygen is highly electronegative.

Meaning it really wants electrons.

It loves electrons. It aggressively pulls electrons away from other elements to form oxides. So iron, for example, easily oxidizes to form silicate minerals, and those make up the bulk of Earth's mantle and crust.

So an oxidized Earth is kind of like, well, it's like a rusting piece of iron sitting out in the.

Brain right now that oxygen is grabbing onto everything.

That's a perfect analogy. Actually, Earth is basically fully rusted.

But mercury isn't rusting.

No, not at all. Mercury formed way closer to the proto Sun in a part of the early Solar nebula where the oxygen fugacity that the availability we talked about was incredibly low.

So no free oxygen, almost none. It's the most chemically reduced terrestrial planet we have in the Solar System. In this kind of environment, elements don't lose their electrons to oxygen to keep them right. The whole system favors retaining electrons. So without enough oxygen around to bond with the iron in this big molten protoplanet, the way the iron behaves shifts dramatically.

Okay, but wait, let me push back on this a little bit. We know as a planetary fact that Mercury has a disproportionately massive iron core, right, Like, the core is huge compared to the rest of the planet.

Oh, absolutely massive. It's essentially a giant iron cannonball with a thin, little rocky shell.

Right. So if it has this giant iron core, how can its crust be so incredibly poor and iron? Where did it all go?

That is the magic of the reduced state, because the iron wasn't bonding with oxygen to form light silicate rocks in the mantle. It stayed as unbound heavy metallic iron. Oh I see, and gravity just took over. In a totally molten young planet, all that heavy unbound iron just sank rapidly right to the center.

Wow, So it basically filtered itself out.

Exactly, the extreme reducing conditions caused super efficient planetary differentiation. The iron partitioned almost completely into the core, and it left behind a mantle that is just stripped bear of iron.

That makes so much sense, but that brings up a really big chemical mystery, then the sulfur mystery. Exactly. If the mantle has no iron but it has tons of sulfur, what is the sulfur doing Because here on Earth, sulfur is a home wrecker, but it specifically loves iron, right Yeah.

On Earth, sulfur is what we call a chalcophile element. It has a super strong affinity for iron. It constantly seeks out iron to form these dense sulfide melts.

But if all the iron on mercury already sank to the center.

Then the sulfur in the outer layers the mantle is basically stranded. It's stuck up there without its favorite bonding partner.

So it has to find someone else to dance with.

Exactly that total absence of iron forces a huge shift in the mineral pathways. On Earth, sulfur and iron get together in four minerals like pyrite, which you know as fool's gold. But on mercury, the sulfur has to interact with the next best things available, and in this highly reduced iron poor molten rock, sulfur starts bonding aggressively with magnesium.

Calcium, magnesium and calcium.

Yeah, and that is a pathway that is almost entirely shut down on Earth. We just don't see it much here, But on mercury it becomes the dominant chemical mechanism.

Okay, so sulfur is suddenly best friends with magnesium and calcium. But how does that atomic level chemistry actually change the physical magma? Like, what is it doing to the rock itself?

It completely changes the structural integrity of the magma. Think about how molden rock where silicon melt is built in a normal Earth magma, the structure is this really complex network of silicon, oxygen tetrahedrol.

Okay, silicon and oxygen linking up right.

Oxygen acts as the glue or the bridging ion. It links these structures together into a really robust three dimensional web. So it's strong, very strong, and that interconnected web dictates everything. It controls how thick and viscous the magma is, its density, and crucially its liquidst temperature.

The liquidst temperature meaning the exact point where it stops being liquid and starts forming solid crystals exactly.

So if you have all this sulfur for forming complexes with magnesium and calcium on mercury, it isn't just floating around doing nothing. It actively attacks that silicon oxygen framework.

It's actively breaking it. Like Okay, think about Earth's magma as a building. The oxygen atoms are these strong steel bolts holding the steel beams together. I like that, But on molcury, because of all this weird chemistry, those steel bolts get replaced by a softer metal, yes, exactly, like lead or something. So it fundamentally compromises the rigidity of the whole building. It's way easier to collapse.

That's spot on. The sulfur actively substitutes for oxygen in the network because sulfur is in the same group as oxygen on the periodic table, it can technically fit into those same spots. It's like an impostor a very clumsy imposter. Sulfur is a much larger ion than oxygen, and it doesn't hold onto electrons as tightly, So when sulfur forces its way in and replaces a bridging oxygen atom, it bends the bond.

Angles, it warps the structure, It.

Totally warps it. The bond between sulfur and silic is just much weaker and way easier to break than the normal oxygen silicon bond, so the milt to polymizes.

To polymerizes, meaning it just falls apart.

It breaks up those long, strong chains. Now, a strong oxidized magma on Earth needs a huge amount of heat just to stay liquid. But if you introduce sulfur and break all those bonds, you're lowering the amount of energy.

Needed, which means it stays liquid at much cooler temperatures.

Exactly the thermal energy required to prevent it from freezing into solid rock drops significantly.

Okay, so that's the theory, But how do we actually know this Because we haven't brought any rocks back from Mercury.

No, we haven't. And that's where the lab work comes.

In, right, because you can't just run a computer simulation and call it a day. You have to prove it. You have to physically cook a rock and see what happens.

Yeah, you have to subject the exact chemical recipe of mercury's mantle to the massive pressures and temperatures deep inside the planet.

Which brings us to this mind blowing research from twenty twenty six. This was done at Rice University.

Yes, pecifically by a postdoctoral researcher named Yushen Zang working in roj Deep Dusk Gupta's lab. They wanted to physically replicate.

Early mercury, but again, no rocks for mercury. So they needed a stand in proxy and.

They found the perfect proxy, the in Darsch meteorite.

Okay, we have to talk up the backstory of this meteorite because it is just incredible to me that this rock fell out of the sky in Azerbaijan in eighteen ninety one.

Eighteen ninety one, way before we even knew what mercury really was.

Yeah, it's just astounding. A rock falls from space in the nineteenth century sits in a drawer somewhere for one hundred and thirty years, and now we're using it to simulate an unreachable planet. Why this specific rock though, What makes in dark so special.

In darts is a very rare type of rock called an ensteatite chondrite. They make up only about two percent of all meteorites that hit Earth.

Wow, so very rare, extremely.

Rare, and their chemistry is key. When we look at their isotopes like oxygen, titanium, and calcium, they look a lot like the rocks in the Earth Moon system, which tells us they formed in inner solar nebula.

Toast of the Sun.

Right. But unlike Earth rocks, their mineral makeup is aggressively reduced.

There's that word again, reduced, Right.

They're packed with minerals that simply cannot exist if there's any free oxygen around. Minerals like oldamite, which is calcium sulfide, and nineiningerite, which is a magnesium iron sulfide.

So this meteorite basically has the exact same weird low iron high sulfur recipe as mercury's crust.

It's practically a perfect match for the messenger data. It is a surviving piece of the exact type of material that clump together to build mercury.

Okay, so Zang and the team at Rice University have this piece of the in darch meteorite. But you can't just stick it in a microwave. How do you simulate the inside of a planet? How do you cook this rock at those kinds of pressures without destroying the laboratory?

Oh?

It requires some serious heavy duty engineering. They use what's called a multianvil press.

Multi anvil sounds heavy, it's massive. These machines generate gigab has scals of pressure.

You get pascals. What does that feel life? It's basically equivalent to the crushing weight of hundreds of kilometers of solid planetary rock pressing down on you all at once.

Oh wow, Okay, So how does the machine do that?

They take the rock, sample, powder it, and put it inside this tiny capsule, usually made a graphite or a special metal alloy. Then they put that tiny capsule inside an octaegal pressure medium, basically an eight sided block. Then massive hydraulic rams drive heavy tungsten carbide andles together from

That is insane. Maintaining that kind of heat and pressure without the whole thing just exploding.

The precision is unbelievable, but they have to do it to track exactly when the rock melts and when it freezes. They map the liquidus and the solidus.

It's the solidus being the point where it's totally hundred percent solid rock.

Right, So they get the rock to the exact pressure and temperature they want, wait for the chemistry to balance out, and then they do a rabid quench, a.

Quench like plunging a hot sword into water.

Essentially, Yes, they instantly drop the temperature while keeping the crushing pressure on. It freezes the molten rock into glass in a split second.

So it basically takes a chemical snapshot of what the magma look like at that exact depth and temperature exactly.

And then they slice it open and look at it under an electron microprobe.

And so the big reveal when they finally looked at these cooked, quenched samples of this mercury like rock. What did they actually see? Did the sulfur do what they thought it would?

It did exactly what they predicted. The data was crystal clear. The sulfur aggressively substituted into the silicate network, weakened it and actively suppressed crystallization.

So the anti freeze theory worked.

It worked perfectly. These reduced sulfur rich melts stayed liquid at significantly lower temperatures than an equivalent Earth rock would have.

Which brings up that great quote from Rajdeep Dascupta, the head of the lab.

Oh I love this quote. He said, what water or carbon does to magmatic evolution on Earth, sulfur does on mercury.

It's such a perfect summary because on Earth, if you add water to hot rock deep underground, like at a subduction zone, it acts as a flux. It lowers the melting point and triggers volcanoes.

Right The water physically gets into the rock structure and breaks up the.

Polymers, like pouring salt on an icy road exactly.

It forces the ice to melt even though it's freezing outside. On mercury, sulfur is the salt. It's the planetary anti freeze.

But the difference is water on Earth is kind of localized. It happens at specific tectonic plates. On Mercury, this sulfur is everywhere. It's built into the whole planet.

Yes, it operates globally. It depressed the melting point across the entire mantle of the planet.

Okay, so let's zoom out. Now we've got the lab data. We know the atomic bonds are weaker. We know the magma stays liquid at cooler temperatures. How does it's that atomic level chemistry translate to the huge planet wide features we see on Mercury today.

Well, if the magma stays liquid longer at cooler temperatures, it drastically extends the life span of Mercury's early magma oceans.

So instead of freezing solid quickly, the planet just sloshed around as a giant ball of liquid rock for millions of years, longer than we thought exactly.

And because the magma had a weaker structureless polymerized, it was also runnier, less.

Viscous like water instead of honey.

Right, and a running mantle moves heat really efficiently. It convects faster, pulling heat from the core to the surface.

Wait, if it's moving heat to the surface faster, wouldn't the planet cool down faster. That seems like a contradiction.

It does seem like one. Yeah, but remember the anti freeze effect. Even though the planet is losing heat rapidly, the sulfur ensures the rock simply refuses to solidify until the temperature drops incredibly low.

Oh man, So it's cooling off, but it's trapped in a liquid.

State precisely, and this extended period of cooling as a liquid completely changes what kind of rocks eventually form. As the temperature slowly drops toward that new super low freezing point. The very first minerals to pop out of the liquid and crystallize are those sulfur compounds.

The calcium and magnesium sulfides we talked about earlier oldamite and nininjurite.

Yes, because they crystallized early while the mantle was still fluid, they were able to concentrate in the upper.

Crust, which perfectly explains why messengers saw all that calcium and magnesium on the surface exactly.

The entire surface composition is a direct result of this prolonged low temperature crystallization that is.

So incredibly elegant. It just ties everything together.

It really does. It also explains Mercury's volcanic history, oh.

Right, because Mercury has these huge, smooth volcanic planes.

Right, vast plains of cured lava that sit on top of older, cratered terrain, which means these massive volcanic eruptions happened relatively late in the planet's youth.

But if Mercury was an Earth like rock, it's so small it should have cooled and formed a thick, solid crust really early. The volcano should have choked off and died exactly.

The thermal models for a normal rocky planet of Mercury size say the volcano should have shut down quickly, but the sulfur kept the engine running. It kept the mantle partially melted and fluid enough to erupt onto the surface long after it should have frozen solid.

Okay, but this leads me to another question, a pushback.

Actually, let's hear it.

If the sulfur kept the magma hot and fluid for such a long time, how does that fit with the low bait.

Scarps us the scarps?

Yeah, for people who don't know mercury is covered in these massive cliffs and ridges called scarps. They look like wrinkles. The planet literally contracted and shrank like a raisin.

A giant iron filled raisin.

Right, But you can't wrinkle a liquid. If the mantle was fluid for so long, a shrinking core wouldn't cause the surface to snap and form cliffs. The liquid would just adjust. So how did the scarps form?

That is a brilliant point. The mechanics have scarp formed absolutely require a rigid, solid, thick outer shell lithosphere. So the timeline is everything here. The global contraction, the shrinking that made those cliffs. It had to happen primarily after the sulfur rich magma ocean finally cooled past its super low freezing point and locked up.

Oh I see, So it stayed liquid for a long time, letting the volcanoes erupt but eventually even the anti freeze couldn't stop it from freezing exactly.

The temperature eventually dropped low enough that even those weakened sulfur silicate networks froze solid.

And once it formed that thick, hard shell.

Once that rigid lithosphere was in place, the massive iron core underneath continued to slowly cool and shrink over billions.

Of years, and the crust had nowhere to go right, it.

Induced immense compressive stress on the solid rock above it. The crust had no choice but to snap, fail catastrophically, and thrust up over itself to create those huge cliffs.

So the timing of the whole tectonic history is dictated by the atomic behavior of sulfur.

Yes, down to the very atoms, and it goes even deeper. It affects the magnetic field too.

Wait, really, the sulfur on the rocks affects the magnetic field of the core.

Oh. Absolutely, Mercury has a global magnetic field generated by a churning, convecting liquid iron core, a geodynamo.

Just like Earth.

Just like Earth, But for a geodynamo to work, you need a very specific rate of heat escaping from the core into the mantle. If the mantle acts like a thick thermal blanket, the core stops churning and the magnetic field dies.

But we just said the sulfur made the mantle runnier and more efficient at moving heat.

Exactly by proving that the sulfur rich mantle stayed fluid and less viscous for longer. The researchers basically re rode the equations for how heat moves across that boundary. The chemistry of the rock directly controlled the cooling rate of the iron core.

Which means the sulfur literally dictated the strength and the longevity of the magnetic field we still measure today.

It's all connected. The microscopic chemistry of the rock is inextricably linked to the massive geophysical engine inside the planet.

That is just phenomenal. It's a cohesive start to finish explanation for every weird thing about mercury, the surface chemistry, the extended volcanoes, the cliffs, the magnetic field, it all tracks back to sulfur.

It's a total paradigm shift, and it really validates the incredible work done by the Rice University team.

Supported by NASA grants and the Rice Space Institute Center for Planetary Origins to Habitability, which you know is a long name, but they are doing vital work.

Vital work because honestly, this goes way beyond mercury.

Yes, this is what I really want to get into. Why does this single case study matter for the rest of the universe.

Because it exposes our severe terrestrial bias, our Earth centric trap exactly because we live here. All our instruments are math, our thermodynamic assumptions. They are all grounded in the oxidized, iron rich chemistry of Earth.

So when we look at other planets, we just assume they're built the same way.

We project our local rules onto the rest of the galaxy. And Mercury just proved that if you apply an oxidized model to a highly reduced planet, you get totally wrong answers. You predict the wrong thermal history, the wrong structure, everything.

And we are discovering so many planets right now.

Thousands of them with advanced telescopes like James Web. We aren't just finding rocky exoplanets, super earths and sub neptunes. We're actually starting to look at their atmospheres and figure out what they're made of.

And they aren't all like Earth, not at all.

Many of them orbit stars with totally different chemistry in primordial discs that are vastly different from our own. Statistically, a huge number of these rocky exoplanets formed under highly reduced conditions.

Meaning the universe is likely just teeming with these sulfur dominated, iron poor worlds absolutely teeming with them, which is kind of ironic. Right. By turning our focus inward to the absolute smallest closest planet to our Sun, we've basically built a decoder ring to understand the farthest most alien exoplanets in the galaxy.

That's a great way to put it, because if we look at a distant super Earth through a telescope and try to model its interior using Earth math, we will fail.

We'd calculate the wrong melting points, the wrong crust thickness.

The wrong timeline for when it might release gases into an atmosphere. We would completely misunderstand its capacity to be a stable, maybe even habitable environment.

But thanks to the data from crushing that little in darch media.

Right, we now have the actual thermodynamic framework. We have the correct phase boundaries to model the geology of reduced exoplanets accurately.

It's incredible. It lets us predict when a sulfur rich planet light years away will solidify, when its volcanoes will stop, and how its crust will deform.

We can anticipate that its surface might be covered in old mite instead of basalt. We can adjust our models for its magnetic field. We are learning how to read the geological history of worlds we can't even see clearly yet.

It's just a remarkable progression of knowledge. To summarize this whole journey, we started with one anomalous, weirdly reduced rock that fell in Azerbaijan in eighteen ninety one.

The indarge and statype chondrite.

Right, and by recognizing that its chemistry matched the weird data from mercury, researchers in twenty twenty six used it as a proxy. They subjected it to gigapascals of pressure and thousands of degrees of heat.

They mapped the phase boundaries of a sulfur rich.

Mantle and prove that sulfur acts as a planetary anti freeze. It depalmerizes the melt, lowers the energy needed to stay liquid, and pushes the freezing point way down.

Which subsequently rewrites the entire geological timeline of mercury. It explains the sulfide crust, the long lasting magma oceans, the late forming scarps, and the magnetic dynamo.

And more importantly, it proves how much our understanding of the universe is biased by our own backyard. Sometimes just swapping out one single element, oxygen for sulfur, can rewrite an entire planet's history.

It changes the architecture from the atomic scale all the way up to the global features.

Which leaves us with a really provoked could have thought to end on today? We now know the profound consequences of swapping oxygen for sulfur. But sulfur is just one variable, just one of many. Right as we keep looking at distant star systems, we are going to find protoplanetary discs with entirely alien elemental recipes. If a relatively simple swap like sulfur can create a world as bizarre as mercury, what else is out there?

The possibilities are endless.

What happens to a planet's mantle if it's heavily enriched with carbon? How does a planet dissipate heat if it's dominated by exotic refractory metals. What kind of wild, unpredicted rocks are currently shaping worlds where the initial chemistry forces elements into configurations we haven't even thought to synthesize in a lab.

It's mind boggling.

Mercury is proof that our familiar Earth chemistry is just one specific lucky outcome in a universe that is capable of building planets through an almost infinite number of distinct alien pathways.