New experiments have shown that the core of Mars formed much faster than Earth’s core, thanks to molten iron and nickel sulfides seeping down through solid rock and into the center of the Red Planet.
Planets are layered, somewhat like an onion. The surface upon which we stand is the crust, which sits atop the mantle. Much deeper, and we find a solid outer core and a molten inner core, the spinning of which can generate a global magnetic field.
Planetary scientists call this layering “differentiation,” in the sense that different elements were able to differentiate themselves from each other. Heavier elements, particularly iron and nickel, typically sink to the hearts of planets, while lighter silicate elements remain in outer layers. However, scientists have typically assumed that for iron and nickel to be able to sink into a planetary core, a planet’s interior has to be molten, melted primarily by the heat released by the radioactive decay of aluminium-26 and possibly iron-56.
This is almost certainly how Earth‘s core formed, at least, in a process that scientists estimate took a billion years or longer. But Mars presents a blip in this story. Martian meteorites contain radioisotopic evidence that is sensitive to the formation of Mars’ core, and this evidence points to that core forming not in billions of years, but in just a few million years after the birth of the solar system. The implication of this seems to be that Mars grew far more quickly than Earth, but formation models of the solar system have struggled to replicate this.
Now, scientists at NASA Johnson Space Center’s Astromaterials Research and Exploration Science (ARES) Division think they have the answer. They may have figured out how Mars could have formed its core so quickly without experiencing any anomalous growth spurts early on.
About 4.5 billion to 4.6 billion years ago, the planets coalesced out of a disk of gas and dust that encircled the sun, called a protoplanetary disk. The infant sun’s gravity pulled the heaviest elements and minerals, including iron and nickel, into the inner sanctum of the disk. Meanwhile, the lighter materials such as water and hydrogen resided in the outer parts of the disk.
The place where Mars formed sat somewhere in-between those sections. There was still plenty of iron and nickel in its vicinity, but there was also room for lighter elements such as oxygen and sulfur. The team at ARES realized this could have had an influence on how Mars’ core formed, so they put it to the test. In doing so, they produced the first direct evidence that molten iron and nickel sulfides can seep through tiny cracks between minerals in solid rock, ultimately accumulating in a planet’s core after only a few million years, long before radioactive decay turned the interior molten.
The scientists, led by Sam Crossley who has since moved from ARES to the University of Arizona in Tucson, conducted high-temperature experiments at NASA Johnson’s Experimental Petrology Lab, heating samples of sulfate-rich rock in excess of 1,020 degrees Celsius, which is hot enough to melt sulfides — but not silicate rock. They then probed the heated samples at the space center’s X-ray computed tomography lab to see if the sulfides had percolated through the solid rock.
“We could actually see in full 3D renderings how the sulfide melts were moving through the experimental sample, percolating in cracks between other minerals,” Crossley said in a statement.
It’s all well and good demonstrating this in controlled conditions inside a laboratory, but could it really take place in the bowels of a planetary body? To be sure, the team had to double check their hypothesis against material that really was once part of a planetary body.
“We took the next step and searched for forensic chemical evidence of sulfide percolation in meteorites,” said Crossley. “By partially melting synthetic sulfides infused with trace platinum-group metals, we were able to reproduce the same unusual chemical patterns found in oxygen-rich meteorites, providing strong evidence that sulfide percolation occurred under those conditions in the early solar system.”
However, identifying those trace platinum-group metals, specifically iridium, osmium, palladium, platinum and ruthenium, without destroying the samples required some clever techniques devised by ARES researcher Jake Setera.
“To confirm what the 3D visualizations were showing us, we needed to develop an appropriate laser ablation method that could trace the platinum group-elements in these complex experimental samples,” Setera said in the statement.
Setera’s method found that the passage of molten sulfides through solid rock left residues of these platinum-group metals in the samples in quantities that matched those found in certain chondritic meteorites.
“It confirmed our hypothesis — that in a planetary setting, these dense melts would migrate to the center of a body and form a core, even before the surrounding rock began to melt,” said Crossley.
This model of core formation would apply to all significant large bodies residing in that middle region of the protoplanetary disk, not just Mars. That said, given the puzzle of Mars’ formation, the findings potentially answers some fundamental questions about the earliest days of the Red Planet, and makes the prediction that Mars’ core should be rich in sulfur. And you know what sulfur smells like? Rotten eggs.
The research was published on April 4 in the journal Nature Communications.
