Scientists have long believed that rocky planets like Earth and Mars formed their cores through a process that required intense heat.
In that traditional view, planets melted from the inside out, causing heavy metal to sink to the center and form a dense core.
But a new study led by Sam Crossley from the University of Arizona’s Lunar and Planetary Laboratory is turning that idea on its head.
The research, published in Nature Communications, shows for the first time that molten sulfide—liquid sulfur combined with iron—can seep through solid rock and form a planetary core before the rest of the planet even starts to melt.
This discovery could reshape our understanding of how planets like Mars formed and evolved, especially in the early solar system.
To test their idea, Crossley and his team recreated planetary conditions in the lab using pieces of real meteorites and synthetic minerals.
The experiments were sealed in glass tubes under a vacuum and heated to high temperatures.
They watched how the molten sulfide moved through solid rock using powerful X-ray imaging, revealing tiny melt channels winding their way between mineral grains.
These experiments provided direct evidence that sulfide-rich melts can travel downward, pulled by gravity, and pool at a planet’s center—just like metal does in the traditional core formation model.
The team also studied how certain trace elements, like platinum and iridium, behaved during this process. These elements are usually found in cores and helped confirm that what they were seeing was indeed core formation in action.
This process could be especially important for planets that formed farther from the sun, like Mars, where elements like sulfur and oxygen are more abundant.
These elements act like road salt in winter, lowering the melting point of metal and letting sulfide start to flow at much lower temperatures than pure iron. That means planets like Mars could have formed their cores earlier and more quickly than scientists previously thought—without ever needing to completely melt like Earth did.
The study also raises important questions about how scientists estimate the ages of planetary cores using isotopes like hafnium and tungsten. If sulfur changes the behavior of elements during core formation, the “clocks” we use to date planets might need adjusting.
“This gives us a whole new way to think about how rocky planets form and evolve,” said Crossley. The results are especially timely as NASA prepares for future missions to the moon and Mars.
Knowing how cores form can help scientists interpret spacecraft data, analyze samples from other worlds, and piece together the story of our solar system’s earliest days.
