Research

This research explores how hydrogen affects the stability of iron–sulfur (Fe–S) alloys at the extreme pressures and temperatures found in the deep interiors of Mars and Mars-like exoplanets. While meteorites tell us that Mars's core likely contains iron, sulfur, and some nickel, recent seismic data from NASA’s InSight mission suggests that Mars's core is larger and less dense than expected—hinting that more light elements like hydrogen may be present.

To understand this, we conducted high-pressure, high-temperature experiments on Fe–S materials in hydrogen-varying environments. Our findings show that hydrogen can destabilize certain Fe–S compounds and promote the formation of hydrogen-rich metallic liquids. These results help explain why Mars's core might still be mostly molten today and offer new clues about the interior makeup of similar rocky planets beyond our solar system.

Hydrogen-rich sub-Neptune exoplanets are thought to host thick atmospheres that interact dynamically and chemically with their rocky interiors. At the boundary between H2 envelopes and silicate-metal cores, pressures of tens of gigapascals and temperatures of several thousand kelvin enable hydrogen to react with rock-forming elements, potentially altering interior composition and density. Here we investigate these processes through laser-heated diamond-anvil cell experiments on Mg-Fe mixtures in H2 rich media at 19-45 GPa and up to 3311 K. In situ synchrotron X-ray diffraction reveals the formation of Mg2FeH6 with a cubic perovskite-type structure, which remains stable upon quenching. Fitting the pressure-volume data yields a bulk modulus of 62(2) GPa with a pressure derivative of 5.1(2). Using these parameters, mass-radius calculations show that a planet composed of Mg2FeH6 + 25% H2O produces radii 2-5 times smaller than of those of a rocky (2MgO + Fe) planet surrounded by a 5% H2 envelope. This correspondence demonstrates that hydrogen chemically incorporated into hydrides can replicate the bulk densities of volatile-rich sub-Neptunes without requiring thick gaseous envelopes. Such ingassing of hydrogen at the envelope-core boundary may therefore reconcile the wide range of observed sub-Neptune densities, implying that part of the hydrogen inventory can be stored in solid hydrides within the deep interior.