Session EE03: Planetary Science

High-Pressure Chemistry Linking Dry and Wet Planets

It has been believed that water-rich planets are formed outside the snow line. However, discovery of many close-in transiting sub-Neptune exoplanets, if at least some of them are water rich, challenges the notion. Therefore, sub-Neptunes have been believed to be large Earth-like planets with thick hydrogen rich atmosphere and hence dry. We have conducted a series of experiments under the pressure-temperature conditions expected at the boundary between hydrogen-rich atmosphere and rocky interior of a sub-Neptune at the GSECARS sector of Advanced Photon Source. Micro-second pulsed laser heating enables us to melt silicate in a hydrogen medium at high pressures. The reaction was monitored in situ using gated synchrotron X-ray diffraction. The recovered samples were analyzed with Raman spectroscopy and electron microscopy. We found reaction between hydrogen and magma which can unlock oxygen from silicate magma and enable oxygen to react with hydrogen to form water. The chemical reaction can convert a dry hydrogen-rich planet to a water-rich planet inside the snow line. H2/H2O ratio controls the degree of the conversion, explaining diverse mass-radius relations found among sub-Neptune exoplanets. We also found that high pressure promotes mixing between metal and hydrogen and mixing between oxide and water. Such mixing can result in a radial compositional gradient in the interiors of volatile-rich planets, which is different from well-differentiated layered interior structures of rocky planets. This dataset will advance our knowledge on atmosphere-interior interactions in large exoplanets, providing key data for understanding astrophysical measurements of their atmospheres.   

Reduction of Silicates by Hydrogen at High Pressure-Temperature – Experimental Evidence for the Formation of Hot Wet Sub-Neptunes

Sub-Neptunes – planets of radius ~1.75-3.5 R_Earth – with orbital periods of <100 days are the most abundant type of observed exo-planets [1]. Their mass-radius relations can be modeled as either dry, rocky worlds overlain by a thick hydrogen atmosphere, or water-rich planets, with the latter favored due to their orbit within the ice line requiring migration or delivery of water-rich materials from beyond the ice line [2]. We conducted experiments in the pulsed-laser-heated diamond-anvil-cell loaded with silicate samples in a hydrogen medium at pressures of 6-42 GPa and temperatures of 1650-4050 K. Pulsed laser-heating (along with inert gasket coatings) was used to mitigate H-diffusion out of the sample chamber, a challenge that previously made high P-T studies with pure H infeasible. We found that H reduces Si⁴⁺ and Fe²⁺ in silicate melt to metal, releasing O which in turn forms H₂O. This reaction can convert a dry rocky planet with a hydrogen atmosphere into a water rich planet from within, even inside the snow line. Depending on the degree of this reaction, a wide range of H₂ to H₂O ratios are possible, offering a new explanation for the diverse mass-radius relations in sub-Neptune populations.

[1] Fulton, B. J., et al. AJ 154.3 (2017)

[2] Bean, J. L., et al. JGR: Planets 126.1 (2021)   

In situ X-ray diffraction study of high-pressure phase transitions in iron oxide under shock loading

The high-pressure-temperature (P-T) behavior of iron oxides is a topic of considerable interest both due to their importance in Earth’s interior as well as their role as analogs for understanding ultrahigh-P magnesium silicates phase transitions in super-Earth interiors. Hugoniot data for hematite (Fe₂O₃) and magnetite (Fe₃O₄) both show significant densification across a broad mixed-phase region between 40-80 GPa. However, the structures of the high-P phases along the Hugoniot are unknown. In contrast to the limited shock data, there has been significant interest in understanding iron oxides from diamond cell (DAC) experiments, and recent studies have shown complex polymorphism under static high-P-T conditions. However, these results are discrepant, and it is unclear which static high-P phases, if any, form under shock compression. To address this, we carried out an in situ X-ray diffraction study to investigate phase transformations in hematite and magnetite under laser-shock loading at the Dynamic Compression Sector at the Advanced Photon Source. Our results demonstrate that the high-P phase on the hematite Hugoniot corresponds to the θ-Fe₂O₃ structure while magnetite transforms to a Th₃P₄-phase, both of which are structures that have been reported in recent laser-heated DAC experiments.   

X-ray diffraction measurements across the melt line in shocked nickel

We have investigated elemental bulk nickel at high shock pressures and temperatures using in situ x-ray diffraction and the velocimetry technique, VISAR, with the goal of bracketing the onset of melt under shock conditions. We observe a solid compressed fcc-Ni phase up to pressures as high as 500 GPa. In the range of approximately 300–375 GPa, we see evidence for the onset of melt, with full melt achieved above 500 GPa. The experiments were conducted at the Matter in Extreme Conditions endstation at the SLAC National Accelerator Laboratory using a flat-top laser drive. Experimental results were supported by 1D hydrodynamic simulations, providing particle velocity, transit time, pressure, and density data that agree well with experiment.

Our work presents the first in situ x-ray diffraction data on shock-compressed nickel up to ~500 GPa and is particularly relevant to computational and experimental researchers studying nickel and other dense metals. The pressures we reached are some of the highest ever reported for shocked nickel, and our identification of a solid compressed phase up to 500 GPa is significantly higher than expected by the majority of melt lines that have been proposed for nickel in the literature.