[Paper Review] From stars to diverse mantles, melts, crusts and atmospheres of rocky exoplanets

Claire Marie Guimond, Haiyang Wang+4 more|arXiv (Cornell University)|Apr 23, 2024

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper develops a predictive framework linking stellar compositions to the mineralogical and atmospheric diversity of rocky exoplanets, using thermodynamic modeling of mantle mineral assemblages, partial melting processes, and crust formation. It demonstrates that small variations in stellar composition can lead to significant differences in planetary bulk composition, crustal mineralogy, and detectable atmospheric signatures, enhancing the sensitivity of future observations to exoplanetary diversity.

ABSTRACT

This review is focused on describing the logic by which we make predictions of exoplanetary compositions and mineralogies, and how these processes could lead to compositional diversity among rocky exoplanets. We use these predictions to determine the sensitivity of present-day and future observations to detecting compositional differences between rocky exoplanets and the four terrestrial planets. First, we review data on stellar abundances and infer how changes in composition may manifest themselves in the expected bulk compositions of rocky exoplanets (section 2). Converting this information in mass-radius relationships requires calculation of the stable mineral assemblages at a given temperature-pressure-composition (T-P-X), an exercise we describe in section 3. Should the planet be hot enough to engender partial melting of the mantle, then these liquids are likely to rise to the surface and erupt to form planetary crusts; the possible compositional and mineralogical variability of which we examine in section 4. Finally, the expected spectroscopic responses of such crusts are examined in section 5.

Motivation & Objective

• To establish a predictive chain from stellar abundances to planetary compositions and mineralogy in rocky exoplanets.
• To quantify how variations in stellar composition translate into differences in planetary bulk composition and mantle mineral assemblages.
• To model the formation of planetary crusts through mantle partial melting and assess resulting compositional and mineralogical variability.
• To evaluate the spectroscopic detectability of crustal compositions and their atmospheric signatures in present and future exoplanet observations.

Proposed method

• Using observed stellar abundances as proxies for protoplanetary disk compositions to infer bulk planetary compositions.
• Applying thermodynamic calculations to determine stable mineral assemblages at varying temperature, pressure, and composition (T-P-X) conditions in planetary mantles.
• Modeling partial melting of mantles under high-temperature conditions to predict melt compositions and their potential ascent to form planetary crusts.
• Simulating the resulting crustal mineralogy and its spectral signatures for comparison with observational data from telescopes.
• Integrating these models to predict detectable differences in planetary atmospheres linked to crustal outggassing and surface processes.
• Validating the framework against the terrestrial planets (Earth, Venus, Mars, Mercury) to calibrate predictions for exoplanets.

Experimental results

Research questions

• RQ1 How do variations in stellar composition translate into detectable differences in the bulk compositions of rocky exoplanets?
• RQ2 What mineralogical assemblages are expected in exoplanetary mantles under different T-P-X conditions derived from stellar abundances?
• RQ3 How does partial melting of exoplanetary mantles influence the composition and mineralogy of resulting planetary crusts?
• RQ4 What spectroscopic signatures can be expected from exoplanetary crusts, and how detectable are they with current and future telescopes?
• RQ5 To what extent can observed differences in exoplanet bulk compositions and atmospheres be attributed to variations in initial stellar compositions?

Key findings

• Small changes in stellar abundance ratios—particularly in Mg/Si and Fe/Si—can lead to significant differences in predicted exoplanetary bulk compositions and mantle mineralogy.
• Planets forming from stars with higher Mg/Si ratios are predicted to have more enstatite-rich mantles, while lower ratios favor ferropericlase-dominated assemblages.
• Partial melting of these varied mantles produces melts with distinct compositions, leading to crusts enriched in feldspathic, ultramafic, or felsic materials depending on the starting composition.
• The resulting crusts exhibit distinct mineralogical and spectral signatures, with feldspar-rich crusts showing stronger absorption features in the 1–3 μm range.
• These spectral differences are potentially detectable with upcoming space telescopes such as the James Webb Space Telescope and future missions like LUVOIR.
• The framework successfully reproduces key characteristics of the terrestrial planets, validating its predictive power for exoplanetary systems.