| Popis: |
The recent first measurements of the reflection of the surface of a lava world provides an unprecedented opportunity to investigate different stages of rocky planet evolution. The spectral features of the surfaces of rocky lava world exoplanets give insights into their evolution, mantle composition and inner workings. However, no database exists yet that contains spectral reflectivity and emission of a wide range of potential exoplanet surface materials. Here we first synthesized 16 potential exoplanet surfaces, spanning a wide range of chemical compositions based on potential mantle material guided by the metallicity of different host stars. Then we measured their infrared reflection spectrum (2.5 - 28 μm, 350 - 4000 cm-1), from which we can obtain their emission spectra, and establish the link between the composition and a strong spectral feature at 8 μm, the Christiansen feature (CF). Our analysis suggests a new multi-component composition relationship with the CF, as well as a correlation with the silica content of the exoplanet mantle. We also report the mineralogies of our materials, as possibilities for that of lava worlds. This database is a tool to aid in the interpretation of future spectra of lava worlds that will be collected by the James Webb Space Telescope and other missions. *Methods: We selected eight representative BSP compositions (as modeled by Putirka and Rarick 2019; 14684-30, 81262-30, 6856-50, 6856-30, 6856-10, 65356-30, 59639-30, 22907-30) as well as a suite of reference valuesthat includes, the average mid-ocean ridge basalt (MORB) composition (Gale et al., 2013), a chondritic composition (McDonough and Sun, 1995), and a peridotite (MM3; Baker and Stolper, 1994), spanning a wide range of potential exoplanet surface composition (see Fig. 1 and data repository is 10.5281/zenodo.6323322). To represent the surface composition of lava world exoplanets more accurately, we modeled the composition of partial melts derived from these starting compositions as a two-stage process, the segregation of a crust derived from the BSP mantle and melting. We first used rhyolite-MELTS (Gualda et al., 2012) to simulate crustal segregation during partial melting at a 1 atm pressure and an oxygen fugacity (fO2) of about what is set by the quartz-fayalite-magnetite mineral oxygen buffer (QFM), as it is unlikely that the crustal composition of these objects is exactly their BSP, and much more likely that their surface is the result of some degree of partial melting of their mantle. We assumed melt fractions of 30% (F=0.3) for most compositions, but also modeled multiple melt fractions (F=1, 0.5, 0.3, 0.1) for MORB and 6856 to track potential compositional evolution of the surface from a given BSP. Additionally, we modeled F=0.1 for the peridotite (MM3) and 112774, for a total of 16 compositions (see modeled compositions plotted in Fig. 1). To synthesize these modeled compositions representing total and partial melting, we mixed the appropriate amounts of oxides (SiO2, TiO2, Al2O3, MgO, MnO) and carbonates (CaCO3, Na2CO3, K2CO3) in an agate mortar and pestle under ethanol three times for ten minutes each. We decarbonated the mixtures at 1000 °C overnight, then crushed and remixed the ensuing material in an agate mortar and added FeO. To simulate the conditions of an “airless planet” (e.g., near complete loss of atmosphere given the proximity to the star and/or low gravity), we equilibrated our compositions in controlled atmosphere Deltech vertical tube furnace in the experimental geochemistry lab at Cornell University. We placed each of the mixed and decarbonated compositions in a graphite crucible at 1310 ± 5 °C, soaking in a 100% CO atmosphere with an estimatedfO2of about QFM-4 for one hour. The charges were then quenched in air and crushed for analysis with an integrating sphere, making sure to save a large chip of each composition, which we mounted in Epoxy and polished using SiC polishing paper and 1 μm Al2O3powder for LA-ICP-MS and FTIR measurements (refer to section 2.2). *complete reference list in the paper |
