Rocky Planet or Water World? Observability of Low-density Lava World Atmospheres

Abstract

Super-Earths span a wide range of bulk densities, indicating a diversity in interior conditions beyond that seen in the solar system. In particular, an emerging population of low-density super-Earths may be explained by volatile-rich interiors. Among these, low-density lava worlds have dayside temperatures that are high enough to evaporate their surfaces, providing a unique opportunity to probe their interior compositions and test for the presence of volatiles. In this work, we investigate the atmospheric observability of low-density lava worlds. We use a radiative-convective model to explore the atmospheric structures and emission spectra of these planets, focusing on three case studies with high observability metrics and substellar temperatures spanning ∼1900–2800 K: HD 86226 c, HD 3167 b, and 55 Cnc e. Given the possibility of mixed volatile and silicate interior compositions for these planets, we consider a range of mixed volatile and rock-vapor atmospheric compositions. This includes a range of volatile fractions and three volatile compositions: water-rich (100% H 2 O), water with CO 2 (80% H 2 O+20% CO 2 ), and a desiccated O-rich scenario (67% O 2 +33% CO 2 ). We find that spectral features due to H 2 O, CO 2 , SiO, and SiO 2 are present in the infrared emission spectra as either emission or absorption features, depending on dayside temperature, volatile fraction, and volatile composition. We further simulate JWST secondary-eclipse observations for each of the three case studies, finding that H 2 O and/or CO 2 could be detected with as few as ∼five eclipses. Detecting volatiles in these atmospheres would provide crucial independent evidence that volatile-rich interiors exist among the super-Earth population.