# NIR-Driven Moist Upper Atmospheres of Synchronously Rotating Temperate Terrestrial Exoplanets

## TL;DR

This study uses 3D climate modeling to explore how water vapor distribution in the upper atmospheres of tidally locked Earth-sized exoplanets affects the detectability of water signatures in transmission spectra, influenced by stellar flux and spectral type.

## Contribution

It provides new insights into the vertical water vapor transport driven by radiative heating and cloud effects in synchronously rotating exoplanets, enhancing understanding of H₂O detectability.

## Key findings

- Water vapor in upper atmospheres increases with incident flux, but more gradually than 1D models suggest.
- Upper atmospheric water vapor correlates with near-infrared incident flux, affecting spectral signatures.
- Higher irradiation can amplify H₂O spectral features, easing observational detection.

## Abstract

H$_2$O is a key molecule in characterizing atmospheres of temperate terrestrial planets, and observations of transmission spectra are expected to play a primary role in detecting its signatures in the near future. The detectability of H$_2$O absorption features in transmission spectra depends on the abundance of water vapor in the upper part of the atmosphere. We study the three-dimensional distribution of atmospheric H$_2$O for synchronously rotating Earth-sized aquaplanets using the general circulation model (GCM) ROCKE-3D, and examine the effects of total incident flux and stellar spectral type. We observe a more gentle increase of the water vapor mixing ratio in response to increased incident flux than one-dimensional models suggest, in qualitative agreement with the climate-stabilizing effect of clouds around the substellar point previously observed in GCMs applied to synchronously rotating planets. However, the water vapor mixing ratio in the upper atmosphere starts to increase while the surface temperature is still moderate. This is explained by the circulation in the upper atmosphere being driven by the radiative heating due to absorption by water vapor and cloud particles, causing efficient vertical transport of water vapor. Consistently, the water vapor mixing ratio is found to be well-correlated with the near-infrared portion of the incident flux. We also simulate transmission spectra based on the GCM outputs, and show that for the more highly irradiated planets, the H$_2$O signatures may be strengthened by a factor of a few, loosening the observational demands for a H$_2$O detection.