# Stellar Activity Effects on Moist Habitable Terrestrial Atmospheres Around M dwarfs

## TL;DR

This study investigates how stellar UV activity affects water vapor detectability in the atmospheres of Earth-sized planets around M dwarfs, finding that high UV flux may not hinder water detection in transmission spectra.

## Contribution

It introduces a 1-D photochemical model incorporating stellar UV effects to assess water vapor detectability, expanding previous climate models by including chemical complexity and UV photolysis impacts.

## Key findings

- High stellar UV activity does not significantly reduce water vapor detectability in transmission spectra.
- Water features are strongest in JWST MIRI wavelength range, near the systematic noise floor.
- Well-mixed atmospheres up to 1 mbar retain detectable water features despite UV photolysis.

## Abstract

Transit spectroscopy of terrestrial planets around nearby M dwarfs is a primary goal of space missions in coming decades. 3-D climate modeling has shown that slow-synchronous rotating terrestrial planets may develop thick clouds at the substellar point, increasing the albedo. For M dwarfs with Teff > 3000 K, such planets at the inner habitable zone (IHZ) have been shown to retain moist greenhouse conditions, with enhanced stratospheric water vapor (fH2O > 1E-3) and low Earth-like surface temperatures. However, M dwarfs also possess strong UV activity, which may effectively photolyze stratospheric H2O. Prior modeling efforts have not included the impact of high stellar UV activity on the H2O. Here, we employ a 1-D photochemical model with varied stellar UV, to assess whether H2O destruction driven by high stellar UV would affect its detectability in transmission spectroscopy. Temperature and water vapor profiles are taken from published 3-D climate model simulations for an IHZ Earth-sized planet around a 3300 K M dwarf with an N2-H2O atmosphere; they serve as self-consistent input profiles for the 1-D model. We explore additional chemical complexity within the 1-D model by introducing other species into the atmosphere. We find that as long as the atmosphere is well-mixed up to 1 mbar, UV activity appears to not impact detectability of H2O in the transmission spectrum. The strongest H2O features occur in the JWST MIRI instrument wavelength range and are comparable to the estimated systematic noise floor of ~50 ppm.