The Hubble Space Telescope's near-UV and optical transmission spectrum of Earth as an exoplanet

TL;DR

This study used Hubble to observe Earth's transmission spectrum during a lunar eclipse, focusing on near-UV wavelengths to identify ozone features critical for exoplanet habitability assessments.

Contribution

First near-UV transmission spectrum of Earth obtained during a lunar eclipse, providing insights into ozone detection relevant for exoplanet studies.

Findings

Detected spectral features of ozone and Rayleigh scattering in Earth's spectrum.

Near-UV spectra appeared featureless due to eclipse timing and atmospheric absorption.

Methodology for extracting Earth's transmission spectrum during lunar eclipses.

Abstract

We observed the 2019 January total lunar eclipse with the Hubble Space Telescope's STIS spectrograph to obtain the first near-UV (1700-3200 $\r{A}$) observation of Earth as a transiting exoplanet. The observatories and instruments that will be able to perform transmission spectroscopy of exo-Earths are beginning to be planned, and characterizing the transmission spectrum of Earth is vital to ensuring that key spectral features (e.g., ozone, or O$_3$) are appropriately captured in mission concept studies. O$_3$ is photochemically produced from O$_2$, a product of the dominant metabolism on Earth today, and it will be sought in future observations as critical evidence for life on exoplanets. Ground-based observations of lunar eclipses have provided the Earth's transmission spectrum at optical and near-IR wavelengths, but the strongest O$_3$ signatures are in the near-UV. We describe the observations and methods used to extract a transmission spectrum from Hubble lunar eclipse spectra, and identify spectral features of O$_3$ and Rayleigh scattering in the 3000-5500 \r{A} region in Earth's transmission spectrum by comparing to Earth models that include refraction effects in the terrestrial atmosphere during a lunar eclipse. Our near-UV spectra are featureless, a consequence of missing the narrow time span during the eclipse when near-UV sunlight is not completely attenuated through Earth's atmosphere due to extremely strong O$_3$ absorption and when sunlight is transmitted to the lunar surface at altitudes where it passes through the O$_3$ layer rather than above it.