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

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 Å) 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 Å 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. INTRODUCTIONAs we approach the era of directly characterizing the atmospheres of Earth-sized exoplanets, considerable preparation is underway to determine how we would recognize signatures of habitability or life from many parsecs away using techniques like transit spectroscopy and direct imaging. A major component of these preparations is the evaluation of biosignatures, the remotely observable features