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 $\mathring{A}$) 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 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.

27 pages, 14 figures, 1 table, published in the Astronomical Journal

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• Observability of ultraviolet N I lines in the atmosphere of transiting Earth-like planets