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arXiv · 2008.01837

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

Allison Youngblood·Giada N. Arney·Antonio García Muñoz·John T. Stocke·Kevin France·Aki Roberge
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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.

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Allison Youngblood, Giada N. Arney, Antonio García Muñoz, John T. Stocke, Kevin France, Aki Roberge. 2020-08-04. The Hubble Space Telescope's near-UV and optical transmission spectrum of Earth as an exoplanet. https://doi.org/10.3847/1538-3881%2Faba0b4

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