Transmission spectrum of Venus as a transiting exoplanet

TL;DR

This paper presents a theoretical transmission spectrum of Venus's atmosphere during transit, serving as a proxy for characterizing Earth-like exoplanets and aiding in their observational discrimination.

Contribution

It provides the first detailed model of Venus's transmission spectrum across 0.1-5 μm, useful for exoplanet atmospheric studies during transits.

Findings

UV absorption by CO2 produces a ~20 ppm transit signal.

Mie scattering by sulfuric acid droplets causes ~5 ppm extinction from 0.2-2.7 μm.

Spectral features can distinguish Venus-like planets from Earth-like habitable worlds.

Abstract

On 5-6 June 2012, Venus will be transiting the Sun for the last time before 2117. This event is an unique opportunity to assess the feasibility of the atmospheric characterisation of Earth-size exoplanets near the habitable zone with the transmission spectroscopy technique and provide an invaluable proxy for the atmosphere of such a planet. In this letter, we provide a theoretical transmission spectrum of the atmosphere of Venus that could be tested with spectroscopic observations during the 2012 transit. This is done using radiative transfer across Venus' atmosphere, with inputs from in-situ missions such as Venus Express and theoretical models. The transmission spectrum covers a range of 0.1-5 {\mu}m and probes the limb between 70 and 150 km in altitude. It is dominated in UV by carbon dioxide absorption producing a broad transit signal of ~20 ppm as seen from Earth, and from 0.2 to 2.7 {\mu}m by Mie extinction (~5 ppm at 0.8 {\mu}m) caused by droplets of sulfuric acid composing an upper haze layer above the main deck of clouds. These features are not expected for a terrestrial exoplanet and could help discriminating an Earth-like habitable world from a cytherean planet.