The Effects of Refraction and Forward Scattering on Exoplanet Transit Transmission Spectroscopy

Abstract

Transit transmission spectroscopy may provide the first glimpse at the atmosphere of an Earth-like exoplanet, and therefore the first chance to detect habitability markers and biosignatures, or signs of life. Within the next 10-15 years, the James Webb Space Telescope and large-aperture ground-based telescopes like the European Extremely Large Telescope (E-ELT) will come on line. JWST and E-ELT will have much greater collecting areas than existing space and ground-based telescopes, respectively, and will therefore allow for lower noise levels to be reached in transit transmission observations. With lower noise levels, it could be possible to characterize the atmospheres of Earth-sized, transiting planets orbiting in their star's habitable zones. Additionally, with that same increase in precision, physical effects that could previously be ignored may need to be considered in the near future. In light of this, I propose that refraction and forward scattering need to be considered when modeling transit transmission spectra, especially when comparing models to observations from future telescopes. I present the first model for transmission spectroscopy that includes the physical effects of refraction and forward scattering, and examine the effects of both on in-transit spectra and transit light curves. The model uses a backwards Monte Carlo ray tracing module coupled with an existing radiative transfer model (SMART). I validate the model against observations of the Earth from satellite data and lunar eclipse observations, and against theoretical and empirical estimates of the Venus scattering footprint. I find that refraction can reduce the detectability of spectral absorption features by setting a maximum tangent pressure that can be probed. This effect of refraction is small for planets whose orbital diameter is less than 12 times the diameter of the stellar disk (Rstar/a > 5º, i.e., close-in planets). However, for Earth-like planets orbiting Sun-like stars, refraction can severely reduce the detectability of biosignatures and habitability markers. Refraction also leads to changes in the transit light curve. I show that the wavelength-dependence of these changes can be used to obtain altitude-dependent spectra of exoplanets. In addition, a detection of refracted light in the out-of-transit light curve would indicate a lack of optically-thick hazes above the 1 mbar pressure level because light is refracted most at pressures greater than 1 mbar. Forward scattering increases the total flux seen in the in-transit spectrum, relative to models that include only extinction from aerosols. This may be an important effect for modeling planets whose orbital diameter is less than ~6 times the diameter of the stellar disk (Rstar/a > 10º, i.e., very close-in planets), such as hot Jupiters, but will be negligible for potentially habitable planets. Light can also be forward scattered when the planet is out-of-transit, and a detection of out-of-transit scattered light would be strongly suggestive of aerosols in the exoplanet atmosphere. Furthermore, a quantification of out-of-transit scattered light would allow for upper limits to be placed on the scattering asymmetry parameter, which in turn could be used to constrain the aerosol particle size distribution and composition. The model described above was used to simulate Earth-like planets to identify two signals as potential habitability markers and biosignatures: absorption from O2 dimer molecules and transient sulfate aerosols. Dimer molecules could be used to estimate atmospheric pressure, which is an important parameter for determining if liquid water could be present on the surface of an exoplanet, a requirement for habitability. Absorption from dimer molecules is more strongly dependent on density (and therefore pressure) than absorption from non-dimer molecules, and I show that combining absorption strengths from both types of features may allow limits to be placed on the surface or cloud-top pressure. O2 dimer molecules could also be biosignatures, and I show that O2 dimer features may be the most detectable O2 features in transit transmission for Earth-like exoplanets. The second signal I propose as a habitability marker and biosignature is transient sulfate aerosols. On Earth, explosive volcanism injects SO2 into the stratosphere, where it forms H2SO4 aerosols with lifetimes from months to years. The rapid rise and gradual decrease of aerosols could be indicative of a volcanic eruption on an exoplanet. For Earth, volcanism, and more broadly, geological activity, is important for the origin of life and for maintaining habitability. Therefore, the detection of a transient sulfate aerosol signal would suggest a geologically active planet, and increase the likelihood of that planet being habitable. Furthermore, there exist some scenarios in which O2 and O3 can build up abiotically if volcanic outgassing rates are much lower than those on the present-day Earth. Therefore, a co-detection of O2 or O3 with a transient aerosol signal may aid in resolving the ambiguity between biotic and abiotic O2 or O3.