Thesis Topic: Tracing biosignatures on planet Earth with high-resolution near-infrared spectropolarimetry

Thesis Supervisor: Michael Sterzik


The search for life elsewhere in the universe has become a sensible, while highly ambitious, scientific goal. Several thousands of exoplanets have been found during the last decade, and the quest for "Earth-like" planets, habitable zones, and, ultimately, life on extrasolar planets has already started. A pre-requisit to interpret images or spectra of (Earth- like) extrasolar planets as expected from future space missions or ground-based ELTs is the detailed knowledge of how our Earth looks like from space. 

From the ground, the globally integrated spectrum of Earth can be measured by observing the light reflected by the daytime Earth onto the dark side of the lunar surface (aka Earthshine). Earthshine observations intrinsically yield disk-integrated Earth spectra, not contain spatial information, and resemble spatially unresolved observations of a truly Earth-like exoplanet.

Our group has pioneered observations and interpretation of (polarization) spectra of Earthshine as key to probe the Earth as an exoplanet. Using the VLT and FORS2, it was possible to discern changes in Earth's cloudiness, to determine that the planet is partially covered by oceans, and to detect the Vegetation Red Edge, a biosignature caused by chlorophyll (Sterzik, Bagnulo, Palle, 2012, Nature, 483, 64). Both surface (ocean versus different lands) and atmospheric properties (cloud composition, hazes and aerosols, cloud deck heights and mixing ratios) could be constrained. 

A quantitative explanation of Earthshine spectra requires highly advanced Monte Carlo radiative transfer model to simulate full-Stokes radiative transfer in the atmosphere of Earth in full 3D geometry. We therefore actively develop and apply the most advanced scattering models of Earth to explain numerous properties of its continuum and line spectrum (Emde et al, 2017, A&A 605, A2). 

Using this methodology, we can infer the existence of weak and strong biomarkers. For example, we can model the spectral and temporal variability of Earthshine spectra originating in a combination of changing ground surface viewing sceneries caused by Earth rotation, and cloud coverage caused by the dynamics of atmospheric weather and climate systems (Sterzik et al., 2019, A&A 622, A41). We can also quantitatively retrieve cloud properties of Earth (refraction index, optical depth and particle sizes) by modeling the cloudbow observations (Sterzik et al., 2020, A&A 639, A89). 

In summary, the extraction of all detectable biomarkers contained in the NIR high-spectral resolution (polarization) spectra of Earthshine observed with CRIRES+ constitutes the main observational goal of this thesis. In addition, numerical simulations of the Earth’s atmosphere are required to model and characterize the sensitivity and specificity of the method. 

It is expected that this analysis opens a novel window for the direct detection of biosignatures of Earth-like planets in the future.