Mia Belle Parkinson

For the foreseeable future, the great distances that separate us from even the most nearby star systems indicate that all measurements of exoplanets must be made through remote sensing techniques. Current technology, like the James Webb Space Telescope (JWST), can obtain transmission spectra of exoplanet atmospheres that help astrobiologists and astronomers determine these planets’ atmospheric conditions. Future missions in the coming decades, like LUVOIR and HabEx, will make it possible to resolve disk-integrated reflectance spectra from exoplanet surfaces in addition to atmospheric features where we could potentially identify habitable and life-bearing worlds. Among surface features, the “vegetation red edge” (VRE), a sharp step-like feature in the reflectance spectrum characteristic of light-harvesting organisms, is the paradigm of a biosignature that might be detectable on exoplanets. However, similar spectral features in biomass have been found from abiotic sources. In astrobiology, microbes and minerals are closely linked – minerals record environmental conditions, supply the surfaces needed to support and preserve organic material, and act as templates for biogeochemical reactions that define life. 

Current research has limitations due to the lack of comprehensive experimentation in capturing reflectance spectra of minerals and microbes, accounting for exoplanet atmospheres, and simulating realistic observations with telescopic noise sources. As astrobiologists interested in the distribution of life in the universe, we also wish to learn from exoplanet spectra how planetary habitability changes through time. Mars (for example) once had liquid water on its surface, and because there might be exoplanets that once had water or even life, observing hydrated minerals may tell us that a planet might have been once habitable, even if it is now desiccated. 

This project prepares for future missions in astrobiology by helping to make the crucial distinction between biology and abiotic processes that mimic life, while examining potential evidence for past planetary habitability. First, by examining NASA’s ECOSTRESS (Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station) spectral database and extracting all available mineral data. Then, taking the microbial spectral data from Hegde et al. (2015) to plot both groups onto a principal component analysis (PCA). The PCA allowed me to study overlaps between spectral features and where several minerals were identified with edge-like features between 500 and 700 nm. After choosing a selection of minerals to examine, I acquired hemispherical reflectance measurements using ASD FieldSpec 4 spectroradiometers from the University of Edinburgh and Cornell University for more reliable comparisons. Finally, I integrated astronomy with mineral spectra by applying the effects of telescope instrument sensitivity, resolution, and noise using NASA’s Planetary Spectrum Generator (PSG) to produce simulated spectroscopic fingerprints of minerals analogous to those we might find on exoplanets. 

Finally, the use of EXO-Prime2 provided by collaborators at Cornell University allowed me to compare our mineral measurements with a modern Earth atmosphere. The results highlight several challenges to the future detection of reflectance biosignatures on exoplanets.