Understanding the Geochemical Context for the Origin of Life in Surface Lakes on Early Earth and Mars

Abstract

The site for the origin of life on Earth remains one of the central unsolved questions in astrobiology. Among the many proposed environments capable of supporting prebiotic chemistry, surface lakes represent a compelling candidate. Through evaporative cycles, lakes can concentrate key elements necessary for the synthesis of biomolecules. For this reason, ancient lakes are also a prime target in the search for life beyond Earth. NASA’s Perseverance rover was sent to an ancient lakebed on Mars to look for signs of ancient life.To refine our understanding of these environments, this dissertation investigates the geochemistry of surface lake environments on early Earth and Mars. First, I examine the most phosphate-rich lakes on Earth, Last Chance Lake and Goodenough Lake in British Columbia, as possible origin-of-life analog environments. Given that phosphorus is an essential element in the formation of phospholipid membranes, nucleotides, and ATP, its concentration is a vital prerequisite for the origin of life. However, until recently, the mechanisms that allow phosphate to concentrate in natural waters have been poorly understood. As part of my thesis, I investigate these phosphate-rich soda lakes to understand how carbonate mineral formation and aqueous speciation enables the accumulation of phosphate by the preferential removal of Ca,Mg, and Na-carbonate evaporites. This work provides the first quantification of vertical, lateral, and seasonal mineralogic gradients in these unique soda lakes. After investigating origin-of-life environments on early Earth, I then turn to Mars. Using the Planetary Instrument for X-ray Lithochemistry onboard the Perseverance rover, I analyze geochemical textures to understand the aqueous alteration that has affected Jezero crater. First, I identify rare manganese alteration minerals in the crater floor that record multiple distinct aqueous episodes transforming the crater over time. This analysis also marks the first observation of despujolsite on Mars, a mineral that typically forms in evaporative alkaline lake settings or acidic hydrothermal environments, both of which could have supported life on ancient Mars. Finally, I investigate the Margin unit, a carbonate-bearing region at the edge of Jezero crater interpreted as a possible ancient shoreline. With micron-scale PIXL maps, I constrain the possible formation mechanisms of the carbonates and widespread silica cement to be low-temperature, incomplete serpentinization of an olivine-rich detrital framework followed by episodes of silicification and carbonation. Together, this work refines our understanding of surface lakes as potential environments for the origin of life on early Earth and Mars.