Spectral reflectance (0.35–2.50 μm) properties of minerals and organic-bearing compounds exposed to current Martian surface conditions

Mars shows unambiguous evidence of once-abundant ancient liquid surface water, driving exploration of the planet to assess habitability and the possibility of life. The global martian surface has been studied using orbital and ground-based observations, revealing mineralogical diversity and widespread evidence of mineral hydration. However, how exactly these ancient, hydrated minerals are affected by the cold, dry, and desolate surface conditions of modern Mars remains poorly understood, which limits our understanding of their original hydration state. To better address Mars surface mineralogy, mineral detectability, and hydration state, we exposed a suite of 27 hydrous and anhydrous minerals and carbonaceous materials to Mars-like surface conditions for 66 days (∼500 Pa, CO₂, ambient temperature). The mineral phases included carbonates, halides, organics, oxides, phyllosilicates (micas and phyllosilicates), sulfates, sulfides, and zeolite. They were selected based on their relevance to past detection on Mars by remote sensing instruments and their importance for habitability and potential biosignatures. The minerals were periodically characterized with reflectance spectroscopy over the 0.35 to 2.50 μm wavelength region through a sealed sapphire glass window. Dehydration occurred for some, but not all, hydrated minerals, with decreased intensity of the hydration bands observed for halloysite, hectorite, illite, kaolinite, montmorillonite, nontronite, and trona. Our data indicated that dehydration can be gradual or abrupt, but none of our samples fully dehydrated, as the 1.90 μm absorption band persisted but became shallower in the seven samples that showed spectral changes. Hydroxyl (OH), as evidenced by a 1.40 μm region absorption feature and metal-OH absorption bands in the 2.00–2.50 μm region, was largely resistant to dehydroxylation under Mars-like surface conditions, with the exception of hectorite and trona. Data from this study is integral for the robust identification and interpretation of martian surface mineralogy observed by remote sensing instruments. The dehydration processes observed in this work suggest similar processes on Mars have likely occurred, which can affect spectroscopic mineral identification based on whether or not hydration absorption features are observed on both natural and abraded surfaces. Thus, this can have major implications for global estimates of the amount of ancient, mineral-bound water that is still present on the planet.