Measurement of the Depth of Penetration of UV Photons into Mars Relevant Rock Samples to Constrain Habitability and Limits of Detection for the SHERLOC Mars 2020 Instrument

Abstract:

We report on the depth of penetration of UV photons into a suite of Mars relevant materials in order to better characterize what constitutes a habitable environment on Mars, as well as to characterize the sensitivities of the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument on the Mars 2020 payload. To date, UV transparency of these rock types have not been fully characterized, and we have performed a systematic study to better constrain the UV attenuation over a wide range of materials relevant to Mars.

At one point during the history of Mars, the surface may have been habitable, with flowing liquid water and access to food and energy sources. As surface conditions changed, it is not unreasonable to assume that life would have migrated into the protected interior of porous rocks, veins, fissures, and the subsurface as a means to protect itself from harsh surface conditions, such as the UV flux that we observe today. Given geological time, the depth that UV light penetrates into the subsurface will play a role in altering and/or effecting the preservation of organic molecule containing biosignatures. However, the extent to which various rock types can shield organic material currently is not well understood.

In addition to constraining the UV-driven habitable “zone”, the data also helps constrain the SHERLOC instrument limits of detection. SHERLOC is a deep UV fluorescence and Raman imaging instrument. This is achieved by spatially scanning a deep UV laser at 248.6 nm to stimulate fluorescence emissions and Raman scattering from the sample. Specifically, fluorescence is generated from electronic transition from aromatic organics and Raman scatter is generated from vibrational bonds from both organics and minerals. Given the excitation wavelength, and the emission/scattering wavelengths (250-350), the mineral transparency will affect the interrogation volume of analysis and thus constrain the limits of detection.

We will report on the attenuation of both Raman and Fluorescence signals through natural rock samples selected based on relevance to Martian surface mineralogy. These samples have been prepared as thin sections of known thickness from 30-1000 μm. Samples analyzed include various naturally occurring basalts, basaltic sandstones and evaporites.