Attenuation of Ultraviolet Radiation in Rocks and Minerals: Implications for Mars Science. Issue 10 (25th October 2019)

• Record Type: Journal Article
• Authors: Carrier, B. L.; Abbey, W. J.; Beegle, L. W.; Bhartia, R.; Liu, Y.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 10(2019)
• Page Start: 2599
• Page End: 2612
• Publication Date: 2019-10-25
• Subjects: Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2018JE005758
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The effects of radiation on the survivability of key biosignatures are a driving factor in exploration strategies throughout the solar system. Ultraviolet (UV) radiation, especially shorter wavelength UVC radiation, is known to be damaging to organisms and to potential organic biosignatures; however, the interaction of UV radiation with minerals and rocks is not well understood. Constraining the survivability of organics and generation of habitable zones requires assessment of physical parameters such as penetration depth of UV photons. This type of information helps to identify to what extent rocks and minerals can provide effective shielding against UV radiation and is especially important on Mars, where the surface chemistry is more oxidizing, and the radiation environment is more extreme than on Earth. Using pressed pellets of natural gypsum, kaolinite, Mars simulant basalt, and welded tuff, we measured the spectral transmittance of each in the wavelength range of 220–400 nm. Although transmittance drops off quickly with depth, detectable levels of UV can penetrate >500 μm in each material. Each substrate allowed higher transmittance of UVC radiation than of longer wavelength UVA/B radiation, possibly as a result of surface reflectance and internal scattering properties. This could result in increased subsurface photolysis of organic compounds and biosignatures. We have used the transmittance data collected herein to constrain the lifetimes of several organic molecules in the Martian subsurface. These results will also have implications for organic analyses to be conducted by Mars 2020 and could be used to better constrain the SHERLOC/Mars 2020 interrogation volume. Plain Language Summary: Some types of radiation, including ultraviolet (UV) radiation, can be damaging to organisms and to organic molecules which could potentially be used as evidence of life. For this reason it is important to understand how rocks and minerals can provide protection against these types of radiation. In this study we prepared pellets of varying thickness made of different types of rocks and minerals relevant to Mars exploration and measured the amount of UV light that was able to pass through them. These data were used in combination with knowledge of the UV environment on Mars in order to determine what the UV radiation dosage would be under various thicknesses of the rocks and minerals we had analyzed. The percentage of UV radiation that was able to pass through the rock and mineral pellets was found to be higher than we expected, which suggests that this type of damaging radiation would be able to penetrate further into rock and mineral environments. This information was then used to determine approximately how long different types of organic molecules could be expected to survive if buried under different depths in rock and mineral materials. Key Points: The interaction of UV photons with rocks and minerals has implications for habitability as well as organic biosignature preservation and detection Transmittance of UV radiation has been determined for four types of natural rocks and minerals that are relevant for Mars exploration UV photons can penetrate to depths greater than anticipated, resulting in a large effective radiation dosage over geological timescales