Impact of UV radiation on the Raman signal of cystine: implications for the detection of S-rich organics on Mars.
Abstract
A unique record of ancient biological processes is possibly preserved in the subsurface of Mars in the form of molecular fossils. This is why both NASA and ESA-Roscosmos are about to send rovers on Mars to search for ancient traces of life likely under the form of organic compounds. For the first time, these rovers will carry Raman instruments that will allow detecting organics on Mars. Diverse pyrolysis products, including S-rich organics were recently detected by SAM onboard Curiosity, confirming the presence of organic compounds in Martian soils. Yet, the surface of Mars is continuously exposed to intense UV radiation detrimental to the preservation of organics. Here, we conducted laboratory experiments to document the impact of UV radiation on the Raman and fluorescence signals of S-rich organic compounds. We also used Fourier-transform infrared (FTIR) and electron spin resonance (ESR) spectroscopies to evaluate the evolution of the chemical structure and the radical content of cystine due to exposure to UV. Results evidence that UV radiation is directly responsible for the increase of disorder and for the creation of electronic defects and radicals within the molecular structure of S-rich organics such as cystine, increasing the contribution of light diffusion processes to the Raman signal. Extrapolation of the present results to long time periods suggests that UV radiation would ultimately be responsible for the total degradation of the Raman signal of cystine, preventing its detection. Yet, because the degradation induced by UV is not instantaneous, it should be possible to detect freshly excavated S-rich organics such as cystine using the Raman instruments onboard the rovers that will soon be exploring Mars. Alternatively, given the very short lifetime of organic fluorescence (nanoseconds) compared to most mineral luminescence (micro to milliseconds), exploiting fluorescence signals might allow the detection of S-rich organics on Mars.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMP027.0006M
- Keywords:
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- 6225 Mars;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6297 Instruments and techniques;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS