Ageing of organic materials at the surface of Mars: A Raman study aboard Perseverance
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Keywords: Mars, organics, UV radiation, Raman, biosignatures
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 Figure 1 (a) Image of the Perseverance rover showing the location of the Ertalyte target and RMI images of the Ertalyte target after 12 and 996 sols since the landing of Perseverance on Mars. (b) Schematic molecular structure of the Ertalyte polymer. (c) Raman spectrum of the Ertalyte target collected using the customised time-resolved Raman spectrometer built by the Cellule Projet @ IMPMC. |  Figure 2 (a) Evolution of the color of the Ertalyte target as a function of the time spent on Mars (sol number). (b) SuperCam Raman spectra (normalised to the total signal) collected on the Ertalyte target every 40 to 70 sols since the landing of Perseverance. The colour code mimics the true colour evolution of the Ertalyte target from sol 026 to sol 996. |  Figure 3 (a) Evolution of the area of the band at 1613 cm−1 as a function of the time spent on Mars (sol number). (b) Air temperature measured at 1.45 m in Jezero crater from sol 020 to sol 400 (from Munguira et al., 2023). (c) Temperature of the primary mirror at the time of SuperCam measurements from sol 500 to sol 1000. |  Figure 4 (a) Time-resolved Raman spectra (normalised to the total signal) collected using the customised time-resolved Raman spectrometer built by the Cellule Projet @ IMPMC on the spare Ertalyte target before and after exposure to UV radiation in the lab for 100, 370, 1370 and 2640 minutes. (b) Evolution of the area of the band at 1613 cm−1 as a function of the irradiation duration. |
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Letter
The Perseverance rover was sent to Jezero crater on Mars to explore an astrobiologically relevant site, characterise its biosignature preservation potential, and collect rock samples to be returned to Earth (Farley et al., 2020
Farley, K.A. Williford, K.H., Stack, K.M., Bhartia, R., Chen, A. et al. (2020) Mars 2020 Mission Overview. Space Science Reviews 216, 142. https://doi.org/10.1007/s11214-020-00762-y
; Haltigin et al., 2022Haltigin, T., Hauber, E., Kminek, G., Meyer, M.A., Agee, C.B. et al. (2022) Rationale and Proposed Design for a Mars Sample Return (MSR) Science Program. Astrobiology 22, S-27–S-56. https://doi.org/10.1089/ast.2021.0122
). Determining the nature and the origin (biogenic or abiotic) of the organic compounds possibly trapped in these rocks will be required to assess whether or not there has been life on Mars (Viennet et al., 2019Viennet, J.-C., Bernard, S., Le Guillou, C., Jacquemot, P., Balan, E., Delbes, L., Rigaud, B., Georgelin, T., Jaber, M. (2019) Experimental clues for detecting biosignatures on Mars. Geochemical Perspectives Letters 12, 28–33. https://doi.org/10.7185/geochemlet.1931
; Bosak et al., 2021Bosak, T., Moore, K.R., Gong, J., Grotzinger, J.P. (2021) Searching for biosignatures in sedimentary rocks from early Earth and Mars. Nature Reviews Earth & Environment 2, 490–506. https://doi.org/10.1038/s43017-021-00169-5
; McMahon and Cosmidis, 2021McMahon, S., Cosmidis, J. (2021) False biosignatures on Mars: anticipating ambiguity. Journal of the Geological Society 179, jgs2021-050. https://doi.org/10.1144/jgs2021-050
; Ansari, 2023Ansari, A.H. (2023) Detection of organic matter on Mars, results from various Mars missions, challenges, and future strategy: A review. Frontiers in Astronomy and Space Sciences 10, 1075052. https://doi.org/10.3389/fspas.2023.1075052
; Criouet et al., 2023Criouet, I., Viennet, C.-J., Balan, E., Baron, F., Buch, A. et al. (2023) Experimental investigations of the preservation/degradation of microbial signatures in the presence of clay minerals under Martian subsurface conditions. Icarus 406, 115743. https://doi.org/10.1016/j.icarus.2023.115743
). Yet, the thin CO2 atmosphere of Mars and the lack of a magnetic field expose the Martian surface to high doses of electromagnetic (UV and γ) and particle (protons, neutrons and high Z atoms) radiation (Patel et al., 2002Patel, M.R., Zarnecki, J.C., Catling, D.C. (2002) Ultraviolet radiation on the surface of Mars and the Beagle 2 UV sensor. Planetary and Space Science 50, 915–927. https://doi.org/10.1016/S0032-0633(02)00067-3
; Hassler et al., 2014Hassler, D.M., Zeitlin, C., Wimmer-Schweingruber, R.F., Ehresmann, B., Rafkin, S. et al. (2014) Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover. Science 343, 1244797–1244797. https://doi.org/10.1126/science.1244797
), which may chemically and structurally alter organic compounds. With a present day Martian UV flux similar to that of early Earth (Cockell et al., 2000Cockell, C. et al. (2000) The Ultraviolet Environment of Mars: Biological Implications Past, Present, and Future. Icarus 146, 343–359. https://doi.org/10.1006/icar.2000.6393
), a number of studies have investigated the impact of UV exposure, mainly relying on gas chromatography mass spectrometry (GCMS) and/or Fourier transform infrared spectroscopy (FTIR) to monitor the evolution of organic compounds (e.g., Fornaro et al., 2018Fornaro, T., Steele, A., Brucato, J. (2018) Catalytic/Protective Properties of Martian Minerals and Implications for Possible Origin of Life on Mars. Life 8, 56. https://doi.org/10.3390/life8040056
). These studies have shown that even short exposure to UV (corresponding to less than hundreds of sols on Mars) leads to the photodecomposition of organic compounds, such as amino acids (ten Kate et al., 2005ten Kate, I.L., Garry, J.R.C., Peeters, Z., Quinn, R., Foing, B., Ehrenfreund, P. (2005) Amino acid photostability on the Martian surface. Meteoritics & Planetary Science 40, 1185–1193. https://doi.org/10.1111/j.1945-5100.2005.tb00183.x
), carboxylic acids (Stalport et al., 2009Stalport, F., Coll, P., Szopa, C., Cottin, H., Raulin, F. (2009) Investigating the Photostability of Carboxylic Acids Exposed to Mars Surface Ultraviolet Radiation Conditions. Astrobiology 9, 543–549. https://doi.org/10.1089/ast.2008.0300
), nucleobases (Fornaro et al., 2013Fornaro, T., Brucato, J.R., Pace, E., Guidi, M.C., Branciamore, S., Pucci, A. (2013) Infrared spectral investigations of UV irradiated nucleobases adsorbed on mineral surfaces. Icarus 226, 1068–1085. https://doi.org/10.1016/j.icarus.2013.07.024
), urea (Poch et al., 2014Poch, O., Kaci, S., Stalport, F., Szopa, C., Coll, P. (2014) Laboratory insights into the chemical and kinetic evolution of several organic molecules under simulated Mars surface UV radiation conditions. Icarus 242, 50–63. https://doi.org/10.1016/j.icarus.2014.07.014
) or PAHs (Stalport et al., 2019Stalport, F., Rouquette, L., Poch, O., Dequaire, T., Chaouche-Mechidal, N., et al. (2019) The Photochemistry on Space Station (PSS) Experiment: Organic Matter under Mars-like Surface UV Radiation Conditions in Low Earth Orbit. Astrobiology 19, 1037–1052. https://doi.org/10.1089/ast.2018.2001
). Such photodecomposition is generally interpreted as the result of UV-driven, Fenton-like reactions producing radicals, as predicted by Benner et al. (2000)Benner, S.A., Devine, K.G., Matveeva, L.N., Powell, D.H. (2000) The missing organic molecules on Mars. Proceedings of the National Academy of Sciences 97, 2425–2430. https://doi.org/10.1073/pnas.040539497
.Raman spectroscopy has recently become available on Mars with Perseverance carrying both the SuperCam time-resolved Raman (Maurice et al., 2021
Maurice, S., Wiens, R. C., Bernardi, P., Cais, P., Robinson, S., et al. (2021) The SuperCam Instrument Suite on the Mars 2020 Rover: Science Objectives and Mast-Unit Description. Space Science Reviews 217, 1–108. https://doi.org/10.1007/s11214-021-00807-w
; Wiens et al., 2021Wiens, R.C., Maurice, S., Robinson, S.H., Nelson, A.E., Cais, P. et al. (2021) The SuperCam Instrument Suite on the NASA Mars 2020 Rover: Body Unit and Combined System Tests. Space Science Reviews 217. https://doi.org/10.1007/s11214-020-00777-5
) and the deep UV SHERLOC Raman (Bhartia et al., 2021Bhartia, R., Beegle, L.W., DeFlores, L., Abbey, W., Razzell Hollis J. et al. (2021) Perseverance’s Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) Investigation. Space Science Reviews 217, 58. https://doi.org/10.1007/s11214-021-00812-z
) spectrometers. This has motivated specific experimental studies to document the effect of UV radiation on the Raman signals of organics and minerals (Megevand et al., 2021Megevand, V., Viennet, J. C., Balan, E., Gauthier, M., Rosier, P., et al. (2021) Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars. Astrobiology 21, 566–574. https://doi.org/10.1089/ast.2020.2340
; Fox et al., 2023Fox, A.C., Jakubek, R.S., Eigenbrode, J.L. (2023) Changes in the Raman and Fluorescence Spectroscopic Signatures of Irradiated Organic‐Mineral Mixtures: Implications for Molecular Biosignature Detection on Mars. Journal of Geophysical Research: Planets 128, e2022JE007624. https://doi.org/10.1029/2022JE007624
; Clavé et al., 2024Clavé, E., Beyssac, O., Bernard, S., Royer, C., Lopez-Reyes, G. et al. (2024) Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance. Scientific Reports 14, 11284. https://doi.org/10.1038/s41598-024-61494-5
; Royer et al., 2024Royer, C., Bernard, S., Beyssac, O., Balan, E., Forni, O., Gauthier, M., Morand, M., Garino, Y., Rosier, P. (2024) Impact of UV radiation on the Raman and infrared spectral signatures of sulfates, phosphates and carbonates: Implications for Mars exploration. Icarus 410, 115894. https://doi.org/10.1016/j.icarus.2023.115894
). For instance, combining time-resolved Raman, FTIR and ESR (electron spin resonance) spectroscopies, Megevand et al. (2021)Megevand, V., Viennet, J. C., Balan, E., Gauthier, M., Rosier, P., et al. (2021) Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars. Astrobiology 21, 566–574. https://doi.org/10.1089/ast.2020.2340
showed that exposure to UV leads to the increase of structural disorder and to the creation of electronic defects and/or radicals within the molecular structure of S-rich organic compounds, such as cystine. Yet, such experimental studies may not have completely mimicked Martian conditions. In fact, results of experiments conducted both on Mars and in the laboratory on Earth showed that the Raman signal of the Apatite (TAPAG) SuperCam calibration target changed faster in the laboratory than on Mars when exposed to equivalent doses of UV (Clavé et al., 2024Clavé, E., Beyssac, O., Bernard, S., Royer, C., Lopez-Reyes, G. et al. (2024) Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance. Scientific Reports 14, 11284. https://doi.org/10.1038/s41598-024-61494-5
). This is possibly a result of relaxation periods absent in the laboratory but existing on Mars (periods of shades, day/night cycles), and/or from differences in pressure, temperature, humidity or radiation conditions (Clavé et al., 2024Clavé, E., Beyssac, O., Bernard, S., Royer, C., Lopez-Reyes, G. et al. (2024) Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance. Scientific Reports 14, 11284. https://doi.org/10.1038/s41598-024-61494-5
). A similar study conducted on organic materials exposed to actual Martian conditions, i.e. on board Perseverance, thus appears necessary to properly determine what would/should be expected to be found in Martian rocks using Raman spectroscopy.One of the onboard SuperCam calibration targets (Manrique et al., 2020
Manrique, J.A., Lopez-Reyes, G., Cousin, A., Rull, F., Maurice, S., et al. (2020) SuperCam Calibration Targets: Design and Development. Space Science Reviews 216, 1–27. https://doi.org/10.1007/s11214-020-00764-w
; Cousin et al., 2022Cousin, A., Sautter, V., Fabre, C., Dromart, G., Montagnac, G. et al. (2022) SuperCam calibration targets on board the perseverance rover: Fabrication and quantitative characterization. Spectrochimica Acta Part B: Atomic Spectroscopy 188, 106341. https://doi.org/10.1016/j.sab.2021.106341
) is a 100 % organic target made of polyethylene terephthalate (PET - (C10H8O4)n, a.k.a. the Ertalyte® target; Fig. 1), which has allowed conducting a 1000 sols long ageing experiment under actual Martian conditions. This target is made of a thermoplastic polymer intrinsically resistant to UV and exhibiting a chemical structure including aromatic, aliphatic and ester/carboxylic functional groups (Fig. 1). Such molecular groups are expected to be found in biogenic organic compounds trapped in ancient rocks (Bernard et al., 2021Bernard, S., Criouet, I., Alleon, J. (2021) Recognizing Archean Traces of Life: Difficulties and Perspectives. Encyclopedia of Geology. Elsevier, 211–219. https://doi.org/10.1016/B978-0-08-102908-4.00184-3
). The Ertalyte target carried by Perseverance is 100 % crystalline and was machined at IMPMC in Paris out of an Ertalyte® rod provided by Mitsubishi Chemical Advanced Materials. The surface roughness was reduced through gentle polishing, making it the same size as the other SuperCam targets (1 cm diameter, 5 mm thickness). The Raman signal of the Ertalyte target exhibits a number of intense Raman bands (Fig. 1), at 633 cm−1 (ring mode), 859 cm−1 (ring C-C and C(O)-O), 998 cm−1 (C-C), 1095 cm−1 (ring C-C, C(O)-O and C-C), 1180 cm−1 (C-H and C-C), 1288 cm−1 (C(O)-O), 1460 cm−1 (C-H), 1613 cm−1 (ring mode), 1727 cm−1 (C=O), 2966 cm−1 (C-H) and 3083 cm−1 (O-H) (Manrique et al., 2020Manrique, J.A., Lopez-Reyes, G., Cousin, A., Rull, F., Maurice, S., et al. (2020) SuperCam Calibration Targets: Design and Development. Space Science Reviews 216, 1–27. https://doi.org/10.1007/s11214-020-00764-w
; Cousin et al., 2022Cousin, A., Sautter, V., Fabre, C., Dromart, G., Montagnac, G. et al. (2022) SuperCam calibration targets on board the perseverance rover: Fabrication and quantitative characterization. Spectrochimica Acta Part B: Atomic Spectroscopy 188, 106341. https://doi.org/10.1016/j.sab.2021.106341
).Figure 1 (a) Image of the Perseverance rover showing the location of the Ertalyte target and RMI images of the Ertalyte target after 12 and 996 sols since the landing of Perseverance on Mars. (b) Schematic molecular structure of the Ertalyte polymer. (c) Raman spectrum of the Ertalyte target collected using the customised time-resolved Raman spectrometer built by the Cellule Projet @ IMPMC.
On Mars, the Ertalyte target has been systematically imaged using the SuperCam Remote Micro-Imager (RMI), and analysed using the SuperCam Raman every 40 to 70 sols from sol 26 (after the landing of Perseverance on Feb 18th, 2021) up to sol 996, providing a consistent set of data over almost 1000 sols. Briefly, the SuperCam Raman relies on a pulsed Nd:YAG laser and a frequency doubler to produce a 532 nm pulsed green beam collimated towards the sample. A repetition rate of 10 Hz is used with 9 mJ per pulse deposited on a ∼1 cm diameter spot, yielding an irradiance at the sample surface ranging from 1.1010 to 5.1010 W.m−2, i.e. well below the threshold for laser-induced damage (Fau et al., 2019
Fau, A., Beyssac, O., Gauthier, M., Meslin, P.Y., Cousin, A. et al. (2019) Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy. Spectrochimica Acta Part B: Atomic Spectroscopy 160, 105687. https://doi.org/10.1016/j.sab.2019.105687
). The transmission spectrometer has a compact design including diffraction gratings, an intensifier, and relay optics that focus light onto a CCD, yielding a spectral resolution of about 10–12 cm−1 for the Raman window, ranging from 165 to 7065 cm−1. Each spectrum shown in the present study corresponds to the accumulation of 100 laser shots collected using a gate of 100 ns, with the laser powered at 110 A and an intensifier gain of 3200. Of note, given the variability of the total signal collected for each shot, spectra have been normalised to the total signal received on the spectrometers (Figs. S-1 and S-2). SuperCam VIS spectra of the Ertalyte target were also collected every 80 to 100 sols (Fig. S-3). In addition, the Raman spectrum of the sample holder paint has been measured on the same sols from sol 450 to serve as a reference insensitive to UV radiation (the sample holder paint has not degraded over time; Fig. S-4). The strong variability in the total signal collected on the sample holder paint from one sol to another is instrumental as it is directly correlated to the temperature of the primary mirror of the SuperCam telescope, which is directly related to the temperature of the laser (Fig. S-5).In parallel, irradiation experiments were conducted in the laboratory on Earth, in a dedicated Martian chamber and using a customised time-resolved Raman spectrometer, both built by the Cellule Projet @ IMPMC (Megevand et al., 2021
Megevand, V., Viennet, J. C., Balan, E., Gauthier, M., Rosier, P., et al. (2021) Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars. Astrobiology 21, 566–574. https://doi.org/10.1089/ast.2020.2340
; Clavé et al., 2024Clavé, E., Beyssac, O., Bernard, S., Royer, C., Lopez-Reyes, G. et al. (2024) Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance. Scientific Reports 14, 11284. https://doi.org/10.1038/s41598-024-61494-5
; Royer et al., 2024Royer, C., Bernard, S., Beyssac, O., Balan, E., Forni, O., Gauthier, M., Morand, M., Garino, Y., Rosier, P. (2024) Impact of UV radiation on the Raman and infrared spectral signatures of sulfates, phosphates and carbonates: Implications for Mars exploration. Icarus 410, 115894. https://doi.org/10.1016/j.icarus.2023.115894
). In this chamber, a spare Ertalyte target was exposed to UV radiation at 0 °C under a primary vacuum (<1 mbar). The UV flux was produced by a 150 W arc lamp equipped with a high pressure Xenon bulb (UXL-150SP - LOT-ORIEL), delivering UV with a pattern similar to that of the Martian surface radiation spectrum (Royer et al., 2024Royer, C., Bernard, S., Beyssac, O., Balan, E., Forni, O., Gauthier, M., Morand, M., Garino, Y., Rosier, P. (2024) Impact of UV radiation on the Raman and infrared spectral signatures of sulfates, phosphates and carbonates: Implications for Mars exploration. Icarus 410, 115894. https://doi.org/10.1016/j.icarus.2023.115894
), a DUV Grade Fused Silica window allowing for rejection of most of the IR radiation (down to 2.89 μm). Time-resolved Raman spectra of the pristine and irradiated Ertalyte target were measured directly in the Martian chamber (from 200 to 2000 cm−1 with a spectral resolution of about 10–12 cm−1) after 100, 370, 1370 and 2640 minutes. Each spectrum corresponds to the accumulation of 106 laser shots collected using a 2 ns ICCD gate centred on the 1.2 ns laser pulse. The irradiance of the laser, distributed over a spot of ∼6 mm in diameter at the surface of the Ertalyte target, did not exceed 1010 W.m-2, thereby preventing laser-induced damage (Fau et al., 2019Fau, A., Beyssac, O., Gauthier, M., Meslin, P.Y., Cousin, A. et al. (2019) Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy. Spectrochimica Acta Part B: Atomic Spectroscopy 160, 105687. https://doi.org/10.1016/j.sab.2019.105687
). To describe more quantitatively the evolution of the Raman signals of the Ertalyte irradiated on Mars and in the laboratory on Earth, the main Raman bands (at 1613 and 1727 cm−1) were fitted (after normalisation to the total signal) with pure Gaussian functions (with the background classically modelled by a linear function in the range 1550–1880 cm−1) using the SuperCam Display Spectra 4.0 developed by the SuperCam team at IRAP.While white at the time of landing, the Ertalyte target has turned brown with time as revealed by the RMI images (Fig. 1). White balancing the RMIs using the sample holder paint, a clear browning of the Ertalyte target with time can be observed (Fig. 2), consistent with the evolution of its VIS signal (Fig. S-3). The Raman spectrum of the Ertalyte target has also changed with time (Fig. 2), with a modification of the shape of the background and a shift of its maximum from 1500 to 2000 cm−1. No new peak was detected, suggesting that no new Raman active compounds were produced, or were in quantities insufficient to be detectable. Of note, the Raman spectrum of the Ertalyte target still exhibits all the features observed in the spectrum of pristine Ertalyte, even after 996 sols on Mars. This indicates that a certain volume of pristine Ertalyte is still contributing to the signal. Still, the contribution of the Raman signal to the total signal has decreased by a factor of 5 over the first 500 sols (Fig. 2). At first sight, the evolution of the area of the band at 1613 cm−1 (normalised to the total signal) follows a first order logarithmic law with time (Fig. 3). Yet, a closer look reveals that three stages of evolution can be identified: an early stage up to sol 280, a second stage from sol 280 to sol 480 and a last stage starting from sol 480 (Fig. 3).
Figure 2 (a) Evolution of the color of the Ertalyte target as a function of the time spent on Mars (sol number). (b) SuperCam Raman spectra (normalised to the total signal) collected on the Ertalyte target every 40 to 70 sols since the landing of Perseverance. The colour code mimics the true colour evolution of the Ertalyte target from sol 026 to sol 996.
Figure 3 (a) Evolution of the area of the band at 1613 cm−1 as a function of the time spent on Mars (sol number). (b) Air temperature measured at 1.45 m in Jezero crater from sol 020 to sol 400 (from Munguira et al., 2023
Munguira, A., Hueso, R., Sánchez‐Lavega, A., De La Torre‐Juarez, M., Martínez, G. M., et al. (2023) Near Surface Atmospheric Temperatures at Jezero From Mars 2020 MEDA Measurements. Journal of Geophysical Research: Planets 128, e2022JE007559. https://doi.org/10.1029/2022JE007559
). (c) Temperature of the primary mirror at the time of SuperCam measurements from sol 500 to sol 1000.From sol 26 to sol 280, the total signal (dominated by the background) has been increasing while the contribution of the Raman signal has been decreasing (as shown by the evolution of the area of the band at 1613 cm−1 normalised to the total signal; Figs. 3 and S-6). This evolution is synchronous with the yellowing/browning of the Ertalyte target, which has likely increased the opacity of the target and limited the penetration of the laser, thereby reducing the contribution of the pristine Ertalyte to the total signal. A similar evolution of the Raman signal of the Ertalyte target is observed for irradiation experiments conducted in the lab, i.e. a decrease of the contribution of the Raman signal to the total signal with increasing duration of exposure to UV (Fig. 4). This suggests that the radiative environment on the surface of Mars can be simulated in the laboratory using UV only. Yet, kinetics are very different; the contribution of the Raman signal to the total signal has decreased by a factor of 5 over 500 sols on Mars versus over only 1270 minutes in the lab, corresponding to only a few sols assuming day/night cycles. The causes of such discrepancies may reside in the atmospheric pressure and temperature conditions, higher temperature enhancing the effect of exposure to UV (François-Heude et al., 2014
François-Heude, A., Richaud, E., Desnoux, E., Colin, X. (2014) Influence of temperature, UV-light wavelength and intensity on polypropylene photothermal oxidation. Polymer Degradation and Stability 100, 10–20. https://doi.org/10.1016/j.polymdegradstab.2013.12.038
; Gogotov and Barazov, 2014Gogotov, I.N., Barazov, S.Kh. (2014) The Effect of Ultraviolet Light and Temperature on the Degradation of Composite Polypropylene. International Polymer Science and Technology 41, 55–58. https://doi.org/10.1177/0307174X1404100313
), or in some sort of relaxation occurring during periods of non-exposure to UV that remains to be investigated (Clavé et al., 2024Clavé, E., Beyssac, O., Bernard, S., Royer, C., Lopez-Reyes, G. et al. (2024) Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance. Scientific Reports 14, 11284. https://doi.org/10.1038/s41598-024-61494-5
).Figure 4 (a) Time-resolved Raman spectra (normalised to the total signal) collected using the customised time-resolved Raman spectrometer built by the Cellule Projet @ IMPMC on the spare Ertalyte target before and after exposure to UV radiation in the lab for 100, 370, 1370 and 2640 minutes. (b) Evolution of the area of the band at 1613 cm−1 as a function of the irradiation duration.
Mechanistically, such evolutions of the colour and the Raman signal of the Ertalyte target can be explained by the UV-induced production of electronic defects and/or radicals (Grossetête et al., 2000
Grossetête, T., Rivaton, A., Gardette, J.L., Hoyle, C.E., Ziemer, M., Fagerburg, D.R., Clauberg, H. (2000) Photochemical degradation of poly(ethylene terephthalate)-modified copolymer. Polymer 41, 3541–3554. https://doi.org/10.1016/S0032-3861(99)00580-7
; Sang et al., 2020Sang, T., Wallis, C.J., Hill, G., Britovsek, G.J.P. (2020) Polyethylene terephthalate degradation under natural and accelerated weathering conditions. European Polymer Journal 136, 109873. https://doi.org/10.1016/j.eurpolymj.2020.109873
; Jamalzadeh and Sobkowicz, 2022Jamalzadeh, M., Sobkowicz, M.J. (2022) Review of the effects of irradiation treatments on poly(ethylene terephthalate). Polymer Degradation and Stability 206, 110191. https://doi.org/10.1016/j.polymdegradstab.2022.110191
; Rostampour et al., 2024Rostampour, S., Cook, R., Jhang, S.-S., Li, Y., Fan, C., Sung, L.-P. (2024) Changes in the Chemical Composition of Polyethylene Terephthalate under UV Radiation in Various Environmental Conditions. Polymers 16, 2249. https://doi.org/10.3390/polym16162249
). In fact, UV exposure of polymers has been shown to induce Norrish type I and type II reactions producing radicals via chain scission (Grossetête et al., 2000Grossetête, T., Rivaton, A., Gardette, J.L., Hoyle, C.E., Ziemer, M., Fagerburg, D.R., Clauberg, H. (2000) Photochemical degradation of poly(ethylene terephthalate)-modified copolymer. Polymer 41, 3541–3554. https://doi.org/10.1016/S0032-3861(99)00580-7
; Sang et al., 2020Sang, T., Wallis, C.J., Hill, G., Britovsek, G.J.P. (2020) Polyethylene terephthalate degradation under natural and accelerated weathering conditions. European Polymer Journal 136, 109873. https://doi.org/10.1016/j.eurpolymj.2020.109873
; Jamalzadeh and Sobkowicz, 2022Jamalzadeh, M., Sobkowicz, M.J. (2022) Review of the effects of irradiation treatments on poly(ethylene terephthalate). Polymer Degradation and Stability 206, 110191. https://doi.org/10.1016/j.polymdegradstab.2022.110191
; Rostampour et al., 2024Rostampour, S., Cook, R., Jhang, S.-S., Li, Y., Fan, C., Sung, L.-P. (2024) Changes in the Chemical Composition of Polyethylene Terephthalate under UV Radiation in Various Environmental Conditions. Polymers 16, 2249. https://doi.org/10.3390/polym16162249
). Norrish type I reactions mainly involve the production of radicals via ester bond cleavage, whereas Norrish type II reactions involve hydrogen atom abstraction via photolysis, eventually inducing a cyclisation process (Grossetête et al., 2000Grossetête, T., Rivaton, A., Gardette, J.L., Hoyle, C.E., Ziemer, M., Fagerburg, D.R., Clauberg, H. (2000) Photochemical degradation of poly(ethylene terephthalate)-modified copolymer. Polymer 41, 3541–3554. https://doi.org/10.1016/S0032-3861(99)00580-7
; Sang et al., 2020Sang, T., Wallis, C.J., Hill, G., Britovsek, G.J.P. (2020) Polyethylene terephthalate degradation under natural and accelerated weathering conditions. European Polymer Journal 136, 109873. https://doi.org/10.1016/j.eurpolymj.2020.109873
; Jamalzadeh and Sobkowicz, 2022Jamalzadeh, M., Sobkowicz, M.J. (2022) Review of the effects of irradiation treatments on poly(ethylene terephthalate). Polymer Degradation and Stability 206, 110191. https://doi.org/10.1016/j.polymdegradstab.2022.110191
; Rostampour et al., 2024Rostampour, S., Cook, R., Jhang, S.-S., Li, Y., Fan, C., Sung, L.-P. (2024) Changes in the Chemical Composition of Polyethylene Terephthalate under UV Radiation in Various Environmental Conditions. Polymers 16, 2249. https://doi.org/10.3390/polym16162249
). Of note, a similar evolution of the colour and the Raman signal was also observed in the laboratory for cystine upon exposure to UV and interpreted as resulting from the creation of electronic defects and/or radicals acting as coloured centres (Megevand et al., 2021Megevand, V., Viennet, J. C., Balan, E., Gauthier, M., Rosier, P., et al. (2021) Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars. Astrobiology 21, 566–574. https://doi.org/10.1089/ast.2020.2340
). Such an increasing concentration in electronic defects and/or radicals upon exposure to UV explains the yellowing/browning of the Ertalyte target and the evolution of its Raman signal. Consistently, electron spin resonance (ESR) data, collected on Ertalyte irradiated for 2640 minutes, show a new population of defects/radicals absent from the pristine Ertalyte (Fig. S-7).From sol 290 to sol 480, the data collected on the Ertalyte target define a new stage of evolution. In contrast to the previous stage, the total signal has remained rather constant from sol 280 to sol 480 (Fig. S-6), while the contribution of the Raman signal to the total signal has decreased even more than during the first stage (Fig. 3). This could be an instrumental effect; the temperature of the primary mirror (which is directly related to the temperature of the laser) was rather low on sols 395, 439 and 479, and lower signal is generally collected in such cases (Fig. S-6). Yet, this was not the case for sols 281 and 341 (Fig. S-6). With higher temperature conditions enhancing the effect of exposure to UV (François-Heude et al., 2014
François-Heude, A., Richaud, E., Desnoux, E., Colin, X. (2014) Influence of temperature, UV-light wavelength and intensity on polypropylene photothermal oxidation. Polymer Degradation and Stability 100, 10–20. https://doi.org/10.1016/j.polymdegradstab.2013.12.038
; Gogotov and Barazov, 2014Gogotov, I.N., Barazov, S.Kh. (2014) The Effect of Ultraviolet Light and Temperature on the Degradation of Composite Polypropylene. International Polymer Science and Technology 41, 55–58. https://doi.org/10.1177/0307174X1404100313
), a more likely explanation would be related to the evolution of the mean daily temperature. In fact, air temperature has increased on sol 280 compared to the first several hundred sols, from ∼ 240 K to ∼ 255 K, due to the beginning of the warm season with Mars approaching its perihelion (Fig. 3; Munguira et al., 2023Munguira, A., Hueso, R., Sánchez‐Lavega, A., De La Torre‐Juarez, M., Martínez, G. M., et al. (2023) Near Surface Atmospheric Temperatures at Jezero From Mars 2020 MEDA Measurements. Journal of Geophysical Research: Planets 128, e2022JE007559. https://doi.org/10.1029/2022JE007559
). Such increase in temperature has likely triggered a second stage of UV-induced defect/radical creation, further limiting the contribution of pristine Ertalyte to the total signal. Note that this is definitely not a temperature-related instrumental effect, since higher temperatures lead to higher signals, not the opposite, as demonstrated in the laboratory (Fig. S-8).Lastly, the Raman signals collected on the Ertalyte target observed during the third stage (from sol 480 to sol 1000) exhibit a significant variability that is also observed for the signal collected on the sample holder paint on the same sols and seems correlated with the temperature of the primary mirror (Fig. 2 and S-6). Because the relationship between the signal measured and the primary mirror temperature is not linear, it is difficult to take into account this instrumental contribution, preventing the identification of any significant evolution of the Raman signal of the Ertalyte. Instead, as supported by the halting of the browning process from sol 480, it can be assumed that the evolution of the Ertalyte has either stopped or reached a steady state. A steady state would imply that the degradation of the Ertalyte is still ongoing even after 1000 sols on Mars, with some material loss compensating for the progress of radiation damage, resulting in a constant volume of irradiated/damaged Ertalyte contributing to the signal. Determining whether the evolution of the Ertalyte has stopped or reached a steady state cannot be done from the data available.
Altogether, the good agreement between the data collected on Mars and the data collected in the laboratory on Earth suggests that using only UV can be considered as a first order approximation of the radiative environment of the surface of Mars. This means that laboratory studies may provide valuable insights into what organic materials would/should be expected to be found in Martian rocks having been exposed at the surface of Mars. Of note, a Raman signal is still collected after 1000 sols, but 1000 sols remain negligible compared to geological times and, in addition to its intrinsic resistance to UV, the Ertalyte target is a 5 mm thick, 100 % organic target, such quality and quantity not being anticipated for any organic material in Martian rocks. Plus, additional parameters may enhance the degradation of organic compounds upon exposure to UV at the surface of Mars (e.g., Wadsworth and Cockell, 2017
Wadsworth, J., Cockell, C.S. (2017) Perchlorates on Mars enhance the bacteriocidal effects of UV light. Scientific Reports 7, 4662. https://doi.org/10.1038/s41598-017-04910-3
; Fornaro et al., 2018Fornaro, T., Steele, A., Brucato, J. (2018) Catalytic/Protective Properties of Martian Minerals and Implications for Possible Origin of Life on Mars. Life 8, 56. https://doi.org/10.3390/life8040056
). Overall, the present results suggest that prolonged exposure at the surface will eventually cause the severe degradation of organic materials trapped in Martian rocks, making their detection and identification rather challenging.top
Acknowledgements
Special thanks go to the Mars 2020 Science, Engineering, Hardware and Operation Teams who allowed this work to be conducted, as well as to the members of the Cellule Projet @ IMPMC who built the Mars chamber and the customized time-resolved Raman spectrometer used in this study. Authors acknowledge support from NASA, JPL-Caltech, LANL, CNES, CNRS, SU and MNHN. SB and OB acknowledge funding from CNRS, SU and MNHN through the Emergence and ATM programs. JMM, JA and KC acknowledge funding from the Spanish Agency for Research AEI/MCIN/FEDER (MCIN/AEI/10.13039/501100011033/FEDER,UE), through the Grant PID2022-142750OB-I00. TF acknowledges funding from the ASI/INAF (Agreement 2023-3-HH). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Editor: Tanja Bosak
top
References
Ansari, A.H. (2023) Detection of organic matter on Mars, results from various Mars missions, challenges, and future strategy: A review. Frontiers in Astronomy and Space Sciences 10, 1075052. https://doi.org/10.3389/fspas.2023.1075052
Show in context Determining the nature and the origin (biogenic or abiotic) of the organic compounds possibly trapped in these rocks will be required to assess whether or not there has been life on Mars (Viennet et al., 2019; Bosak et al., 2021; McMahon and Cosmidis, 2021; Ansari, 2023; Criouet et al., 2023).
View in article
Benner, S.A., Devine, K.G., Matveeva, L.N., Powell, D.H. (2000) The missing organic molecules on Mars. Proceedings of the National Academy of Sciences 97, 2425–2430. https://doi.org/10.1073/pnas.040539497
Show in context Such photodecomposition is generally interpreted as the result of UV-driven, Fenton-like reactions producing radicals, as predicted by Benner et al. (2000).
View in article
Bernard, S., Criouet, I., Alleon, J. (2021) Recognizing Archean Traces of Life: Difficulties and Perspectives. Encyclopedia of Geology. Elsevier, 211–219. https://doi.org/10.1016/B978-0-08-102908-4.00184-3
Show in context Such molecular groups are expected to be found in biogenic organic compounds trapped in ancient rocks (Bernard et al., 2021).
View in article
Bhartia, R., Beegle, L.W., DeFlores, L., Abbey, W., Razzell Hollis J. et al. (2021) Perseverance’s Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) Investigation. Space Science Reviews 217, 58. https://doi.org/10.1007/s11214-021-00812-z
Show in context Raman spectroscopy has recently become available on Mars with Perseverance carrying both the SuperCam time-resolved Raman (Maurice et al., 2021; Wiens et al., 2021) and the deep UV SHERLOC Raman (Bhartia et al., 2021) spectrometers.
View in article
Bosak, T., Moore, K.R., Gong, J., Grotzinger, J.P. (2021) Searching for biosignatures in sedimentary rocks from early Earth and Mars. Nature Reviews Earth & Environment 2, 490–506. https://doi.org/10.1038/s43017-021-00169-5
Show in context Determining the nature and the origin (biogenic or abiotic) of the organic compounds possibly trapped in these rocks will be required to assess whether or not there has been life on Mars (Viennet et al., 2019; Bosak et al., 2021; McMahon and Cosmidis, 2021; Ansari, 2023; Criouet et al., 2023).
View in article
Clavé, E., Beyssac, O., Bernard, S., Royer, C., Lopez-Reyes, G. et al. (2024) Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance. Scientific Reports 14, 11284. https://doi.org/10.1038/s41598-024-61494-5
Show in context This has motivated specific experimental studies to document the effect of UV radiation on the Raman signals of organics and minerals (Megevand et al., 2021; Fox et al., 2023; Clavé et al., 2024; Royer et al., 2024).
View in article
In fact, results of experiments conducted both on Mars and in the laboratory on Earth showed that the Raman signal of the Apatite (TAPAG) SuperCam calibration target changed faster in the laboratory than on Mars when exposed to equivalent doses of UV (Clavé et al., 2024).
View in article
This is possibly a result of relaxation periods absent in the laboratory but existing on Mars (periods of shades, day/night cycles), and/or from differences in pressure, temperature, humidity or radiation conditions (Clavé et al., 2024).
View in article
In parallel, irradiation experiments were conducted in the laboratory on Earth, in a dedicated Martian chamber and using a customised time-resolved Raman spectrometer, both built by the Cellule Projet @ IMPMC (Megevand et al., 2021; Clavé et al., 2024; Royer et al., 2024).
View in article
The causes of such discrepancies may reside in the atmospheric pressure and temperature conditions, higher temperature enhancing the effect of exposure to UV (François-Heude et al., 2014; Gogotov and Barazov, 2014), or in some sort of relaxation occurring during periods of non-exposure to UV that remains to be investigated (Clavé et al., 2024).
View in article
Cockell, C. et al. (2000) The Ultraviolet Environment of Mars: Biological Implications Past, Present, and Future. Icarus 146, 343–359. https://doi.org/10.1006/icar.2000.6393
Show in context Yet, the thin CO2 atmosphere of Mars and the lack of a magnetic field expose the Martian surface to high doses of electromagnetic (UV and γ) and particle (protons, neutrons and high Z atoms) radiation (Patel et al., 2002; Hassler et al., 2014), which may chemically and structurally alter organic compounds. With a present day Martian UV flux similar to that of early Earth (Cockell et al., 2000), a number of studies have investigated the impact of UV exposure, mainly relying on gas chromatography mass spectrometry (GCMS) and/or Fourier transform infrared spectroscopy (FTIR) to monitor the evolution of organic compounds (e.g., Fornaro et al., 2018).
View in article
Cousin, A., Sautter, V., Fabre, C., Dromart, G., Montagnac, G. et al. (2022) SuperCam calibration targets on board the perseverance rover: Fabrication and quantitative characterization. Spectrochimica Acta Part B: Atomic Spectroscopy 188, 106341. https://doi.org/10.1016/j.sab.2021.106341
Show in context One of the onboard SuperCam calibration targets (Manrique et al., 2020; Cousin et al., 2022) is a 100 % organic target made of polyethylene terephthalate (PET - (C10H8O4)n, a.k.a. the Ertalyte® target; Fig. 1), which has allowed conducting a 1000 sols long ageing experiment under actual Martian conditions.
View in article
The Raman signal of the Ertalyte target exhibits a number of intense Raman bands (Fig. 1), at 633 cm−1 (ring mode), 859 cm−1 (ring C-C and C(O)-O), 998 cm−1 (C-C), 1095 cm−1 (ring C-C, C(O)-O and C-C), 1180 cm−1 (C-H and C-C), 1288 cm−1 (C(O)-O), 1460 cm−1 (C-H), 1613 cm−1 (ring mode), 1727 cm−1 (C=O), 2966 cm−1 (C-H) and 3083 cm−1 (O-H) (Manrique et al., 2020; Cousin et al., 2022).
View in article
Criouet, I., Viennet, C.-J., Balan, E., Baron, F., Buch, A. et al. (2023) Experimental investigations of the preservation/degradation of microbial signatures in the presence of clay minerals under Martian subsurface conditions. Icarus 406, 115743. https://doi.org/10.1016/j.icarus.2023.115743
Show in context Determining the nature and the origin (biogenic or abiotic) of the organic compounds possibly trapped in these rocks will be required to assess whether or not there has been life on Mars (Viennet et al., 2019; Bosak et al., 2021; McMahon and Cosmidis, 2021; Ansari, 2023; Criouet et al., 2023).
View in article
Farley, K.A. Williford, K.H., Stack, K.M., Bhartia, R., Chen, A. et al. (2020) Mars 2020 Mission Overview. Space Science Reviews 216, 142. https://doi.org/10.1007/s11214-020-00762-y
Show in context The Perseverance rover was sent to Jezero crater on Mars to explore an astrobiologically relevant site, characterise its biosignature preservation potential, and collect rock samples to be returned to Earth (Farley et al., 2020; Haltigin et al., 2022).
View in article
Fau, A., Beyssac, O., Gauthier, M., Meslin, P.Y., Cousin, A. et al. (2019) Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy. Spectrochimica Acta Part B: Atomic Spectroscopy 160, 105687. https://doi.org/10.1016/j.sab.2019.105687
Show in context The irradiance of the laser, distributed over a spot of ∼6 mm in diameter at the surface of the Ertalyte target, did not exceed 1010 W.m-2, thereby preventing laser-induced damage (Fau et al., 2019).
View in article
Fornaro, T., Brucato, J.R., Pace, E., Guidi, M.C., Branciamore, S., Pucci, A. (2013) Infrared spectral investigations of UV irradiated nucleobases adsorbed on mineral surfaces. Icarus 226, 1068–1085. https://doi.org/10.1016/j.icarus.2013.07.024
Show in context These studies have shown that even short exposure to UV (corresponding to less than hundreds of sols on Mars) leads to the photodecomposition of organic compounds, such as amino acids (ten Kate et al., 2005), carboxylic acids (Stalport et al., 2009), nucleobases (Fornaro et al., 2013), urea (Poch et al., 2014) or PAHs (Stalport et al., 2019).
View in article
Fornaro, T., Steele, A., Brucato, J. (2018) Catalytic/Protective Properties of Martian Minerals and Implications for Possible Origin of Life on Mars. Life 8, 56. https://doi.org/10.3390/life8040056
Show in context Yet, the thin CO2 atmosphere of Mars and the lack of a magnetic field expose the Martian surface to high doses of electromagnetic (UV and γ) and particle (protons, neutrons and high Z atoms) radiation (Patel et al., 2002; Hassler et al., 2014), which may chemically and structurally alter organic compounds. With a present day Martian UV flux similar to that of early Earth (Cockell et al., 2000), a number of studies have investigated the impact of UV exposure, mainly relying on gas chromatography mass spectrometry (GCMS) and/or Fourier transform infrared spectroscopy (FTIR) to monitor the evolution of organic compounds (e.g., Fornaro et al., 2018).
View in article
Plus, additional parameters may enhance the degradation of organic compounds upon exposure to UV at the surface of Mars (e.g., Wadsworth and Cockell, 2017; Fornaro et al., 2018).
View in article
Fox, A.C., Jakubek, R.S., Eigenbrode, J.L. (2023) Changes in the Raman and Fluorescence Spectroscopic Signatures of Irradiated Organic‐Mineral Mixtures: Implications for Molecular Biosignature Detection on Mars. Journal of Geophysical Research: Planets 128, e2022JE007624. https://doi.org/10.1029/2022JE007624
Show in context This has motivated specific experimental studies to document the effect of UV radiation on the Raman signals of organics and minerals (Megevand et al., 2021; Fox et al., 2023; Clavé et al., 2024; Royer et al., 2024).
View in article
François-Heude, A., Richaud, E., Desnoux, E., Colin, X. (2014) Influence of temperature, UV-light wavelength and intensity on polypropylene photothermal oxidation. Polymer Degradation and Stability 100, 10–20. https://doi.org/10.1016/j.polymdegradstab.2013.12.038
Show in context The causes of such discrepancies may reside in the atmospheric pressure and temperature conditions, higher temperature enhancing the effect of exposure to UV (François-Heude et al., 2014; Gogotov and Barazov, 2014), or in some sort of relaxation occurring during periods of non-exposure to UV that remains to be investigated (Clavé et al., 2024).
View in article
With higher temperature conditions enhancing the effect of exposure to UV (François-Heude et al., 2014; Gogotov and Barazov, 2014), a more likely explanation would be related to the evolution of the mean daily temperature.
View in article
Gogotov, I.N., Barazov, S.Kh. (2014) The Effect of Ultraviolet Light and Temperature on the Degradation of Composite Polypropylene. International Polymer Science and Technology 41, 55–58. https://doi.org/10.1177/0307174X1404100313
Show in context The causes of such discrepancies may reside in the atmospheric pressure and temperature conditions, higher temperature enhancing the effect of exposure to UV (François-Heude et al., 2014; Gogotov and Barazov, 2014), or in some sort of relaxation occurring during periods of non-exposure to UV that remains to be investigated (Clavé et al., 2024).
View in article
With higher temperature conditions enhancing the effect of exposure to UV (François-Heude et al., 2014; Gogotov and Barazov, 2014), a more likely explanation would be related to the evolution of the mean daily temperature.
View in article
Grossetête, T., Rivaton, A., Gardette, J.L., Hoyle, C.E., Ziemer, M., Fagerburg, D.R., Clauberg, H. (2000) Photochemical degradation of poly(ethylene terephthalate)-modified copolymer. Polymer 41, 3541–3554. https://doi.org/10.1016/S0032-3861(99)00580-7
Show in context Mechanistically, such evolutions of the colour and the Raman signal of the Ertalyte target can be explained by the UV-induced production of electronic defects and/or radicals (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
In fact, UV exposure of polymers has been shown to induce Norrish type I and type II reactions producing radicals via chain scission (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
Norrish type I reactions mainly involve the production of radicals via ester bond cleavage, whereas Norrish type II reactions involve hydrogen atom abstraction via photolysis, eventually inducing a cyclisation process (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
Haltigin, T., Hauber, E., Kminek, G., Meyer, M.A., Agee, C.B. et al. (2022) Rationale and Proposed Design for a Mars Sample Return (MSR) Science Program. Astrobiology 22, S-27–S-56. https://doi.org/10.1089/ast.2021.0122
Show in context The Perseverance rover was sent to Jezero crater on Mars to explore an astrobiologically relevant site, characterise its biosignature preservation potential, and collect rock samples to be returned to Earth (Farley et al., 2020; Haltigin et al., 2022).
View in article
Hassler, D.M., Zeitlin, C., Wimmer-Schweingruber, R.F., Ehresmann, B., Rafkin, S. et al. (2014) Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover. Science 343, 1244797–1244797. https://doi.org/10.1126/science.1244797
Show in context Yet, the thin CO2 atmosphere of Mars and the lack of a magnetic field expose the Martian surface to high doses of electromagnetic (UV and γ) and particle (protons, neutrons and high Z atoms) radiation (Patel et al., 2002; Hassler et al., 2014), which may chemically and structurally alter organic compounds. With a present day Martian UV flux similar to that of early Earth (Cockell et al., 2000), a number of studies have investigated the impact of UV exposure, mainly relying on gas chromatography mass spectrometry (GCMS) and/or Fourier transform infrared spectroscopy (FTIR) to monitor the evolution of organic compounds (e.g., Fornaro et al., 2018).
View in article
Jamalzadeh, M., Sobkowicz, M.J. (2022) Review of the effects of irradiation treatments on poly(ethylene terephthalate). Polymer Degradation and Stability 206, 110191. https://doi.org/10.1016/j.polymdegradstab.2022.110191
Show in context Mechanistically, such evolutions of the colour and the Raman signal of the Ertalyte target can be explained by the UV-induced production of electronic defects and/or radicals (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
In fact, UV exposure of polymers has been shown to induce Norrish type I and type II reactions producing radicals via chain scission (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
Norrish type I reactions mainly involve the production of radicals via ester bond cleavage, whereas Norrish type II reactions involve hydrogen atom abstraction via photolysis, eventually inducing a cyclisation process (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
Manrique, J.A., Lopez-Reyes, G., Cousin, A., Rull, F., Maurice, S., et al. (2020) SuperCam Calibration Targets: Design and Development. Space Science Reviews 216, 1–27. https://doi.org/10.1007/s11214-020-00764-w
Show in context One of the onboard SuperCam calibration targets (Manrique et al., 2020; Cousin et al., 2022) is a 100 % organic target made of polyethylene terephthalate (PET - (C10H8O4)n, a.k.a. the Ertalyte® target; Fig. 1), which has allowed conducting a 1000 sols long ageing experiment under actual Martian conditions.
View in article
The Raman signal of the Ertalyte target exhibits a number of intense Raman bands (Fig. 1), at 633 cm−1 (ring mode), 859 cm−1 (ring C-C and C(O)-O), 998 cm−1 (C-C), 1095 cm−1 (ring C-C, C(O)-O and C-C), 1180 cm−1 (C-H and C-C), 1288 cm−1 (C(O)-O), 1460 cm−1 (C-H), 1613 cm−1 (ring mode), 1727 cm−1 (C=O), 2966 cm−1 (C-H) and 3083 cm−1 (O-H) (Manrique et al., 2020; Cousin et al., 2022).
View in article
Maurice, S., Wiens, R. C., Bernardi, P., Cais, P., Robinson, S., et al. (2021) The SuperCam Instrument Suite on the Mars 2020 Rover: Science Objectives and Mast-Unit Description. Space Science Reviews 217, 1–108. https://doi.org/10.1007/s11214-021-00807-w
Show in context Raman spectroscopy has recently become available on Mars with Perseverance carrying both the SuperCam time-resolved Raman (Maurice et al., 2021; Wiens et al., 2021) and the deep UV SHERLOC Raman (Bhartia et al., 2021) spectrometers.
View in article
McMahon, S., Cosmidis, J. (2021) False biosignatures on Mars: anticipating ambiguity. Journal of the Geological Society 179, jgs2021-050. https://doi.org/10.1144/jgs2021-050
Show in context Determining the nature and the origin (biogenic or abiotic) of the organic compounds possibly trapped in these rocks will be required to assess whether or not there has been life on Mars (Viennet et al., 2019; Bosak et al., 2021; McMahon and Cosmidis, 2021; Ansari, 2023; Criouet et al., 2023).
View in article
Megevand, V., Viennet, J. C., Balan, E., Gauthier, M., Rosier, P., et al. (2021) Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars. Astrobiology 21, 566–574. https://doi.org/10.1089/ast.2020.2340
Show in context This has motivated specific experimental studies to document the effect of UV radiation on the Raman signals of organics and minerals (Megevand et al., 2021; Fox et al., 2023; Clavé et al., 2024; Royer et al., 2024).
View in article
For instance, combining time-resolved Raman, FTIR and ESR (electron spin resonance) spectroscopies, Megevand et al. (2021) showed that exposure to UV leads to the increase of structural disorder and to the creation of electronic defects and/or radicals within the molecular structure of S-rich organic compounds, such as cystine.
View in article
In parallel, irradiation experiments were conducted in the laboratory on Earth, in a dedicated Martian chamber and using a customised time-resolved Raman spectrometer, both built by the Cellule Projet @ IMPMC (Megevand et al., 2021; Clavé et al., 2024; Royer et al., 2024).
View in article
Of note, a similar evolution of the colour and the Raman signal was also observed in the laboratory for cystine upon exposure to UV and interpreted as resulting from the creation of electronic defects and/or radicals acting as coloured centres (Megevand et al., 2021).
View in article
Munguira, A., Hueso, R., Sánchez‐Lavega, A., De La Torre‐Juarez, M., Martínez, G. M., et al. (2023) Near Surface Atmospheric Temperatures at Jezero From Mars 2020 MEDA Measurements. Journal of Geophysical Research: Planets 128, e2022JE007559. https://doi.org/10.1029/2022JE007559
Show in context (b) Air temperature measured at 1.45 m in Jezero crater from sol 020 to sol 400 (from Munguira et al., 2023).
View in article
In fact, air temperature has increased on sol 280 compared to the first several hundred sols, from ∼ 240 K to ∼ 255 K, due to the beginning of the warm season with Mars approaching its perihelion (Fig. 3; Munguira et al., 2023).
View in article
Patel, M.R., Zarnecki, J.C., Catling, D.C. (2002) Ultraviolet radiation on the surface of Mars and the Beagle 2 UV sensor. Planetary and Space Science 50, 915–927. https://doi.org/10.1016/S0032-0633(02)00067-3
Show in context Yet, the thin CO2 atmosphere of Mars and the lack of a magnetic field expose the Martian surface to high doses of electromagnetic (UV and γ) and particle (protons, neutrons and high Z atoms) radiation (Patel et al., 2002; Hassler et al., 2014), which may chemically and structurally alter organic compounds. With a present day Martian UV flux similar to that of early Earth (Cockell et al., 2000), a number of studies have investigated the impact of UV exposure, mainly relying on gas chromatography mass spectrometry (GCMS) and/or Fourier transform infrared spectroscopy (FTIR) to monitor the evolution of organic compounds (e.g., Fornaro et al., 2018).
View in article
Poch, O., Kaci, S., Stalport, F., Szopa, C., Coll, P. (2014) Laboratory insights into the chemical and kinetic evolution of several organic molecules under simulated Mars surface UV radiation conditions. Icarus 242, 50–63. https://doi.org/10.1016/j.icarus.2014.07.014
Show in context These studies have shown that even short exposure to UV (corresponding to less than hundreds of sols on Mars) leads to the photodecomposition of organic compounds, such as amino acids (ten Kate et al., 2005), carboxylic acids (Stalport et al., 2009), nucleobases (Fornaro et al., 2013), urea (Poch et al., 2014) or PAHs (Stalport et al., 2019).
View in article
Rostampour, S., Cook, R., Jhang, S.-S., Li, Y., Fan, C., Sung, L.-P. (2024) Changes in the Chemical Composition of Polyethylene Terephthalate under UV Radiation in Various Environmental Conditions. Polymers 16, 2249. https://doi.org/10.3390/polym16162249
Show in context Mechanistically, such evolutions of the colour and the Raman signal of the Ertalyte target can be explained by the UV-induced production of electronic defects and/or radicals (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
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In fact, UV exposure of polymers has been shown to induce Norrish type I and type II reactions producing radicals via chain scission (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
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Norrish type I reactions mainly involve the production of radicals via ester bond cleavage, whereas Norrish type II reactions involve hydrogen atom abstraction via photolysis, eventually inducing a cyclisation process (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
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Royer, C., Bernard, S., Beyssac, O., Balan, E., Forni, O., Gauthier, M., Morand, M., Garino, Y., Rosier, P. (2024) Impact of UV radiation on the Raman and infrared spectral signatures of sulfates, phosphates and carbonates: Implications for Mars exploration. Icarus 410, 115894. https://doi.org/10.1016/j.icarus.2023.115894
Show in context This has motivated specific experimental studies to document the effect of UV radiation on the Raman signals of organics and minerals (Megevand et al., 2021; Fox et al., 2023; Clavé et al., 2024; Royer et al., 2024).
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In parallel, irradiation experiments were conducted in the laboratory on Earth, in a dedicated Martian chamber and using a customised time-resolved Raman spectrometer, both built by the Cellule Projet @ IMPMC (Megevand et al., 2021; Clavé et al., 2024; Royer et al., 2024).
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The UV flux was produced by a 150 W arc lamp equipped with a high pressure Xenon bulb (UXL-150SP - LOT-ORIEL), delivering UV with a pattern similar to that of the Martian surface radiation spectrum (Royer et al., 2024), a DUV Grade Fused Silica window allowing for rejection of most of the IR radiation (down to 2.89 μm).
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Sang, T., Wallis, C.J., Hill, G., Britovsek, G.J.P. (2020) Polyethylene terephthalate degradation under natural and accelerated weathering conditions. European Polymer Journal 136, 109873. https://doi.org/10.1016/j.eurpolymj.2020.109873
Show in context Mechanistically, such evolutions of the colour and the Raman signal of the Ertalyte target can be explained by the UV-induced production of electronic defects and/or radicals (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
In fact, UV exposure of polymers has been shown to induce Norrish type I and type II reactions producing radicals via chain scission (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
View in article
Norrish type I reactions mainly involve the production of radicals via ester bond cleavage, whereas Norrish type II reactions involve hydrogen atom abstraction via photolysis, eventually inducing a cyclisation process (Grossetête et al., 2000; Sang et al., 2020; Jamalzadeh and Sobkowicz, 2022; Rostampour et al., 2024).
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Stalport, F., Coll, P., Szopa, C., Cottin, H., Raulin, F. (2009) Investigating the Photostability of Carboxylic Acids Exposed to Mars Surface Ultraviolet Radiation Conditions. Astrobiology 9, 543–549. https://doi.org/10.1089/ast.2008.0300
Show in context These studies have shown that even short exposure to UV (corresponding to less than hundreds of sols on Mars) leads to the photodecomposition of organic compounds, such as amino acids (ten Kate et al., 2005), carboxylic acids (Stalport et al., 2009), nucleobases (Fornaro et al., 2013), urea (Poch et al., 2014) or PAHs (Stalport et al., 2019).
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Stalport, F., Rouquette, L., Poch, O., Dequaire, T., Chaouche-Mechidal, N., et al. (2019) The Photochemistry on Space Station (PSS) Experiment: Organic Matter under Mars-like Surface UV Radiation Conditions in Low Earth Orbit. Astrobiology 19, 1037–1052. https://doi.org/10.1089/ast.2018.2001
Show in context These studies have shown that even short exposure to UV (corresponding to less than hundreds of sols on Mars) leads to the photodecomposition of organic compounds, such as amino acids (ten Kate et al., 2005), carboxylic acids (Stalport et al., 2009), nucleobases (Fornaro et al., 2013), urea (Poch et al., 2014) or PAHs (Stalport et al., 2019).
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ten Kate, I.L., Garry, J.R.C., Peeters, Z., Quinn, R., Foing, B., Ehrenfreund, P. (2005) Amino acid photostability on the Martian surface. Meteoritics & Planetary Science 40, 1185–1193. https://doi.org/10.1111/j.1945-5100.2005.tb00183.x
Show in context These studies have shown that even short exposure to UV (corresponding to less than hundreds of sols on Mars) leads to the photodecomposition of organic compounds, such as amino acids (ten Kate et al., 2005), carboxylic acids (Stalport et al., 2009), nucleobases (Fornaro et al., 2013), urea (Poch et al., 2014) or PAHs (Stalport et al., 2019).
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Viennet, J.-C., Bernard, S., Le Guillou, C., Jacquemot, P., Balan, E., Delbes, L., Rigaud, B., Georgelin, T., Jaber, M. (2019) Experimental clues for detecting biosignatures on Mars. Geochemical Perspectives Letters 12, 28–33. https://doi.org/10.7185/geochemlet.1931
Show in context Determining the nature and the origin (biogenic or abiotic) of the organic compounds possibly trapped in these rocks will be required to assess whether or not there has been life on Mars (Viennet et al., 2019; Bosak et al., 2021; McMahon and Cosmidis, 2021; Ansari, 2023; Criouet et al., 2023).
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Wadsworth, J., Cockell, C.S. (2017) Perchlorates on Mars enhance the bacteriocidal effects of UV light. Scientific Reports 7, 4662. https://doi.org/10.1038/s41598-017-04910-3
Show in context Plus, additional parameters may enhance the degradation of organic compounds upon exposure to UV at the surface of Mars (e.g., Wadsworth and Cockell, 2017; Fornaro et al., 2018).
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Wiens, R.C., Maurice, S., Robinson, S.H., Nelson, A.E., Cais, P. et al. (2021) The SuperCam Instrument Suite on the NASA Mars 2020 Rover: Body Unit and Combined System Tests. Space Science Reviews 217. https://doi.org/10.1007/s11214-020-00777-5
Show in context Raman spectroscopy has recently become available on Mars with Perseverance carrying both the SuperCam time-resolved Raman (Maurice et al., 2021; Wiens et al., 2021) and the deep UV SHERLOC Raman (Bhartia et al., 2021) spectrometers.
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Supplementary Information
The Supplementary Information includes:
- Methods
- Figures S-1 to S-8
- Tables S-1
- Supplementary Information References
Download the Supplementary Information (PDF)
Figures
Figure 1 (a) Image of the Perseverance rover showing the location of the Ertalyte target and RMI images of the Ertalyte target after 12 and 996 sols since the landing of Perseverance on Mars. (b) Schematic molecular structure of the Ertalyte polymer. (c) Raman spectrum of the Ertalyte target collected using the customised time-resolved Raman spectrometer built by the Cellule Projet @ IMPMC.
Figure 2 (a) Evolution of the color of the Ertalyte target as a function of the time spent on Mars (sol number). (b) SuperCam Raman spectra (normalised to the total signal) collected on the Ertalyte target every 40 to 70 sols since the landing of Perseverance. The colour code mimics the true colour evolution of the Ertalyte target from sol 026 to sol 996.
Figure 3 (a) Evolution of the area of the band at 1613 cm−1 as a function of the time spent on Mars (sol number). (b) Air temperature measured at 1.45 m in Jezero crater from sol 020 to sol 400 (from Munguira et al., 2023
Munguira, A., Hueso, R., Sánchez‐Lavega, A., De La Torre‐Juarez, M., Martínez, G. M., et al. (2023) Near Surface Atmospheric Temperatures at Jezero From Mars 2020 MEDA Measurements. Journal of Geophysical Research: Planets 128, e2022JE007559. https://doi.org/10.1029/2022JE007559
). (c) Temperature of the primary mirror at the time of SuperCam measurements from sol 500 to sol 1000.
Figure 4 (a) Time-resolved Raman spectra (normalised to the total signal) collected using the customised time-resolved Raman spectrometer built by the Cellule Projet @ IMPMC on the spare Ertalyte target before and after exposure to UV radiation in the lab for 100, 370, 1370 and 2640 minutes. (b) Evolution of the area of the band at 1613 cm−1 as a function of the irradiation duration.