EXOA7
Understanding how the planetary environment has influenced the evolution of life and how biological processes have changed the environment is an essential part of any study of the origin and search for signs of life. A central issue in the research on the emergence of life is the paradoxical role of water in pre-biotic chemistry. In fact,on the one hand, water is essential for all known life, on the other hand it is highly destructive for key biomolecules such as nucleic and polypeptides. Earth analogues experiments/instruments test and/or simulation campaigns and limits of life studies are included as well as one of the main topics of this session.
Major Space Agencies identified planetary habitability and the search for evidence of life as a key component of their scientific missions in the next two decades. The development of instrumentation and technology to support the search for complex organic molecules/sings of life/biosignatures and the endurance of life in space environments is critical to define unambiguous approaches to life detection over a broad range of planetary environments. A truly interdisciplinary approach is needed to delve into the core of the issue of emergence of life, because in addition to physics and chemistry it is also need to deploy a number of other sciences. We rely on contribution coming from mathematical or philosophical perspectives not only on astrobiology moreover we think that a part of the answers may lie in scientists who working on cancer research, genetics, space exploration paleontology who are not necessarily involved in this field.
Session assets
Introduction
Planetary Protection aims to protect both Earth and other solar system bodies from contamination by possible life forms. Protecting other bodies from terrestrial contamination (also known as forward contamination) has been a high priority for space agencies when sending spacecraft into the solar system. Even with planetary protection protocols in place to limit the prelaunch bioburden, spacecraft are being launched with terrestrial microorganisms on the surface – for example, the Perseverance Rover had a pre-launch bioburden of 3.86 x 104 spores (1). This work estimates the forward contamination of Mars from terrestrial microorganisms by modelling the survivability of bacillus subtills under several biocidal effects on the Martian surface.
Methodology
The model used in this work, called the Mars Microbial Survival (MMS) model, is adapted from similar microbial survival models for the lunar surface (2) and interplanetary space (3). Here, we modeled the effects that UVC irradiation, low pressure and temperature synergism, biotoxic soils, and desiccation might have on Earth microorganisms on the surface of Mars at fourteen historical landing/crash sites.
First, the biocidal effects were modelled for Mars-relevant conditions. For example, the UVC irradiation (200 – 280 nm) at the Martian surface was calculated using a radiative transfer model at the time of year for each landing site. This included time and location dependent parameters such as the UVC flux at the top of the atmosphere and the nominal dust loading conditions. The UVC irradiation incident on horizontal surfaces of each spacecraft was calculated for the first 24-hours after landing. To capture one Mars Year on the surface, the UVC irradiation received over one sol at each landing site was modelled in increments of Ls = 10, and interpolated to cover one Mars Year. In addition to the UVC light incident on horizontal surfaces, surfaces tilted 90 from the horizontal, facing north, south, east, and west were considered. The irradiation values were then converted to bioburden reductions using the method from Schuerger and Moores (2019), which adapted laboratory studies of B. subtilis into bioburden reduction plots.
Results
The UVC irradiation incident on horizontal surfaces for the first 24-hours on the Martian surface for eight missions is shown in Figure 1a, and the resulting bioburden reductions are shown in Figure 1b. Note that the UVC irradiation shows 24-hours starting at sunrise (Fig. 1a), but the bioburden reductions are dependent on the specific landing time of the spacecraft (Fig. 1b).
Figure 1: (a) UVC irradiation on flat surfaces for the first sol on Mars at 8 landing sites. (b) Bioburden reductions from UVC after one sol on the surface
The biphasic bioburden reduction as reported in (2) is seen in Figure 1b. Upon landing, all spacecraft except Pathfinder, which landed in the middle of the night, underwent a rapid inactivation of microbes from UVC exposure. After the initial inactivation of approx. ‒4 logs in < 2 hrs, a slower decrease began at approximately –4.1 log reductions. For Pathfinder, the initial, rapid inactivation occurred when the sun rose approximately 10 hours after landing. Horizontal plots in Fig. 1b indicate no bioburden losses during night-time hours. After the first 24 hours on Mars, sun-facing surfaces of each spacecraft had a bioburden reduction of approx. –20 logs, indicating UVC rapidly sterilized external surfaces of each spacecraft.
Figure 2 shows the bioburden reductions from UVC after one Mars Year on the surface for fourteen spacecraft. After one year on the surface, every spacecraft accumulated thousands of log-reductions on horizontal and upward facing surfaces. Despite the proximity to the northern and southern poles, Mars Phoenix Lander and Mars Polar Lander still underwent at least –4000 log reductions on sun-facing surfaces. We found that surfaces tilted away from the horizontal had a smaller bioburden reduction but were still able to accumulate several thousand log-reductions on every surface over one Mars Year.
Figure 2: Bioburden reductions from UVC inactivation on horizontal surfaces after one year on the surface of Mars
On the internal components of the spacecraft, low-pressure and temperature synergism rapidly inactivated microbes. If the internal temperature of a spacecraft was kept at 313 K, a –12 log reduction (referred to as the Sterility Assurance Level) was reached within a few sols. Colder components, such as those kept around 233 K, had a –2 log bioburden reduction after one Mars Year (668 sols), meaning colder components are sterilized much more slowly than warmer components. Below 210 K, when low-pressure and temperature cease to have synergism---and low-pressure acts alone as the dominant biocidal effect---the time to one Sterility Assurance Level is greater than 25 Mars Years. Only Mars 2, Mars 3, and Mars 6 have been on the surface for the time required to sterilize the insides of the spacecraft from low-pressure alone.
The results from the MMS imply that there are likely terrestrial microorganisms that have not yet been inactivated on the surface of Mars today. However, due to the highly biocidal nature of UVC irradiation, the outsides of the spacecraft are likely sterilized. Microbes within the spacecraft themselves, or microbes that dislodged upon landing and became covered under UVC-attenuating regolith, are likely to not yet be affected by the biocidal factors of the Martian surface. With these results, the MMS helps provide a first-order analysis on the likelihood of forward contamination at historical landing sites on Mars, which may be applied to future landing sites of interest.
References
(1) Cooper M, Chen F, Guan L, Hinzer AA, Kazarians G, Ly C, et al. Planetary Protection Implementation and Verification Approach for the Mars 2020 Mission. Astrobiology. 2023 Aug;23(8):825–34.
(2) Schuerger AC, Moores JE, Smith DJ, Reitz G. A Lunar Microbial Survival Model for Predicting the Forward Contamination of the Moon. Astrobiology. 2019 Jun;19(6):730–56.
(3) Moores JE, Schuerger AC. A Cruise-Phase Microbial Survival Model for Calculating Bioburden Reductions on Past or Future Spacecraft Throughout Their Missions with Application to Europa Clipper. Astrobiology. 2020 Dec;20(12):1450–64.
How to cite: Bischof, G., Moores, J. E., Ordinaria, R., and Schuerger, A. C.: Estimating the Forward Contamination of Mars with a Microbial Survival Model by Calculating Bioburden Reductions on Past Mars Spacecraft, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-131, https://doi.org/10.5194/epsc-dps2025-131, 2025.
Introduction
Titan’s cryovolcanism may extrude, onto its surface, water-ammonia cryolavas [1]. Once flowing on the surface, these water-ammonia solutions can hydrolyze Titan’s organic aerosols to produce amino acids [2]. This is most likely to occur on Titan’s polar regions, where cryovolcanic geological features are most common [3]. Unfortunately, NASA’s upcoming Dragonfly lander is due to explore equatorial locations [4]. The distance between the high-latitude cryovolcanic features and Dragonfly’s equatorial landing site may prevent the lander from detecting cryo-volcanogenic prebiotic molecules.
Herein, we explore the possibility of cryo-volcanogenic amino acids being transported from Titan’s poles to its equator. This transportation scenario requires two steps. First, the molecules are made airborne when gases dissolved in the cryolavas – such as methane, carbon dioxide, and ammonia – coalesce into bubbles which, reaching the cryolava free surface, burst and project droplets into the atmosphere [5]. These water-ammonia aerosols rapidly freeze, encapsulating the prebiotic molecules. Second, the icy aerosols, bearing the prebiotic molecules, are transported by the pole-to-pole atmospheric Hadley circulation. This atmospheric cell upwells at the summer pole and subsequently circulates towards the opposite pole, through the equator. Amino acids produced in the cryovolcanic regions of the seasonal summer pole can thus accumulate on the equatorial sand dunes.
Concerningly, the upper branch of the Hadley circulation may sit as high as 600 km [6], in Titan’s mesosphere, exposing the aerosols to radiation. The success of the two-step transportation thus depends on the survivability of the amino acids to the irradiation felt throughout. We assessed their survivability by comparing the expected photodegradation rates of the amino acids to the pole-to-equator transportation timescale.
Results
Alanine survivability: We irradiated nanolayers of alanine and glycine kept at 90 K and coated with water-ammonia (95:5) ice layers (∼200-nm thick) with a broadband solar simulator (Figure 1). These conditions mimic the environment of the water-ammonia icy aerosols. Based on atmospheric radiative fluxes modeled by us, we calculated an alanine photodegradation half-life of 38.9 ± 8.5 Titan-days in Titan’s mesosphere.
Whilst the velocity of the meridional circulation on Titan’s mesosphere remains to be measured directly, global circulation models estimate it to be within 3–5 cm s−1 [7], [8]. Dynamical timescales of the meridional circulation are obtained by dividing the meridional scale (equivalent to Titan’s radius) by the mean meridional wind velocity. An average meridional wind velocity of 4 cm−1 translates to a dynamical timescale of about 47 Titan-days, similar to the alanine half-life. The similarity between the two timescales implies that half of the alanine molecules would survive their journey from the pole to the equator through Titan’s mesosphere. Nevertheless, as discussed in reference [9], the aerosols are likely subject to faster meridional winds and lower photodegradation rates than we considered, further increasing the expected survivability of alanine.
Glycine survivability and the role of alanine in it: Under the same conditions, the photodegradation half-life of glycine was 535.0 ± 492.7 Titan-days. Being more photostable than alanine, glycine is very likely to survive the meridional transport towards the equator.
We also irradiated nanolayers made up of a 1:1 mixture of alanine and glycine to understand if a close packing of alanine and glycine in the icy aerosols would influence their individual photodegradation behaviors. The presence of alanine decreased the half-life of glycine to 54.2 ± 13.8 Titan-days, in a ten-times faster photodegradation than in the pure glycine sample. This evolution contrasts with that of the alanine photodegradation half-life, which remained the same in the pure alanine and mixture samples (Figure 2). The contrasting behaviors could not be explained by the alanine-glycine sample deposition morphology, infrared signatures, or electronic properties [10].
Through computational methods, we understood that the accelerated photodegradation rate of glycine is due to a reduction in the environment polarity. A slight decrease in environment polarity from εglycine = 18 to εalanine = 15 – produced by the amino acids co-deposition – stabilizes the transition state of the glycine decarboxylation reaction enough to explain large variations in its photo-decarboxylation rate. The alanine photodegradation rate is not, on the other hand, as sensitive to the environment polarity, which is consistent with the unchanged alanine half-life measured experimentally.
Conclusions
Dragonfly may find alanine and glycine molecules produced in the polar cryovolcanic regions. In Titan’s summer pole, after the water-ammonia cryolavas hydrolyze the atmospheric haze into amino acids, the exsolution of dissolved gases can produce icy aerosols encapsulating the amino acids. The icy aerosols would then be transported to the equator and accumulate within the equatorial dunes.
Further, the co-deposition of alanine and glycine induced the latter to photodegrade ten-times faster. This teaches us that by exclusively considering the effects of inorganic surfaces, we disregard a significant fraction of photochemical fates. The effect of organic interactions in the photochemistry of prebiotic molecules deserves further exploration.
Acknowledgements
The authors acknowledge funding by Fundação para a Ciência e Tecnologia (UIDB/00100/2020, UIDP/00100/2020, LA/P/0056/ 2020, UIDB/04565/2020, UIDP/04565/2020, LA/P/ 0140/2020, and 2021.04932.BD), the Ministry of Economics and Energy, Germany (50WB2023 and 50WB2323), the Einstein Foundation Berlin (IPF-2018-469), the Volkswagen Foundation (Freigeist Program), INAF (RSN3 “ORSO” C63C23001250005), the U.S. Department of Energy (DE-AC52-07NA27344), and the European Research Council (804144, ERC-ALIFE).
References
[1] Mitri, G. et al. Icarus 196, 216–224 (2008)
[2] Brassé, C. et al. Astrobiology 17, 8–26 (2017)
[3] Wood, C. A. et al. J Geophys Res Planets 125, e2019JE006036 (2020)
[4] Barnes, J. W. et al. Planetary Science Journal 2, 130 (2021)
[5] Cordier, D. et al. J Geophys Res Planets 129, e2023JE008248 (2024)
[6] Teanby, N. A. et al. Nature 491, 732–735 (2012)
[7] Lebonnois, S. et al. in Titan 122–157 (Cambridge University Press, 2014)
[8] Achterberg, R. K. et al. Icarus 194, 263–277 (2008)
[9] Gonçalves, D. et al. ACS Earth Space Chem 9, 715–728 (2025)
[10] Gonçalves, D. et al. ACS Earth Space Chem 9, 356–368 (2025)
Figure 1. Sample structure (A, not to scale), irradiation setup (B), and exploded view of the sample in the sample holder (C).
Figure 2. Relative abundances of alanine and glycine plotted against irradiation time, in the three samples: (pure) alanine, (pure) glycine, and alanine-glycine (1:1 mixture).
How to cite: Gonçalves, D., Hofmann, F., Wipf, S., Giovanni Urso, R., Bocková, J., Meinert, C., Brandon Rimmer, P., Dutta Stroscio, G., Goldman, N., Elsaesser, A., Pedras, B., and Martins, Z.: Atmospheric survivability of amino acids produced in the cryovolcanic regions of Titan, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-192, https://doi.org/10.5194/epsc-dps2025-192, 2025.
Introduction: Methane on Mars was first detected using ground-based telescopes [1] and Martian orbiters [2] in the early 2000s. Since then, the Mars Science Laboratory (Curiosity Rover) has detected additional methane in the Martian atmosphere using the Sample Analysis at Mars (SAM) instrument [3]. Observations of atmospheric methane have prompted further investigation into the putative biotic or abiotic mechanisms of its production. Potential mechanisms include modern and/or ancient microbial methanogenesis, deep subsurface geothermal processes, abiotic Fischer-Tropsch Type reactions after serpentinization, or thermogenesis of organics [4]. Our presentation will explore the plausibility of modern microbial processes as a contributing production mechanism to the methane emissions presently observed on Mars.
Modelling Production of Methane: The generation rate of methane is set by the methane flux derived at Gale of 1.5 x 10-10 kg m-2 sol-1 [5]. While this value is agnostic as to the methane production mechanism, it can be converted to a maximum bioburden (assuming all methane is produced biogenically) by combining it with experiments performed by Harris and Schuerger [6] which outline the production of methane from Methanosarcina barkeri under simulated Martian surface conditions (0°C, 7-12 mbar, CO2 dominated gas mixture). Results from [6] demonstrated CH4 production, although genes involved in the hydrogenotrophic pathway of methanogenesis were significantly downregulated compared to ideal M. barkeri growth conditions (30°C, 1500 mbar, 80:20 H2:CO2). The full experiment involved 6 runs of methane production rate measurements, each run varying by atmospheric composition, atmospheric pressure, and temperature. Based on these experiments, we created a model of methane production by methanogenesis (Fig 1.). This figure displays a comparison of modeled versus measured methane production rates using values obtained by [6]. Temperature, CO2, H2, and the methanogen surface density (cells m-2) serve as adjustable model constraints to simulate a range of environmental scenarios, including Mars- or Earth-like conditions. Variations in these parameters have yielded promising results.
Predicted Bioburden Below 30m in the Subsurface: With methanogens located below the annual temperature wave (at least 30m below the surface) we can solve for the total bioburden of methanogens required (Fig 2.). The temperature and depth axes were defined using a thermal gradient of 5 K/km [7] starting from the mean surface temperature of Gale Crater and extending to approximately 400 K consistent with experimental findings that methanogenesis may occur at temperatures exceeding 100°C [8]. Depth may then be determined using the value for the thermal gradient.
Figure 2 illustrates the dependance on pH2 bioavailability for increased bioproductivity of methanogens. At greater temperatures and availability of pH2, fewer methanogens are required for methane production using a rate constant of 1.05 x 10-13 cells m-2 s-1 [5]. Conversely, at low temperatures and availability of H2, more methanogens are needed using the same rate constant. Kral et al. [9] [10] identified H2 as a primary electron donor driving methanogenic metabolism under simulated Martian conditions.
Examining the Evolution of Atmospheric Concentrations using an Atmospheric Box Model: Closer than 30m to the surface, methane production from methanogens will vary daily and annually as the temperature to which they are exposed changes. Rapid changes in methane emitted requires the production model to be coupled to a destruction model. Thus, to examine how the atmospheric concentration near the surface changes in response, a box model was developed to demonstrate the production/destruction couple and the resulting CH4 concentrations. The destruction mechanism used to model the observed CH4 measurements was outlined by [11] and involved perchlorate-rich Martian soils that when activated by UV irradiation oxidized adsorbed alkanes.
To set up our simplified box model a 1 m2 homogeneous atmospheric column was created containing CO2 at 610 Pa, an H2 abundance of 15 ppmv and surface temperatures given by [12]. These conditions along with a methanogen cell count of 1.0 x 1017 cells m-2 produce the blue line shown in Figure 3 below.
The orange line utilized the same CO2 and H2 abundances, however the temperature was adjusted to reflect conditions at approximately 30 m depth, where temperatures corresponded to the thermal average of Gale Crater.
At surface conditions (Fig 3), the modeled atmospheric methane was slowly generated overnight when temperatures were coolest and when the destruction process was inhibited by lack of sunlight. During the day, there was a steep decline in methane observed in methane concentration until concentrations become so low that destruction was ineffective. After the lowest methane was achieved at 0.5 sol, methane concentrations steadily rose throughout the day as ground temperatures increased. The population of methanogens was kept at 1.0 x 1017 cells m-2 for the duration of the simulations. Cell counts increased to 8.0 x 1017 m-2 when simulating conditions at depth (~30 m). Here, similar overall diurnal trends were observed; however, minimum methane concentration was reduced by nearly an order of magnitude by midday and was likely attributable to the increased maximum methane concentration prior to sunrise, enhancing the efficiency of the destruction mechanism following sunrise.
Conclusions: Diurnal patterns observed in Figure 3 are distinct from patterns expected solely by atmospheric processes, warranting further investigation into possible sources and sinks of methane on Mars. The development of an accurate model that can predict methane concentrations, rate of methanogenesis, and/or methanogen cell counts are crucial to understanding if methanogenic metabolism is a possible source for the methane currently observed on Mars.
References: [1] Formisano et al. (2004) Sci 306(5702), 1758–1761 [2] Mumma et al. (2009) Sci 323(5917), 1041-1045 [3] Webster et al. (2014) Sci 347(6220), 415-417 [4] Oehler and Etiope Astrobio 17(12), 1233-1264 [5] Moores et al. (2019) GRL 46(16), 9430-9438 [6] Harris and Schuerger (2025) Sci Rep 15(2880), 1-15 [7] Jones et al. (2011) Astrobio 11(10), 1017-1033 [8] Takai et al. (2008) Proc of the NAS 105(31), 10949–10954 [9] Kral et al. (2011) PSS 59(2-3), 264–70 [10] Kral et al. (2004) Origins of Life and Evolution of the Biosphere 34(6), 615–626 [11] Zhang et al. (2022) Icarus 376(114832):1–15 [12] Martinez et al. (2017) Space Sci Rev 212(1-2):295–338
How to cite: Marincic, I., Moores, J., Harris, R., and Schuerger, A.: Numerical Modelling of Surface/Subsurface Methanogen Bioburden at Gale Crater Using Methane Production and Destruction Processes, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-389, https://doi.org/10.5194/epsc-dps2025-389, 2025.
1. Introduction
Mars’ surface is currently one of the environments in the solar system, where the research about past prebiotic chemistry is the more active, because Mars gathered the conditions required for the emergence of life at the time it arose on Earth (3.7-4 Ga) [1].
In this context, the Curiosity rover landed in Gale crater in 2012. This ancient lake shows stratified geological units which favors the study of different periods in the history of the crater. Onboard the Curiosity rover, a gas chromatograph mass spectrometer (GCMS) instrument, part of Sample Analysis at Mars (SAM) experiment, has the objective to detect and identify organic and inorganic molecules in surface samples collected by the rover. In the recent years, Curiosity has explored a strata enriched in sulfates, such as magnesium sulfates or iron sulfates, on its ascent of Mount Sharp [2]. Like other minerals and inorganic phases, sulfates may contain organic matter and protect it from the harsh surface environmental conditions. Likewise, sulfates can play a role in the chemical extraction of organic matter by the sample preparation techniques used by SAM, i.e. pyrolysis, chemical derivatization and thermochemolysis [3]. To support the treatment and the interpretation of the data provided by SAM, it is of primary importance to perform laboratory experiments mimicking the sample treatments and operating conditions used by SAM.
In this frame, we performed a systematic analysis of a variety of synthetic and natural samples containing both sulfates and organic molecules by reproducing the SAM analytical conditions, with a specific interest for magnesium and iron sulfates detected in Gale crater.
2. Samples and experimental set up
Considering the complexity of natural samples to infer possible chemical interactions between organic molecules and sulfates that may induce the production of detected S-bearing compounds [3][4], a first set of synthetic samples was made and studied. The samples were made by mixing individual sulfates and organic molecules representative of chemical species either suspected to be present on Mars or of interest for astrobiology. These results were then compared with natural Martian analogs. For the synthetic samples, undecanoic acid and benzoic acid were selected because both of them are possible precursors of molecules detected by SAM in Cumberland samples [5][6]. Naphthalene was selected as a possible molecule delivered by meteoritic influx and valine as an amino acid of interest for astrobiology. Regarding the sulfate phase, Fe-sulfate and Mg-sulfate have been used because they were both detected in Gale crater with different instruments onboard Curiosity. Typical synthetic samples were made by mixing the organic compound at 10 wt% ratio in sulfates in aquese phase.
Samples analyses were performed with a laboratory set up simulating the analytic pathway and operating condition of SAM, based on a gas chromatograph mass spectrometer coupled with an oven pyrolyzer (figure 1). This last one allows to reproduce the pyrolysis conditions of the SAM instrument. The pyrolyser ramp up to 850 °C with a ramp of 35 °C min-1 [7]. The gases released by the sample are then trapped and focused at the GC column inlet cooled with liquid nitrogen during the whole duration of the pyrolysis. The gases are then quickly released to the chromatograph by stopping the cryocooling. Finally, to complete the scope of this study, the samples were also analyzed using the two other sample preparation techniques used in SAM, i.e., thermochemolysis with tetramethylammonium hydroxide (25% in methanol), and derivatization with a 4:1 mix of N,N-methyltert-butyl-dimethylsilyltrifluoroacetamide:N,N-dimethylformamide. In this last case the reaction was done ex situ and the resulting reaction mixture was injected as a liquid sample directly into the GC using a syringe injection.
Figure 1: Schematic of the experimental pathway from the pyrolysis of the sample to the detection of the molecules
3. Results
The first results obtained by the pyro-GCMS reveal interactions between sulfate and organic compounds during the pyrolysis by the production of S-bearing compounds or sulfurization of the organic molecule, except in the case of naphthalene.
As an example of results, laboratory experiments done with a mixture of undecanoic acid and Mg-sulfate, using the SAM-like pyrolysis conditions, show two important features. First, when pyrolyzed at 850°C in presence of sulfates, undecanoic acid seems more inclined to create aromatic compounds. This could result from the acid being trapped in the sulfate’s crystal and being released at high temperature resulting in a cyclisation of the acid. Second, the acid does react with the salt to produce, especially, thiophene based compounds (figure 2).
Figure 2: Chromatogram obtained from GCMS analysis of undecanoic acid after a pyrolysis at 850 °C, with a heating ramp of 35 °C min-1. The molecules represented on the upper chromatogram were found in presence of magnesium sulfates. The control using undecanoic acid alone is display on the lower chromatogram.
It is also interesting to notice, that with and without sulfates, undecanoic acid is degraded in smaller alkanes and oxidized up to CO2. As a consequence, small alkane chains, aromatic molecules, and thiophene based compounds detected from those analyses are coherent with SAM detection and may be a clue to understand the interaction of aliphatic carboxylic acid with sulfates.
In this presentation, we will give an overview of the results obtained in this study for all the samples, and we will conclude on the consequences for the results obtained with the SAM experiment.
4. Acknowledgements
SAM-GC team acknowledges support from the French Space Agency (CNES), National French Council (CNRS), and DIM ORIGINS of Région Ile de France.
References
[1] Wordsworth, R. D. (2016). Annual Review of Earth and Planetary Sciences, 44, 381-408.
[2] Sutter et al., (2017). Journal of Geophysical Research: Planets, 122(12), 2574-2609.
[3] Millan et al., (2022). Journal of Geophysical Research: Planets, 127(11), e2021JE007107.
[4] Eigenbrode et al., (2018). Science, 360(6393), 1096-1101.
[5] Freissinet et al., (2015). Journal of Geophysical Research: Planets, 120(3), 495–514.
[6] Freissinet et al., (2025). Proceedings of the National Academy of Sciences, 122(13), e2420580122.
[7] Mahaffy et al., (2012). Space Science Reviews, 170, 401-478.
How to cite: Govekar, T., Szopa, C., Freissinet, C., Millan, M., Buch, A., and Boulesteix, D.: Analysis of organic molecules in the presence of sulfates with gas chromatography mass spectrometry to interpret Curiosity data, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1356, https://doi.org/10.5194/epsc-dps2025-1356, 2025.
Scientific Rationale:
Life on Earth exhibits a fundamental molecular dissymmetry arising from homochirality – the exclusive use of one enantiomer of chiral molecules in biochemistry. This universal trait of biogenic macromolecules (proteins, DNA, most pigments) is a unique characteristic of life (Cahn et al.,1956; Blackmond,2010). For example, the backbone of terrestrial DNA is composed of only right-handed (D) sugars, and proteins consist solely of left-handed (L) amino acids. Incorporating both enantiomers (a racemic mixture) into biopolymers would disrupt the formation of stable, functional structures, so life’s chemistry has evolved to be strictly single-handed.
Chirality’s Interaction with Light:
A direct consequence of molecular homochirality is that living matter interacts uniquely with electromagnetic waves. As Louis Pasteur already discovered in 1848, “living matter” can rotate the plane of linearly polarized light (Pasteur,1848). Much later, circular dichroism, i.e. the differential absorption of left- vs. right-handed circularly polarized light, was observed. This means that at specific wavelengths, biomolecules may preferentially absorb one circular polarization state over the other, imparting a net circular polarization to transmitted or reflected light (Wald,1957; Velluz et al.,1965).
Fig.1: A) Illustration of circular polarizance. B) Example for circular polarization signals of a leaf in reflectance (upper panel) and cyanobacteria in transmittance (lower panel).
Circular Polarization as a Biosignature:
Following studies have revealed that even when initially unpolarized light (such as sunlight or starlight) is scattered from a surface containing chiral biopigments, it can acquire a faint but distinct circular polarization signature (Pospergelis,1969; Wolstencroft,1974; see Fig.1A). Crucially, the spectral pattern of this induced circular polarization correlates with the absorption bands of specific biological molecules: peaks in the degree of circular polarization coincide with wavelengths where pigments absorb, providing a fingerprint of life’s molecular dissymmetry (Kemp et al.,1971; Swedlund et al.,1972; Sparks et al.,2005; Patty et al.,2019; see Fig.1B). Importantly, circular polarization biosignatures have no known abiotic false positives. Additionally, because this effect does not require a pre-polarized light source (only an initially unpolarized illumination is needed), it is highly advantageous for remote sensing of life on other worlds (Kemp et al.,1987; Wolstencroft et al.,2004).
Observational Evidence:
Spectropolarimetric observations on Earth have validated this concept. Previous studies have measured circular polarization signals from a variety of living samples – ranging from photosynthetic microorganisms and biofilms to tree leaves and entire vegetation canopies – all of which contain homochiral biopolymers or pigments (Sparks et al.,2009; Patty et al.,2021; Mulder et al.,2022). Notably, airborne and ground-based instruments have successfully detected these signals remotely, distinguishing biotic surfaces from inorganic backgrounds. These observations confirm that circular spectropolarimetry can reliably indicate the presence of life, reinforcing its value as a biosignature detection method.
Biosignatures in Icy Environments:
We extend this biosignature approach to icy worlds. Moons such as Enceladus and Europa eject plume particles from subsurface oceans that could contain microbial life frozen within water-ice grains. However, the presence of water ice (and ice mixed with salts) might modify or obscure polarization signals. Ice and frost are known to strongly influence the linear polarization of reflected light (Poch et al.,2018), which raises the question: will the circular polarization signature of embedded microbes remain discernible in an icy matrix? While no known abiotic process produces a narrow-banded circular polarization signal, multiple scattering in ice could depolarize the light and dampen the signal’s intensity or shift its spectral features, potentially reducing the diagnostic power of the biosignature. To investigate this, we designed experiments to quantify how microbial circular polarization signals behave when microbes are encapsulated in ice particles analogous to those from icy moon plumes.
Experimental Approach:
We produced microbe-laden ice analog particles using the Setup for Production of Icy Planetary Analogues B (SPIPA-B) apparatus (Pommerol et al.,2019). SPIPA-B employs an ultrasonic nebulizer to spray droplets of salt brine (with suspended microorganisms) into liquid nitrogen. The rapid flash-freezing in liquid N₂ yields tiny ice spheres with microbes embedded throughout, closely mimicking the formation of plume ice grains under Enceladus-like conditions. The frozen samples were then transferred to the POLarimeter for ICE Samples (POLICES) chamber (Poch et al.,2018; see Fig.2) for analysis. The POLICES facility maintains cryogenic temperatures for the sample and continuously purges the environment with dry nitrogen gas to prevent frost or condensation, creating stable ice conditions for optical measurements.
Using this setup, we measured the light scattered from the icy samples with a highly sensitive, full-Stokes, dual PEM polarimeter by Hinds Instruments with a variable monochromatic light source (see Fig.2). This allowed us to obtain circular polarization spectra of the microbe-bearing ice across visible wavelengths under controlled laboratory conditions. The spectral measurements capture any circular polarizance imparted by the embedded microbes, enabling us to assess how the signal is altered by surrounding ice.
Fig.2: Illustration of the POLICES chamber. 1) Sample holder with liquid nitrogen cooling, 2) sample with variable azimuthal angle, 3) monochromatic light source with a variable phase angle, 4) dual PEM polarimeter, 5) closed chamber purged with dry nitrogen.
Implications:
The results of this experiment (to be presented at EPSC 2025) will elucidate the extent to which ice scattering affects the circular polarization biosignature of microorganisms. This knowledge is crucial for the design of future life-detection missions. If circular polarization signals can penetrate the glare of ice, they could be sought in situ during flybys or plume-sampling missions to astrobiologically relevant sites on icy moons. Conversely, understanding any signal attenuation by ice will help in setting realistic detection limits for those missions. In summary, circular polarization arising from molecular homochirality represents a powerful and uniquely reliable remote-sensing biosignature, and our study advances its applicability to the icy domains that are prime targets in the search for extraterrestrial life.
How to cite: Grone, J., Patty, L., Brandenburg, L., Pommerol, A., Rimle, S., and Demory, B.-O.: Detecting Life in Ice: Circular Polarization as a Remote Biosignature, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1799, https://doi.org/10.5194/epsc-dps2025-1799, 2025.