The main goal of the Viking mission was to look for signs of current or past life on Mars (Biemann et al., 1977; Mazur et al., 1978). Viking found clear evidence of the presence of water vapour and water ice, as well as strong inferences that there was once a significant amount of underlying permafrost on Mars. But it found no indication of existing liquid water with the high chemical potential necessary for the survival of terrestrial life, which is in line with the known pressure-temperature relations on the planet’s surface (Mazur et al., 1978). The elemental analysis of the atmosphere and regolith revealed that the chemical elements typically thought to be necessary for terrestrial life are present (Mazur et al., 1978). Viking’s GCMS studies carried by Biemann et al. (1977) and Mazur et al. (1978) found no unambiguous indigenous organic compound. According to Biemann et al. (1977) and Mazur et al. (1978), minor traces of volatile chloro-hydrocarbons detected in Martian regolith samples (Table 1) by the Viking’s GCMS (Figure 1) was a result of terrestrial contamination. Furthermore, they stated that if volatile organic compounds were present in the regolith samples, they were either limited to substances like methane, which were undetectable or only detectable at low sensitivity levels, or they were present in concentrations below the detection limit of the instrument.

The reasons for the negative results of Viking’s GCMS in detecting the presence of indigenous organic compounds were debated in subsequent studies (Benner et al., 2000; Glavin et al., 2013). According to Benner et al. (2000) and Glavin et al. (2013) the Viking’s GCMS design was not optimized for the detection of certain potential hydrocarbons found in Martian meteorites (Sephton et al., 2002) which could have been present in the Martian samples. Benner et al. (2000) re-examined the Viking’s GCMS results in light of what is known about the oxidation of organic compounds more generally as well as the makeup of organics that are likely to arrive on Mars via meteorite. They came to the conclusion that non-volatile salts of benzenecarboxylic acids, as well as possibly oxalic and acetic acid, should be metastable intermediates of meteoritic hydrocarbons under oxidizing conditions. For the Viking’s GCMS, the salts of these non-volatile organic acids would have been largely undetectable, as the organic molecule must first go through a GC in order to access the MS, which is only possible for volatile molecules. Under pyrolysis, oxalic acid produces carbon dioxide, carbon monoxide, and water (Lewis et al., 2021). In fact, all of these were detected in the Viking’s GCMS, but all these molecules are also present in the Martian atmosphere. Thus, Benner et al. (2000) and Mahaffy (2008) presented three possible reason for the failure of Viking’s GCMS in detecting indigenous organic matter in the Martian regolith’s: 1) the design of the gas-processing system that safeguarded the vacuum ion pumps decreased the GCMS’s sensitivity for light organic compounds by a factor of 1,000; 2) the Viking GC column would not have transmitted numerous kinds of polar organic compounds, such as carboxylic acids, which are probably oxidation products of aliphatic and aromatic hydrocarbons; 3) instead of less permeable materials that would have been better protected from oxidants, the Viking sample-acquisition device could only collect loosely consolidated particles samples.

After the discovery of perchlorate in the Martian arctic soils by the Phoenix lander (Hecht et al., 2009), Navarro-González et al. (2010) ran an analogous experiment in which they heated the Mars-like soil from the Atacama desert containing 32 ppm of organic carbon with 1% magnesium perchlorate and demonstrated that nearly all of the organic present in the samples were decomposed to CO₂ and water, with the production of chloromethane and dichloromethane in trace amounts. Navarro-Gonzalez reconsidered the analysis results and suggested the burning of Martian soil sample would essentially decompose all the organics into water and CO₂. Based on this experiment they developed a chemical kinetics model for the degree of oxidation and chlorination of organics in the Viking oven and re-analysed the Viking results. According to Navarro-González et al. (2010) re-analysis results, ≤0.1% perchlorate and 1.5–6.5 ppm organic carbon were present in the sample of Viking landing site 1, and ≤1% perchlorate and 0.7–2.6 ppm organic carbon were present in the sample of Viking landing site 2 (Table 1).

Phoenix mission touched down on Mars’ polar surface in March 2008 on a valley floor covered by the Scandia Formation, which is thought to be an ancient deposit from the Amazonian period (Arvidson et al., 2008; Smith et al., 2009). This formation encircles the northern edge of a shield volcano called Alba Patera (Arvidson et al., 2008). Phoenix heated soil and ice samples in the Thermal and Evolved-Gas Analyzer (TEGA) (Figure 2) in order to look for organic compounds (Boynton et al., 2009). For a mass range of 2–140 daltons, TEGA was made up of eight differential scanning calorimeter (DSC) ovens that were connected with a mass spectrometer (Boynton et al., 2009). The TEGA DSC monitored endothermic and exothermic processes when samples were heated from ambient temperature to roughly 1,000°C. Throughout the heating process, the mass spectrometer simultaneously detected all evolved gases, including any organic compounds and their byproducts. The evolved gas analysis (EGA) using Phoenix TEGA data did not reveal any organic fragments (Lauer et al., 2009; Ming et al., 2009). A perchlorate salt was found in the soil of Mars by the Microscopy, Electrochemical and Conductivity Analyzer (MECA) and Wet Chemistry Lab (WCL) (Hecht et al., 2009). It is probable that the O₂ generated during the thermal decomposition of the perchlorate salt ignited organic fragments evolved in the temperature range of 300°C–600°C. During the heating, CO₂ was produced as a byproduct of the burning of organic molecules. According to Ming et al. (2014), the possible causes of low temperature release of CO₂ in Phoenix TEGA could be desorption of adsorbed CO₂, thermal breakdown of Fe- and Mg-carbonates, and combustion of organic molecules in the Martian soils.

This is one of the strongest and long-standing missions which landed on Mars (in August 2012) in Aeolis Palus area inside Gale Crater. The main goal of Curiosity is to search the possible traces of life on Mars using the Sample Analysis Mars (SAM) lab suite it carried (Figure 3). SAM onboard the Curiosity rover has the capability to detect complex hydrocarbon molecules having astrobiological implication, by coupling the EGA and GCMS with two type of wet chemistry methods: 1) organic derivatization by adding N-methyl-N-(ter-butyldimethylsilyl) trifluoroacetamide (MTBSTFA) and dimethylformamide (DMF) in 4:1 by volume, 2) organic derivatization by adding 25% tetramethylammonium hydroxide (TMAH) in methanol (Millan et al., 2022a; Millan et al., 2022b). Glavin et al. (2013) reported first SAM-EGA-GCMS data of a Martian solid sample, from Rocknest aeolian deposit of Gale Crater and provided an extensive discussion on the possible origins of chlorinated hydrocarbons detected by SAM. Their studies detected several chlorinated chemical species above EGA background concentration including hydrochloric acid, chloromethane, and dichloromethane (Table 1). As these chlorinated organic species were not detected in the blank EGA run of SAM, the source of this organic carbon was combustion of reduced carbon in the presence of oxygen from oxychlorine. In the blank run 1,3-bis (1,1-dimethylethyl)-1,1,3,3-tetramethyldisiloxane and tert-butyldimethylsilanol were detected by GCMS. These two compounds are the fragmentary products of MTBSTFA (Glavin et al., 2013; Leshin et al., 2013; Millan et al., 2022a; Millan et al., 2022b). Therefore, it is assumed that the MTBSTFA and DMF vapor was perhaps leaked and reacted either with terrestrial or Martian water during the pyrolysis. A major difference between the peaks of 1,3-bis(1,1-dimethylethyl)-1,1,3,3-tetramethyldisiloxane for blank run and Rocknest sample run was that during the pyrolysis of the Rocknest sample, the peak intensity of 1,3-bis(1,1-dimethylethyl)-1,1,3,3-tetramethyldisiloxane sharply decreased with increase in O₂ peak. This according to Glavin et al. (2013) was a result of MTBSTFA decomposition along with the chlorate minerals present in the samples which may have contributed to some chlorinated hydrocarbons. However, GCMS analysis of Rocknest samples after pyrolysis detected chlorinated hydrocarbons such as chloromethane, dichloromethane, trichloromethane, choromethylpropene, and chlorobenzene (Table 1) significantly above the background level recorded in black run (Glavin et al., 2013). The concentration of chlorohydrocarbons detected by EGA-GCMS was more than 50 times higher than detected by the Viking-GCMS (Biemann et al., 1977). But, this comparison of detected concentration by the Viking and SAM is less relevant as unlike SAM, Viking did not carry derivatizing agent MTBSTFA. Nevertheless EGA-GCMS analysis at various pyrolysis temperature cut (>500°C) demonstrated that light chlorinated hydrocarbons can be effectively purged from SAM gas processing unit between the analyses. Chlorobenzene was also detected in the blank GCMS run of SAM and to some level above in the Rocknest samples. Its most likely source has been traced to thermal degradation of SAM organic trap (Tenax TA) containing 2,6-diphenylphenylene oxide which can produce Benzene, Toluene, and lesser amount of biphenyl. Hence its sourcing from Rocknest sample was suggested unlikely (Glavin et al., 2013).

Based on the results from EGA-GCMS, Freissinet et al. (2015) reported detection of 150–300 ppb chlorobenzene and up to 70 ppb C2-C4 dichloroalkanes in several locations of the Cumberland drill hole in the Sheepbed Mudstone at Yellowknife Bay (Table 1). Their findings indicate that when combined with EGA-GCMS data from multiple scooped and drilled samples, blank runs, and laboratory analogs, the relatively high concentrations of dichloroalkanes and chlorobenzene cannot be a result of the instrument’s internal sourcing. The Sheepbed Mudstone may contain chlorohydrocarbons as such (Szopa et al., 2020), but they are thought to be the products of chemical reactions occurring during pyrolysis between Martian oxychloride and the sample’s organic compounds. The finding of chlorinated organic molecules close to the surface indicates that reduced organic material with covalent bonds has endured despite the presence of several oxidants and high-energy radiation exposure due to the thin Martian atmosphere. Thus, the outcomes of the Freissinet et al. (2015) studies made a significant contribution to identifying possible preservation windows for reduced organic molecules on the Martian surface. In addition, this perspective on ancient Mars offers some context for a potentially habitable environment and represents a first step toward comprehending the existence and variety of potential prebiotic or biotic chemical fingerprints.

To circumvent the perchlorate degradation product of organic matter in the Martian sample analysis, Eigenbrode et al. (2018) analyzed the gases produced exclusively above 400°C, as salt present in the Martian regolith breakdown during heating to a temperature of 200°C in the SAM. Their results provided unambiguous evidence of organic molecules such as thiophenic, aromatic, and aliphatic hydrocarbon from the drill samples of Gale Crater. These sulfur-bearing hydrocarbons are considered to have originated from Mars surface (i.e. igneous, hydrothermal, atmospheric, or biological) and indicates presence of refractory organic materials in the samples (Freissinet et al., 2015; Eigenbrode et al., 2018).

The MTBSTFA derivatization experiment on Mars performed on Ogunquit Beach dune samples of Gale Crater detected several molecules containing benzoic acid, phenol, ammonia and phosphoric acid structures (Millan et al., 2022a; Millan et al., 2022b). Such molecules were either sourced from organic molecules on surface and subsurface environment due to strong oxidizing condition or from MTBSTFA present in SAM background or both (Millan et al., 2022a; Millan et al., 2022b). First TMAH experiment on Mars was performed at Mary Anning (MA) drill site in the Glen Torridon region of Gale Crater that detected a range of hydrocarbons e.g., benzene, toluene, trimethyl- and tetramethyl-benzene, naphthalene, methylnaphthalene, pentamethyl-benzene, benzoic acid methyl ester, dimethyl-, trimethyl-, and tetramethyl-benzenamine, dihydronaphthalene, 2-butyl-thiophene, and benzothiophene (Millan et al., 2022a; Millan et al., 2022b) (Table 1). As TMAH is a methylating agent that break polar bonds and methylate pyrolysis products in situ, suggests that hydrocarbons might have derived from macromolecules which was methylated by TMAH thermochemolysis. In this way pentamethyl benzene could be a product of multimethylation during TMAH reaction. Benzoic acid is detected both in SAM background and Mars samples, therefore its TMAH methylated product benzoic acid methyl ester is of currently unknown sources. The benzenamine may have originated on Mars or it could be a result of benzene reacting again with the N in TMAH. In addition, thiophene has been found in both refractory and mature organic phases in Martian meteorites (Steele et al., 2016; Eigenbrode et al., 2018). Hence, thiophenes found on Mars are thought to be pyrolysis breakdown products from various macromolecules.

The Perseverance rover (Figure 4) was launched by NASA in 2020 (Farley et al., 2020) in an effort to determine the potential habitability of Mars and possible evidence of past life. This mission landed (on February 2021) at Jazero Crater, a 45 km diameter impact structure in the Nili Fossae region of Mars, where a lake and river delta existed approximately 3.5 billion years ago (Mangold et al., 2007; Goudge et al., 2012; Goudge et al., 2015; Goudge et al., 2017; Farley et al., 2020). The main instrument onboard Perseverance to detect signs of past life is SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals) (Hollis et al., 2021; Hollis et al., 2022). SHERLOC (Figure 4) uses a deep ultraviolet (DUV) laser to detect fluorescence emissions from aromatic compounds, as well as Raman scattered photons to identify organic functional groups, chemicals, and minerals (Bhartia et al., 2021). This enables SHERLOC to improve Martian studies by analyzing the abundance and variety of organic matter at the fine scale in different minerals to comprehend the potential for organic matter preservation in the Martian surface soils/outcrops, and help resolving organic biosignatures such as hopanes, steranes, and other organic macromolecules (Abbey et al., 2017; Bhartia et al., 2021). SHERLOC’s micro-mapping mode allows non-destructive, sub-picogram sensitive organic detection (Bhartia et al., 2010) to identify smaller-scale features. In order to confirm that all the observed spectral characteristics are intrinsic to the material, the spectrometer’s dust cover will occasionally be opened, and the instrument laser will be fired into the starry night sky (Bhartia et al., 2021). This method makes it possible to assess and eliminate any slight spectrum contamination from the optics or signature from Mars dust pollution deposited on optics from the data processing pipeline. Raman and Fluorescence data from first 208 Martian days of Perseverance on Mars has recently been published by Scheller et al. (2022). These analysis were conducted on three rock targets in two lithological units of the Jazero Crater floor. Reports of detailed scanning of the three samples by Scheller et al. (2022) demonstrated presence of carbonate, sulfate, phosphate, amorphous silicate bearing minerals in association with aromatic organic compounds most likely single- and double-ring aromatics. Carbonation of olivine in presence of briny water has been interpreted as an origin of carbonate minerals, whereas sulfate and perchlorate in the subsurface samples are indicated to have formed by precipitation from briny water. Presence of aromatic organic compounds in all the three targeted rocks is suggested by DUV analysis however, Raman analysis was unable to detect presence of organic compounds most likely due to insufficient organic matter concentration.

For more than 4 decades, the planetary science community has placed a high focus on Mars Sample Return (MSR) (Beaty et al., 2019). By analyzing Mars samples in terrestrial laboratories, we could gain a greater understanding of Mars than we could achieve with just in situ missions. Perseverance is capable of collecting samples to return to Earth. In support of its goal to collect sample for possible future Earth return, Perseverance carries an entirely new sampling and catching subsystem (SCS) (Townsend et al., 2022). SCS is used to collect rock core and regolith samples into individual ultraclean and sterile tubes, to photo-document and assess the collected volume of each sample, and to hermetically seal each tube. The sample tube can be manipulated through a sophisticated internal robotic mechanism called the Adaptive Catching Assembly (ACA) (Townsend et al., 2022), which can place the tube on the surface individually, at any time, and in one or more locations from which they can be retrieved. To maximize the likelihood of success of the Mars Sample Return (MSR) campaign duplicates have been collected for each particular target rock. One copy will be deposited at the end of the prime mission in the Jazero deposit identified at Three Forks, to ensure their availability for return to Earth in case Perseverance fail in the future. The second copy will be carried in the rover during its extended mission, with aim of collecting additional samples that the rover will directly deliver to the lander of the retrieval mission. The Mars 2020 science team has identified two hypothetical sample caches in order to examine: 1) the geology of the Jezero Crater, which will be collected during the prime mission, and, 2) the ancient crust outside of the Jezero Crater, which will be collected during a potential extended mission (Herd et al., 2022). The recent successful sample collection by the Perseverance rover and ongoing work by the National Aeronautics and Space Administration (NASA) and European Space Agency (ESA) on developing missions that could retrieve and transport the samples to Earth have put MSR on track to become a reality (Kminek et al., 2022).

Comets, asteroids, meteorites, and interplanetary dust particles deliver organic compounds to planetary surfaces abiotically (Chyba and Sagan, 1992; Ehrenfreund et al., 2011). Based on scaling the terrestrial influx to a Martian scenario and measurements of the Martian atmosphere, the estimated current influx of abiotic organic molecules to the surface of Mars is around 100–300 tons per year (Eigenbrode et al., 2018). Given that majority of the Martian surface is covered by billions of years old rocks (Carr and Head, 2010), it must have received abundance of organic molecules over this time span. But various Mars lander missions so far found it challenging to detect organic molecules unambiguously on the Martian surface. This suggests a more rapid degradation (by ionic radiation or oxidizing agents) of organic matter than accumulation. However, the crystal structures of minerals found on Mars, such as sulfates and clay minerals, may retain organic molecules, shielding them from the hostile environment (Keil and Mayer, 2014).

The results of the Mars missions to date mostly point to the existence of powerful oxidants and ultraviolet light, which can significantly shorten the residence period of organic matter on the Martian surface and represent the biggest barrier to its identification using in situ techniques. However, recent evidences of organic compounds from SAM observations (Glavin et al., 2013; Freissinet et al., 2015; Eigenbrode et al., 2018; Millan et al., 2022a; Millan et al., 2022b) demonstrate the possibilities and ways to detect in situ organic matter on Mars. Moreover, Applin et al. (2015) found that oxalate minerals are particularly susceptible to accumulation at the surface, as their experiments indicate they are as stable to UV radiation as sulfates and carbonates. Other significant degrading agents for organic materials on Mars are solar energetic particles and galactic cosmic rays which are more powerful than UV as its influence can extends much deeper (up to 9 m) in the subsurface than UV light (Pavlov et al., 2002; Dartnell et al., 2007b; a; Pavlov et al., 2012; Pavlov et al., 2022).

According to Fornaro et al. (2018), it is extremely difficult to develop a general mechanism to predict the behavior of Martian minerals. However, an accurate simulation of Martian environment considering all the key factors would contribute significantly to understanding the trends. Studies of organic matter association with different mineralogy have shown that some minerals aid in organic preservation while others speed up their degradation (Fornaro et al., 2018; Brucato and Fornaro, 2019). For instance, Poch et al. (2015) found that when adenine and glycine molecules were exposed to mid-UV irradiation in iron (III) smectite clay and non-tronite at very high molecule-mineral mass ratios, the effectiveness of the two organic molecules’ photodecomposition was reduced by a factor of 5. This suggests not only physical shielding by non-tronite but also organic molecules stabilization by their interaction with the minerals allow recombination of dissociated organic molecule fragments (Poch et al., 2015). Furthermore, dos Santos et al. (2016) studied preservation of 25 amino acids in various minerals like olivine, enstatite, goethite, hematite, gypsum, jarosite, labradorite, augite, montmorillonite, non-tronite, and saponite, as well as basaltic lava. In their studies, clay minerals were found to exhibit amino acid-protective properties. The typical characteristics of clay minerals, such as their high surface area, which promotes molecular adsorption, their small pore sizes, which prevent radiation from penetrating, and their ideal interlayer sites for the accommodation of organic compounds, which create a shielded environment against external agents, were used to explain this behavior (dos Santos et al., 2016). Such minerals have been detected at several sites on Mars (Bishop et al., 2008; Ehlmann et al., 2009; Ehlmann et al., 2011a; Ehlmann et al., 2011b; Noe Dobrea et al., 2012; Carter et al., 2013; Ehlmann and Edwards, 2014; Vaniman et al., 2014; Adeli et al., 2015; Michalski et al., 2015) which has also been seen as a trace of past active hydrosphere. Millan et al. (2022b) discovered that highest diversity of organic molecules in Martian soil was associated with samples having highest proportion of clay and calcium sulfate minerals. These results comply with the recent findings that phyllosilicate clay minerals show better preservation potential for organic molecules, especially in active Fe-Mn redox environment (Gasda et al., 2022).

Detection of high abundance of thiophenic and other sulfur containing organic carbon in the Murray Formation mudstone (Confidence Hills) infer the possibility that sulfurization of organic matter during deposition or during early diagenesis would also have added their preservation (Eigenbrode et al., 2018). Observations from Earth’s natural environment have indicated that sulfurization of organic matter increases their refractory properties by reducing reactive functional groups and adding small cross links between small unstable molecules (Aubrey et al., 2006; Kotler et al., 2008). Incidentally, studies of terrestrial organic-mineral interaction have demonstrated better preservation of organic molecules by entrapment within the crystal structure (Aubrey et al., 2006; Kotler et al., 2008). Significant capacity to store organic molecules has also been seen in terrestrial hypersaline halite- and perchlorate subsurface deposits, which are common on Mars (Fernández-Remolar et al., 2013).

Applin et al. (2015), who reinterpreted the results collected by various pyrolysis-based instruments aboard Viking, Phoenix, and Curiosity, have recently reported the potential existence of oxalates on Mars in various regions (Biemann et al., 1977; Boynton et al., 2009; Mahaffy et al., 2012; Ming et al., 2014). Furthermore, a re-analysis of a Martian mudstone by Applin et al. (2015) using X-ray diffraction (XRD) of the Mars Science Laboratory (MSL) onboard Curiosity rover revealed that the data are compatible with the existence of refractory Ca, Fe, or Mg oxalates at low levels. It has been shown that oxalates can be produced from other organic compounds through both biological (Johnston and Vestal, 1993; Baran, 2014) and abiotic pathways (Durand and Nicaise, 1980; Peltzer et al., 1984). By interacting with aqueous cations, oxalate in solution forms weddellite, whewellite, and glushinskite, which are extremely resistant to chemical and physical degradation (Wilson et al., 1980; Chen et al., 2000; Frost and Weier, 2003; Frost, 2004). According to Cheng et al. (2016) oxalate formation may be a widespread process in the Martian surface condition and potentially play a key role in global carbon cycling and organic matter geochemistry.