The simulations for each site (Fig. 8, 9, 10) show that the depths corresponding to same concentration of methane do not change much from site to site. For example, 100 ppbv corresponds to a range of depths from 0.008 to 0.012.
In these simulations, one exception is the observation frs00041a28 in Nili Fossae area which shows greater depths with respect to the other sites. This could be related to the viewing geometry or season/hour or a combination of these variables, see Fig. 12.
The depths of the deepest pixels in these featured clusters in CRISM observations show different values that goes from − 0.013 to -0.057 (Table 3).
To avoid, as more as possible, misinterpretation of false absorptions due to unknown artifacts the threshold for depth to consider was set at µ + 5σ of depth maps. The thresholds range from 0.0136 to 0.0237 (Table 4).
Comparing the plots of the simulated depths with the threshold derived from the depth map statistics, we could say that, for concentrations lower than 180–600 ppbv, depending on the considered site, would increase the chance to find a false absorption.
Good candidates but artefacts
Among the known artifacts of CRISM, there is an optical effect due to out of band leakage in zone 3 of the IR order sorting filter. This leakage peaks appears at 3.4 µm (Murchie et al., 2007). However, this kind of artifacts generates positive signal peaks.
However, we consider that absorptions or a great part of them could represent an unknown artifact. In fact, although we carefully analyzed the noise typical of each image, we have considered that these absorptions could be a new artifact similarly to a probable artifact found in CRISM data at 3.18 µm (Viviano-Beck et al., 2014). Furthermore, it could be another possible artifact, similar to ones found by Leask et al., (2018) named “spurious absorptions or absorption-like features”. This artifact consists of absorptions on over 20 channels, showing gradual shoulders from the continuum value. In our data, to exclude, at least, it was an artifact introduced from the I/F calibration, we analyzed radiance and I/F data, before the atmospheric correction. As it can be seen in the example of Fig. 12, the featured pixel of the image frs00029b2 shows the 3.3 absorption both in radiance spectrum and in I/F. Therefore, 3.3 µm absorption is not related to the I/F correction.
Good candidates, potential methane spikes?
Despite all the precautions, there is still the possibility that the feature identified is related with an artifact, however, some of the identified locations could indeed be localized methane sources. If some of the clusters were related to methane emissions, their findings would strengthen the hypothesis of localized sources of methane in the subsurface. In fact, data on Martian methane concentrations include background values (Webster et al., 2018), spikes (Mumma et al., 2009), non-detections (Korablev et al., 2019) and seasonality (Moore et al., 2018). The results of this investigation can well fit with sources of methane in form of gas seepages (Etiope and Oheler, 2019). In this work we find that if methane whiffs were present as emissions from gas seepages, in the selected dataset, CRISM could detect the methane spectral features for concentrations > 180–600 ppb, depending on the site.
Unfortunately, it is not possible to calculate the flux of this potential source, because CRISM does not collect data periodically in fixed areas, being conceived for studying the mineralogy of Mars surface. However, the clusters that satisfied the two criteria we set for potential methane detection in CRISM data, consist of few pixels, 4–15. For each cluster, the value of the pixel with a deepest absorption is considered and listed in table 3. In general, the remaining pixels in the cluster show shallower absorptions. Which means that if these absorptions were methane, the clusters could represent diffusion of gas in the atmosphere from a source point, or a diffusion by a more spread source on the surface.
The concentrations found during this investigation are high respect to previous spikes and plumes detections, but remains in the order of hundreds of ppbv, also we do not have precise information regarding the time of methane removal/sinks from the atmosphere. In fact, the oxidation process on Mars destroys methane in about 300 years (Summers et al., 2002; Atreya et al., 2007). This mechanism is too long to explain, for example, the detection of the last spike of 21 ppbv by MSL (AbSciCon 24–28 June 2019) and the non-detection of Mars Express and ExoMars TGO (ESA’s Mars orbiters did not see latest Curiosity methane burst; Korablev et al., 2019), after some hours on the same area.
Several hypotheses were formulated for a shorter lifetime of CH4 that include gas-solids reactions, (Jensen et al., 2014; Holmes et al., 2015) however, the faster mechanism proposed for methane removal is the oxidation CH4 by the action of hydrogen peroxide in the regolith (Lefevre and Forget, 2009). This mechanism could shorten the methane life from 200 days to few hours near the surface.
If in some of the featured pixels observed, the 3.3 µm band was due to increase of methane gas, then it would be one of the few detections of 3.3 µm by an imaging spectrometer. The imaging spectrometer VIMS onboard Cassini mission detected a strong deep band of methane at 3.3 µm on Titan’s upper atmosphere. The estimation of abundance in this case was around 1.4% of the atmosphere (Maltagliati et al., 2014).
On the Earth, in 2010, a field experiment was performed at the former Rocky Mountain Oilfield Testing Center (RMOTC), Wyoming (USA), in which controlled flow rates of methane were released on surface and subsurface to simulate anthropogenic and natural sources. Simultaneously, the spectral imager on the SEBASS platform flew at 462 m and 762 m over these artificial sources of methane. The 3.3 µm band was detected on few pixels on surface and, in one station, also on the subsurface source (Scafutto et al., 2018).
Finally, very recently, the mid-infrared channel (MIR) of Atmospheric Chemistry Suite spectrometer onboard Trace Gas Orbiter (Korablev, et al., 2018) detected new bands in the range of methane absorptions, around 3.3 mm. These new bands were attributed to both ozone (Olsen et al., 2020) and to magnetic dipole and electric quadrupole 01111-00001 (ν2 + ν3) absorption bands of the main CO2 isotopologue (Trokhimovskiy et al., 2020).
The spectral features we observed at 3.3 mm in CRISM data could be also assigned to magnetic dipole CO2 absorption bands. Nevertheless, some of the differences between our investigation on CRISM data and ACS results stand in the geometry of the scene and location of investigations. The variation of latitude and geometry of the scene correspond also to variation in temperatures and pressures. In this work, we analyzed CRISM data acquired at Nadir whereas data from ACS where collected at solar occultation conditions. Moreover, we focused the investigation on CRISM data at mid latitudes; the ACS spectrometer focused to northern latitudes (> 65°N). However, currently, the new bands of ozone and CO2 magnetic dipole are not integrated in the HITRAN database. Consequently, it is not possible to model the absorption of CO2 and O3 at 3.3 mm with the PSG tool.
Organic matter and PAH’s
Some clusters that show absorptions at longer wavelengths, could be related to aliphatic hydrocarbons such as methane, as well as other aliphatic compounds that show absorptions at about 3.3–3.6 µm.
For example, aliphatic compounds were individuated in the spectral features of the comet 67P/Churyumov-Gerasimenko nucleus by VIRTIS spectrometer during the Rosetta mission (Raponi et al., 2020; Capaccioni et al., 2015).
Aliphatic features similar to kerite and asphaltite at 3.38 to 3.50 µm were found in the spectra of the crater Ernutet on Ceres asteroid by VIR spectrometer onboard Dawn mission.
Beside aliphatic compounds also aromatic hydrocarbons have been found in different planetary environments and materials. In particular, polycyclic aromatic hydrocarbons (PAHs) have been found in the organic fraction in carbonaceous chondrites (CCs) (Sephton et al. 1998; Botta and Bada 2002; Sephton 2002). Moreover, PAHs include up to 20% of the carbon material in the interstellar medium (ISM) (Allamandola et al. 1985). Signatures of PAHs have recently been identified in the atmosphere of Titan (López-Puertas et al., 2013).
Campbell et al., 2018 investigated on hydrocarbons detection on Mars South Polar Cap although the feature at 3.3 µm was difficult to interpret due to the strong absorption by the CO2 ice, Oancea et al., 2012.
Even if studies (Dartnell et al., 2012; Pavlov et al., 2012) on Mars surface revealed a short lifetime and rapid degradation for PAH’s in the shallow surface due to UV and ionizing radiations the eventual occasional occurrence of PAH’s on surface images and the related 3.3 µm band absorption in CRISM data can be linked to PAH’s bearing impacting bodies on Mars surface or in correspondence of fresh crater outcrops (Blanco et al., 2018).
Implications for ExoMars2022 and other rover missions
In 2022 the Exomars mission (Vago et al., 2017) will deliver the Kazachock surface platform and the Rosalind Franklin rover on Mars surface that will host several instruments onboard.
On the rover, almost all these instruments will provide data on eventual C-H compounds and organic molecules. Therefore, for what concerns Oxia Planum the results of this investigation would potentially be compared.
Once landed the rover, the Infrared Spectrometer for ExoMars (ISEM) will work coupled with the PanCam camera to select interesting sites for biosignatures. It has a spectral range of 1.15 to 3.3 µm with a spectral resolution 3.3 µm at 1.15 µm and 28 nm at 3.30 µm. As already seen, the range just end at the value of C-H absorption at 3.3 µm, therefore a potential comparison could be done, also looking for absorptions in the 2.2–2.4 µm range of C-H compounds. At micrometric scale, MicrOmega (Micro observatoire pour la mineralogie, l'eau, les glaces et l'activité) -IR will analyse in situ the powder material derived from crushed samples collected by the rover's core drill MaMISS. MicrOmega-IR has an IR range from 0.95 to 3.65 µm in 320 channels of about 8 nm of spectral resolution.
The analyses of samples collected in a depth up to 2 m by MaMISS will be very useful. In fact, either sample extracted will be not so much irradiated and damaged as the surface materials, and this increases the probability to find organic compounds. Moreover, in case of methane, this will be rapidly detected in the original abundance with respect to methane detected on the surface, which is mixed and/or removed from the Mars near surface atmosphere. This would be potentially possible, searching for other C-H absorptions at also in the region from 2.2 to 2.6 um, range that is characterized by combination and overtone bands (Cloutis, 1989).
Finally, visible and NIR data on crushed samples will be compared with data from Raman Laser Spectrometer (RLS) that will permit the identification of minerals and the detection of different organic functional groups to be successively analyzed by the Mars Organic Molecules Analyzer (MOMA), (Rull et al., 2017). One of the major goals of the MOMA analyzer will be to assess whether the potential organic compounds detected are biogenic or abiogenic (Goetz et al., 2018).