Free Access
Issue
A&A
Volume 649, May 2021
Article Number L1
Number of page(s) 5
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202140868
Published online 30 April 2021

© ESO 2021

1. Introduction

Phosphate () is an essential molecule for all forms of life on Earth. Phosphine (PH3) is a very reactive, rapidly photolyzed, and highly variable component of the terrestrial atmosphere. Found at parts per trillion by volume (pptv) to parts per billion (ppbv) volume mixing ratios (VMRs), it accounts for approximately 10% of Earth’s atmospheric phosphorus (Glindemann et al. 2003; Elm et al. 2017). It does not play a direct role in Earth’s biology but acts as an exchange path for phosphorous, being emitted from the biosphere, oxidized to phosphoric acid (H3PO4), and then taken up again in biological systems. The dominant sources of phosphine in the terrestrial atmosphere are emissions from soils, sediments, and manures (Glindemann et al. 2005a). In contrast, abiotic PH3 production may be ongoing in the hot and reducing deep atmospheres of Saturn and Jupiter (e.g. Bregman et al. 1975; Burgdorf et al. 2004; Fletcher et al. 2009).

Phosphine may play a role in the anaerobic biological systems of anoxic environments, where it will not be readily oxidized (Bains et al. 2019; Sousa-Silva et al. 2020). One such proposed environment is the cloud decks of Venus (Seager et al. 2020). Thus, when the first detection of a phosphine absorption line (at 1.123 mm) in the Venusian atmosphere was announced (Greaves et al. 2020a), it was hailed as evidence of a possible microbial habitat in the upper atmosphere of Venus. The result has since been criticized on technical grounds (Snellen et al. 2020; Villanueva et al. 2020; Thompson 2021; Akins et al. 2021; Lincowski et al. 2021), and other observations did not corroborate it (Encrenaz et al. 2020; Trompet et al. 2021); however, the original team stands by their findings (Greaves et al. 2020b,c) and has reported revised mixing ratios of 5–20 ppbv.

On Mars, methane is another possible biomarker that has been tentatively detected (Formisano et al. 2004; Krasnopolsky et al. 2004; Mumma et al. 2009; Webster et al. 2015; Giuranna et al. 2019), though not confirmed by the ExoMars Trace Gas Orbiter (TGO; Korablev et al. 2019), and thus measurements of phosphine may be able to help identify its source. For example, identifying methane and phosphine would hint towards a biological source of methane, whereas finding methane alongside sulphur species would imply a geologic source of both trace gases.

The primary objective of the ExoMars TGO mission is to search for undetected trace gases diagnostic of active geological or biogenic processes (Vago et al. 2015). To this end, it carries two spectrometer suites, the Atmospheric Chemistry Suite (ACS; Korablev et al. 2018) and the Nadir and Occultation for Mars Discovery (NOMAD; Vandaele et al. 2018), which both offer unprecedented sensitivity due to their fine spectral resolutions and solar occultation technique. Phosphine (PH3) was not an initial target gas of ExoMars and was not proposed as a possible biomarker prior to the arrival of the TGO in Martian orbit in 2018 (Seager et al. 2016; Schwieterman et al. 2018). However, some of the strongest infrared absorption lines that do not overlap with the CO2ν3 absorption band lie within the spectral range of the ACS mid-infrared channel (ACS MIR), providing an opportunity to search for the signature of PH3 in the atmosphere of Mars.

So far, neither the biomarker CH4 nor the related gases C2H4 and C2H6 have been observed with ACS or NOMAD (Korablev et al. 2019; Knutsen et al. 2021; Montmessin et al. 2021). Here, we report new upper limits for phosphine in the atmosphere of Mars of 0.2 ppbv above 10 km.

2. Observations

ACS is a suite of three spectrometers: The thermal infrared channel (TIRVIM) is a compact Fourier transform spectrometer with a spectral range of 1.7–17 μm; the near-infrared channel (NIR) combines an echelle grating with an acousto-optical tunable filter and covers the range 1–1.7 μm; and ACS MIR is a cross-dispersion spectrometer consisting of an echelle grating and a secondary, steerable reflecting diffraction grating. ACS MIR has the finest spectral resolution of any remote sensing mission to Mars to date (λλ ∼ 30 000) and covers a spectral range of 2400–4400 cm−1 (2.2–4.2 μm; Korablev et al. 2018).

ACS MIR operates in solar occultation mode, recording a series of spectra with tangent height spacings of 1–2 km by using the Sun as a light source and observing the limb of the atmosphere. The secondary diffraction grating separates diffraction orders that would otherwise overlap in the infrared spectral region. Observations are measured with a two-dimensional detector array. The horizontal axis corresponds to the wavenumber, and the vertical axis corresponds firstly to the diffraction order and secondly to the vertical field of view of the instrument. Each order appears as a stripe across the detector, the width of which covers around 20 rows of pixels, each row producing a unique spectrum recorded ∼0.2 km above or below the adjacent rows. Thus, at every tangent height several spectra are recorded for each order, representing a vertical range of several kilometres. Details about the instrument and data processing, as well as example detector image frames, can be found in Korablev et al. (2018); Korablev et al. (2019); Trokhimovskiy et al. (2020), and Olsen et al. (2021a).

The absorption signature for PH3 can be sought by using secondary grating position 9, which provides access to diffraction orders 142–152, covering the spectral range 2377–2560 cm−1; these are the lowest wavenumbers available to ACS MIR. Figure 1a shows modelled PH3 absorption features with ACS MIR spectral resolution, indicating the locations of the most significant PH3 absorption features over orders 142–143. Figure 1b shows example spectra from the same orders recorded during one occultation. At each tangent height, a single spectrum was extracted for each order. The change in baseline transmittance is mainly due to increasing aerosol extinction as tangent heights approach the surface. The left side of the spectrum is dominated by the strong ν3 vibration-rotation band of CO2, the continuum absorption of which also impacts the transmittance of adjacent orders. While the strongest PH3 lines directly overlap the CO2ν3 band, prominent lines free of interference remain in order 143, which is where this study focuses (lines in order 144 are too weak, and interference from CO2 is too strong in order 142; see Fig. 1).

thumbnail Fig. 1.

Spectral range of ACS MIR containing PH3 absorption features. Panel a: modelled transmission spectrum contributions from 10 ppbv of PH3 from a 20 km tangent height. Panel b: single spectra extracted from orders 142–145 for 12 consecutive tangent heights (2–26 km) in the lower atmosphere of Mars using ACS MIR secondary grating position 9. All visible absorption lines result from CO2, which also impacts the broad continuum that decreases transmission towards lower wavenumbers, where the CO2ν3 vibration-rotation band is centred. Boxes in panel a indicate the spectral windows covering PH3 absorption features in order 143 used to determine upper limits.

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The line strengths of the strongest PH3 lines in this spectral region are about 1.5 × 10−19 in the 2016 edition of the HITRAN database, which is used in the forward modelling in this study (Gordon et al. 2017, we note that Greaves et al. 2020a used the ExoMol line list for high temperatures). This is comparable to those used by ACS MIR and others to search for methane. Figure 2 shows the line strengths of the ν3 band of methane and the ν3 band of phosphine, with the spectral ranges of diffraction orders used to search for them indicated. The expectation, then, is that ACS MIR should have roughly similar sensitivity to both trace gases, of the order of less than a part per billion. Upper limits for methane from the ACS MIR data are lower than for PH3, around tens to hundreds of pptv (Korablev et al. 2019).

thumbnail Fig. 2.

HITRAN 2016 line strengths for the ν3 vibration-rotation band of CH4 (panel a) and the ν3 vibration-rotation band of PH3 (panel b). The spectral ranges of important ACS MIR diffraction orders are indicated in grey.

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The ExoMars TGO entered Mars orbit in late 2016, and its nominal science phase commenced in April 2018. It is approaching 1.5 Mars years (MYs) of coverage (three Earth years). The dataset analysed here consists of all 192 usable occultations recorded between Ls = 140–350° in both MY 34 and 35 (we note that there was a period between MYs in which position 9 was not used). The majority of observations are at high northern or southern latitudes (around 60°N and −60°S), but several low latitude observations were also made.

3. Results

To make the most accurate measurements of rare trace gases possible, we exploited the vertical field of view of the ACS MIR instrument and considered 12 spectra for diffraction order 143 from each detector frame recorded at each observation in the occultation series. Each spectrum has a unique tangent altitude, with corresponding temperatures and pressures, and some also have a unique instrument line shape (ILS) that needed to be fitted prior to VMR retrievals. The ILS was impacted by an optical aberration that caused the total absorption to be divided into a primary and secondary image (Alday et al. 2019; Olsen et al. 2021a) that had to be accounted for. The accuracy of retrievals using a fitted ILS for ACS MIR was verified by comparing retrieved vertical profiles of temperature, pressure, water vapour, and carbon monoxide with simultaneous observations made with ACS NIR (Alday et al. 2019; Fedorova et al. 2020; Olsen et al. 2021a).

To best account for the observational differences affecting adjacent spectra, each of the 12 detector rows from which spectra were extracted was treated as a unique solar occultation sequence and analysed independently. The upper limits were derived from the mean of the 12 retrieved VMR vertical profiles. This method was developed to examine the weak signature of HDO in the upper atmosphere towards the hygropause but has effectively been used to study hydrogen chloride (HCl) Olsen et al. (2021b), the first novel trace gas discovered by ACS MIR (Korablev et al. 2021). Vertical profiles of temperature and pressure are independently measured by simultaneous observations with ACS NIR, which observes a CO2 band at 1.38 μm (7250 cm−1). Spectral fitting is done with the Jet Propulsion Laboratory’s Gas Fitting software suite (GFIT or GGG) (Sen et al. 1996; Irion et al. 2002; Wunch et al. 2011).

Vertical profiles of a fitted gas’s VMR were retrieved by inverting the matrices of line-of-sight optical paths through the atmospheric layers with the matrices of estimated number densities derived from the spectral fits at each altitude. Two fitting windows covering the strongest features in order 143 were used (2394.95–2401.53 cm−1 and 2401.52–2405.84 cm−1; see Fig. 1). Uncertainties were computed from the Jacobian matrix of partial derivatives. Additional details about spectral fitting and VMR retrievals can be found in Olsen et al. (2020); Olsen et al. (2021a), and Korablev et al. (2021), and details about the analysis method used here can be found in Olsen et al. (2021b).

Figure 3a shows an example of the ACS MIR data and helps to illustrate the PH3 analysis technique. For one occultation, at a tangent height near 13 km, each row extracted from order 143 in the detector frame has been normalized and is shown. Also shown is the mean spectrum, with greatly reduced instrument noise (the root mean square of fitting residuals is reduced from ∼0.0014 to 0.0008). The observed absorption lines are due to CO2, and two best-fit lines are shown, for which PH3 abundances were fixed in order to illustrate where PH3 features would be found. We note that the noise to the left, near 2396 cm−1, is larger than elsewhere. This is because it is near the edge of the detector frame and has a lower signal strength. We also note that there appears to be a systematic error fitting the CO2 lines from left to right; this is due to the normalization of the spectra, which can safely remove the impact of aerosols under normal circumstances but should not be performed in this region due to the CO2 continuum near the CO2ν3 band. Normalization was not used when computing detection limits; it is only used in Fig. 3a to facilitate the comparison of the spectra and to illustrate the line locations of PH3 absorption features, the impact that 10 ppbv of PH3 would have, and the noise level of ACS MIR spectra.

thumbnail Fig. 3.

Example ACS MIR spectra and upper limits for PH3. Panel a: normalized spectra for diffraction order 143 extracted from a single detector frame observed near 13 km tangent height. The lines in various shades of grey are the individual normalized spectra, the black line is a mean spectrum, and the dashed and solid purple lines are best-fit lines to the mean spectrum with two fixed quantities of PH3, 5 and 10 ppbv, respectively. Panel b: 1σ detection limits derived from non-normalized spectra (shades of grey) and from the mean spectrum (black). The standard error computed from all 12 sets of spectra is also shown, as in Fig. 4. The detection limit derived near 13 km is 0.5 ppbv for the entire occultation and 1.5–2.5 ppbv for the individual extracted detector rows.

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thumbnail Fig. 4.

Vertical profiles of the standard error of the weighted mean for 192 ACS MIR solar occultations observed between Ls = 140–350° in MY 34 and 35. The lines are coloured by the Gaussian kernel density estimate for the entire dataset, with the highest density of data appearing as orange and yellow.

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To determine whether PH3 was detected, we considered the set of 12 retrieved VMR vertical profiles. We computed the weighted mean, using the uncertainty (σ) of each retrieval as weights (1/σ2), and computed the standard error of the weighted mean. Figure 3b shows the VMR vertical profile retrieval uncertainties for the 12 sets of solar occultation spectra from which those shown in Fig. 3a were taken. Also shown are the retrieval uncertainties derived using the mean spectra, which are lower than those of the individual spectra due to the reduced noise, but still greater than the standard error of the weighted mean, which is the uncertainty of the mean retrieved VMR vertical profile.

If the retrieved VMR was greater than three or five times the standard error, we considered it to be a possible detection. We would then verify it by examining the ensemble of spectra and mean spectra at each altitude. However, no candidate detections were found.

In the absence of any detection, we interpreted the vertical profile of the standard error of the weighted mean as an upper limit to the detection. These 1σ upper limits are shown in Fig. 4 for the entire ACS MIR dataset recorded thus far using secondary grating position 9. Each profile is coloured by the density of observations, derived using a Gaussian kernel density estimation for the entire dataset. The density is low below 10 km due to differences in the lowest observable tangent height. That is, only a fraction of the occultations are clearly able to probe the lowest layers of the atmosphere due to aerosol extinction (dust is present during most of the observation period). Above 50 km, the differences are related to the total number density, which varies with the latitude and time of observation.

4. Discussion and conclusions

The highest density of derived upper limits occurs near 30 km and at a detection limit of ∼0.6 ppbv. The lowest values are < 0.2 ppbv and are reached at altitudes between 10 and 35 km. These are comparable to the reported upper limits in Korablev et al. (2019) for CH4 of 0.15 ppbv. While several ACS MIR observations led to CH4 detection limits of the order of tens of pptv, we must note that the R-branch CH4 lines available are stronger than the PH3 lines in Fig. 3 (e.g. compare to Fig. 2), and they are more towards the centre of their diffraction orders, which leads to a higher signal-to-noise ratio. Furthermore, potential CH4 lines are not subject to the strong absorption of the CO2ν3 continuum, which effectively reduces the line depth relative to the noise. We also note that, relative to the Venusian observations of 20 ppbv, the line strengths of PH3 in the ACS MIR spectral range (∼1.5 × 10−19) are much larger than that of the single line claimed in Venus’s atmosphere (1.27 × 10−22) Greaves et al. (2020a).

Had PH3 been detected at Mars, a biological origin would not be certain. The Martian environment is not anoxic, unlike the environments proposed by Sousa-Silva et al. (2020), and biogenic (or other) PH3 may be removed by rapid oxidation by the hydroxyl radical (OH), itself a by-product of water vapour photolysis.

While ACS MIR observations have not detected any sulphur species indicative of a geological source of methane, they have detected hydrogen chloride, which may have a magmatic or volcanic source (Korablev et al. 2021). However, ACS MIR has also not detected any signatures of methane in the atmosphere of Mars (Montmessin et al. 2021). Phosphorous is found on the surface of Mars (e.g. Gellert et al. 2004; Rieder et al. 2004), and phosphate has been identified in Martian meteorites (e.g. McCubbin & Nekvasil 2008; Agee et al. 2013). Though not favoured, there are abiotic processes that may release PH3 gas from the surface (Glindemann et al. 2004, 2005b), and volcanic processes have even been proposed to explain its presence on Venus (Truong & Lunine 2020).

Thus, while PH3 has been proposed as a possible biomarker for anoxic exoplanetary environments, it is not observed in the atmosphere of Mars, where the biomarker CH4 had previously been reported (although we note that these reports remain controversial and have not been corroborated by the ExoMars TGO). Due to the highly oxidative state of the Martian atmosphere, however, its presence was perhaps unexpected. An upper limit of 0.2–0.6 ppbv is set for the atmosphere of Mars between 10 and 45 km.

Acknowledgments

The ACS investigation was developed by the Space Research Institute (IKI) in Moscow, and the Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS/CNRS) in Paris. The investigation was funded by Roscosmos, the National Centre for Space Studies of France (CNES) and the Ministry of Science and Education of Russia. The GGG software suite is maintained at JPL (tccon-wiki.caltech.edu). This work was funded by the UK Space Agency (ST/T002069/1, ST/V002295/1). All spectral fitting was performed by KSO using the GGG software suite. The interpretation of the results was done by KSO, AT, ASB, PGJI, MP, OK, FM, and FL. The processing of ACS spectra is done at IKI by AT and at LATMOS by LB. Input and aid on spectral fitting were given by JA, DAB, AF, and CFM. The ACS instrument was designed, developed, and operated by AP, AS, AT, FM, and OK. Vertical profiles of measured temperature and pressure were generated by AF.

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All Figures

thumbnail Fig. 1.

Spectral range of ACS MIR containing PH3 absorption features. Panel a: modelled transmission spectrum contributions from 10 ppbv of PH3 from a 20 km tangent height. Panel b: single spectra extracted from orders 142–145 for 12 consecutive tangent heights (2–26 km) in the lower atmosphere of Mars using ACS MIR secondary grating position 9. All visible absorption lines result from CO2, which also impacts the broad continuum that decreases transmission towards lower wavenumbers, where the CO2ν3 vibration-rotation band is centred. Boxes in panel a indicate the spectral windows covering PH3 absorption features in order 143 used to determine upper limits.

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In the text
thumbnail Fig. 2.

HITRAN 2016 line strengths for the ν3 vibration-rotation band of CH4 (panel a) and the ν3 vibration-rotation band of PH3 (panel b). The spectral ranges of important ACS MIR diffraction orders are indicated in grey.

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In the text
thumbnail Fig. 3.

Example ACS MIR spectra and upper limits for PH3. Panel a: normalized spectra for diffraction order 143 extracted from a single detector frame observed near 13 km tangent height. The lines in various shades of grey are the individual normalized spectra, the black line is a mean spectrum, and the dashed and solid purple lines are best-fit lines to the mean spectrum with two fixed quantities of PH3, 5 and 10 ppbv, respectively. Panel b: 1σ detection limits derived from non-normalized spectra (shades of grey) and from the mean spectrum (black). The standard error computed from all 12 sets of spectra is also shown, as in Fig. 4. The detection limit derived near 13 km is 0.5 ppbv for the entire occultation and 1.5–2.5 ppbv for the individual extracted detector rows.

Open with DEXTER
In the text
thumbnail Fig. 4.

Vertical profiles of the standard error of the weighted mean for 192 ACS MIR solar occultations observed between Ls = 140–350° in MY 34 and 35. The lines are coloured by the Gaussian kernel density estimate for the entire dataset, with the highest density of data appearing as orange and yellow.

Open with DEXTER
In the text

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