Spectrometer Onboard Mars Express Research Articles

Independent confirmation of a methane spike on Mars and a source region east of Gale Crater

Reports of methane detection in the Martian atmosphere have been intensely debated. The presence of methane could enhance habitability and may even be a signature of life. However, no detection has been confirmed with independent measurements. Here, we report a firm detection of 15.5 ± 2.5 ppb by volume of methane in the Martian atmosphere above Gale Crater on 16 June 2013, by the Planetary Fourier Spectrometer onboard Mars Express, one day after the in situ observation of a methane spike by the Curiosity rover. Methane was not detected in other orbital passages. The detection uses improved observational geometry, as well as more sophisticated data treatment and analysis, and constitutes a contemporaneous, independent detection of methane. We perform ensemble simulations of the Martian atmosphere, using stochastic gas release scenarios to identify a potential source region east of Gale Crater. Our independent geological analysis also points to a source in this region, where faults of Aeolis Mensae may extend into proposed shallow ice of the Medusae Fossae Formation and episodically release gas trapped below or within the ice. Our identification of a probable release location will provide focus for future investigations into the origin of methane on Mars. A methane spike 15.5 ± 2.5 parts per billion by volume was detected in the Martian atmosphere above Gale Crater on 16 June 2013 by Mars Express, independently confirming the debated in situ observation by the Curiosity rover a day earlier.

Mesospheric CO2 ice clouds on Mars observed by Planetary Fourier Spectrometer onboard Mars Express

We have investigated mesospheric CO2 ice clouds on Mars through analysis of near-infrared spectra acquired by Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEx) from MY 27 to MY 32. With the highest spectral resolution achieved thus far in the relevant spectral range among remote-sensing experiments orbiting Mars, PFS enables precise identification of the scattering peak of CO2 ice at the bottom of the 4.3 μm CO2 band. A total of 111 occurrences of CO2 ice cloud features have been detected over the period investigated. Data from the OMEGA imaging spectrometer onboard MEx confirm all of PFS detections from times when OMEGA operated simultaneously with PFS. The spatial and seasonal distributions of the CO2 ice clouds detected by PFS are consistent with previous observations by other instruments. We find CO2 ice clouds between Ls = 0° and 140° in distinct longitudinal corridors around the equatorial region (± 20°N). Moreover, CO2 ice clouds were preferentially detected at the observational LT range between 15–16 h in MY 29. However, observational biases prevent from distinguishing local time dependency from inter-annual variation. PFS also enables us to investigate the shape of mesospheric CO2 ice cloud spectral features in detail. In all cases, peaks were found between 4.240 and 4.265 μm. Relatively small secondary peaks were occasionally observed around 4.28 μm (8 occurrences). These spectral features cannot be reproduced using our radiative transfer model, which may be because the available CO2 ice refractive indices are inappropriate for the mesospheric temperatures of Mars, or because of the assumption in our model that the CO2 ice crystals are spherical and composed by pure CO2 ice.

Search for hydrogen peroxide in the Martian atmosphere by the Planetary Fourier Spectrometer onboard Mars Express

We searched for hydrogen peroxide (H2O2) in the Martian atmosphere using data measured by the Planetary Fourier Spectrometer (PFS) onboard Mars Express during five martian years (MY27–31). It is well known that H2O2 plays a key role in the oxidizing capacity of the Martian atmosphere. However, only a few studies based on ground-based observations can be found in the literature. Here, we performed the first analysis of H2O2 using long-term measurements by a spacecraft-borne instrument. We used the ν4 band of H2O2 in the spectral range between 359cm−1 and 382cm−1 where strong features of H2O2 are present around 362cm−1 and 379cm−1. Since the features were expected to be very weak even at the strong band, sensitive data calibrations were performed and a large number of spectra were selected and averaged. We made three averaged spectra for different seasons over relatively low latitudes (50°S–50°N). We found features of H2O2 at 379cm−1, whereas no clear features were detected at 362cm−1 due to large amounts of uncertainty in the data. The derived mixing ratios of H2O2 were close to the detection limits: 16±19ppb at Ls=0–120°, 35±32ppb at Ls=120–240°, and 41±28ppb at Ls=240–360°. The retrieved value showed the detection of H2O2 only for the third seasonal period, and the values in the other periods provided the upper limits. These long-term averaged abundances derived by the PFS generally agreed with the ones reported by ground-based observations. From our derived mixing ratio of H2O2, the lifetime of CH4 in the Martian atmosphere is estimated to be several decades in the shortest case. Our results and sporadic detections of CH4 suggest the presence of strong CH4 sinks not subject to atmospheric oxidation.

Annual survey of water vapor vertical distribution and water–aerosol coupling in the martian atmosphere observed by SPICAM/MEx solar occultations

The vertical distribution of water vapor is a very important diagnostic to determine the physical and chemical processes that drive the martian water cycle. Yet, very few direct measurements have been performed so far, and our knowledge of the H2O vertical distribution on Mars relies on General Circulation Models (GCMs). The study presented here follows for the first time the evolution of water vapor profile during a martian year. 120 profiles, obtained by the SPICAM spectrometer onboard Mars Express with the solar occultations technique, are retrieved. They cover the northern spring-summer season and the southern spring of Mars Year (MY) 29. The seasonal evolution of H2O mixing ratio vertical distribution reveals its strong dynamism, especially during southern spring. There are significant discrepancies with the predictions of the General Circulation Model developed at the Laboratoire de Météorologie Dynamique (LMD-GCM). The LMD-GCM underestimates the water vapor content in the middle atmosphere. The measured profiles also exhibit often abrupt temporal variations and a greater variety of shapes, with the frequent presence of detached layers. We believe that the model underestimates the strength of the coupling between water vapor and aerosols, whose slant optical depth profile is obtained by SPICAM simultaneously with H2O. The SPICAM measurements can be grouped according to the mutual behavior of the two profiles. Individual features are often related too. The presence of water supersaturation and of correlated aerosol–water detached layers highlights the role of water ice clouds as a favorable location for the dust–water coupling. The water vapor vertical distribution is more reactive than expected to regional perturbations, which can propagate rapidly through the atmosphere, create abrupt water vapor and aerosol upsurges and influence the large-scale vertical evolution of these two constituents. This phenomenon has been observed thrice during MY29. The martian annual water cycle revealed by the SPICAM profiles exhibits a different behavior with respect to nadir observations. This result suggests a generally weak connection between the upper atmosphere and the lower atmospheric layers, to whom the nadir measurements are most sensitive and that are not resolved by SPICAM occultations, and hints at a significant influence of surface-atmosphere interactions on the water cycle.

Computing uncertainties in ionosphere‐airglow models: I. Electron flux and species production uncertainties for Mars

The ionization and excitation of atoms and molecules in the upper atmospheres of the Earth and planets are computed by a number of physical models. From these calculations, quantities measurable by dedicated satellite experiments such as airglow and electron fluxes can be derived. It is then possible to compare model and observation to derive more fundamental physical properties of the upper atmospheres, for example, the density as a function of altitude. To ensure the accuracy of these retrieval techniques, it is important to have an estimation of the uncertainty of these models and to have ways to account for these uncertainties. The complexity of kinetic models for computing the secondary production of excited state species (including ions) makes it a difficult evaluation, and studies usually neglect or underestimate it. We present here a Monte‐Carlo approach to the computation of model uncertainties. As an example, we studied several aspects of the model uncertainties in the upper atmosphere of Mars, including the computed secondary electron flux and the production of the main ion species. Our simulations show the importance of improving solar flux models, especially on the energy binning and on the photon impact cross sections, which are the main sources of uncertainties on the dayside. The risk of modifying cross sections on the basis of aeronomical observations is highlighted for the case of Mars, while accurate uncertainties are shown to be crucial for the interpretation of data from the particle detectors onboard Mars Global Surveyor. Finally, it shows the importance of AtMoCiad, a public database dedicated to the evaluation of aeronomy cross section uncertainties. A detailed study of the resulting emissions cross sections uncertainties is the focus of a forthcoming paper (Gronoff et al., 2012) in which the outputs discussed in the present paper are used to compute airglow uncertainty, and the overall result is compared with the data from the SPICAM UV spectrometer onboard Mars Express.

Annual survey of water vapor behavior from the OMEGA mapping spectrometer onboard Mars Express

We present here the annual behavior of atmospheric water vapor on Mars, as observed by the OMEGA spectrometer on board Mars Express during its first martian year. We consider all the different features of the cycle of water vapor: temporal evolution, both at a seasonal and at a diurnal scale; longitudinal distribution; and the vertical profile, through the variations in the saturation height. We put our results into the context of the current knowledge on the water cycle through a systematic comparison with the already published datasets. The seasonal behavior is in very good agreement with past and simultaneous retrievals both qualitatively and quantitatively, within the uncertainties. The average water vapor abundance during the year is ∼10 pr. μm, with an imbalance between northern and southern hemisphere, in favor of the first. The maximum of activity, up to 60 pr. μm, occurs at high northern latitudes during local summer and shows the dominance of the northern polar cap within the driving processes of the water cycle. A corresponding maximum at southern polar latitudes during the local summer is present, but less structured and intense. It reaches ∼25 pr. μm at its peak. Global circulation has some influence in shaping the water cycle, but it is less prominent than the results from previous instruments suggest. No significant correlation between water vapor column density and local hour is detected. We can constrain the amount of water vapor exchanged between the surface and the atmosphere to few pr. μm. This is consistent with recent results by OMEGA and PFS-LW. The action of the regolith layer on the global water cycle seems to be minor, but it cannot be precisely constrained. The distribution of water vapor on the planet, after removing the topography, shows the already known two-maxima system, over Tharsis and Arabia Terra. However, the Arabia Terra increase is quite fragmented compared with previous observations. A deep zone of minimum separates the two regions. The saturation height of water vapor is mainly governed by the variations of insolation during the year. It is confined within 5–15 km from the surface at aphelion, while in the perihelion season it stretches up to 55 km of altitude.

A study of the bound water, water ice, and frost distribution over the Martian surface: Treatment and correcting of the data of observations with the OMEGA spectrometer onboard Mars Express

The results of the analysis of the spectral observations of Mars carried out with the OMEGA spectrometer onboard the Mars Express spacecraft are presented. The data from one of the spectrometer’s channels working in the near-IR spectral range (0.93–2.69 µm) were analyzed. This range includes the characteristic absorption bands of both condensed water phases (ice and frost) and bound water contained in hydrated minerals of the Martial soil. From the 1.93-µm band indicating the presence of these minerals, global maps of the bound-water index have been made. They show a noticeable latitude dependence of the index: the largest values refer to high latitudes (>60°), while they gradually diminish toward the equator. Seasonal variations of the spectral index obtained by the 1.93-µm band are connected with the hydration-dehydration processes occurring in hydrogenous minerals when the temperature of the soil and the relative humidity in the near-surface atmospheric layer are changing. The evolution of the spectral absorption bands of water ice (1.2 and 1.5 µm) dependent on the season testifies to the changes in the microstructure of the surface layer in the North ploar cap caused by the sublimate re-crystallization processes in the ice sheet. The spatial pattern of the location of the areas where the microstructure most quickly grows could be formed under the influence of the stationary atmospheric waves.

Ferric oxides in East Candor Chasma, Valles Marineris (Mars) inferred from analysis of OMEGA/Mars Express data: Identification and geological interpretation

The mineralogical composition of the Martian surface is constrained by analyzing the data of the OMEGA visible and near infrared imaging spectrometer onboard Mars Express. Ferric signatures had previously been reported in Valles Marineris, Margaritifer Terra, and Terra Meridiani. Here we use three independent data reduction methods (Spectral Angle Mapper, a modified Spectral Mixture Analysis and Modified Gaussian Model) to detect and map ferric oxides in East Candor Chasma, a part of Valles Marineris. Ferric oxides in East Candor Chasma are concentrated in scattered formations. MOLA altimetry indicates that the ferric oxides are preferentially located in topographic lows. THEMIS, HRSC and MOC images show that the ferric oxide spectral signatures are systematically correlated with superficial deposits of low albedo, located at the foot of, or resting on Interior Layered Deposits (ILDs). This spatial distribution suggests that ferric oxides are genetically linked to ILDs. Gravity and wind‐driven remobilization of ferric oxides previously formed in the ILDs can explain their accumulation around the ILDs.

Analysis of OMEGA/Mars Express data hyperspectral data using a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM): Methodology and first results

The mineralogical composition of the Martian surface is investigated by a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM) of the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) imaging spectrometer onboard Mars Express. OMEGA has fully covered the surface of the red planet at medium to low resolution (2–4 km per pixel). Several areas have been imaged at a resolution up to 300 m per pixel. One difficulty in the data processing is to extract the mineralogical composition, since rocks are mixtures of several components. MELSUM is an algorithm that selects the best linear combination of spectra among the families of minerals available in a reference library. The best fit of the observed spectrum on each pixel is calculated by the same unmixing equation used in the classical Spectral Mixture Analysis (SMA). This study shows the importance of the choice of the input library, which contains in our case 24 laboratory spectra (endmembers) of minerals that cover the diversity of the mineral families that may be found on the Martian surface. The analysis is restricted to the 1.0–2.5 μm wavelength range. Grain size variations and atmospheric scattering by aerosols induce changes in overall albedo level and continuum slopes. Synthetic flat and pure slope spectra have therefore been included in the input mineral spectral endmembers library in order to take these effects into account. The selection process for the endmembers is a systematic exploration of whole set of combinations of four components plus the straight line spectra. When negative coefficients occur, the results are discarded. This strategy is successfully tested on the terrestrial Cuprite site (Nevada, USA), for which extensive ground observations exist. It is then applied to different areas on Mars including Syrtis Major, Aram Chaos and Olympia Undae near the North Polar Cap. MELSUM on Syrtis Major reveals a region dominated by mafic minerals, with the oldest crustal regions composed of a mixing between low-calcium pyroxenes (LCPs) (orthopyroxenes (OPx)) and high-calcium pyroxenes (HCPs) (clinopyroxenes (CPx)). The Syrtis volcanic edifice appears depleted in LCP (OPx) and enriched in HCP (CPx), which is consistent with materials produced with a lower partial fusion degree at an age younger to the surrounding crust. Strong olivine signatures are found between the two calderas Nili Patera and Meroe Patera and in Nili Fossae. A strong signature of iron oxides is found within Aram Chaos, with a spatial distribution also consistent with thermal emission spectrometer (TES). Gypsum is unambiguously detected in the northern polar region, in agreement with the study of Langevin et al. [2005. Sulfates in the north polar region of Mars detected by OMEGA/Mars Express. Science 307(5715), 1584–1586]. Our results show that the linear spectral unmixing provides good first order results in a variety of mineralogical contexts, and can therefore confidently be used on a wider scale to analyze the complete archive of OMEGA data.

Detection of Martian dust clouds by SPICAM UV nadir measurements during the October 2005 regional dust storm

The SPICAM UV spectrometer onboard Mars Express observed two regional dust storms: on October 24, 2005, near Argyre Planitia and on July 9, 2005, in Hellas Planitia. The measurements were performed in nadir mode. The spectral domain 210–310 nm is considered. A comparison with a prior orbit which passed above almost the same path 6 days before the storm near Argyre Planitia has shown a significant decrease of the radiance factor. The difference grows at shorter wavelengths and shows significant dust absorption in the UV. The data have been used to retrieve the dust optical thickness τ, single scattering albedo w and asymmetry factor g. The fitting of the spectra by a radiative transfer code has revealed a set of solutions which could be constrained by Mie modeling of the parameters τ, w, and g for a gamma particle size distribution. The following estimates of the dust optical parameters are obtained: w = 0.64 ± 0.04 at λ = 300 nm and decreases to 0.6 ± 0.045 at 213 nm, g = 0.86 ± 0.03 at 300 nm and slightly grows to 0.88 ± 0.04 at 213 nm, τ = 2.5 ± 0.5 for the first dust event and τ = 2.0 ± 0.5 for the second one. The obtained values of single scattering albedo are consistent with the Hubble Space Telescope and Mariner 9 results.

Results of measurements with the Planetary Fourier Spectrometer onboard Mars Express: Clouds and dust at the end of southern summer. A comparison with OMEGA images

We discuss the results of measurements made with the Planetary Fourier Spectrometer (PFS) onboard the Mars Express spacecraft. The data were obtained in the beginning of the mission and correspond to the end of summer in the southern hemisphere of Mars (Ls ∼ 340°). Three orbits are considered, two of which passed through volcanoes Olympus and Ascraeus Mons (the height above the surface is about +20 km), while the third orbit intersects lowland Hellas (−7 km). The influence of the relief on the properties of the aerosol observed is demonstrated: clouds of water ice with a visual optical thickness of 0.1–0.5 were observed above volcanoes, while only dust was found during the observations (close in time) along the orbit passing through Hellas in low and middle latitudes. This dust is homogeneously mixed with gas and has a reduced optical thickness of 0.25±0.05 (at v = 1100 cm−1). In addition to orographic clouds, ice clouds were observed in this season in the northern polar region. The clouds seen in the images obtained simultaneously by the mapping spectrometer OMEGA confirm the PFS results. Temperature inversion is discovered in the north polar hood below the level 1 mbar with a temperature maximum at about 0.6 mbar. This inversion is associated with descending movements in the Hadley cell.