Characterization of dust activity on Mars from MY27 to MY32 by PFS-MEX observations

We present spatial and temporal distributions of dust on Mars from Ls = 331 in MY26 until Ls = 80 in MY33 retrieved from the measurements taken by the Planetary Fourier Spectrometer (PFS) aboard Mars Express. In agreement with previous observations, large dust opacity is observed mostly in the southern hemisphere spring/summer and particularly over regions of higher terrain and large topographic variation. We present a comparison with dust opacities obtained from Thermal Emission Spectrometer (TES) - Mars Global Surveyor (MGS) measurements. We found good consistency between observations of two instruments during overlapping interval (Ls = 331 in MY26 until Ls = 77 in MY27). We found a different behavior of the dust opacity with latitude in the various Martian years (inter-annual variations). A global dust storm occurred in MY28. We observe a different spatial distribution, a later occurrence and dissipation of the dust maximum activity in MY28 than in other Martian years. A possible precursor signal to the global dust storm in MY 28 is observed at Ls = 200 - 235 especially over west Hellas. Heavy dust loads alter atmospheric temperatures. Due to the absorption of solar radiation and emission of infrared radiation to space by dust vertically non-uniformly distributed, a strong heating of high atmospheric levels (40 - 50 km) and cooling below around 30 km are observed.Comment: in press in Icarus. 47 pages, 15 figure

Daily dust variation from the PFS MEx observations

We collected over 7 Martian years (MY) of data observed by the Planetary Fourier Spectrometer (PFS) to present a daily variation of dust content in the Martian atmosphere. We found three typical behaviors of dust opacities with LT (local time). The most peculiar variation was observed when global dust storms (MYs 28 and 34) or particularly strong regional storms (MY 29) occurred on Mars. Here, large dust opacities were measured at 10 LT (MY 34) and 11 LT (MY 28). Then, relatively small values of dust opacities were found in the evening (20 LT). The non-dusty season, particularly near northern summer solstice, was characterized by a deep minimum of the total dust opacity at late night/early morning, while small variations around the mean value were observed during daytime. The clear trend of dust was observed over both hemispheres during early morning. We noted elevated dust opacities in the second half of the year compared to the non-dusty season in all Martian years without global dust storms. The daily variation of three types of storms occurring in moderately dusty conditions was also investigated. Dust in A storms was present in the atmosphere at all LTs and was mostly confined to the southern hemisphere. The maximum of dust opacities in B storms was found at 15–17 LT, close to the South Pole. C storms were mainly constrained to southern latitudes and occurred from the late morning to midday

Response of dust on thermal emission spectra observed by Planetary Fourier Spectrometer (PFS) on-board Mars Express (MEX)

38-44The thermal emission spectra have provided many useful insights about the Martian atmosphere and surface. The interpretation of the thermal emission spectra can give us information about atmospheric temperature, pressure, mineralogy and presence of atmospheric constituents including their isotopes. In the present work, we have analysed the thermal emission data for dust storm season on Mars. The signature of dust in the thermal emission spectra for Martian Year (MY) 28 confirm the presence (Ls=280o and 300o) and the absence (Ls=240o and 320o) of the dust storm at latitude range 0o-10oS, 10o-20oS and 20o-30oS. We have compared our results with earlier mission data with thermal emission measurements made by Planetary Fourier Spectrometer (PFS) on-board Mars Express (MEX) between wave numbers 250-1400 cm-1. We have observed features at wave numbers 600-750 cm-1 and 900-1200 cm-1 due to absorptions by CO2 and dust respectively. We have obtained brightness temperatures from thermal emission spectra by inverting the Planck function. The maximum brightness temperature ~280o K was measured at Ls=240o when Mars received a large amount of solar radiation at perihelion. The minimum brightness temperature ~ 220o K was observed at Ls=320o in the absence of dust storm. In presence of dust storm, thermal emission spectra and brightness temperatures were reduced by factors of ~ 3.0 and ~1.3, respectively, between wave numbers 900-1200 cm-1 in comparison to that observed in absence of dust storm

Response of dust on thermal emission spectra observed by Planetary Fourier3 Spectrometer (PFS) on-board Mars Express (MEX)

The thermal emission spectra have provided many useful insights about the Martian atmosphere and surface. The interpretation of the thermal emission spectra can give us information about atmospheric temperature, pressure, mineralogy and presence of atmospheric constituents including their isotopes. In the present work, we have analysed the thermal emission data for dust storm season on Mars. The signature of dust in the thermal emission spectra for Martian Year (MY) 28 confirms presence (Ls=280o and 300o) and absence (Ls=240o and 320o) of the dust storm at latitude range 0o-10oS, 10o-20oS and 20o-30oS. We have compared our results with earlier mission data with thermal emission measurements made by Planetary Fourier Spectrometer (PFS) on-board Mars Express (MEX) between wave numbers 250-1400 cm-1. We have observed features at wave numbers 600-750 cm-1 and 900-1200 cm-1 due to absorptions by CO2 and dust respectively. We have obtained brightness temperatures from thermal emission spectra by inverting the Planck function. The maximum brightness temperature ~280 K is measured at Ls=240o when Mars received a large amount of solar radiation at perihelion. The minimum brightness temperature ~ 220o K is observed at Ls=320o in the absence of dust storm. In presence of dust storm thermal emission spectra and brightness  temperatures are reduced by factors of ~ 3.0 and ~1.3 respectively between wave numbers 900-1200 cm-1 in comparison to that observed in absence of dust storm

Modeling infrared thermal emissions on Mars during dust storm of MY28: PFS/MEX observation

We have analysed thermal emission spectra obtained from Planetary Fourier Spectrometer (PFS) onboard Mars Express (MEX) for Martian Year (MY) 28 in presence and absence of dust storm at low latitude. A radiative transfer model for dusty atmosphere of Mars is developed to estimate the thermal emission spectra at latitude range 0-10oS, 10-20oS and 20-30oS. These calculations are made at Ls=240o, 280o, 300o, and 320o between wave numbers 250-1400 cm-1. We have also retrieved brightness temperatures from thermal emission spectra by inverting the Planck function. The model reproduces the observed features at wave numbers 600-750 cm-1 and 900-1200 cm-1 due to absorptions by CO2 and dust respectively. In presence of dust storm thermal emission spectra and brightness temperature are reduced by a factor of ~ 2 between wave numbers 900-1200 cm-1. The altitude profiles of dust concentration are also estimated for different aerosol particles of sizes 0.2 to 3 micron. The best fit to the PFS measurements is obtained in presence of aerosol particle of size 0.2 micron

Similarities and Differences of Global Dust Storms in MY 25, 28, and 34

To better understand the dust cycle on Mars during years with planet-encircling dust storms, we analyze the last three events that took place in Mars Year (MY) 25, MY 28, and MY 34. Global dust storms that occurred in MY 25 and MY 34 (June 2018) were taking place during equinox, while the MY 28 storm had an onset after perihelion. Before the expansion phase of the MY 25 and MY 34 storms, we find similar regions (northern rim of Hellas, Arabia Terra, and Utopia Planitia) where dust is present. Possible precursor dust storms over Hellas and the southern polar cap edges were observed during MY 28 as a component of background dust activity. These features are not found in equinoctial dust storms on this scale. Dust during the MY 25 and MY 34 storms encircled the entire planet by the similar season (Ls = 193°). The MY 34 storm is characterized by a shorter decay phase compared to the events in MY 25 and MY 28. Dust opacity is correlated with atmospheric temperatures at 0.5 mbar and nighttime surface temperatures, while daytime surface temperatures are anticorrelated with dust opacity

PFS/MEX limb observations of 4.3-μm CO2 non-LTE emission in the atmosphere of Mars

We present PFS-MEX limb observations of the CO2 non-local thermodynamic equilibrium (non-LTE) emission at 4.3 μm in the atmosphere of Mars collected in more than six Martian years. With unprecedented spatial and temporal coverage, and relatively high spectral resolution, this unique dataset promises to improve our understanding of the upper atmosphere of Mars. The former allows analyses of the emission as a function of tangent altitude, solar zenith angle, season, latitude, local time, and thermal condition of the atmosphere. The latter allows unambiguous identification of several emission bands of different isotopologues. We selected observations in the altitude range 50-200 km. No emission was detected for altitudes higher than 170 km. The spectral shape of the non-LTE emission changes dramatically with the altitude of the tangent point, reflecting the different contribution of the major and minor CO2 bands and isotopologues to the total emission at different heights. For altitudes higher than 130 km the observed spectrum is dominated by the second hot (SH) bands of the main isotopologue 12C16O2 (also referred to as 626 SH). At lower altitudes, the contribution of the isotopic 13C16O2 second hot bands (636 SH) to the observed spectrum gradually increases, and is maximum around 70-80 km. Similar consideration apply to the fourth hot bands of the 12C16O2 (626 FRH), and particularly those from the (2001x) levels, whose contribution is maximum around 80-90 km. The 626 SH bands can be observed up to an altitude 160-170 km, and their emission is peaked around 120-130 km. The 626 FRH and 636 SH bands are not observed above 130-140 km. Both the first hot (FH) and the fundamental band (FB) of the main isotopologue show a peculiar behavior. Indeed, these emissions can be observed at all altitudes, from 50 km up to 170 km. The intensity of the FH band increases linearly with decreasing height, while the intensity of the FB band is essentially constant at all altitudes, and rapidly decreases above 150 km. For a fixed altitude, the solar zenith angle (SZA) is the main parameter affecting the intensity and the spectral shape of the non-LTE emission. For SZA between 0 and 40° the intensity of the emission does not show significant variations. For SZAs larger than 40° the observed emission decreases rapidly with increasing SZA, following a cosine-like relation. The different illumination also affects the spectral shape of the non-LTE emission spectrum. High incidence angles tend to increase the relative contribution of weaker bands compared to stronger/optically thicker bands. For a fixed SZA, we found variation of the intensity of the emission with local time, in response to variations of the thermal structure of the atmosphere. Latitudinal variation of the intensity of the CO2 non-LTE are also investigated. The maximum intensity is observed around the sub-solar latitudes, where the solar flux is maximum. The intensity of the emission and the altitude at which the maximum emission is observed also changes with the season. The altitude where the maximum intensity of the 626 SH bands is observed decreases from 120-130 km at the perihelion (Ls = 251°), down to ∼85 km at the southern winter solstice (Ls = 90°). This is explained by the variability of the thermal structure (scale heights) of the Martian atmosphere with the season, as a response to the changing solar flux. The altitude of a given pressure level depends on the thermal structure of the atmosphere which, in turn, depends on the season. On the contrary, the pressure level of the peak emission does not depend on the scale heights, as it is mainly controlled by the CO2 column density above the peak. These results, while on one hand confirm and provide more insights and constraints to some aspects of the non-LTE processes on Mars, on the other hand further stimulate and challenge current theoretical models, possibly bringing closer the moment in which the measurements could be inverted to derive important information about the upper mesosphere and lower thermosphere of Mars

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 359 cm-1 and 382 cm-1 where strong features of H2O2 are present around 362 cm-1 and 379 cm-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 379 cm-1, whereas no clear features were detected at 362 cm-1 due to large amounts of uncertainty in the data. The derived mixing ratios of H2O2 were close to the detection limits: 16 ± 19 ppb at Ls = 0-120°, 35 ± 32 ppb at Ls = 120-240°, and 41 ± 28 ppb 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. <P /

Seasonal variation of the HDO/H2O ratio in the atmosphere of Mars at the middle of northern spring and beginning of northern summer

We present the seasonal variation of the HDO/H2O ratio caused by sublimation-condensation processes in a global view of the martian water cycle. The HDO/H2O ratio was retrieved from ground-based observations using high-dispersion echelle spectroscopy of the Infrared Camera and Spectrograph (IRCS) of the Subaru telescope. Coordinated joint observations were made by the Planetary Fourier Spectrometer (PFS) onboard Mars Express (MEX). The observations were performed during the middle of northern spring (Ls = 52°) and at the beginning of summer (Ls = 96°) in Mars Year 31. The retrieved latitudinal mean HDO/H2O ratios are 4.1 ± 1.4 (Ls = 52°) and 4.4 ± 1.0 (Ls = 96°) times larger than the terrestrial Vienna Standard Mean Ocean Water (VSMOW). The HDO/H2O ratio shows a large seasonal variation at high latitudes. The HDO/H2O ratio significantly increases from 2.4 ± 0.6 wrt VSMOW at Ls = 52° to 5.5 ± 1.1 wrt VSMOW at Ls = 96° over the latitude range between 70°N and 80°N. This can be explained by preferential condensation of HDO vapor during the northern fall, winter, and spring and sublimation of the seasonal polar cap in the northern summer. In addition, we investigated the geographical distribution of the HDO/H2O ratio over low latitudes at the northern spring in the longitudinal range between 220°W and 360°W, including different local times from 10 h to 17 h. We found the HDO/H2O ratio has no significant variation (5.1 ± 1.2 wrt VSMOW) over the entire range. Our observations suggest that the HDO/H2O distribution in the northern spring and summer seasons is mainly controlled by condensation-induced fractionation between the seasonal northern polar cap and the atmosphere

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