Mapping of Mars O2 1.27 μm dayglow at four seasonal points

The O₂ dayglow at 1.27 μm is formed by high-altitude ozone on Mars and is a sensitive tracer of Mars photochemistry. Mapping of this dayglow using the IRTF/CSHELL long-slit spectrograph requires the extraction of weak emission lines against a strong continuum of the reflected solar light. Some new tools are suggested to improve the data processing. The observed O₂ dayglow intensities at L_S=67°, 112°, 148°, and 173° show a decrease from late spring (aphelion) to fall equinox by a factor of ≈5 at low latitudes (±30°). This decrease agrees with that predicted by a model of Clancy and Nair (1996, J. Geophys. Res. 101 (12) 12785-12790), although the dayglow intensities are weaker than those based on that model. The measured dayglow variations with latitude are rather low at L_S=67°, 112°, and 148° and unexpectedly high at 173°. The dayglow intensity peaks near noon and is smaller at 9:00 and 16:30 LT by a factor of 2. Some data on the ozone profile near aphelion are obtained from a combination of the dayglow and ozone observations. It is hardly possible to detect the O₂ night airglow at 1.27 μm on Mars using the existing ground-based and on-orbit instruments. The O₂ dayglow intensity as a function of latitude and season from aphelion to fall equinox has been obtained. Our goal is to extend this distribution to the full martian year and get a database for Mars photochemistry to complement the MGS/TES observations of water vapor, atmospheric temperature, and dust and ice aerosol.     

Long-term spectroscopic observations of Mars using IRTF/CSHELL: Mapping of O2 dayglow, CO, and search for CH4

Long-term spectroscopic observations of the O₂ dayglow at 1.27 μm result in a map of the latitudinal and seasonal behavior of the dayglow intensity for the full martian year. The O₂ dayglow is a sensitive tracer of Mars' photochemistry, and this map reflects variations of Mars' photochemistry at low and middle latitudes. It may be used to test photochemical models. Long-term observations of the CO mixing ratio have been also combined into the seasonal-latitudinal map. Seasonal and latitudinal variations of the mixing ratios of CO and the other incondensable gases (N₂, Ar, O₂, and H₂) discovered in our previous work are caused by condensation and sublimation of CO₂ to and from the polar regions. They reflect dynamics of the atmosphere and polar processes. The observed map may be used to test global circulation models of the martian atmosphere. The observed global abundances of CO are in reasonable agreement with the predicted variations with the 11-year solar cycle. Despite the perfect observing conditions, methane has not been detected using the IRTF/CSHELL with a 3σ upper limit of 14 ppb. This upper limit does not rule out the value of 10 ppb observed using the Canada-France-Hawaii Telescope and the Mars Express Planetary Fourier Spectrometer.     

“Methane on Mars: A product of H2O photolysis in the presence of CO”: Response to V.A. Krasnopolsky

The detection of CH₄ in the martian atmosphere, at a mixing ratio of about 10 ppb, prompted Krasnopolsky et al. [Krasnopolsky, V.A., Maillard, J.P., Owen, T.C., 2004. Icarus 172, 537-547] and Krasnopolsky [Krasnopolsky, V.A., 2006. Icarus 180, 359-367] to propose that the CH₄ is of biogenic origin. Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320-322] proposed that CH₄ can be formed in the martian atmosphere by photolysis of H₂O in the presence of CO. We based our arguments on a clear demonstration that CH₄ is formed in our experiments, and on thermodynamic equilibrium calculations, which show that CH₄ formation is favored even in the presence of oxygen at a mixing ratio 1.3×¹⁰⁻³, as observed on Mars. In the present comment, Krasnopolsky [Krasnopolsky, V.A., 2007. Icarus, in press (this issue)] presents his arguments against the suggestion of Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320-322], based on the effect of O₂ on CH₄ formation, the absence of kinetic pathways for CH₄ formation and on the inadequacy of thermodynamic equilibrium calculations to describe the martian atmosphere. In this rebuttal we demonstrate that experiments with molecular oxygen at a ratio of O₂/CO₂=(8.9-17)×¹⁰⁻³, exceeding the martian ratio, still form CH₄. Thermodynamic equilibrium calculations replicate the experimental CH₄ mixing ratio to within a factor of 1.9 and demonstrate that CH₄ production is favored in the martian atmosphere, which is obviously not in thermodynamic equilibrium. Consequently, we do not find the presence of methane to be a sign of biological activity on Mars.     

Photochemistry of the martian atmosphere: Seasonal, latitudinal, and diurnal variations

There is a significant progress in the observational data relevant to Mars photochemistry in the current decade. These data are not covered by and sometimes disagree with the published models. Therefore we consider three types of models for Mars photochemistry. A steady-state model for global-mean conditions is currently the only way to calculate the abundances of long living species (H₂, O₂, and CO). However, our model does not fit the observed CO abundance using gas-phase chemistry and reasonable values of heterogeneous loss of odd hydrogen on the water ice aerosol. The second type of the calculated models is steady-state models for local conditions. The MGS/TES data on temperature profiles, H₂O, and dust are input parameters for these models. The calculations have been made for nine seasonal points spread over the martian year and for twelve latitudes with a step of 10° for each season. The only adopted heterogeneous reaction is a weak loss of H₂O₂ on water ice with probability of 5×¹⁰⁻⁴. The results are in good agreement with the recent observations of the O₂ dayglow at 1.27 μm and the O₃ and H₂O₂ abundances. Global maps of the seasonal and latitudinal behavior of these species have been made. The third type of models is a time-dependent model for local conditions. These models show that odd hydrogen quickly converts to H₂O₂ at the nighttime and the chemistry is switched off while the association of O, the heterogeneous loss of H₂O₂, and eddy diffusion continue. This requires significant changes in the global-mean and local steady-state models discussed above, and these changes have been properly done. The calculated diurnal variations of Mars photochemistry are discussed. The martian photochemistry at low and middle latitudes is significantly different in the aphelion period at LS=10°-130° from that in the remaining part of the year.     

Hydrogen peroxide on Mars: evidence for spatial and seasonal variations

Hydrogen peroxide (H₂O₂) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10¹⁵-10¹⁶ cm⁻². However, a stringent upper limit of the H₂O₂ abundance on Mars (9×10¹⁴ cm⁻²) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112°). Here we report the detection of H₂O₂ on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring (Ls=206°) when the global water vapor abundance was ∼10 pr. μm. The spatial distribution of H₂O₂ shows a maximum in the morning around the sub-solar latitude. The mean H₂O₂ column density (6×10¹⁵ cm⁻²) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 (Ls=254°), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).     

Simultaneous mapping of H2O and H2O2 on Mars from infrared high-resolution imaging spectroscopy

New maps of martian water vapor and hydrogen peroxide have been obtained in November-December 2005, using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infra Red Telescope facility (IRTF) at Mauna Kea Observatory. The solar longitude Ls was 332° (end of southern summer). Data have been obtained at 1235-1243 cm⁻¹, with a spectral resolution of 0.016 cm⁻¹ (R=8×¹⁰⁴). The mean water vapor mixing ratio in the region [0°-55° S; 345°-45° W], at the evening limb, is 150±50 ppm (corresponding to a column density of 8.3±2.8 pr-μm). The mean water vapor abundance derived from our measurements is in global overall agreement with the TES and Mars Express results, as well as the GCM models, however its spatial distribution looks different from the GCM predictions, with evidence for an enhancement at low latitudes toward the evening side. The inferred mean H₂O₂ abundance is 15±10 ppb, which is significantly lower than the June 2003 result [Encrenaz, T., Bézard, B., Greathouse, T.K., Richter, M.J., Lacy, J.H., Atreya, S.K., Wong, A.S., Lebonnois, S., Lefèvre, F., Forget, F., 2004. Icarus 170, 424-429] and lower than expected from the photochemical models, taking in account the change in season. Its spatial distribution shows some similarities with the map predicted by the GCM but the discrepancy in the H₂O₂ abundance remains to be understood and modeled.     

Radio occultation measurements and MGCM simulations of Kelvin waves on Mars

We have derived new results concerning thermal tides on Mars from a combination of radio occultation measurements and numerical simulations by a Mars General Circulation Model (MGCM). This investigation exploits a set of concurrent observations by Mars Express (MEX) and Mars Global Surveyor (MGS) in mid-2004, when the season on Mars was midspring in the northern hemisphere. The MEX occultations sampled the atmosphere near the evening terminator at latitudes ranging from 54° N to 15° S. The MGS occultations provided complementary coverage near the morning terminator at latitudes of 35° N and 71° S. The geopotential field derived from these measurements contains distinctive modulation caused by solar-asynchronous thermal tides. Through careful analysis of the combined observations, we characterized two prominent wave modes, obtaining direct solutions for some properties, such as the amplitude and phase, as well as constraints on others, such as the period, zonal wave number, and meridional structure. We supplemented these observations with MGCM simulations. After evaluating the performance of the MGCM against the measurements, we used the validated simulation to deduce the identity of the two tidal modes and to explore their behavior. One mode is a semidiurnal Kelvin wave with a zonal wave number of 2 (SK2), while the other is a diurnal Kelvin wave with a zonal wave number of 1 (DK1). Both modes are known to be close to resonance in the martian atmosphere. Our observations of the SK2 are more complete and less ambiguous than any previous measurement. The well-known DK1 is the dominant solar-asynchronous tide in the martian atmosphere, and our results confirm and extend previous observations by diverse instruments.     

Comparison of HIPWAC and Mars Express SPICAM observations of ozone on Mars 2006-2008 and variation from 1993 IRHS observations

Ozone is a tracer of photochemistry in the atmosphere of Mars and an observable used to test predictions of photochemical models. We present a comparison of retrieved ozone abundances on Mars using ground-based infrared heterodyne measurements by NASA Goddard Space Flight Center’s Heterodyne Instrument for Planetary Wind And Composition (HIPWAC) and space-based Mars Express Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) ultraviolet measurements. Ozone retrievals from simultaneous measurements in February 2008 were very consistent (0.8 μm-atm), as were measurements made close in time (ranging from <1 to >8 μm-atm) during this period and during opportunities in October 2006 and February 2007. The consistency of retrievals from the two different observational techniques supports combining the measurements for testing photochemistry-coupled general circulation models and for investigating variability over the long-term between spacecraft missions. Quantitative comparison with ground-based measurements by NASA/GSFC’s Infrared Heterodyne Spectrometer (IRHS) in 1993 reveals 2-4 times more ozone at low latitudes than in 2008 at the same season, and such variability was not evident over the shorter period of the Mars Express mission. This variability may be due to cloud activity.     

Observations of non-LTE emission at 4-5 microns with the planetary Fourier spectrometer abord the Mars Express mission

We report on PFS-MEX (Planetary Fourier Spectrometer on board Mars Express) limb observations of the non-Local Thermodynamic Equilibrium emission by CO and CO₂ isotopic molecules. The CO emission is observed peaking at altitudes lower than the CO₂ emission peak. Two orbits have been considered, which explore latitudes from 75 to 15° N, located in local time at 11:30 and 06:40, and with Ls=138° and 168°, respectively. In general in the season considered (northern summer) the emission intensity increases going to lower latitudes. The peak emission height is also decreasing with decreasing latitude. The CO₂ isotopic molecules are emitting radiance out of proportion with respect to the normal isotopic abundance, which surely indicates a strong contribution from a large number of much weaker CO₂ bands, a result that will demand careful theoretical modeling. By comparison with Hitran data base we can identify, among the emitting bands, the second hot band for the 626 and 636 molecule, while for the 628 and 627 emission from the third hot bands are very possible. Other minor bands or lines are also observed in emission for the first time in Mars. In one of the two orbits considered, the orbit 1234 of MEX, we also observe at altitudes 80-85 km scattered radiation, with indication of CO₂ ice aerosols as scattering centers. At the same altitude the Pathfinder descending measurements show a temperature that allows CO₂ condensation. Pathfinder measurements were at 03:00 local time, while our observations are for orbit 1234 showing CO₂ ice signature at 11:30 local time. These non-LTE limb emissions, with their unprecedented spectral resolution in this portion of the near infrared and their sensitivity and geographical coverage, will represent in our opinion an excellent data set for testing current theoretical models of the martian upper atmosphere.     

Infrared imaging spectroscopy of Mars: H2O mapping and determination of CO2 isotopic ratios

High-resolution infrared imaging spectroscopy of Mars has been achieved at the NASA Infrared Telescope Facility (IRTF) on June 19-21, 2003, using the Texas Echelon Cross Echelle Spectrograph (TEXES). The areocentric longitude was 206°. Following the detection and mapping of hydrogen peroxide H₂O₂ [Encrenaz et al., 2004. Icarus 170, 424-429], we have derived, using the same data set, a map of the water vapor abundance. The results appear in good overall agreement with the TES results and with the predictions of the Global Circulation Model (GCM) developed at the Laboratory of Dynamical Meteorology (LMD), with a maximum abundance of water vapor of 3±1.5×¹⁰⁻⁴(17±9 pr-μm). We have searched for CH₄ over the martian disk, but were unable to detect it. Our upper limits are consistent with earlier reports on the methane abundance on Mars. Finally, we have obtained new measurements of CO₂ isotopic ratios in Mars. As compared to the terrestrial values, these values are: (¹⁸O/¹⁷O)[M/E] = 1.03 ± 0.09; (¹³C/¹²C)[M/E] = 1.00 ± 0.11. In conclusion, in contrast with the analysis of Krasnopolsky et al. [1996. Icarus 124, 553-568], we conclude that the derived martian isotopic ratios do not show evidence for a departure from their terrestrial values.     

O2 1.27 μm emission maps as derived from OMEGA/MEx data

We present a method to derive the 2D maps of the O₂ (a¹Δ_g) airglow emission at 1.27 μm from the OMEGA/MEx nadir observations. The OMEGA imaging capabilities allow monitoring the 2D distribution, daily and seasonal variation of the O₂ emission intensities with a detection limit of 4 MR. The highest values, of the order of ∼31 MR, are found on the south pole for 11 h < LT < 13 h, during the early spring (186° < Ls < 192°) of martian year (MY) 27, according to the Mars Year numbering scheme of Clancy et al. [Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. Mars aerosol studies with the MGS TES emission phase function observations: Optical depths, particle sizes, and ice cloud types versus latitude and solar longitude. J. Geophys. Res. 108. doi: 10.1029/2003JE002058]. In the polar regions the day-by-day variability, associated with polar vortex turbulences, is obtained of the order of 30-50% as predicted by the model [Lefévre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. Three-dimensional modeling of ozone on Mars. J. Geophys. Res. 109, E07004. doi: 10.1029/2004JE002268] and found by SPICAM [Perrier, S., Bertaux, J.-L., Lebonnois, S., Korablev, O., Fedorova, A., 2006. Global distribution of total ozone on Mars from SPICAM/MEX UV measurements. J. Geophys. Res. 111, E09S06. doi: 10.1029/2006JE002681]. In the considered set of data a maximum of the O₂ emission is observed between 11 h and 15 h LT in the latitude range 70-85° during early spring on both hemispheres, while for the southern autumn-winter season a maximum is found between 50° and 60° in the southern hemisphere for MY28. Increase of intensity of the O₂ emission observed from Ls 130° to 160° at southern high latitudes may be explained by increase of solar illumination conditions in the maps acquired during the considered period.Atmospheric waves crossing the terminator on the southern polar regions are observed for the first time during the MY28 early spring. The spatial scale of the waves ranges from 100 to 130 km, and the intensity fluctuations are of the order of 4MR.This study confirms the high potentiality of O₂ (a¹Δ_g) day glow as a passive tracer of the martian atmosphere dynamics at high latitudes.     

The density of the upper martian atmosphere measured by Lyman-α absorption with Mars Express SPICAM

Absorption of interplanetary Lyman-α emission by Mars’ nightside lower thermosphere was observed by Mars Express Spectrometer for Investigation of Characteristics of the Atmosphere of Mars (SPICAM), and is analyzed to derive the CO₂ density at 110 km during a martian year. The observed density seasonal variability is consistent with recent observations obtained by stellar occultations, proving that this method, though not as accurate as stellar occultations could be used complementary to them to characterize large variations of thermospheric density on Mars and provide a better spatial coverage by Lyman-α imagery.     

Metastable methane clathrate particles as a source of methane to the martian atmosphere

The observations of methane made by the PFS instrument onboard Mars Express exhibit a definite correlation between methane mixing ratio, water vapor mixing ratio, and cloud optical depth. The recent data obtained from ground-based telescopes seem to confirm the correlation between methane and water vapor. In order to explain this correlation, we suggest that the source of gaseous methane is atmospheric, rather than at the solid surface of the planet, and that this source may consist of metastable submicronic particles of methane clathrate hydrate continuously released to the atmosphere from one or several clathrate layers at depth, according to the phenomenon of “anomalous preservation” evidenced in the laboratory. These particles, lifted up to middle atmospheric levels due to their small size, and therefore filling the whole atmosphere, serve as condensation nuclei for water vapor. The observed correlation between methane and water vapor mixing ratios could be the signature of the decomposition of the clathrate crystals by condensation-sublimation processes related to cloud activity. Under the effect of water condensation on crystal walls, metastability could be broken and particles be eroded, resulting in a subsequent irreversible release of methane to the gas phase. Using PFS data, and according to our hypothesis, the lifetime of gaseous methane is estimated to be smaller than an upper limit of 6 ± 3 months, much smaller than the lifetime of 300 yr calculated from atmospheric chemical models. The reason why methane has a short lifetime might be the occurrence of heterogeneous chemical decomposition of methane in the subsurface, where it is known since Viking biology experiments that oxidants efficiently decompose organic matter. If true, it is shown by using existing models of H₂O₂ penetration in the regolith that methane could prevent H₂O₂ from penetrating in the subsurface, and further oxidizing the soil, at depths larger than a few millimeters. The present source of methane clathrate, acting over the last few hundred thousand or million years, could have given rise to the thin CO₂-ice layer covering the permanent water ice south polar cap. The hypothesis proposed in this paper requires, to be validated, a number of laboratory experiments studying the stability of methane clathrates in martian atmospheric conditions, and the kinetics and amplitude of clathrate particle erosion in presence of condensing water vapor. Detailed future observations of methane, and associated modeling, will allow to more accurately quantify the production rate of methane clathrate, its temporal variability at seasonal scale, and possibly to locate the source(s) of clathrates at the surface.     

Viking observation of water vapor on Mars: Revision from up-to-date spectroscopy and atmospheric models

A reanalysis of the Mars Atmospheric Water Detector (MAWD, Viking 1 and 2 Orbiters) Planetary Data System (PDS) data set (Jakosky, B.M., Farmer, C.B. [1982]. J. Geophys. Res. 87 (B4), 2999-3019) is presented taking into account a new spectroscopic database and improved atmospheric model assumptions. Starting from HITRAN 2004 edition and later (Rothman, L.S., and 29 colleagues [2005]. J. Quant. Spectrosc. Radiat. Trans. 96, 139-204), the number of lines in the 1.38-μm band has been significantly increased, and their parameters have been modified. The implication of this new spectroscopic data and atmospheric model based on Martian Climate Database (MCD, Forget, F., Hourdin, F., Fournier, R., Hourdin, C., Talagrand, O., Collins, M., Lewis, S.R., Read, P.L., Huot, J.-P. [1999]. J. Geophys. Res. 104 (E10), 24155-24176) gives a significant impact on the H₂O retrieval: the total H₂O abundance after the reanalysis has decreased twofold in all seasons and most of geographic locations. Overall decrease is associated with larger cumulative strength of the band in HITRAN 2004; low saturation height of water profiles imposed by MCD significantly contributes to decrease of summer polar maximum. Revised MAWD data are compared with later H₂O measurements on Mars Global Surveyor (MGS) and Mars-Express (MEX). A good agreement with SPICAM/MEX near-IR (1.38-μm band) measurements is found. However, both sets of near-IR measurements are systematically below TES/MGS results obtained in thermal infrared, with a factor of 1.5-2.0. This difference might be associated with remaining ambiguity of the near IR spectral data, and of line broadening in CO₂ in both spectral ranges. The reanalyzed MAWD data are in much better agreement with later measurements suggesting more homogeneous, and significantly dryer water cycle on Mars, with no signature of change between the Viking epoch (MY12-14) and MGS-Mars-Express measurements (MY24-29).     

Airglow on Mars: Some model expectations for the OH Meinel bands and the O2 IR atmospheric band

This work presents model calculations of the diurnal airglow emissions from the OH Meinel bands and the O₂ IR atmospheric band in the neutral atmosphere of Mars. A time-dependent photochemical model of the lower atmosphere below 80 km has been developed for this purpose. Special emphasis is placed on the nightglow emissions because of their potential to characterize the atomic oxygen profile in the 50-80 km region. Unlike on Earth, the OH Meinel emission rates are very sensitive to the details of the vibrational relaxation pathway. In the sudden death and collisional cascade limits, the maximum OH Meinel column intensities for emissions originating from a fixed upper vibrational level are calculated to be about 300 R, for transitions v^′=9→v^′?8, and 15,000 R, for transitions v^′=1→v^′=0, respectively. During the daytime the 1.27 μm emission from O₂(), primarily formed from ozone photodissociation, is of the order of MegaRayleighs (MR). Due to the long radiative lifetime of O₂(), a luminescent remnant of the dayglow extends to the dark side for about two hours. At night, excited molecular oxygen is expected to be produced through the three body reaction O + O + CO₂. The column emission of this nighttime component of the airglow is estimated to amount to 25 kR. Both nightglow emissions, from the OH Meinel bands and the O₂ IR atmospheric band, overlap in the 50-80 km region. Photodissociation of CO₂ in the upper atmosphere and the subsequent transport of the atomic oxygen produced to the emitting layer are revealed as key factors in the nightglow emissions from these systems. The Mars 5 upper constraint for the product [H][O₃] is revised on the basis of more recent values for the emission probabilities and collisional deactivation coefficients.     

THEMIS observations of Mars aerosol optical depth from 2002-2008

We use infrared images obtained by the Thermal Emission Imaging System (THEMIS) instrument on-board Mars Odyssey to retrieve the optical depth of dust and water ice aerosols over more than 3.5 martian years between February 2002 (MY 25, Ls=330°) and December 2008 (MY 29, Ls=183°). These data provide an important bridge between earlier TES observations and recent observations from Mars Express and Mars Reconnaissance Orbiter. An improvement to our earlier retrieval [Smith, M.D., Bandfield, J.L., Christensen, P.R., Richardson, M.I., 2003. J. Geophys. Res. 108, doi:10.1029/2003JE002114] to include atmospheric temperature information from THEMIS Band 10 observations leads to much improved retrievals during the largest dust storms. The new retrievals show moderate dust storm activity during Mars Years 26 and 27, although details of the strength and timing of dust storms is different from year to year. A planet-encircling dust storm event was observed during Mars Year 28 near Southern Hemisphere Summer solstice. A belt of low-latitude water ice clouds was observed during the aphelion season during each year, Mars Years 26 through 29. The optical depth of water ice clouds is somewhat higher in the THEMIS retrievals at ∼5:00 PM local time than in the TES retrievals at ∼2:00 PM, suggestive of possible local time variation of clouds.     

Solar activity variations of thermospheric temperatures on Mars and a problem of CO in the lower atmosphere

Long-term MGS drag density observations at 390 km reveal variations of the density with season L_S (by a factor of 2) and solar activity index F_10.7 (by a factor of 3 for F_10.7 = 40-100). According to Forbes et al. (Forbes, J.M., Lemoine, F.G., Bruinsma, S.L., Smith, M.D., Zhang, X. [2008]. Geophys. Res. Lett. 35, L01201, doi:10.1029/2007GL031904), the variation with F_10.7 reflects variations of the exospheric temperature from 192 to 284 K. However, the derived temperature range corresponds to variation of the density at 390 km by a factor of 8, far above the observed factor of 3. The recent thermospheric GCMs agree with the derived temperatures but do not prove their adequacy to the MGS densities at 390 km. A model used by Forbes et al. neglects effects of eddy diffusion, chemistry and escape on species densities above 138 km. We have made a 1D-model of neutral and ion composition at 80-400 km that treats selfconsistently chemistry and transport of species with F_10.7, T_∞, and [CO₂]_80 km as input parameters. Applying this model to the MGS densities at 390 km, we find variation of T_∞ from 240 to 280 K for F_10.7 = 40 and 100, respectively. The results are compared with other observations and models. Temperatures from some observations and the latest models disagree with the MGS densities at low and mean solar activity. Linear fits to the exospheric temperatures are T_∞ = 122 + 2.17F_10.7 for the observations, T_∞ = 131 + 1.46F_10.7 for the latest models, and T_∞ = 233 + 0.54F_10.7 for the MGS densities at 390 km. Maybe the observed MGS densities are overestimated near solar minimum when they are low and difficult to measure. Seasonal variations of Mars’ thermosphere corrected for the varying heliocentric distance are mostly due to the density variations in the lower and middle atmosphere and weakly affect thermospheric temperature. Nonthermal escape processes for H, D, H₂, HD, and He are calculated for the solar minimum and maximum conditions.Another problem considered here refers to Mars global photochemistry in the lower and middle atmosphere. The models gave too low abundances of CO, smaller by an order of magnitude than those observed. Our current work shows that modifications in the boundary conditions proposed by Zahnle et al. (Zahnle, K., Haberle, R.M., Catling, D.C., Kasting, J.F. [2008]. J. Geophys. Res. 113, E11004, doi:10.1029/2008JE003160) are reasonable but do not help to solve the problem.     

Interferometric millimeter observations of water vapor on Mars and comparison with Mars Express measurements

We present interferometric mapping of the 225.9-GHz HDO and 203.4-GHz lines on Mars obtained with the IRAM Plateau de Bure facility (PdBI). The observations were performed during martian year 28 (MY28), at L_s=320.3° for the HDO line, and at L_s=324.3° for the line. The HDO line is detected at the eastern (morning) and western (evening) limbs of the northern hemisphere, corresponding to a water column density in the range 3-6 pr.-μm. The line is not detected, which is compatible with the column densities derived from the HDO line. Quasi-simultaneous far infrared measurements obtained by the Planetary Fourier Spectrometer (PFS) onboard the Mars Express spacecraft confirm our PdBI results, yielding a 5±1 pr.-μm meridionally constant water column abundance.Such a low water abundance during the southern mid-autumn of MY28 does not correspond to the standard martian climatology as observed during the previous years. It was however already retrieved from near-infrared observations performed by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter spacecraft [Smith, M.D., Wolff, M.J., Clancy, R.T., Murchie, S.L. 2009. CRISM observations of water vapor and carbon monoxide. J. Geophys. Res. 114, doi: 10.1029/2008JE003288]. Our observations thus confirm that the planet-encircling dust storm that occurred during MY28 significantly affected the martian water cycle. Our observations also demonstrate the usefulness of interferometric submillimeter observations to survey the martian water cycle from ground-based facilities.     

The magnetic field near Mars: A comparison between a hybrid model, Mars Global Surveyor and Mars Express observations

Mars Express (MEX) does not carry its own magnetometer which complicates interpretation of ASPERA-3/MEX ion measurements. The direction of the interplanetary magnetic field (IMF) is especially important because it, among other things, determines the direction of the convective electric field and orientation of the cross tail current sheet and tail lobes. In this paper we present a case study to show the properties of the magnetic field near Mars in a quasi-neutral hybrid (QNH) model at the orbits where the Mars Global Surveyor (MGS) has made measurements, present a method to derive the IMF clock angle by comparing fields in a hybrid model and the direction of the magnetic field measured by MGS by deriving the IMF clock angle. We also use H⁺ ring velocity distribution observations upstream of the bow shock measured by the IMA/ASPERA-3 instrument on board MEX spacecraft. These observations are used to indirectly provide the orientation of the IMF. We use a QNH model (HYB-Mars) where ions are modeled as particles while electrons form a mass-less charge neutralizing fluid. We found that the direct MGS and non-direct IMA observations of the orientation magnetic field vectors in non-crustal magnetic field regions are consistent with the global magnetic field draping pattern predicted by the global model.     

Comment on “Methane on Mars: A product of H2O photolysis in the presence of CO” by A. Bar-Nun and V. Dimitrov

Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320-322] suggested a sequence of reactions to form methane on Mars. These reactions are based on the study of products in the N₂-CO-H₂O mixture irradiated at 185 nm. The suggested scheme was not quantitatively justified by chemical kinetics. One of the key reactions is effectively blocked by O₂ in the martian atmosphere, and another key reaction does not exist. There are no pathways for effective formation of methane in the martian atmosphere.