F. Forget, F. Montmessin, J.-L. Bertaux, F. González-Galindo, S. Lebonnois, E. Quémerais, A. Reberac, E. Dimarellis, and M. A. López-Valverde. Density and temperatures of the upper Martian atmosphere measured by stellar occultations with Mars Express SPICAM. Journal of Geophysical Research (Planets), 114:1004, 2009. [ bib | DOI | PDF version | ADS link ]

We present one Martian year of observations of the density and temperature in the upper atmosphere of Mars (between 60 and 130 km) obtained by the Mars Express ultraviolet spectrometer Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM). Six hundred sixteen profiles were retrieved using stellar occultations technique at various latitude and longitude. The atmospheric densities exhibit large seasonal fluctuations due to variations in the dust content of the lower atmosphere which controls the temperature and, thus, the atmospheric scale height, below 50 km. In particular, the year observed by SPICAM was affected by an unexpected dust loading around Ls = 130deg which induced a sudden increase of density above 60 km. The diurnal cycle could not be analyzed in detail because most data were obtained at nighttime, except for a few occultations observed around noon during northern winter. There, the averaged midday profile is found to slightly differ from the corresponding midnight profile, with the observed differences being consistent with propagating thermal tides and variations in local solar heating. About 6% of the observed mesopause temperatures exhibits temperature below the CO₂ frost point, especially during northern summer in the tropics. Comparison with atmospheric general circulation model predictions shows that the existing models overestimate the temperature around the mesopause (above 80 to 100 km) by up to 30 K, probably because of an underestimation of the atomic oxygen concentration which controls the CO₂ infrared cooling.

F. Lefèvre, J.-L. Bertaux, R. T. Clancy, T. Encrenaz, K. Fast, F. Forget, S. Lebonnois, F. Montmessin, and S. Perrier. Heterogeneous chemistry in the atmosphere of Mars. Nature, 454:971-975, 2008. [ bib | DOI | PDF version | ADS link ]

Hydrogen radicals are produced in the martian atmosphere by the photolysis of water vapour and subsequently initiate catalytic cycles that recycle carbon dioxide from its photolysis product carbon monoxide. These processes provide a qualitative explanation for the stability of the atmosphere of Mars, which contains 95 per cent carbon dioxide. Balancing carbon dioxide production and loss based on our current understanding of the gas-phase chemistry in the martian atmosphere has, however, proven to be difficult. Interactions between gaseous chemical species and ice cloud particles have been shown to be key factors in the loss of polar ozone observed in the Earth's stratosphere, and may significantly perturb the chemistry of the Earth's upper troposphere. Water-ice clouds are also commonly observed in the atmosphere of Mars and it has been suggested previously that heterogeneous chemistry could have an important impact on the composition of the martian atmosphere. Here we use a state-of-the-art general circulation model together with new observations of the martian ozone layer to show that model simulations that include chemical reactions occurring on ice clouds lead to much improved quantitative agreement with observed martian ozone levels in comparison with model simulations based on gas-phase chemistry alone. Ozone is readily destroyed by hydrogen radicals and is therefore a sensitive tracer of the chemistry that regulates the atmosphere of Mars. Our results suggest that heterogeneous chemistry on ice clouds plays an important role in controlling the stability and composition of the martian atmosphere.

T. Encrenaz, T. K. Greathouse, M. J. Richter, B. Bézard, T. Fouchet, F. Lefèvre, F. Montmessin, F. Forget, S. Lebonnois, and S. K. Atreya. Simultaneous mapping of H ₂O and H ₂O ₂ on Mars from infrared high-resolution imaging spectroscopy. Icarus, 195:547-556, 2008. [ bib | DOI | PDF version | ADS link ]

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 L was 332deg (end of southern summer). Data have been obtained at 1235-1243 cm ^-1, with a spectral resolution of 0.016 cm ^-1 ( R=8×10). The mean water vapor mixing ratio in the region [0deg-55deg S; 345deg-45deg W], at the evening limb, is 15050 ppm (corresponding to a column density of 8.32.8 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 H ₂O ₂ abundance is 1510 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 H ₂O ₂ abundance remains to be understood and modeled.

J.-L. Bertaux, O. Korablev, S. Perrier, E. Quémerais, F. Montmessin, F. Leblanc, S. Lebonnois, P. Rannou, F. Lefèvre, F. Forget, A. Fedorova, E. Dimarellis, A. Reberac, D. Fonteyn, J. Y. Chaufray, and S. Guibert. SPICAM on Mars Express: Observing modes and overview of UV spectrometer data and scientific results. Journal of Geophysical Research (Planets), 111:10, 2006. [ bib | DOI | PDF version | ADS link ]

This paper is intended as an introduction to several companion papers describing the results obtained by the SPICAM instrument on board Mars Express orbiter. SPICAM is a lightweight (4.7 kg) UV-IR dual spectrometer dedicated primarily to the study of the atmosphere of Mars. The SPICAM IR spectrometer and its results are described in another companion paper. SPICAM is the first instrument to perform stellar occultations at Mars, and its UV imaging spectrometer (118-320 nm, resolution ˜1.5 nm, intensified CCD detector) was designed primarily to obtain atmospheric vertical profiles by stellar occultation. The wavelength range was dictated by the strong UV absorption of CO₂ (λ 200 nm) and the strong Hartley ozone absorption (220-280 nm). The UV spectrometer is described in some detail. The capacity to orient the spacecraft allows a great versatility of observation modes: nadir and limb viewing (both day and night) and solar and stellar occultations, which are briefly described. The absolute calibration is derived from the observation of UV-rich stars. An overview of a number of scientific results is presented, already published or found in more detail as companion papers in this special section. SPICAM UV findings are relevant to CO₂, ozone, dust, cloud vertical profiles, the ozone column, dayglow, and nightglow. This paper is particularly intended to provide the incentive for SPICAM data exploitation, available to the whole scientific community in the ESA data archive, and to help the SPICAM data users to better understand the instrument and the various data collection modes, for an optimized scientific return.

F. Montmessin, E. Quémerais, J. L. Bertaux, O. Korablev, P. Rannou, and S. Lebonnois. Stellar occultations at UV wavelengths by the SPICAM instrument: Retrieval and analysis of Martian haze profiles. Journal of Geophysical Research (Planets), 111:9, 2006. [ bib | DOI | PDF version | ADS link ]

Observations made by the SPICAM ultraviolet spectrometer on board the Mars Express orbiter are presented. We focus on several hundreds of atmospheric profiles which have been collected over 3/4 of a Martian year by making use of the stellar occultation technique. The typical structure of the Martian haze possesses at least one discrete layer (60% of all cases) standing over an extended portion wherein opacity continuously increases down to the surface. Differences of morphology are, however, noted between profiles observed near the equator and profiles collected elsewhere. The Martian haze exhibits a pronounced seasonal signal manifested by variations of the maximum elevation at which particles are observed. For reasons related to both convective activity and changes in the hygropause level, cold regions display a much lower hazetop than warm regions. Using UV spectrometry data, we put constraints on haze microphysical properties. Vertical variations of particle size are keyed to variations of opacity; e.g., an increase of particle size is systematically observed near extinction peaks. This is the likely consequence of cloud formation which results into a local increase of particle cross section. Despite marked differences of aerosol profiles between low and high latitudes, haze properties above 60 km remain invariant, possibly reflecting the long-term presence of a background submicronic particle population. Several profiles have been analyzed in more detail to extract properties of detached cloud layers lofted above 40 km. Their optical depth ranges between 0.01 and 0.1 in the visible. Estimation of cloud particle size is technically restricted because of SPICAM wavelength sampling, but it generally yields a minimum radius value of about 0.3 μm, while several estimates are consistent with a robust 0.1-0.2 μm. This crystal size, significantly smaller than the 1 to 4 μm associated with recently classified type I and II clouds, suggests that a different class of clouds, henceforth type III clouds, can be extracted from our data. Observations made in the southern winter polar night indicate a very distinct aerosol behavior where particles are less abundant (τ 0.1), confined to lower heights (vertical profile consistent with a Conrath parameter exceeding 0.04) and made of particles having a radius on the order of 0.1 μm. This shows that the Martian polar night is a region with a very clean atmosphere and with a distinct type of aerosols.

S. Perrier, J. L. Bertaux, F. Lefèvre, S. Lebonnois, O. Korablev, A. Fedorova, and F. Montmessin. Global distribution of total ozone on Mars from SPICAM/MEX UV measurements. Journal of Geophysical Research (Planets), 111:9, 2006. [ bib | DOI | PDF version | ADS link ]

The dual UV/IR spectrometer SPICAM on board the European mission Mars Express is dedicated to monitoring the Martian atmosphere and has recorded spectra for more than one Martian year, from January 2004 to April 2006, over a large range of latitudes and longitudes. SPICAM UV spectra were recorded on the day side in a nadir geometry, in the 110-320 nm range, allowing measurement of ozone absorption around 250 nm. The method used to retrieve column-integrated ozone quantities is described. A full radiative transfer forward model of the radiance factor is used in an iterative loop to fit the data with four parameters: the surface albedo at 210 and 300 nm, the dust opacity, and the total ozone column. The analysis of the complete data set is presented. The global climatology of ozone on Mars is retrieved for the first time with spatial and temporal coverage. The most significant findings are (1) large increases in the ozone column density at high latitudes during late winter-early spring of each hemisphere that totally disappear during summer, (2) a large variability of the northern spring content related to the polar vortex oscillations, (3) low ozone columns in the equatorial regions all year long, and (4) local variations of the ozone column related to topography, mainly above Hellas Planitia. A good overall agreement is obtained comparing SPICAM data to predictions of a Chemical General Circulation Model. However, significant discrepancies in total abundances are found near northern spring when ozone reaches its annual peak. These results will help further understanding of the dynamics and chemistry of Mars atmosphere.

S. Lebonnois, E. Quémerais, F. Montmessin, F. Lefèvre, S. Perrier, J.-L. Bertaux, and F. Forget. Vertical distribution of ozone on Mars as measured by SPICAM/Mars Express using stellar occultations. Journal of Geophysical Research (Planets), 111:9, 2006. [ bib | DOI | PDF version | ADS link ]

The ultraviolet spectrometer of the SPICAM instrument on board the European Mars Express mission has performed stellar occultations to probe the atmosphere. Vertical profiles of ozone are retrieved from inversion of transmission spectra in the altitude range 20-30 to 70 km. They are analyzed here as functions of latitude and season of the observations. These occultations have been monitored on the night side, from northern spring equinox (L_s = 8deg) to northern winter solstice (L_s = 270deg). The profiles show the presence of two ozone layers: (1) one located near the surface, the top of which is visible below 30 km altitude, and (2) one layer located in the altitude range 30 to 60 km, a feature that is highly variable with latitude and season. This layer is first seen after L_s = 11deg, and the ozone abundance at the peak tends to increase until L_s ˜ 40deg, when it stabilizes around 6-8 × 10⁹ cm^-3. After southern winter solstice (L_s ˜ 100deg), the peak abundance starts decreasing again, and this ozone layer is no longer detected after L_s ˜ 130deg. A recent model (Lefèvre et al., 2004) predicted the presence of these ozone layers, the altitude one being only present at night. Though the agreement between model and observations is quite good, this nocturnal altitude layer is present in SPICAM data over a less extended period than predicted. Though a possible role of heterogeneous chemistry is not excluded, this difference is probably linked to the seasonal evolution of the vertical distribution of water vapor.

F. Montmessin, J.-L. Bertaux, E. Quémerais, O. Korablev, P. Rannou, F. Forget, S. Perrier, D. Fussen, S. Lebonnois, A. Rébérac, and E. Dimarellis. Subvisible CO ₂ ice clouds detected in the mesosphere of Mars. Icarus, 183:403-410, 2006. [ bib | DOI | PDF version | ADS link ]

The formation of CO ₂ ice clouds in the upper atmosphere of Mars has been suggested in the past on the basis of a few temperature profiles exhibiting portions colder than CO ₂ frost point. However, the corresponding clouds were never observed. In this paper, we discuss the detection of the highest clouds ever observed on Mars by the SPICAM ultraviolet spectrometer on board Mars Express spacecraft. Analyzing stellar occultations, we detected several mesospheric detached layers at about 100 km in the southern winter subtropical latitudes, and found that clouds formed where simultaneous temperature measurements indicated that CO ₂ was highly supersaturated and probably condensing. Further analysis of the spectra reveals a cloud opacity in the subvisible range and ice crystals smaller than 100 nm in radius. These layers are therefore similar in nature as the noctilucent clouds which appear on Earth in the polar mesosphere. We interpret these phenomena as CO ₂ ice clouds forming inside supersaturated pockets of air created by upward propagating thermal waves. This detection of clouds in such an ultrararefied and supercold atmosphere raises important questions about the martian middle-atmosphere dynamics and microphysics. In particular, the presence of condensates at such high altitudes begs the question of the origin of the condensation nuclei.

K. Fast, T. Kostiuk, T. Hewagama, M. F. A'Hearn, T. A. Livengood, S. Lebonnois, and F. Lefèvre. Ozone abundance on Mars from infrared heterodyne spectra. II. Validating photochemical models. Icarus, 183:396-402, 2006. [ bib | DOI | PDF version | ADS link ]

Ozone is an important observable tracer of martian photochemistry, including odd hydrogen (HO _x) species important to the chemistry and stability of the martian atmosphere. Infrared heterodyne spectroscopy with spectral resolution 10 provides the only ground-based direct access to ozone absorption features in the martian atmosphere. Ozone abundances were measured with the Goddard Infrared Heterodyne Spectrometer and the Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility on Mauna Kea, Hawai'i. Retrieved total ozone column abundances from various latitudes and orbital positions ( L=40deg, 74deg, 102deg, 115deg, 202deg, 208deg, 291deg) are compared to those predicted by the first three-dimensional gas phase photochemical model of the martian atmosphere [Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. J. Geophys. Res. 109, doi:10.1029/2004JE002268. E07004]. Observed and modeled ozone abundances show good agreement at all latitudes at perihelion orbital positions ( L=202deg, 208deg, 291deg). Observed low-latitude ozone abundances are significantly higher than those predicted by the model at aphelion orbital positions ( L=40deg, 74deg, 115deg). Heterogeneous loss of odd hydrogen onto water ice cloud particles would explain the discrepancy, as clouds are observed at low latitudes around aphelion on Mars.

T. Encrenaz, B. Bézard, T. Owen, S. Lebonnois, F. Lefèvre, T. Greathouse, M. Richter, J. Lacy, S. Atreya, A. S. Wong, and F. Forget. Infrared imaging spectroscopy of Mars: H ₂O mapping and determination of CO ₂ isotopic ratios. Icarus, 179:43-54, 2005. [ bib | DOI | PDF version | ADS link ]

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 206deg. Following the detection and mapping of hydrogen peroxide H ₂O ₂ [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 31.5×10(179 pr-μm). We have searched for CH ₄ 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 CO ₂ 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.

T. Encrenaz, B. Bézard, T. K. Greathouse, M. J. Richter, J. H. Lacy, S. K. Atreya, A. S. Wong, S. Lebonnois, F. Lefèvre, and F. Forget. Hydrogen peroxide on Mars: evidence for spatial and seasonal variations. Icarus, 170:424-429, 2004. [ bib | DOI | PDF version | ADS link ]

Hydrogen peroxide (H ₂O ₂) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10 ¹⁵-10 ¹⁶ cm ^-2. However, a stringent upper limit of the H ₂O ₂ abundance on Mars (9×10 ¹⁴ 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=112deg). Here we report the detection of H ₂O ₂ on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring ( Ls=206deg) when the global water vapor abundance was 10 pr. μm. The spatial distribution of H ₂O ₂ shows a maximum in the morning around the sub-solar latitude. The mean H ₂O ₂ column density (6×10 ¹⁵ 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=254deg), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).

F. Lefèvre, S. Lebonnois, F. Montmessin, and F. Forget. Three-dimensional modeling of ozone on Mars. Journal of Geophysical Research (Planets), 109:7004, 2004. [ bib | DOI | PDF version | ADS link ]

We present the first three-dimensional model simulations of ozone on Mars. The model couples a state-of-the-art gas-phase photochemical package to the general circulation model developed at Laboratoire de Météorologie Dynamique (LMD). The results do not contradict the classical picture of a global anticorrelation between the ozone (O₃) and water vapor columns. However, the quantitative approach shows significant departures from this relationship, related to substantial orbital variations in the O₃ vertical distribution. Over the period L_s = 180deg-330deg, low-latitude to midlatitude O₃ is essentially confined below 20 km, has a weak diurnal cycle, and is largely modulated by topography. During the rest of the year (L_s = 330deg-180deg) the model predicts the formation of an O₃ layer at 25-70 km altitude, characterized by nighttime densities about one order of magnitude larger than during the day. Throughout the year, high-latitude O₃ peaks near the surface and reaches maximum integrated amounts (˜40 μm-atm) in the winter polar vortex, with considerable (30 to 50%) dynamically induced day-to-day variations. The most stringent comparison to date with O₃ observational data reveals contrasted results. A good quantitative agreement is found in the postperihelion period (L_s = 290deg-10deg), but the model fails to reproduce O₃ columns as large as those measured near aphelion (L_s = 61deg-67deg). Current uncertainties in absorption cross sections and gas-phase kinetics data do not seem to provide credible explanations to explain this discrepancy, which may suggest the existence of heterogeneous processes.