2006 .

V. Formisano, F. Angrilli, G. Arnold, S. Atreya, K. H. Baines, G. Bellucci, B. Bezard, F. Billebaud, D. Biondi, M. I. Blecka, L. Colangeli, L. Comolli, D. Crisp, M. D'Amore, T. Encrenaz, A. Ekonomov, F. Esposito, C. Fiorenza, S. Fonti, M. Giuranna, D. Grassi, B. Grieger, A. Grigoriev, J. Helbert, H. Hirsch, N. Ignatiev, A. Jurewicz, I. Khatuntsev, S. Lebonnois, E. Lellouch, A. Mattana, A. Maturilli, E. Mencarelli, M. Michalska, J. Lopez Moreno, B. Moshkin, F. Nespoli, Y. Nikolsky, F. Nuccilli, P. Orleanski, E. Palomba, G. Piccioni, M. Rataj, G. Rinaldi, M. Rossi, B. Saggin, D. Stam, D. Titov, G. Visconti, and L. Zasova. The planetary fourier spectrometer (PFS) onboard the European Venus Express mission. Planetary and Space Science, 54:1298-1314, 2006. [ bib | DOI | PDF version | ADS link ]

The planetary fourier spectrometer (PFS) for the Venus Express mission is an infrared spectrometer optimized for atmospheric studies. This instrument has a short wavelength (SW) channel that covers the spectral range from 1700 to 11400 cm -1 (0.9-5.5 μm) and a long wavelength (LW) channel that covers 250-1700 cm -1 (5.5-45 μm). Both channels have a uniform spectral resolution of 1.3 cm -1. The instrument field of view FOV is about 1.6 deg (FWHM) for the short wavelength channel and 2.8 deg for the LW channel which corresponds to a spatial resolution of 7 and 12 km when Venus is observed from an altitude of 250 km. PFS can provide unique data necessary to improve our knowledge not only of the atmospheric properties but also surface properties (temperature) and the surface-atmosphere interaction (volcanic activity). PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from larger distances. Each measurements takes 4.5 s, with a repetition time of 11.5 s. By working roughly 1.5 h around pericentre, a total of 460 measurements per orbit will be acquired plus 60 for calibrations. PFS is able to take measurements at all local times, enabling the retrieval of atmospheric vertical temperature profiles on both the day and the night side. The PFS measures a host of atmospheric and surface phenomena on Venus. These include the:(1) thermal surface flux at several wavelengths near 1 μm, with concurrent constraints on surface temperature and emissivity (indicative of composition); (2) the abundances of several highly-diagnostic trace molecular species; (3) atmospheric temperatures from 55 to 100 km altitude; (4) cloud opacities and cloud-tracked winds in the lower-level cloud layers near 50-km altitudes; (5) cloud top pressures of the uppermost haze/cloud region near 70-80 km altitude; and (6) oxygen airglow near the 100 km level. All of these will be observed repeatedly during the 500-day nominal mission of Venus Express to yield an increased understanding of meteorological, dynamical, photochemical, and thermo-chemical processes in the Venus atmosphere. Additionally, PFS will search for and characterize current volcanic activity through spatial and temporal anomalies in both the surface thermal flux and the abundances of volcanic trace species in the lower atmosphere. Measurement of the 15 μm CO 2 band is very important. Its profile gives, by means of a complex temperature profile retrieval technique, the vertical pressure-temperature relation, basis of the global atmospheric study. PFS is made of four modules called O, E, P and S being, respectively, the interferometer and proximity electronics, the digital control unit, the power supply and the pointing device.

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 (Ls = 8deg) to northern winter solstice (Ls = 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 Ls = 11deg, and the ozone abundance at the peak tends to increase until Ls ˜ 40deg, when it stabilizes around 6-8 × 109 cm-3. After southern winter solstice (Ls ˜ 100deg), the peak abundance starts decreasing again, and this ozone layer is no longer detected after Ls ˜ 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.

M. Hirtzig, A. Coustenis, E. Gendron, P. Drossart, A. Negrão, M. Combes, O. Lai, P. Rannou, S. Lebonnois, and D. Luz. Monitoring atmospheric phenomena on Titan. Astronomy Astrophysics, 456:761-774, 2006. [ bib | DOI | PDF version | ADS link ]

For the past 8 years (1998-2005), we have used adaptive optics imaging (with VLT/NACO and CFHT/PUEO) to explore Titan's atmosphere, which is currently scrutinized in situ by the Cassini-Huygens mission. In the course of our work, we have found variations, such as as seasonal and diurnal effects, as well as temporary features in the southern polar region. The north-south asymmetry is shown to have changed since 2000 in the near-IR and to be currently organized in a brighter northern than southern pole. We study this evolution here. With our data, we also have new significant statistical evidence of diurnal effects in Titan's stratosphere, with a brighter (as much as 19%) morning limb appearing in our images in many cases, when the phase effect is expected on the evening side. The southern bright feature is probably a time-limited seasonal and/or meteorological phenomenon, revolving around the south pole (confined in its motion within the 80degS parallel) and located somewhere in the upper troposphere (18-40 km of altitude). Its behavior and possible nature are discussed here.

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.

P. Rannou, F. Montmessin, F. Hourdin, and S. Lebonnois. The Latitudinal Distribution of Clouds on Titan. Science, 311:201-205, 2006. [ bib | DOI | PDF version | ADS link ]

Clouds have been observed recently on Titan, through the thick haze, using near-infrared spectroscopy and images near the south pole and in temperate regions near 40degS. Recent telescope and Cassini orbiter observations are now providing an insight into cloud climatology. To study clouds, we have developed a general circulation model of Titan that includes cloud microphysics. We identify and explain the formation of several types of ethane and methane clouds, including south polar clouds and sporadic clouds in temperate regions and especially at 40deg in the summer hemisphere. The locations, frequencies, and composition of these cloud types are essentially explained by the large-scale circulation.