D. V. Titov

Water vapour in the atmosphere of Mars

Abstract Water vapour on Mars has always been in the focus of Martian research. A set of fundamental questions concerning the past and present of the planet is closely related to the peculiarities of its water cycle. Spacecraft and ground-based observations revealed complex behavior of the water vapour. This gas appears to be the most variable minor constituent in the atmosphere of Mars. Its seasonal and latitudinal variations reach a factor of 10. They are driven by condensation and sublimation of the polar caps, exchange with regolith and surface frost, and advective transport by the general circulation. Both seasonal changes and observed spatial and diurnal H 2 O variability imply that the regolith is one of the main reservoirs of atmospheric water on Mars. The vertical distribution of water vapour is affected by the vertical transport, condensation and regolith-atmosphere exchange and can be strongly inhomogeneous. Current numerical models succeed to reproduce the water cycle in general. However some important details remain unclear. This paper reviews our knowledge about the Martian water cycle and discusses the open questions and future studies.

A mapping of martian water sublimation during early northern summer using OMEGA/Mars Express

The OMEGA imaging spectrometer aboard Mars Express has been used to map the water vapor abundance over the martian surface, from the analysis of the 2.6 μm band of H 2 O. As a preliminary result of this study, we present water vapor maps in the northern hemisphere at the time of the northern polar cap sublimation (Ls = 94-112 deg). The maps show a mean H 2 O mixing ratio of about 2-3 x 10 -4 at a latitude of 40N, and in the range of 5 x 10 -4 -10 -3 at 60N-80N latitudes. The corresponding mean H 2 O column densities are about 25 pr-μm at 40N and between 40 and 60 pr-μm at 60N-80N, with uncertainties of about 30 percent. Our results are in agreement with previous results by MAWD/Viking and TES/MGS for latitudes up to 60N, but seem to indicate lower values at high latitude. However they are still globally consistent in view of our error bars.

Observations of water vapour anomaly above Tharsis volcanoes on Mars in the ISM (Phobos-2) experiment

Abstract Observations of Mars with the infrared imaging spectrometer ISM on board the Phobos -2 spacecraft provided more than 40,000 spectra in the wavelength range 0.7–3.2 μm with a spatial resolution of about 20 km. These spectra bear features of both atmospheric and surface origin. Measurements in the water vapour bands near 1.9 and 2.6 μm along the flanks of huge Martian volcanoes provided the data on the H 2 O spatial variations above them. The retrieved mixing ratio was about 200 ppm above the volcanoes, which was about a factor of five higher than that found above the surrounding terrains. The origin of the observed water vapour anomaly is likely to be connected to the volcanoes and could be explained by the regolith-atmosphere water vapour exchange. On the basis of a new theoretical approach to the process of diffusion of adsorbed water from the regolith to the atmosphere, a model of the observed anomaly was proposed. It implies an intensive vapour desorption during the day hours from the volcanic regolith, which was assumed to be different in composition from that of surrounding highlands.

Venus Express: highlights of the nominal mission

Venus Express is the first European (ESA) mission to the planet Venus. Its main science goal is to carry out a global survey of the atmosphere, the plasma environment, and the surface of Venus from orbit. The payload consists of seven experiments. It includes a powerful suite of remote sensing imagers and spectrometers, instruments for in-situ investigation of the circumplanetary plasma and magnetic field, and a radio science experiment. The spacecraft, based on the Mars Express bus modified for the conditions at Venus, provides a versatile platform for nadir and limb observations as well as solar, stellar, and radio occultations. In April 2006 Venus Express was inserted in an elliptical polar orbit around Venus, with a pericentre height of ∼250 km and apocentre distance of ∼66000 km and an orbital period of 24 hours. The nominal mission lasted from June 4, 2006 till October 2, 2007, which corresponds to about two Venus sidereal days. Here we present an overview of the main results of the nominal mission, based on a set of papers recently published in Nature, Icarus, Planetary and Space Science, and Geophysical Research Letters.

Evidences of the Regolith-Atmosphere Water Exchange on Mars from the ISM (Phobos-2) Infrared Spectrometer Observations

Abstract The infrared imaging spectrometer (ISM) on board Phobos-2 spacecraft provided the most important and valuable data on the atmospheric water content on Mars in the post-Viking period. Some of the results of this experiment are likely to be the evidence of the existence of the regolith-atmosphere water vapour exchange. Release of the water molecules, stored in the regolith in the adsorbed phase, to the atmosphere, induced by the noon temperature rise can cause up to a factor of 3 increase in the atmospheric water amount above the areas with high adsorbing capabilities such as clay regolith. Much less, about 30%, increase is expected above the terrains, covered with basalt grains. Local time variations of atmospheric water amount, observed by ISM, are within these limits. The dependence of water vapour desorption rate on the regolith properties can result in the spatial variations of atmospheric water content above clay and basalt regions. This is assumed to be the reason of a factor of 5±2 increase in the atmospheric water mixing ratio, observed by ISM above the flanks of Martian volcanoes as compared to the surrounding terrains.

PFS for Mars Express: a new approach to study the Martian atmosphere

PFS, a Fourier spectrometer for the study of the Martian atmosphere, will fly on Mars Express. For the first time the study of the 15 μm CO2 absorption band, which gives the vertical temperature-pressure profile, will be carried out simultaneously with the retrieval of the aerosols number density vertical profile using multi bands approach and using the measured ground pressure from the 2 μm CO2 absorption band. It will enable us to get the vertical structure of the Martian atmosphere, the water vapour column abundance and vertical distribution, and the vertical distribution of the dust. A comparison with all the previously flown spectrometers will be presented.

PFS on Mars Express: preparing the analysis of infrared spectra to be measured by the Planetary Fourier Spectrometer

Abstract A radiative transfer simulation and retrieval technique (RRT) is described that will be applied to derive atmospheric and surface properties of Mars from measured infrared signatures recorded by the Planetary Fourier Spectrometer (PFS) onboard the Mars Express mission in 2003. The RRT considers absorption, emission and multiple scattering of thermal and solar radiation by molecular (CO2, H2O, CO) and particulate (palagonite, montmorillonite) species. Precalculated sets of absorption cross-sections which are based on quasi-monochromatic line-by-line calculations are used to predict molecular transmission functions at 2.0 cm −1 spectral resolution in a layered atmosphere. Synthetic infrared spectra have been calculated in the 200– 8000 cm −1 wavenumber range (1.25– 50 μm ) for a variety of atmospheric models and surface conditions. The influence of multiple aerosol scattering on the spectra is investigated on the basis of a successive approximation technique. Numerical results are compared with the spherical harmonics discrete ordinate method. Different retrieval procedures and results of a simultaneous retrieval of vertical temperature profiles and aerosol optical depths as well as surface temperature, pressure and reflectance are discussed. Real measurements performed by the IRIS instrument onboard Mariner 9 are also examined.

The atmosphere of Venus: current knowledge and future investigations

As the Earth’s nearest planetary neighbour, Venus has been studied by ground-based observers for centuries, and has been visited by more than 20 spacecraft. However, in the last decade and a half Venus research has been relatively neglected, despite the fact that a great many major questions about its atmosphere, surface and interior remain unanswered. Several of these questions relate to the unique position of Venus as the Earth’s near twin, in terms of size, density and proximity to the Sun, which led early astronomers to expect an Earth-like environment on the planet, possibly one fit for human habitation, and perhaps even the seat of indigenous life. The picture which has emerged from missions to the planet is quite different, raising questions about the evolution and stability of terrestrial planet environments that are both intriguing and possibly of practical relevance to global change problems on the Earth. This chapter reviews the scientific issues, and goes on to describe two new missions to Venus, the European Venus Express and the Japanese Planet-C orbiters, which will take place in the next few years to address some of them in depth. Other questions will remain unanswered, and further missions will be required, including landing on Venus and sample return.

Planetary Fourier spectrometer: An interferometer for atmospheric studies on board Mars 94 mission

SummaryPFS is a two-channel Fourier spectrometer operating in the infra-red wavelengths between 1.25 and 45 μm. The instrument will be used mainly in the study of the Martian atmosphere. The principal goals are the measurements of the atmospheric temperature and pressure, atmospheric constituents, aerosol and clouds, ground pressure for surface topography, optical and thermophysics properties of the Martian soil. PFS will fly on the Mars 94 spacecraft which should be launched in 1994 and reach the planet in 1995. It is essentially constituted by two different interferometers located in the same box which is divided into two parts. A dichroic placed on the PFS entrance is used to separate the spectral range into two parts, a division needed by the different optical materials which have to be used in each spectral range. The optical layout of the experiment is very compact. Each channel uses two cubic mirrors mounted on an L-structure pivoted on a motor. The motor moves the mechanics and permits the optical-path difference between the arms to be varied. Each interformeter operates in a different spectral range, respectively, between (1.25÷4.8) μm (8000÷2083cm−1) and (6÷45)μm (1666÷220)cm−1). The spectral resolution is 2 cm−1. The entrance aperture area is 30 cm2 per channel and the field of view is 2 and 4 degrees. Every measurement lasts about 4 s. The time and, therefore, the relative optical-path difference for the measurement of every point of the interferogram is given by the zero crossings of the interferogram of a reference monochromatic channel at 1.2 μm which uses a laser diode as source. The two interferograms are double-sided and will have 16384 and 4096 points, respectively, corresponding to spectra of 6250 and 1823 useful points.

Optical properties of the Martian aerosols in the visible spectral range

Abstract Imager for Mars Pathfinder (IMP) obtained data of sky brightness as a function of the scattering angle, wavelength, time of day and Sol. This data set is fitted with model calculations to extract the size distribution, shape and the refractive index of the aerosols suspended in the atmosphere. The inferred optical parameters are discussed in context of diurnal variations and compared to those derived from Viking Landers cameras and Phobos KRFM radiometer data. The effects of the scattering and absorption of the solar radiation by the atmospheric aerosols are discussed in terms of their influence on the spectrophotometry of the Martian surface.