L. V. Zasova

Perennial water ice identified in the south polar cap of Mars.

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.

A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the ‘cryosphere’ above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80–90 km for which we have no explanation.

South-polar features on Venus similar to those near the north pole

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright ‘dipole’ feature surrounded by a cold ‘collar’ at its north pole. The polar dipole is a ‘double-eye’ feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus’ south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express

The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90–120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO2, O2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO2 non-local thermodynamic equilibrium emission at 4.3 µm, extending from 90 to 120 km altitude, and of night-side O2 emission extending from 95 to 100 km. The CO2 emission peak occurs at ∼115 km and varies with solar zenith angle over a range of ∼10 km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O2 peak emission happens at 96 km ± 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted.

First detection of hydroxyl in the atmosphere of Venus

Context. Airglow emissions, such as previously observed from NO and O2(a−X )( 0−0) on Venus, provide insight into the chemical and dynamical processes that control the composition and energy balance in the upper atmospheres of planets. The OH airglow emission has been observed previously only in the Earth’s atmosphere where it has been used to infer atomic oxygen abundances. The O2(a − X )( 0−1) airglow emission also has only been observed in the Earth’s atmosphere, and neither laboratory nor theoretical studies have reached a consensus on its transition probability. Aims. We report measurements of night-side airglow emission in the atmosphere of Venus in the OH (2−0), OH (1−0), O2(a − X )( 0−1), and O2(a − X )( 0−0) bands. This is the first detection of the first three of these airglow emissions on another planet. These observations provide the most direct observational constraints to date on H, OH, and O3, key species in the chemistry of Venus’ upper atmosphere. Methods. Airglow emission detected at wavelengths of 1.40−1.49 and 2.6−3.14 µm in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express spacecraft is attributed to the OH (2−0) and (1−0) transitions, respectively, and compared to calculations from a photochemical model. Simultaneous limb observations of airglow emission in the O2(a − X )( 0−0) and (0−1) bands at 1.27 and 1.58 µm, respectively, were used to derive the ratio of the transition probabilities for these bands. Results. The integrated emission rates for the OH (2−0) and (1−0) bands were measured to be 100 ± 40 and 880 ± 90 kR respectively, both peaking at an altitude of 96 ± 2 km near midnight local time for the considered orbit. The measured ratio of the O2(a −X )( 0−0) and (0−1) bands is 78 ± 8. Conclusions. Photochemical model calculations suggest the observed OH emission is produced primarily via the Bates-Nicolet mechanism, as on the Earth. The observed ratio of the intensities of the O2(a − X )( 0−0) and (0−1) bands implies the ratio of their transition probabilities is 63 ± 6.

Structure of the Venusian atmosphere from surface up to 100 km

The goal of this paper is to summarize the experimental data on the atmosphere of Venus obtained after 1985, when the VIRA (Venus International Reference Atmosphere) or COSPAR model was published. Among the most important results that have appeared since then are the following: measurements of the vertical temperature profile by the VEGA spacecraft with high precision and high altitude resolution; measurements made with balloons of the VEGA spacecraft; radio occultation measurements of Magellan, Venera-15, and Venera-16; and temperature profiles derived from the data of infrared spectrometry obtained by Venera-15. The new result as compared to VIRA is the creation of a model of the atmosphere in the altitude range 55 to 100 km dependent on local time. This model is presented in our paper in tabulated form.

VIRA-2: A review of inputs for updating The Venus International Reference Atmosphere

Abstract VIRA (The Venus International Reference Atmosphere, ed. by A.J.Kliore, V.I. Moroz, and J.M.Keating) was compiled in 1982–1983 and published in 1985 ( Advances in Space Research, V5 , N11). A lot of new data and important findings have appeared over the last decade owing to the experiments on the latest Soviet and US spacecraft and to Earth-based observations. Reanalysis of the previous missions data has also brought some new results. The most essential inputs to the updated VIRA are: 1) new data on the chemical composition (ground based and Galileo observations of the near IR spectra of the Venus nightside emission; VEGA 1, 2 UV in situ spectrometry; Venera 15 infrared spectrometry; reanalysis of Venera 11,13,14 spectrometry; Pioneer Venus entry probes and OIR data), 2) vertical T,P profiles obtained from the VEGA 2 entry probes, IR thermal sounding (Venera 15, Galileo.), and from radio-occultation (Venera 15,16, Pioneer Venus and Magellan Orbiters), horizontal T -profiles from the VEGA 1,2 balloons, 3) winds and turbulence measurements on the balloons, thermal winds retrieved from T -profiles (Venera 15, Galileo); 4) new data on the variability of the cloud structure (Venera 15 IR-spectrometry, VEGA 1,2 entry probes and NIMS observations during the Galileo Venus fly-by).

Water vapour in the middle atmosphere of Venus:: An improved treatment of the Venera 15 ir spectra

In 1983, spectra of Venus in the region of 6–40 μm were measured by means of the Fourier Spectrometer aboard the Venera 15 orbiter. It covered local solar times from 4 am to 10 am and from 4 pm to 10 pm in the latitude range from 65°S up to 87°N. The results of an extended processing and analysis of these data are presented. Time and spatial variations of the water vapour were found. Most of the measurements fall in the range of 5–15 ppm, which is close to earlier results. The effective altitude of sounding is approximately equal to the altitude where the optical depth τ = 1. In the northern hemisphere, which was mainly covered by the measurements, two latitude regions can be distinguished; (A) 20° 60°, which are characterised by different altitudes of the level of τ = 1, 62 and 55 km respectively. Mean mixing ratios near this level in the two regions are almost the same, but the partial pressures and mass densities in the region (B) are 2–4 times greater than those in region (A). In region (A) a weak maximum was detected near 10 am local solar time (17 ppm at φ = 35°) and a minimum—near 10 pm (2ppm at φ = 30°). Region (B) is of inhomogeneous structure, and the retrieved mixing ratio has greater uncertainty and may probably change from the low values up to 30 ppm. In region (A) the water vapour mass density at the level of τ = 1 is 2–4 times greater than the mean density of the water contained in aerosol particles, while in region (B) this ratio may vary in the limits 0.5–5. Although the retrieval of H2O mixing ratio altitude profile from the Venera 15 data appeared to be impossible, indirect indications were found that at least in region (A) the mixing ratio decreases with altitude.

Structure of the venus middle atmosphere: Venera 15 fourier spectrometry data revisited

Abstract The data obtained by Infrared Fourier Spectrometer on board Venera 15 Orbiter are revisited. The new database of temperature and aerosol profiles is created for the altitude range 55–100 km. The main improvements concern the involving of the whole spectral range free from absorption by any gases but CO 2 into the temperature retrieval procedure. Besides the CO 2 15 μm fundamental band, this range also includes the weak hot and isotopic CO 2 bands. HITRAN-96 spectral database was used for calculation of the gaseous absorption coefficients. The diurnal variations at the isobaric levels are investigated. At low latitudes at the altitude h > 85 km a minimal temperature is observed in the afternoon, and a maximal one is on the morning day side. The temperature differences reach 20 K near 0.1 mb level. The temperature difference changes its sign below 1 mb level: in the afternoon it is warmer by more than 10 K than in the morning. The density of the clouds at all latitudes is found to be higher in the afternoon than in the morning. In the coldest parts of the ‘cold collar’ the clouds are found to be composed of the mode 3 particles. The thermal zonal wind field reveals the presence of the midlatitude jet, connected with the ‘cold collar’. The low latitude jet near 85 km, connected with the temperature inversion above this level, is observed. It is also possible that another low latitude jet exists near the cloud tops at low latitudes.

PFS:A FOURIER SPECTROMETER FOR THE STUDY OF MARTIAN ATMOSPHERE

The Planetary Fourier Spectrometer PFS has been designed for the study of the atmosphere and soil of Mars. PFS has two infrared channels: a long wavelength (LW) channel with range 250 - 2000 cm-’ and a short wavelength (SW) channel with range 2000 - 8333 cm-‘. The spectral resolution is 2 cm-‘. Both channels work simultaneously. The field of view is 2” which covers 10 km on the Martian surface being observed from the pericenter at 300 km. The signal to noise ratio is better than 100 in a range of particular scientific interest (at 650 cm-’ , for example). The built-in pointing device allows to study the atmosphere over extreme regions like Hellas Planitia or Olympus Mons.