Oleg Korablev

Discovery of an aurora on Mars

In the high-latitude regions of Earth, aurorae are the often-spectacular visual manifestation of the interaction between electrically charged particles (electrons, protons or ions) with the neutral upper atmosphere, as they precipitate along magnetic field lines. More generally, auroral emissions in planetary atmospheres “are those that result from the impact of particles other than photoelectrons” (ref. 1). Auroral activity has been found on all four giant planets possessing a magnetic field (Jupiter, Saturn, Uranus and Neptune), as well as on Venus, which has no magnetic field. On the nightside of Venus, atomic O emissions at 130.4 nm and 135.6 nm appear in bright patches of varying sizes and intensities, which are believed to be produced by electrons with energy <300 eV (ref. 7). Here we report the discovery of an aurora in the martian atmosphere, using the ultraviolet spectrometer SPICAM on board Mars Express. It corresponds to a distinct type of aurora not seen before in the Solar System: it is unlike aurorae at Earth and the giant planets, which lie at the foot of the intrinsic magnetic field lines near the magnetic poles, and unlike venusian auroras, which are diffuse, sometimes spreading over the entire disk. Instead, the martian aurora is a highly concentrated and localized emission controlled by magnetic field anomalies in the martian crust.

GOMOS on Envisat: an overview

Abstract GOMOS (Global Ozone Monitoring by Occultation of Stars) on board Envisat measures O 3 , NO 2 , NO 3 , neutral density, aerosols, H 2 O, and O 2 , in the stratosphere and mesosphere by detecting absorption of starlight in ultraviolet, visible and near-infrared wavelengths. During bright limb conditions GOMOS will also observe scattered solar radiation. GOMOS will deliver ozone concentration profiles at altitudes 15–100 km with a vertical sampling better than 1.7 km and with a global coverage. As a self-calibrating method stellar occultation measurements provide a basis for a long-term global monitoring of ozone profiles. We will present here the status of the GOMOS instrument and show samples of first results obtained in 2002.

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.

Evidence of Water Vapor in Excess of Saturation in the Atmosphere of Mars

Spacecraft observations reveal more water vapor in the atmosphere of Mars than would be expected if vapor pressure controlled the water vapor profile. The vertical distribution of water vapor is key to the study of Mars’ hydrological cycle. To date, it has been explored mainly through global climate models because of a lack of direct measurements. However, these models assume the absence of supersaturation in the atmosphere of Mars. Here, we report observations made using the SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) instrument onboard Mars Express that provide evidence of the frequent presence of water vapor in excess of saturation, by an amount far surpassing that encountered in Earth’s atmosphere. This result contradicts the widespread assumption that atmospheric water on Mars cannot exist in a supersaturated state, directly affecting our long-term representation of water transport, accumulation, escape, and chemistry on a global scale.

Compact high-resolution spaceborne echelle grating spectrometer with acousto-optical tunable filter based order sorting for the infrared domain from 2.2 to 4.3 μm

A new compact spaceborne high-resolution spectrometer developed for the European Space Agencys Venus Express spacecraft is described. It operates in the IR wavelength range of 2.2 to 4.3 microm and measures absorption spectra of minor constituents in the Venusian atmosphere. It uses a novel echelle grating with a groove density of 4 lines/mm in a Littrow configuration in combination with an IR acousto-optic tunable filter for order sorting and an actively cooled HgCdTe focal plane array of 256 by 320 pixels. It is designed to obtain an instrument line profile of 0.2 cm(-1). First results on optical and spectral properties are reported.

Mars water vapor abundance from SPICAM IR spectrometer: Seasonal and geographic distributions

Received 2 February 2006; revised 7 July 2006; accepted 11 July 2006; published 26 September 2006. [1] The near-IR channel of SPICAM experiment on Mars Express spacecraft is a 800-g acousto-optic tunable filter (AOTF)–based spectrometer operating in the spectral range of 1–1.7 mm with resolving power of � 2000. It was put aboard as an auxiliary channel dedicated to nadir H2O measurements in the 1.37-mm spectral band. This primary scientific goal of the experiment is achieved though successful water vapor retrievals, resulting in spatial and seasonal distributions of H2O. We present the results of H2O retrieval from January 2004 (Ls = 330� ) to December 2005 (Ls = 340� ), covering the entire Martian year. The seasonal trend of water vapor obtained by SPICAM IR is consistent with TES results and reveals disagreement with MAWD results related to south pole maximum. The main feature of SPICAM measurements is globally smaller water vapor abundance for all seasons and locations including polar regions, as compared to other data. The maximum abundance is 50–55 precipitable microns at the north pole and 13–16 precipitable microns (pr mm) at the south pole. The northern tropical maximum amounts to 12–15 pr mm. Possible reasons for the disagreements are discussed.

Vertical Structure and Size Distributions of Martian Aerosols from Solar Occultation Measurements

Solar occultations performed with a spectrometer on board the Soviet spacecraft Phobos 2 (Blamont et al. 1991) provided data on the vertical structure of the Martian aerosols in the equatorial region (0°–20° N latitude) near the northern spring equinox (LS = 0°–20°). All measurements were made close to the evening terminator. Five clouds were detected above 45 km altitude and their vertical structure recorded at six wavelengths between 0.28 and 3.7 μm. They have a small vertical extent (3–6 km) and a vertical optical depth less than 0.03. The thermal structure, as derived from saturated profiles of water vapor observed by our instrument in the infrared, does not allow the CO2 frost point to be reached at cloud altitude, strongly suggesting that cloud particles are formed of H2O ice. Under the assumption of spherical particles, a precise determination of their effective radius, which varies from cloud to cloud and with altitude, is obtained and ranges from 0.15 to 0.85 μm; an estimate of the effective variance of the particle size distribution is ∼ 0.2. The number density of cloud particles at the peak extinction level is ∼1 cm−3. Dust was also observed and monitored at two wavelengths, 1.9 and 3.7 μm, on nine different occasions. The top of the dust opaque layer, defined as the level above which the atmosphere becomes nearly transparent at the wavelengths of observation, is located near 25 km altitude, with variations smaller than ±3 km from place to place. The scale height of dust at this altitude is 3–4 km. The effective radius of dust particles near the top of the opaque layer is 0.95 ± 0.25 μm and increases below with a vertical gradient of ∼0.05 μm km−1. Assuming that particles are levitated by eddy mixing, the eddy diffusion coefficient, K, is found to be ∼106 cm2 sec−1 at 25 km and 105−106 cm2 sec−1 at 50 km using, respectively, dust and cloud observations. An effective variance of 0.25 (±50%) for the dust size distribution is obtained on the basis of a simple theoretical model for the observed vertical gradient of the effective radius of dust particles. Three clouds observed by Viking at midlatitude during the northern summer are reanalyzed. The analysis gives K ≈ 106 cm2 sec−1 below 50 km altitude and at least 107 cm2 sec−1 above. Since the clouds seen from Phobos 2 are observed at twilight, which coincides with the diurnal maximum of the ambient temperature, they can be assumed to be in a steady state. If their thermodynamic state were to vary quickly during the day, our optical thickness at twilight would correspond to unrealistic values in earlier hours when the temperature is lower. Clouds are well fitted by theoretical profiles obtained assuming the steady state. An atmospheric temperature of 165–170 K at ∼50 km is inferred. The negative temperature gradient above the cloud is large (1.5–2 K km−1). A parallel is established between these thin clouds and the polar mesospheric clouds observed on Earth. It is shown that upwelling in equatorial regions at equinox could be a significant factor in levitating cloud particles.

Post‐Phobos model for the altitude and size distribution of dust in the low Martian atmosphere

Four experiments flown on board Phobos 2 provided information on the characteristics of the dust particles suspended in the Martian atmosphere: Auguste (UV-visible-IR spectrometer working in solar occultation geometry), ISM (IR spectrometer measuring the light of the Sun reflected by the planet), Termoskan (scanning radiometer mapping the planetary thermal radiation), KRFM (UV-visible multiphotometer providing limb-to-limb profiles). These experiments, which sounded equatorial regions (20°S–20°N) near the northern spring equinox (LS = 0–20°), are shown to yield a reasonably consistent picture of the dust distribution over the whole altitude range from the ground level, or just above, outside the boundary layer, up to ≈25 km. The vertical profiles of particle volume mixing ratio and effective (projected area-weighted) radius deduced from Auguste measurements, performed in the 15–25 km altitude range, are extrapolated down to the ground by using a simple, physical parameterization of the altitude dependence of dust mixing ratio and radius. This parameterization, which must be understood as describing the vertical distribution of dust in the zonal average and on the mesoscale in latitude, assumes that gravitational settling and eddy diffusion are the only two processes driving vertical dust transport. The vertically averaged effective radius and optical depth of dust particles, as well as vertical profiles of related quantities, are obtained. Optical depth at 1.9-μm wavelength is found to be 0.2 on average, with a typical variation of ±0.1 with time and space. This result is similar to that obtained from ISM spectra analysis. It is also consistent with the Termoskan and KRFM measurements, which yield near-infrared optical depths of 0.12–0.26 and 0.12–0.24, respectively. The particle number density near the surface, as derived from extrapolation of solar occultation profiles, is in the range 1–3 cm-3, in good agreement with Termoskan results (1–2 cm-3). The scale height of the dust volume mixing ratio just above the surface is ≈8–9 km on average, that is, of the same order as the background atmospheric scale height. The vertically averaged effective radius of dust particles is found to lie in the range 1.7±0.2 μm, possibly ≈2 μm in the case of a large effective variance of 0.4. The most likely ISM value is 1.2 μm, with a rather large uncertainty of ±0.4 μm, mainly due to the fact that the spectral dependence of the Minnaert coefficient is not well known. Because ISM data used in the present work were obtained on the Tharsis plateau, at a mean altitude of ≈7 km, the ISM radius must be compared to the Auguste vertically averaged radius for z > 7 km, that is, ≈1.5±0.2 μm. Auguste and ISM radii are therefore consistent at the 1-σ level. Three typical vertical profiles of the dust particle radius and number density, obtained by averaging all solar occultation profiles, including their extrapolated parts below ≈15 km, are proposed as reference models, for three selected values of the effective variance of the particle size distribution (0.10, 0.25, and 0.40).

Composition of the Venus mesosphere measured by Solar Occultation at Infrared on board Venus Express

Solar Occultation at Infrared (SOIR), which is a part of the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) instrument on board Venus Express, combines an echelle-grating spectrometer with an acoustooptical tunable filter. It performs solar occultation measurements in the IR region at a high spectral resolution better than all previously flown planetary spectrometers. The wavelength range probed allows for a detailed chemical inventory of the Venus atmosphere above the cloud layer, with an emphasis on the vertical distribution of the gases. A general description of the retrieval technique is given and is illustrated by some results obtained for CO2 and for a series of minor constituents, such as H2O, HDO, CO, HCl, and HF. Detection limits for previously undetected species will also be discussed.

Preliminary characterization of the upper haze by SPICAV/SOIR solar occultation in UV to mid‐IR onboard Venus Express

The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus/Solar Occultation at Infrared (SPICAV/SOIR) suite of instruments onboard the Venus Express spacecraft comprises three spectrometers covering a wavelength range from ultraviolet to midinfrared and an altitude range from 70 to >100 km. However, it is only recently (more than 1 year after the beginning of the mission) that the three spectrometers can operate simultaneously in the solar occultation mode. These observations have enabled the study of the properties of the Venusian mesosphere over a broad spectral range. In this manuscript, we briefly describe the instrument characteristics and the method used to infer haze microphysical properties from a data set of three selected orbits. Discussion focuses on the wavelength dependence of the continuum, which is primarily shaped by the extinction caused by the aerosol particles of the upper haze. This wavelength dependence is directly related to the effective particle radius (cross section weighted mean radius) of the particles. Through independent analyses for the three channels, we demonstrate the potential to characterize the aerosols in the mesosphere of Venus. The classical assumption that the upper haze is only composed of submicron particles is not sufficient to explain the observations. We find that at high northern latitudes, two types of particles coexist in the upper haze of Venus: mode 1 of mean radius 0.1 ≤ rg ≤ 0.3 μm and mode 2 of 0.4 ≤ rg ≤ 1.0 μm. An additional population of micron-sized aerosols seems, therefore, needed to reconcile the data of the three spectrometers. Moreover, we observe substantial temporal variations of aerosol extinction over a time scale of 24 h.