An Orbiter for the study of the atmosphere, the plasma

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An Orbiter for the study of the atmosphere, the plasma

Transcript Of An Orbiter for the study of the atmosphere, the plasma

Venus Express Mission Definition Report

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ESA-SCI(2001)6
ESA-SCI(2001)6
October 2001

An Orbiter for the study of the atmosphere, the plasma environment, and the surface of Venus

Mission Definition Report
European Space Agency Agence Spatiale Européenne

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Foreword

Venus Express, an Orbiter for the study of the atmosphere, the plasma environment, and the

surface of Venus, is a mission which was proposed to ESA in response to the Call for Ideas to re-use

the Mars Express platform issued in March 2001. Venus Express together with two other missions,

Cosmic DUNE and SPORT Express, was selected by ESA’s Space Science Advisory Committee for a

Mission Definition Study. The industrial study of the three missions was conducted in parallel by

Astrium-SAS (Toulouse, France) from mid-July to mid-October 2001.

The payload included in the Venus Express Study comprises 5 instruments (ASPERA/MEx,

PFS/MEx, SPICAM/MEx, VeRa/Rosetta, VIRTIS/Rosetta) from the Core payload of the original

Proposal and the VENSIS/MEx radar in line with the SSWG recommendation. During the Study it

was found scientifically reasonable and technically feasible to replace the standard Mars Express

engineering Video Monitoring Camera by a scientific instrument, the Venus Monitoring Camera

(VMC).

The Mission Definition Report describes the scientific objectives of the Venus Express mission,

presents selected payload set, and summarizes the results of the Mission Definition Study. This

version of the report covers all science aspects of the mission but contains only a brief summary of the

industrial study. The combined industrial study report for all the three missions is published in a

separate cover. A complete Venus Express Mission Definition Report, including a comprehensive

description of scientific goals, payload, and technical aspects of the spacecraft will be prepared by the

end of 2001.

The Venus Express Study was directly supported by the Science Study Team listed below.

Mission science coordination

D.V. Titov, MPAe, Germany

E. Lellouch, DESPA, France

F.W. Taylor, Oxford University, UK

L. Marinangeli, Universita d’Annunzio, Italy

H. Opgenoorth, IRF-Uppsala, Sweden

Principal Investigators

S. Barabash, IRF-Kiruna, Sweden /PI ASPERA J.-L. Bertaux, Service de Aeronomie, France /Co-PI SPICAM/ P. Drossart, DESPA, France /Co-PI VIRTIS/ V. Formisano, IFSI, Italy /PI PFS/ B. Haeusler, Universitaet der Bundeswehr, Germany /PI VeRa/ O. Korablev, IKI, Moscow, Russia /Co-PI SPICAM/ W.J. Markiewicz, MPAe, Germany /PI VMC/ M. Paetzold, Universitaet zu Koeln, Germany /Co-PI VeRa/ G. Picardi, Infocom Dpt. Univ. of Rome, Italy /PI VENSIS/ G. Piccioni, IAS, Italy /Co-PI VIRTIS/ J. Plaut, JPL/NASA, Pasadena, California, USA J.-A. Sauvaud, CESR-CNRS, France /Co-PI ASPERA/ P. Simon, BISA, Belgium /CO-PI SPICAM/

The ESA members of the Scientific Directorate responsible for the study were: J-P. Lebreton, Study Scientist, Research and Science Support Department (RSSD), ESTEC M. Coradini, Science Planning and Coordination Office, ESA HQ, Paris G. Whitcomb, Future Science Projects and Technology Office, SCI-PF, ESTEC D. McCoy, Mars Express Project Team, SCI-PE, ESTEC.

The Industrial study was lead by: Ch. Koeck (Study Manager), Astrium, France
with support from: S. Kemble (Mission Analysis), Astrium, UK

Venus Express Mission Definition Report
L. Gautret (Payload Interface Engineering), Astrium, France P. Renard (System Engineering), Astrium, France F. Faye (Mars Express expertise), Astrium, France.

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Support was provided by the following colleagues within ESA:

ESOC:

M. Hechler and J. Rodriguez-Canabal, (Mission Analysis); R. Van Holtz, (Ground Segment definition)

ESTEC

A. Chicarro (Mars Express Project Scientist), RSSD/SCI-SO P. Falkner, (payload support), RSSD/SCI-ST P. Martin (Mars Express Deputy Project Scientist), RSSD/SCI-SO J. Romstedt (radiation environment analysis & payload support), RSSD/SCI-ST R. Schmidt (Mars Express Project Manager), SCI-PE J. Sorensen (radiation environment analysis), TOS-EMA P. Wenzel (Head of Solar System Division), RSSD/SCI-SO O. Witasse, (Science support), RSSD/SCI-SO

This report is available in pdf format at: http://solarsystem.estec.esa.nl/Flexi2005/
Requests for further information and additional hard copies of this report should be addressed to:
Jean-Pierre Lebreton: [email protected] Marcello Coradini: [email protected]

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Executive Summary
The first phase of Venus spacecraft exploration (1962-1985) by the Venera, Pioneer Venus and Vega missions established a basic description of the physical and chemical conditions prevailing in the atmosphere, near-planetary environment, and at the surface of the planet. At the same time, they raised many questions on the physical processes sustaining these conditions, most of which remain as of today unsolved. Extensive radar mapping by Venera-15,-16 and Magellan orbiters, combined with earlier glimpses from landers, have expanded considerably our knowledge of Venus’ geology and geophysics. A similar systematic survey of the atmosphere is now in order. This particularly concerns the atmosphere below the cloud tops, which, with the exception of local measurements from descent probes, has escaped detection from previous Venus orbiters. Many problems of the solar wind interaction, in particularly those related to the impact on the planetary evolution are still not resolved. The present proposal aims at a global investigation of Venus’ atmosphere and plasma environment from orbit, and addresses several important aspects of the geology and surface physics.
The fundamental mysteries of Venus are related to the global atmospheric circulation, the atmospheric chemical composition and its variations, the surface-atmosphere physical and chemical interactions including volcanism, the physics and chemistry of the cloud layer, the thermal balance and role of trace gases in the greenhouse effect, the origin and evolution of the atmosphere, and the plasma environment and its interaction with the solar wind. Besides, the key issues of the history of Venusian volcanism, the global tectonic structure of Venus, and important characteristics of the planet’s surface are still unresolved. Beyond the specific case of Venus, resolving these issues is of crucial importance in a comparative planetology context and notably for understanding the long-term climatic evolution processes on Earth.
The above problems can be efficiently addressed by an orbiter equipped with a suite of adequate remote sensing and in situ instruments. Compared with earlier spacecraft missions, a breakthrough will be accomplished by fully exploiting the existence of spectral “windows” in the near-infrared spectrum of Venus’ nightside, discovered in the late ‘80’-s, in which radiation from the lower atmosphere and even the surface escapes to space and can be measured. Thus, a combination of spectrometers, spectro-imagers, and imagers covering the UV to thermal IR range, along with other instruments such as a radar and a plasma analyzer, is able to sound the entire Venus atmosphere from the surface to 200 km, and to address specific questions on the surface that would complement the Magellan investigations. This mission will also tackle still open questions of the plasma environment focusing on the studies of nonthermal atmospheric escape. This issue will be addressed via traditional in situ measurements as well as via innovative ENA (Energetic Neutral Atom) imaging techniques.
The instruments developed for the Mars Express and Rosetta missions are very well suited for this task. The following available instruments: SPICAM – a versatile UV-IR spectrometer for solar/stellar occultations and nadir observations, PFS – a high-resolution IR Fourier spectrometer, ASPERA – a combined energetic neutral atom imager, electron, and ion spectrometer, VIRTIS – a sensitive visible spectro-imager and mid-IR spectrometer, a radio science experiment VeRa, a wide-angle monitoring camera VMC, and subsurface and ionosphere sounding radar VENSIS will form the payload of the proposed Venus Express mission. Taken together, these experiments can address all the broad scientific problems formulated above.
The Mission Definition Study demonstrated the feasibility of the proposed mission to Venus in 2005. The Mars Express spacecraft can accommodate the above mentioned experiments with minor modifications. The launch with Soyuz-Fregat can deliver this payload to a polar orbit around Venus with a pericenter altitude of ~250 km and apocenter of

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~45,000 km. This orbit will provide complete coverage in latitude and local solar time. It is also well suited for atmospheric and surface sounding, as well as the studies based on solar and radio occultations. In comparison to the Pioneer Venus spinning spacecraft, Mars Express is an advanced 3 axis stabilised platform which provides significantly enhanced spectroscopic and imaging capabilities. The proposed duration of the nominal orbital mission is two Venus days (sidereal rotation periods) equivalent to ~500 Earth days.
The Venus Express mission will achieve the following “firsts”: • First global monitoring of the composition of the lower atmosphere in the near IR
transparency “windows”; • First coherent study of the atmospheric temperature and dynamics at different levels of
the atmosphere from the surface up to ~200 km; • First measurements of global surface temperature distribution from orbit; • First study of the middle and upper atmosphere dynamics from O2, O, and NO emissions; • First measurements of the non-thermal atmospheric escape; • First coherent observations of Venus in the spectral range from UV to thermal infrared; • First application of the solar/stellar occultation technique at Venus; • First use of 3D ion mass analyzer, high energy resolution electron spectrometer, and
energetic neutral atom imager; • First sounding of Venusian topside ionospheric structure; • First sounding of the Venus subsurface.
Together with the Mars Express mission to Mars and the Bepi Colombo mission to Mercury, the proposed mission to Venus, through the expected quality of its science results, would ensure a coherent program of terrestrial planets exploration and provide Europe with a leading position in this field of planetary research. The international cooperation formed in the framework of the Mars Express and Rosetta missions will be inherited by the Venus Express and will include efforts of the scientists of European countries, USA, Russia, and Japan. The Venus Express orbiter will play the role of pathfinder for future, more complex missions to the planet, and the data obtained will help to plan and optimize future investigations. Venus studies can have significant public outreach given the exotic conditions of the planet and the interest in comparing Venus to Earth, especially in a context of concern with the climatic evolution on Earth.

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Table of content

1. INTRODUCTION................................................................................................................................................ 8

2. MISSION SCIENCE OBJECTIVES................................................................................................................. 8
2.1 LOWER ATMOSPHERE AND CLOUD LAYER (0 – 60 KM) ................................................................................... 8 2.2 MIDDLE ATMOSPHERE (60 – 110 KM) ........................................................................................................... 12 2.3 UPPER ATMOSPHERE (110 – 200 KM) ............................................................................................................ 13 2.4 PLASMA ENVIRONMENT AND ESCAPE PROCESSES ......................................................................................... 14 2.5 SURFACE AND SURFACE-ATMOSPHERE INTERACTION ................................................................................... 15
3. SCIENTIFIC PAYLOAD ................................................................................................................................. 17
3.1 ASPERA (ANALYZER OF SPACE PLASMAS AND ENERGETIC ATOMS) ......................................................... 17 3.2 PFS (HIGH RESOLUTION IR FOURIER SPECTROMETER) ................................................................................ 18 3.3 SPICAM (UV AND IR SPECTROMETER FOR SOLAR/STELLAR OCCULTATIONS AND NADIR OBSERVATIONS)20 3.4 VERA (VENUS RADIO SCIENCE).................................................................................................................... 22 3.5 VIRTIS (UV-VISIBLE-NEAR IR IMAGING SPECTROMETER) .......................................................................... 23 3.6 VENSIS (LOW FREQUENCY RADAR FOR SURFACE AND IONOSPHERIC STUDIES). ......................................... 25 3.7 VMC (VENUS MONITORING CAMERA) ......................................................................................................... 26 3.8 SYNERGY OF THE PAYLOAD. .......................................................................................................................... 27 3.9 PAYLOAD ACCOMMODATION......................................................................................................................... 28 3.10 MISSION AND PAYLOAD SCHEDULE ............................................................................................................. 29 3.11 PAYLOAD TEAMS ......................................................................................................................................... 29
4 MISSION OVERVIEW...................................................................................................................................... 36
4.1 MISSION SCENARIO ........................................................................................................................................ 36 4.2 LAUNCH, DELTA-V, AND MASS BUDGETS...................................................................................................... 37 4.3 OPERATIONAL ORBIT ..................................................................................................................................... 37 4.4 ORBITAL SCIENCE OPERATIONS ..................................................................................................................... 38 4.5 TELECOMMUNICATIONS................................................................................................................................. 38 4.6 THERMAL CONTROL....................................................................................................................................... 39 4.7 RADIATION REQUIREMENTS........................................................................................................................... 39 4.8 GROUND SEGMENT IMPLEMENTATION AND OPERATIONS SUPPORT............................................................... 39 4.9 MISCELLANEOUS............................................................................................................................................ 40
5. SCIENCE OPERATIONS, DATA ANALYSIS, AND ARCHIVING ......................................................... 40
5.1 SCIENCE OPERATIONS CONCEPT ................................................................................................................... 40 5.2 PRINCIPAL INVESTIGATORS ........................................................................................................................... 40 5.3 INTERDISCIPLINARY SCIENTISTS (IDS) ......................................................................................................... 40 5.4 SCIENCE WORKING TEAM ............................................................................................................................. 40 5.6 SCIENCE OPERATION PLAN............................................................................................................................ 41 5.7 DATA ANALYSIS............................................................................................................................................. 41 5.8 SCIENCE MANAGEMENT PLAN ...................................................................................................................... 41 5.9 COMPLEMENTARY VENUS GROUND-BASED OBSERVATIONS........................................................................ 41
6. PROGRAMMATIC VALIDITY ..................................................................................................................... 41

7. SCIENCE COMMUNICATION AND OUTREACH ................................................................................... 42
7.1 GOALS............................................................................................................................................................ 42 7.2 SCIENTIFIC THEMES ....................................................................................................................................... 42 7.3 IMPLEMENTATION .......................................................................................................................................... 43
8. INTERNATIONAL COOPERATION............................................................................................................ 43

9. REFERENCES................................................................................................................................................... 45

10 ACKNOWLEDGMENTS ................................................................................................................................ 46

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1. Introduction
Since the beginning of the space era, Venus has been an attractive target for planetary science. Our nearest planetary neighbour and, in size, the twin sister of Earth, Venus was expected to be very similar to our planet. However, the first phase of Venus spacecraft exploration (1962-1985) discovered an entirely different, exotic world hidden behind a curtain of dense clouds. The earlier exploration of Venus included a set of Soviet orbiters and descent probes, Veneras 4–16, the US Pioneer Venus mission, the Soviet Vega balloons, the Venera 15, 16 and Magellan radar orbiters, the Galileo and Cassini flybys, and a variety of ground-based observations.
Despite all of this exploration by more than 20 spacecraft, the “morning star” remains a mysterious world. All these studies gave us a basic knowledge of the conditions on the planet, but generated many more questions concerning the atmospheric composition, chemistry, structure, dynamics, surface-atmosphere interactions, atmospheric and geological evolution, and the plasma environment. It is high time to proceed from the discovery phase to a thorough investigation and deep understanding of what lies behind Venus’ complex chemical, dynamical, and geological phenomena.
The data from ground-based observations and previous space missions is very limited in space and time coverage, and, prior to the discovery of the near infrared spectral windows, lacked the capability to sound the lower atmosphere of Venus remotely and study the phenomena hidden behind the thick cloud deck from orbit. Thus a survey of the Venus atmosphere is long overdue. Pioneer Venus, Venera-15, -16, and Magellan provided global comprehensive radar mapping of the surface and investigated its properties. The use of penetrating radar can add a third dimension to the earlier investigations.
While a fully comprehensive exploration of Venus will require, in the long term, in situ measurements from probes, balloons and sample return, so many key questions about Venus remain unanswered that even a relatively simple orbiter mission to the planet can bring a rich harvest of high quality scientific results. The re-use of the Mars Express bus with the payload based on the instruments available from the Mars Express and Rosetta projects is very appropriate in this regard. It offers an excellent opportunity to make major progress in the study of the planet.

2. Mission science objectives
The proposed Venus Express mission covers a broad range of scientific goals including atmospheric physics, subsurface and surface studies, investigation of the plasma environment and interaction of the solar wind with the atmosphere. For clarity we divided the atmosphere into three parts: lower atmosphere (0-60 km), middle atmosphere (60 – 110 km), and upper atmosphere (110 – 200 km). The physics, methods of investigation, and scientific goals are quite different for each atmospheric region. However they all can be studied by a multipurpose remote sensing and in situ payload in the framework of the proposed orbiter mission.
2.1 Lower atmosphere and cloud layer (0 – 60 km)
Structure. Existing observations of the lower atmosphere hidden below the clouds are limited to in situ measurements, acquired by 16 descent probes mostly in equatorial latitudes, by radiooccultations on previous orbiters (Venera 9, 10, 15, 16, Pioneer Venus, and Magellan), and brief glimpses provided by the Galileo and Cassini fly-bys.
The descent probes showed that the temperature structure below 30 km is quite constant all over the planet (Fig. 2.1). However, the temperature structure in the lower scale height is virtually unknown. Mapping the regions of high elevation in sub-micron spectral “windows” at the nightside will determine the surface temperature as a function of altitude (Meadows and Crisp (1996)). Assuming this is equal to the near-surface air temperature, this will allow a determination of the thermal profile and lapse rate in the 0-10 km range and an investigation of its degree of static stability, constraining the dynamics and turbulence in this region. The thermal structure above 35 km altitude will be obtained from radiooccultations with high vertical resolution. Composition. The Venusian atmosphere consists mainly of CO2 and N2 with small amounts of trace gases (Fig. 2.1). Although there is very little observational data, the chemistry of the lower atmosphere is expected to be dominated by the thermal decomposition of sulfuric acid, and cycles that include sulfur and carbon compounds (SO2, CO, COS etc.) and water vapour.

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The discovery of the near IR spectral “windows” (Allen and Crawford, 1984), through which thermal radiation from the lower atmosphere leaks to space, allows us to study the composition of the atmosphere below the clouds on the nightside of the planet. The windows at 2.3 and 1.74 µm sound the atmosphere in broad altitude regions centered at 30-35 km and 20 km respectively, while the windows shortward of 1.2 µm (0.85, 0.9, 1.01, 1.10,and 1.18 µm) probe the first scale height and the surface. The detailed appearance of the windows results from the combined effect of composition, cloud opacity, and thermal structure, including the surface temperature (Taylor et al., 1997). Highresolution observations covering all windows simultaneously, along with physical cloud models, should allow retrieval of all the variables.

Figure 2.1 Structure and main parameters of the lower atmosphere of Venus.
Water vapour is important not only for chemistry but also as a greenhouse gas. The few existing measurements of the H2O abundance in the deep atmosphere show no evidence for variability so far. By mapping simultaneously at several wavelengths, corresponding to radiation originating at different altitudes, it will be possible to probe the H2O profile below the clouds and to search for possible spatial variations, including those that might be the signature of volcanic activity. A precise inventory is also needed to better constrain the origin of the present atmospheric water. The H2O abundance at the surface has strong implications for the stability of some hydrated rocks.
Carbon monoxide is very abundant in the upper atmosphere due to the dissociation of CO2 by solar ultraviolet radiation. It is much less common in the troposphere, but it does there show a definite trend of increasing from equator to pole. The source near the poles could be the downward branch of a Hadley cell transporting CO-rich air from the upper atmosphere, an important diagnostic of the mean meridional circulation. More detailed observations of CO at all levels, latitudes and times are needed to confirm this hypothesis and reveal details of the global-scale dynamics. CO is also a key player in the equilibrium between surface minerals and the atmosphere.
The study of the lower atmosphere composition by means of spectroscopy in the near IR transparency “windows” is one of the main goals of the Venus Express mission. More specific objectives include abundance measurements of H2O, SO2, COS, CO, H2O, HCl, and HF and their horizontal and vertical (especially for H2O) variations, to significantly improve our understanding of the chemistry, dynamics, and radiative balance of the lower atmosphere, and to search for localized volcanic activity. Cloud layer. Venus is shrouded by a 20 km thick cloud layer whose opacity varies between 20 and 40 in the UV, visible and infrared (Fig. 2.1). The clouds are almost featureless in visible light but display prominent markings in the UV-blue spectral region (Fig. 2.2). Earlier observations showed that at least the upper cloud consists of micron size droplets of 75% H2SO4, which is produced by photochemical reactions at the cloud tops. The physical and chemical processes forming the lower clouds are virtually unknown, including major problems like (1) the nature of the UV-blue absorber which produces the features observed from space and absorbs half of the energy received by the

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planet from the Sun, and (2) the origin of the

large solid particles detected by the Pioneer-

Venus probe.

The remote sensing instruments on Venus

Express will sound the structure,

composition, dynamics, and variability of the

cloud layer, including:



Cloud and haze structure and

opacity variations;



Distribution and nature of the UV-

blue absorber;



Measurements of atmospheric

composition which constrain models of

cloud formation and evolution.

Greenhouse effect. The high surface

temperature of about 735 K results from the

powerful greenhouse effect created by the

presence of sulphuric acid clouds and certain gases (CO2, H2O, SO2) in the atmosphere (see Crisp and Titov, 1997). Less than 10%

Figure 2.2 Venus images in the violet filter taken by the Gallileo spacecraft

of the incoming solar radiation penetrates through the atmosphere and heats the surface, but thermal

radiation from the surface and lower atmosphere has a lower probability of escape to space due to the

strong absorption by gas and clouds. The result is about 500K difference between the surface

temperature and that of the cloud tops, an absolute record among the terrestrial planets (Fig. 2.1). The

measurements of outgoing fluxes over a broad spectral range, combined with temporarily and

latitudinally resolved cloud mapping and high resolution spectroscopy in the near IR windows will

give an insight into the roles of radiative and dynamical heat transport, and the various species, in the

greenhouse mechanism.

Atmospheric dynamics. The dynamics of the lower atmosphere of Venus is mysterious. Tracking of

the UV markings, descent probes, and Vega balloons trajectories all showed that the atmosphere is

involved in zonal retrograde super-rotation with wind velocities decreasing from ~100 m/s at the

cloud tops to almost 0 at the surface (Fig. 2.3). At the same time, there appears to be a slower

overturning of the atmosphere from equator to pole, with giant vortices at each pole recycling the air

downwards.

What is most puzzling about the

regime represented by this scenario is how

the atmosphere is accelerated to such high

speeds on a slowly-rotating planet.

Additional questions include (1) whether the

meridional circulation is one enormous

'Hadley' cell extending from the upper

atmosphere to the surface, or a stack of such

cells, or something else altogether; (2) how

the polar vortices couple the two main

components of the global circulation and

why they have such a complex shape and

behaviour; and (3) what the observed (and

observable) distributions of the minor

constituents in Venus' atmosphere, including

the clouds, are telling us about the motions

(Fig.2.4).

All attempts to model the zonal super-

rotation have been unsuccessful so far,

indicating that the basic mechanisms of the

phenomenon are unclear. There is an even Figure 2.3 Zonal winds in the Venus atmosphere

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more basic problem, and that is that we have so few observations of the deep atmosphere that we do

not know what to model. The only real solution is to make new observations to gather a more basic

description of the lower atmosphere and then to think about the problem again.

Pioneer Venus and Venera-15

observations showed that the polar

stratosphere is warmer than the

corresponding levels in the tropics, a

dynamical effect associated with the

zonal super-rotation. We expect the deep

atmosphere to be cooler near the poles.

How much cooler is not known: this will

be a function of the efficiency of the

meridional circulation. In theory the most

efficient regime would be a Hadley

circulation: a single large cell filling each

hemisphere and carrying warm air

polewards and cooler air equatorwards.

The observed movements of the cloud markings would be consistent with such a

Figure 2.4 Venus atmospheric dynamics

regime, but whether it actually exists is

not known. Discontinuities in the vertical temperature gradient, observed by the Venera, Pioneer

Venus, and Vega entry probes at characteristic altitudes, have been interpreted as possible evidence

for a stack of Hadley cells on top of each other, rotating in alternate directions.

Only the North polar vortex has been observed in any detail. It has a double 'eye' surrounded by

a collar of much colder air, the difference in brightness temperature between the two being nearly 100

K at a wavelength of 12.5 :m. The 'dipole' rotates with a period of 2.8 (earth) days. The dynamics of

the vortex was derived from a relatively short period of Pioneer Venus observations - 72 days - and

with ~100 km resolution mapping only every four days. Attempts to model the structure and

dynamical behavior of the vortex have shown only that such limited observations are quite inadequate

for the task. The Venus Express spacecraft orbiting every 10-15 hours and imaging the poles both in

the thermal infrared wavelengths, sensitive to the emission from the cloud tops, and the near-IR

“windows”, which probe much deeper, will allow the production of time- and spatially-resolved

movies which would reveal much more information about its behaviour. Until then, the giant

Venusian polar vortices remain one of the great mysteries of the solar system.

A further attribute of Venus'

atmospheric dynamics, which also defies

explanation, has been revealed in the

post-Pioneer and Venera era following

the discovery of bright near-infrared

markings on the night side of the planet

(Allen and Crawford, 1984). The best

view of these was obtained by the NIMS

on the Galileo spacecraft en route to

Jupiter in February 1990 (Carlson et al,

1991) (Fig. 2.5). The red features are

thermal emission from the surface and

deep atmosphere of Venus, while the

blue markings are regions of relatively

high cloud opacity in the main deck,

obscuring the emission. This shows that

the clouds on Venus are highly

inhomogeneous, both horizontally and

vertically. The origin of the contrasts

must be dynamical and is probably due to

variable condensation of cloud material Figure 2.5 False colour image of lower cloud taken by

in large-scale cumulus-type dynamics in NIMS/Galileo 2.3 µm “window”.
AtmosphereVenusSurfaceStudyMission