venus before venus express

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Planetary and Space Science 54 (2006) 1249–1262 Venus before Venus Express Fredric W. Taylor Atmospheric, Oceanic and Planetary Physics, University of Oxford, Clarendon Laboratory, Oxford OX1 3PU, England Accepted 10 April 2006 Available online 17 August 2006 Abstract An overview is given of current knowledge and mysteries about the planet Venus, with emphasis on those aspects that are intended to be studied with the Venus Express mission following orbit insertion at the planet in March 2006. r 2006 Elsevier Ltd. All rights reserved. Keywords: Venus; Atmosphere; Climate 1. Introduction Venus is the closest planet to the Earth, both in terms of distance and in terms of its physical character. The two planets are almost the same size and mean density (see Table 1), and, so far as we know, have much the same solid-body composition. The largest external differences appear in the absence of a natural satellite around Venus, the slow, retrograde rotation of the solid body of Venus, and the absence of a measurable Venusian planetary magnetic field. Internally, Venus seems to be much more volcanically active than the Earth and, perhaps as a result, has a strikingly dense and hot surface environment that far exceeds in pressure and temperature most of the pre-space age astronomers’ expectations based on models of an Earth-like planet somewhat closer to the Sun. The similarities lead us to expect that we might know the Earth better if we continue to compare it to its neighbour and twin in ever-increasing detail. The differences tell us that there are crucial aspects of solar system formation and planetary evolution, and of geology and climate physics, which we currently fail to appreciate or understand. These motivate us to carry out missions like Venus Express, which, if they are to be most effective, focus on the major unknowns and on the observed properties of Venus that are known but difficult to explain, like the high surface temperature. The goal of this paper is to summarise these mysterious aspects, along with the known facts and some informed speculation about Venus, in the hope and expectation that the new mission will shed light on the mysteries, and increase and enhance our understanding of the facts. 2. The morning and evening star As the brightest of the planets, Venus has always been a much-noted feature of the night, and even the day-time, sky, always appearing within 451 of the Sun by virtue of its inferior orbit relative to the Earth. Many of the parameters listed in Tables 1 and 2 have been known from the early days of scientific observations using telescopes, including the fact that Venus has a very high reflectivity (albedo), apparently due to a thick and ubiquitous cloud cover. At 0.76, the albedo of Venus is two and a half times that of the Earth, more than offsetting the doubling of the solar constant at Venus’ mean distance from the Sun. Since Venus absorbs less radiant energy than Earth, there was no particular reason why early practitioners of what we would now call climate modelling should expect the surface temperature to be massively different from our own, and the popular vision of the surface of Venus often included oceans, deserts and steamy jungles. The Sun, with a disc twice the area it shows at the Earth, was thought to evaporate water efficiently and produce the thick and extensive cloud deck. When the composition of the atmosphere was shown by Adams and Dunham in 1934 ARTICLE IN PRESS www.elsevier.com/locate/pss 0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.04.031 Tel.: +44 1865 272903; fax: +44 1865 272924. E-mail address: [email protected].

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Page 1: Venus before Venus Express

ARTICLE IN PRESS

0032-0633/$ - se

doi:10.1016/j.ps

�Tel.: +44 18

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Planetary and Space Science 54 (2006) 1249–1262

www.elsevier.com/locate/pss

Venus before Venus Express

Fredric W. Taylor�

Atmospheric, Oceanic and Planetary Physics, University of Oxford, Clarendon Laboratory, Oxford OX1 3PU, England

Accepted 10 April 2006

Available online 17 August 2006

Abstract

An overview is given of current knowledge and mysteries about the planet Venus, with emphasis on those aspects that are intended to

be studied with the Venus Express mission following orbit insertion at the planet in March 2006.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Venus; Atmosphere; Climate

1. Introduction

Venus is the closest planet to the Earth, both in terms ofdistance and in terms of its physical character. The twoplanets are almost the same size and mean density (seeTable 1), and, so far as we know, have much the samesolid-body composition. The largest external differencesappear in the absence of a natural satellite around Venus,the slow, retrograde rotation of the solid body of Venus,and the absence of a measurable Venusian planetarymagnetic field. Internally, Venus seems to be much morevolcanically active than the Earth and, perhaps as a result,has a strikingly dense and hot surface environment that farexceeds in pressure and temperature most of the pre-spaceage astronomers’ expectations based on models of anEarth-like planet somewhat closer to the Sun.

The similarities lead us to expect that we might know theEarth better if we continue to compare it to its neighbourand twin in ever-increasing detail. The differences tell usthat there are crucial aspects of solar system formation andplanetary evolution, and of geology and climate physics,which we currently fail to appreciate or understand. Thesemotivate us to carry out missions like Venus Express,which, if they are to be most effective, focus on the majorunknowns and on the observed properties of Venus thatare known but difficult to explain, like the high surfacetemperature. The goal of this paper is to summarise these

e front matter r 2006 Elsevier Ltd. All rights reserved.

s.2006.04.031

65 272903; fax: +44 1865 272924.

ess: [email protected].

mysterious aspects, along with the known facts and someinformed speculation about Venus, in the hope andexpectation that the new mission will shed light on themysteries, and increase and enhance our understanding ofthe facts.

2. The morning and evening star

As the brightest of the planets, Venus has always been amuch-noted feature of the night, and even the day-time,sky, always appearing within 451 of the Sun by virtue of itsinferior orbit relative to the Earth. Many of the parameterslisted in Tables 1 and 2 have been known from the earlydays of scientific observations using telescopes, includingthe fact that Venus has a very high reflectivity (albedo),apparently due to a thick and ubiquitous cloud cover. At0.76, the albedo of Venus is two and a half times that of theEarth, more than offsetting the doubling of the solarconstant at Venus’ mean distance from the Sun. SinceVenus absorbs less radiant energy than Earth, there was noparticular reason why early practitioners of what we wouldnow call climate modelling should expect the surfacetemperature to be massively different from our own, andthe popular vision of the surface of Venus often includedoceans, deserts and steamy jungles. The Sun, with a disctwice the area it shows at the Earth, was thought toevaporate water efficiently and produce the thick andextensive cloud deck. When the composition of theatmosphere was shown by Adams and Dunham in 1934

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Table 1

Venus/Earth comparison (after Williams, 2005)

Venus Earth (Venus/Earth)

Bulk parameters

Mass (1024 kg) 4.8685 5.9736 0.815

Volume (1010 km3) 92.843 108.321 0.857

Equatorial radius (km) 6051.8 6378.1 0.949

Polar radius (km) 6051.8 6356.8 0.952

Volumetric mean radius (km) 6051.8 6371.0 0.950

Ellipticity (polar flattening) 0.000 0.00335 0.0

Mean density (kg/m3) 5243 5515 0.951

Surface gravity at equator (m/s2) 8.87 9.80 0.905

Escape velocity (km/s) 10.36 11.19 0.926

Bond albedo 0.76 0.30 2.53

Visual geometric albedo 0.65 0.367 1.77

Solar irradiance (W/m2) 2613.9 1367.6 1.911

Equivalent blackbody temperature (K) 231.7 254.3 0.911

Topographic range (km) 15 20 0.750

Orbital parameters

Semi major axis (106 km) 108.21 149.60 0.723

Sidereal orbit period (days) 224.701 365.256 0.615

Tropical orbit period (days) 224.695 365.242 0.615

Perihelion (106 km) 107.48 147.09 0.731

Aphelion (106 km) 108.94 152.10 0.716

Synodic period (days) 583.92 — —

Mean orbital velocity (km/s) 35.02 29.78 1.176

Max. orbital velocity(km/s) 35.26 30.29 1.164

Min. orbital velocity (km/s) 34.79 29.29 1.188

Orbit inclination (deg.) 3.39 0.00 —

Orbit eccentricity 0.0067 0.0167 0.401

Sidereal rotation period (h) 5832.5 23.9345 243.686

Length of day (h) 2802.0 24.0000 116.750

Obliquity (deg.) 177.36 23.45 (0.113)

Table 2

Observational parameters

Distance from Earth

Minimum (106 km) 38.2

Maximum (106 km) 261.0

Apparent diameter from Earth

Maximum (seconds of arc) 66.0

Minimum (seconds of arc) 9.7

Maximum visual magnitude �4.6

Mean values at inferior conjunction with Earth

Distance from Earth (106 km) 41.44

Apparent diameter (seconds of arc) 60.2

F.W. Taylor / Planetary and Space Science 54 (2006) 1249–12621250

to be mainly carbon dioxide, soda water oceans became thevogue for Venus.

In the 1950s, it became possible to estimate the surfacetemperature of Venus for the first time using radiotelescopes to measure the intensity of emitted microwaveradiation. At wavelengths of a few centimetres, photonsemitted from the surface of the planet pass almostunaffected through the cloud layers, and can be measuredon Earth. The early results for Venus showed temperaturesof around 400 1C, much too hot for free water or plant life.

The first space mission to Venus, Mariner 2, carried a smallmicrowave radiometer to confirm this measurement fromclose range and show, by observing limb darkening, thatthe intense radiation was indeed coming from the surface,and not from a non-thermal source like the ionosphere.Later, the Soviet ‘Venera’ series of spacecraft made the firstlandings on the planet’s surface, confirming that thetemperature was around 730K, and accompanied by apressure of nearly a hundred Earth atmospheres.At visible wavelengths, the cloud cover on Venus is

complete and impenetrable, with no markings which couldbe associated with continents, oceans or any of the surfacefeatures which abound on the other inner planets. Instead,only extremely subtle and ephemeral markings, and some‘scalloping’ of the terminator which separates the day andnightsides, have been reported by visual observers.Through an ultraviolet filter, like that used in the televisioncameras on Mariner 10, which observed Venus from adistance of 10,000 km in 1973, subtle dark markings appearin the clouds (Fig. 1). In the mid-1980s, it was discoveredthat much more striking contrasts can be observed atcertain wavelengths in the near-infrared (IR) part of thespectrum. These are also due to the clouds, but atconsiderably greater depths, where large-scale meteorolo-gical activity apparently organises the clouds into patterns,

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Fig. 1. An image of Venus through an ultraviolet filter obtained by the

Pioneer Venus Orbiter spacecraft in 1979. The contrasts in the clouds have

been exaggerated by processing, showing the quasi-permanent ‘sideways

Y-shaped’ feature clearly (NASA).

F.W. Taylor / Planetary and Space Science 54 (2006) 1249–1262 1251

rather as on the Earth but in a different and still mysteriousway.

The discovery by D. Allen and colleagues in 1984 ofnear-IR emission from the nightside of Venus provided apowerful new technique for studying the lower atmosphereand surface (see Taylor et al., 1997, for a review). Thisemission, which is most intense within spectral ‘windows’between strong molecular absorption bands in the0.9–2.5 mm wavelength region, is only detected on thenightside, where it is not overwhelmed by the more intensesolar flux reflected from the clouds. Near-IR imaging andspectroscopic observations of this emission by ground-based and spacecraft instruments have been used toinvestigate cloud particle sizes and optical thickness, thewinds within the middle and lower cloud decks, and theabundances of several important trace gases, includingwater vapour, halides, carbon monoxide, sulphur dioxide,and carbonyl sulphide. They have provided new informa-tion about the near-surface temperature lapse rate, and thedeuterium-to-hydrogen ratio. Venus Express will be thefirst mission with optical and spectroscopic experimentsplanned around the exploitation of these windows, with theprospect they offer for further progress in exploring animportant and largely mysterious planetary regime whichpreviously was accessible only to microwave sounding orentry probes.

During the 1991 Venus fly-by of the Galileo spacecraft,en route for Jupiter, it was found that the principaltopographical features on the surface of the planet couldalso be discerned in images obtained at very near-IRwavelengths. These contrasts originate in the temperaturelapse rate of the atmosphere, which causes high features onVenus to appear dark in maps of the thermal emission from

the surface. The very high scattering albedo of the clouddroplets in the near-IR means that the emission can diffuseto space in the spectral windows between the absorptionbands of the main atmospheric constituents. Spectroscopyin these windows allows the abundance of interestingminor constituents of the atmosphere near the surface, likewater vapour and carbon monoxide, to be mapped.Before the first mission to carry a surface-imaging radar

to Venus flew in 1978, progress had been made in obtainingradar pictures of Venus using the large dishes at Goldstoneand Arecibo to transmit and receive the pulses. Regions ofhigh radar reflectivity appear bright, indicative of varia-tions in either the composition or the surface morphology.In other words, bright areas can appear so either becausethey are composed of material which is an intrinsicallybetter reflector, or because the surface is smoother thansurrounding darker areas, or because the surface is tilted tobe more nearly normal to the incoming beam than the localhorizontal. The Pioneer Venus Orbiter was deployed in ahigh inclination orbit and so was able to map a muchgreater fraction of Venus’s surface than is possible whenobserving from the Earth, as well as obtaining betterspatial resolution and relative height information. Mapswere slowly built up strip by strip as the orbit precessedaround the globe, taking a Venus year of 243 days to coverit completely.Veneras 15 and 16 in 1984 also mapped Venus using

radar and improved the spatial resolution to around 1 km.The latest NASA mission to Venus, and the last by anyagency before Venus Express, was Magellan, whichoperated from 1990 to 1994 and was also dedicated toradar imaging. With a resolution of 75–120m, Magellandata produced an explosion of knowledge about thesurface of Venus, taking it from the least to one of thebest-explored terrains in the solar system. Of course, seeingsomething is only the first step towards understanding it,and the radar maps pose as many questions as they answer(Fig. 2).

3. Surface and interior

The Venera landers obtained photographs of the terrainnear the landing sites, revealing a sterile, scorched desertdominated by the boulders that appear strewn about thelandscape. Some of the boulders have dark bands andothers a patchy appearance; many have sharp edges,apparently the result of fracturing by some geologicalprocess. Most likely, the rocks are rubble from the break-up of the ejecta and lava flows associated with volcanicactivity, but what process achieved this fracturing ismysterious, since running water, large daily or seasonaltemperature changes and wind erosion are not available toweather rocks on Venus as they do on Earth. Some of therocks do show evidence of erosion, possibly due tochemical action by acidic vapours in the atmosphere, orto the melting of volatile components of the rocks.

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The rocks rest in one view on a flat, rocky plain and inanother on a mottled layer of soil or gravel (Fig. 3), givingan overall impression rather reminiscent of alluvialmaterial, that is, deposited and modified by flowing liquid.Considering conditions on Venus, it is virtually impossibleto say what processes are at work, or over what timescale,

Fig. 2. Radar map of the surface of Venus obtained by the Magellan

mission in the early 1990s. In this view the N pole is near the centre, the

high (red) feature just below centre is the large continent Aphrodite Terra,

containing the 12-km high mountain Maxwell (white). The blue regions

are depressions, possibly including primordial ocean basins. The largest of

these visible here (above and to the right of centre) is Atalanta Planum. No

evidence has been found for plate tectonics, leaving volcanism as the most

likely process to have shaped the surface of present-day Venus.

Fig. 3. Pictures of the surface of Venus obtained by Venera 14 at 131N, near t

321N, inside the large continent of Beta Regio, on 22 October 1975 (below

geologically recent break-up of volcanic lava flows, while Venera 9 shows what

like sand or soil. The details and ages of the processes that shaped these localiti

underneath the exposed surface. The chemical interaction of surface materials

on Venus today.

from photographs alone. The Venera landers carried g-rayspectrometers that found uranium, thorium and potassiumin the surface rocks in proportions that are consistent witha composition like terrestrial basalt. The density estimatesfrom g-ray backscattering of 2.7–2.9 g/cm3 support thisconclusion. It appears that Venus formed in a mannersimilar to the Moon, Earth and Mars, condensing from amolten protoplanet into shells, with the most fusiblebasaltic minerals making up the crust.Radar altimetry, first from Pioneer Venus then with

higher resolution and coverage from Magellan, revealed asurface that is about 70% smoothly rolling plains, withabout 20% lowland regions. The remaining 10% corre-sponds to the principal highland areas or ‘terrae’ of Venus:Ishtar, Lada, and Aphrodite, and the adjacent Beta,Phoebe and Themis regions making up a fourth majorcontinent. In addition, there is Lakshmi Planum, a 3–4 kmhigh plateau, bordered by mountainous ridges.Ishtar covers an area comparable to Australia and rises

steeply from the surrounding plains at about 701N. Thewestern part is a high plateau (3 km above the mean radiusof Venus) bordered by tall mountains that reach a further3 km in altitude. In the middle of Ishtar stand the Maxwell

Montes, which at 11 km are high enough to tower aboveEarth’s highest mountain, Everest, and are much moresteeply sided. The existence of such a steep and massivemountainous feature on Venus implies processes deepwithin the crust of the planet that created the mountain,and that continue to support its massive bulk. On Earthsuch a feature would most likely result from an energeticcollision between surface plates, trying to move sidewaysinto each other, in which case Maxwell would be analogousto the Himalayas on Earth. On Venus there is little otherevidence for plate tectonics and it seems more likely thatMaxwell is supported by vigorous convection in alarge ‘hot spot’ in the crust. Gravity field experiments

he eastern flank of Phoebe Regio, on 5 March 1982 (top) and Venera 9 at

). The Venera 14 site shows flat, basaltic rocks probably formed by the

seem to be older, more weathered rocks sitting on a bed of finer material,

es remain unknown, as does the stratigraphy and composition of the layers

with the atmosphere may have a key role in explaining the extreme climate

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and bi-static sounding of the more mountainous regions byVenus Express may shed further light.

The reason Venus apparently does not have continent-sized plates, moving relative to each other as they do on theEarth, is not known. Possibly, the low water content orsome other aspect of the composition of the crust, itsthickness, and the high surface temperature, cause it tocrack more easily than on Earth, so major plates break upinstead of moving around. Narrow linear structures(chasmas) and tesserae or tiled terrains can be seen allover the surface of Venus, and these and other tectonicfeatures (including the montes) may be products of thecracking of the crust that occurs in response to convectionin the mantle below.

Aphrodite Terra is the second most prominent highlandregion on Venus, with an area about equal to that of Africain an elongated shape that has been likened to a scorpionstretching for about 10,000 km along and south of theequator. The highlands at the eastern end contain steepvalleys on an enormous scale not found on Earth. Thelargest, Diana Chasma, is comparable to the VallesMarineris on Mars.

The lowlands (Planitiae) are fairly featureless, probablya result of having been flooded by lava relatively recently,in some cases several times with fresh flows partiallycovering the earlier ones. Because of this resurfacing, muchof the surface of Venus appears geologically very young.Dating the surface by the usual method involving the studyof impact craters is made difficult by the thick atmosphere,which prevents the smaller and more frequent impactorsfrom making an observable impression of the surface. Thecraters that have been seen, several hundred in number, arequite pristine in appearance and uniformly distributed overVenus, suggesting that the renewal of the surface has beengeologically recent and planet-wide.

The Magellan images show that most of the surface ofVenus is made up of features attributable to volcanicactivity. The largest volcanoes are the shields, found mostlyin high regions lying 3–5 km above the surrounding areaand featuring lava flows which often extend for hundredsof kilometres from the central caldera. The smallervolcanoes are called anemones, ticks, and arachnids,depending on their appearance. Anemones are charac-terised by ‘hairy’ flow patterns typically 50 km across,radiating out from a central source of magma. Ticks areflat, circular volcanic domes about 25 km in diameterflanked by strongly defined radial ridges and troughs. Thearachnids are volcanic mounds which appear to havecollapsed, cracking the crust and producing a distinctiveinsect-like shape. Similar to ticks but without the ‘legs’ arethe steep-sided, flat-topped volcanoes known as pancake

domes. The shape suggests that the lava that formed themwas of higher viscosity than that emanating from the moreEarth-like large volcanoes, and so did not flow as freely oras far. The nearest terrestrial analogues to pancake domesare on the sea bed, where the surrounding high-densityfluid affects the cooling and solidification of the dome.

There are remarkable river-like features on Venus, someof them extending thousands of kilometres from theirvolcanic sources to the lava-filled flood plains. Several havecut deep, meandering valleys, a process that must haverequired large volumes of liquid to flow over long periodsof time. The fluid involved is obviously not water, butsomething with a melting point that is not too differentfrom the mean surface temperature on Venus. Assumingthis temperature has not varied substantially since thevalleys formed, the most likely candidate would be acarbonate-rich mineral such as carbonatite, although thereare others that cannot be ruled out, even including the lowmelting-point metals like lead or tin. Although the plainsclearly are solid, over most of their area at least, we do notknow for certain that these rivers, if they are different incomposition from the large-scale flows, are not stillrunning in places.Another curiosity is that the higher features on Venus

seem to be not only relatively unweathered, but actuallyplated with something, which gives everything more thanabout 2 km above the mean surface height an unnaturallyhigh radar brightness. Early guesses as to the compositionof these ‘snowcaps’ were made by looking for somethingwhich condenses at the temperature and pressure of theobserved lower altitude boundary. They included the metaltellurium, or the compound iron pyrites (FeS), also knownas fool’s gold from its resemblance to the precious metal.More recent work by Schaefer and Fegley (2004), based onchemical equilibrium arguments, favours a mixture of leadand bismuth sulphides.

4. Atmospheric composition and clouds

A comprehensive and still relevant summary of presentknowledge about the composition of Venus’s atmosphere,plus an account of how the data were obtained, has beengiven by von Zahn et al. (1983)); an update is presented byde Berg et al. (this issue). The abundances derive mainlyfrom mass spectrometer and gas chromatograph measure-ments made on the Pioneer Venus and Venera descentprobes. A summary is included in Table 3.Several of the minor constituents exhibit striking

amounts of temporal and spatial variability, indirectlyrevealing major characteristics of the planet and itsatmospheric circulation and meteorology. Sulphur dioxidewas observed in ultraviolet measurements made by PioneerVenus to show large variations in its abundance near thecloud tops, which Esposito (1984) interpreted as evidencefor active volcanism at the surface. During the Galileo fly-by in 1991, near-IR measurements revealed an equator-to-pole gradient in the abundance of tropospheric carbonmonoxide (Collard et al., 1993), which Taylor (1995)suggested could be characteristic of a hemispherical Hadleycirculation that extended from the lower thermosphere ataround 120 km all the way down to the surface. Finally,water vapour measurements above, below and within thecloud layers show a baffling disparity that is presumably

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Table 3

Atmosphere

Surface pressure 92 bar

Surface density �65 kg/m3

Scale height 15.9 km

Total mass of atmosphere �4.8� 1020 kg

Average temperature 737K (464 1C)

Diurnal temperature range �0

Wind speeds 0.3–1.0m/s (surface)

Mean molecular weight 43.45 g/mol

Atmospheric composition (near

surface, by volume)

Major Carbon dioxide (CO2) 96.5%

Nitrogen (N2) 3.5%

Minor (ppm) Sulphur dioxide (SO2) 150

Argon (Ar) 70

Water (H2O) 20

Carbon monoxide (CO) 17

Helium (He) 12;

Neon (Ne) 7

Fig. 4. Temperature (black curves, bottom scale) and cloud density (grey

curve, top scale) profiles for Venus, based on measurements from several

different instruments on the Pioneer Venus orbiter and entry probes. The

global thermal structure within and beneath the clouds has been sampled

but not mapped in enough detail to understand Venusian meteorology.

Only tentative theories for the extreme greenhouse warming exist, and the

detailed structure, variability and compositional variations in the clouds

are also unknown.

1The estimates from the Soviet landers, up to and including Veneras 11

and 12, was somewhat higher, at 3.457.45% (Moroz et al., 1983).

F.W. Taylor / Planetary and Space Science 54 (2006) 1249–12621254

linked both to cloud formation and dissipation processesand meteorological activity in Venus’ atmosphere (Taylor,2006a, b).

Observations of Venus at visible wavelengths, even fromclose-up, show little or no detail in the clouds, whichappear to the optical astronomer observing from the Earthto form a uniform blanket over the whole planet. However,it was first noted in the 1920s that photographs takenthrough an ultraviolet filter show blotchy dark features inthe cloud. Under the best observing conditions, thedominant pattern in these markings shows a characteristicshape, like a letter ‘Y’ laid sideways (Fig. 1). The contrastsare due to some ultraviolet-absorbing substance that isnon-uniformly dispersed through the clouds. Sulphurdioxide behaves in this way, and is definitely present inspectroscopic observations, but its spectrum does notmatch that of Venus precisely at all wavelengths. Someother material, probably another sulphur compound oreven one of the allotropes of elemental sulphur, which alsoabsorbs ultraviolet more than visible radiation, must becontributing also.

The main cloud deck on Venus extends from about 45 toabout 65 km above the surface, with haze layers above andbelow (Fig. 4). Within this gross structure, detailed layeringoccurs and particles of different sizes congregate atdifferent height levels. The particles range in diameterfrom less than 1 to over 30 mm and tend to a trimodal sizedistribution, with the commonest diameters falling towardsthe ends of the overall range and in the 2–3 mm region.Spectroscopic, polarimetric and other evidence yields acomposition of 75% H2SO4 and 25% H2O for theintermediate size or ‘mode 2’ droplets.

The composition of the smaller, ‘mode 1’ drops isunknown; these form an aerosol haze extending through-out the cloud layer. Most of the mass of the clouds is in thebig ‘mode 3’ drops; these may be more evolved sulphuricacid drops constituting a tail to the mode 2 distribution

curve, or (particularly since there is inconclusive evidenceof a non-spherical shape) a separate mode altogether with adifferent, unknown, composition. The sulphuric aciddroplets are probably formed when H2O and SO2 (thelatter presumably of volcanic origin) combine photoche-mically near the cloud top level. It is difficult to explain thedetails of the size distribution, particularly to explain theexistence of particular modes, and their multiplicity.Compositional contrasts and dynamical effects may be atwork, but at present the observations which wouldelucidate these are in short supply.In spite of their physical depth, the clouds are not

completely opaque at all wavelengths, because they arequite efficient scatterers in the visible and near-IR regionsof the spectrum. The level of illumination in the Venerapictures was higher than had been expected. Even with theSun 601 above the horizon, it was thought that the thickclouds would prevent more than a trace of sunlight fromreaching the ground; instead a light level which has beencompared to that on Earth during a thunderstorm wasdiscovered and the searchlights which the spacecraftcarried were unnecessary. In 1978, radiometers on thePioneer Venus probes measured the solar flux and foundthat 2.5% of the total falling on the planet actually reachesthe surface.1 A corollary of this is that the hot surface andlower atmosphere of Venus, which emit strongly in the IR,can be seen from outside the atmosphere through theclouds in observations made at wavelengths outside regions

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of strong CO2 or H2O absorption. The best study of thisphenomenon, discovered only in the 1980s when the firstphase of dedicated Venus exploration by spacecraft wasessentially over, were made by the Near-Infrared MappingSpectrometer on the Galileo Jupiter Orbiter during itsintermediate flyby of Venus in 1990. Maps of Venusobtained by this instrument show large horizontal varia-tions in the thickness of the main cloud deck inconsiderable detail (see Fig. 5).

The large-scale structure of the clouds, in the horizontaland in the vertical, must be due to a variable balancebetween dynamical transport and the production and lossprocesses for the cloud materials. Except that the sharplower boundary of the entire cloud deck is probably due toevaporation beyond a high temperature threshold, and thatsulphuric acid production is to be expected at the cloudtops, via the reactions:

CO2 þ SO2 þ hn! COþ SO3;

SO3 þH2O! H2SO4:

These processes remain largely mysterious and requirefurther investigation. In addition to the general structure ofthe various layers, specific puzzles are the nature of theabsorber that produces the contrasts seen in UV-blue

Fig. 5. An image of the nightside of Venus in the near-infrared ‘window’

at a wavelength of 2.3mm, obtained by the Near-Infrared Mapping

Spectrometer aboard the Galileo spacecraft during its flyby in February

1990 (Carlson et al., 1991). The details, expressed in false colour, are due

to the lower-level cloud structure, back-lit by thermal emission from the

hot lower atmosphere and surface. The clouds are seen to be highly non-

uniform, leading to brightness variations which range over more than an

order of magnitude from white and red (thin cloud regions) to black and

blue, representing thick clouds. This indicates surprisingly active

meteorological behaviour in the deep atmosphere, where long radiative

and dynamical time constants might be expected to suppress such activity.

images of the planet, and the composition of the large‘mode 3’ particles seen in the Pioneer Venus Large Probedata. Indirect evidence suggests that these are likely to besolid, and therefore not H2O, H2SO4 or even HCl.Another feature of the Venusian clouds which has been

hotly debated is the question of whether or not lightning ispresent. On theoretical grounds, this was thought ratherunlikely, because the clouds are too tenuous, althoughlocalised storms and clouds of volcanic ejecta couldprovide the right conditions. The Galileo observationsfrom February 1990 (Gurnett et al., 1991) provided thehardest experimental evidence to date by detectingimpulsive radio signals in the 100KHz–5.6MHz frequencyrange, for which lightning is the only known source.Lightning was not detected optically, however, in spite of asearch by the Galileo imaging experiment (Belton et al.,1991).Finally, the question of CO2 clouds on Venus is worth a

mention, as an interesting curiosity more than as a crucialaspect of the climate and meteorology of the planet. Themean temperature of the atmosphere above the thin H2SO4

cloud, at altitudes of around 90 km, is in the region of180K (Fig. 3). During the night, the predominantly CO2

atmosphere radiates strongly to space and temperaturesprobably drop below 160K, cold enough for CO2 iceclouds to form. The Pioneer Venus Orbiter InfraredRadiometer noted anomalously strong limb darkeningnear the dawn terminator, between 601 and 951 solarlongitude and up to 151 latitude, that Taylor (1981)tentatively attributed to carbon dioxide (or possibly waterice) cloud formation near the 90 km level. The much moresophisticated and extensive observations expected fromVenus Express and the Japanese Planet-C missions shouldreadily settle this question.

5. Thermal structure and energy balance

Enough sunlight diffuses through the cloud layers toprovide about 17W/cm2 of surface insolation (Tomaskoet al., 1983). This is about 10% of the total absorbed byVenus as a whole, including the atmosphere, assuming afraction A ¼ 0:76 is reflected from the planet without beingabsorbed. Thus heated at and near the surface, the loweratmosphere forms a deep convective region, the tropo-sphere. Within the troposphere the atmosphere cannot coolsignificantly by radiation to space, because the opacity ofthe overlying layers is large. The greenhouse effect,whereby short-wavelength solar radiation heats the loweratmosphere more easily than the longer thermal wave-lengths can cool it, raises the surface temperaturesignificantly above that which would apply on an airlessplanet. The effect is particularly extreme for Venus, wherethe surface temperature must rise to 730K in order to forceenough IR cooling to balance the incoming solar energy.An airless body with the same albedo and heliocentricdistance as Venus would reach equilibrium for a meansurface temperature of only about 230K. Nevertheless,

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Fig. 6. Illustrating some of the processes though to be involved in producing the climate on Venus, which is much more extreme, compared to the Earth,

than anyone expected prior to the first space missions to the planet. It remains a major task to explain how these, and possibly other, factors combine to

give Venus its high surface temperature and pressure, and how that may evolve in the past. Another key question is how the climate will change in the

future, when the volcanoes that stud the surface of Venus, some of them probably very active at present, finally subside. A much more Earth-like regime is

one possibility suggested by models (Taylor, 2006a, b), but there remain many uncertainties.

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measurements by the Pioneer Venus Orbiter of the net IRemission and the total reflected solar energy (Schofield andTaylor, 1982) confirmed that the planet is in overallbalance to within the accuracy of the measurement.

The Pioneer and Venera measurements also showed thatthe net thermal emission from the planet is a much weakerfunction of latitude than it is on the Earth, despite adistribution of incoming energy from the Sun that is morebiased towards the equatorial region by the small obliquityof Venus. The basic reason is undoubtedly that theatmosphere on Venus is an efficient transporter of sensibleand latent heat from equator to poles, not only because it isdense and deep and therefore has a high thermal capacity,but also because it apparently has a less turbulent regimethat allows advection to proceed more freely. Such generalassumptions rest on only very fragmentary evidence,however, and the processes actually at work have yet tobe fully revealed, modelled or understood. The newcapabilities afforded by near-IR sounding of the deepatmosphere, to be fully exploited for the first time by thespectroscopic and imaging instruments on Venus Express,are likely to make great strides in this area.

A detailed understanding of the greenhouse effect onVenus, and of why it finds equilibrium at such anunexpectedly high surface temperature, requires newinsights into the composition of the atmosphere and the

cloud layers, the radiative behaviour of the gases underextremes of temperature and pressure, cloud chemistry andmicrophysics, the role of volcanism, and the chemicalequilibrium between the atmosphere and surface minerals(Fig. 6). Reactions between atmospheric carbon dioxideand carbonate and silicate minerals in the crust may holdthe key to understanding the high surface temperature andpressure, as discussed for instance by Bullock andGrinspoon (1996), following the original suggestion ofUrey (1952). Despite the fact that reactions such as

CaCO3 þ SiO22CaSiO3 þ CO2

reach equilibrium with CO2 at temperatures and pressurestypical of those found on Venus, and would therefore seemto offer a solution to the dilemma of Venus’ extremeclimate, there are many practical questions that remainunresolved about how such a mechanism would operate inpractice (see, for instance, Hashimoto and Abe, 2005).Convection in the troposphere carries energy upwards to

the base of the stratosphere, where radiative cooling tospace can occur strongly. On Venus, this level (thetropopause) occurs about 60 km above the surface (Fig.3). Above the troposphere, in the middle atmosphere ormesosphere, the temperature tends to a constant value withheight, because the atmosphere here is optically thin. To afirst approximation, each layer tends to find the same

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equilibrium temperature, determined by the balancebetween the absorption of upwelling IR from the surfaceand troposphere and cooling to space, if no significantabsorption of direct solar energy takes place. Overall, themean vertical temperature profile on Venus follows thesegeneral expectations and is therefore fairly unremarkable.

The superimposed details observed by thermometers onentry probes and by IR remote sensing from orbitersreveals a great deal of interesting detail, however. Theseinclude (i) discontinuities in the vertical lapse rates andhence the static stability of the atmosphere, in distinct slabsthat correlate between different probes; (ii) a verticallypropagating, solar fixed thermal tide that is predominantlywavenumber 2 in character, accompanied by a variation ofabout 1.5 km during the solar day in mean cloud-topheight, dominated in this case by the wavenumber-1component; (iii) strong latitudinal variations in thetemperature structure in the middle atmosphere,60–100 km, that show a steady increase from equator topole. Dynamical theories have been advanced to explaineach of these, in the form of (i) multiple, stacked Hadley-type circulation cells; (ii) a variant on terrestrial tidaltheory that shows wavenumber 2 propagates more stronglythan wavenumber 1 under model Venus conditions and (iii)pressure gradients associated with the vertical super-rotation and corresponding thermal gradients that are farfrom radiative equilibrium. All of these are exciting ideasthat require considerable elaboration and testing with newand more detailed data, whereupon much can be learnedabout meteorological processes that, like many aspects ofthe Venusian environment, have elements of both theEarth-like and the bizarre.

Above the mesopause the thermosphere begins, a low-density region that takes its name from the fact thattemperature increases with height on the dayside due to theabsorption of mainly ultraviolet photons from the Sun,principally in the extreme ultraviolet portion of thespectrum. Energetic particles in the solar wind alsocontribute, and the actual thermospheric temperaturesvary considerably with solar activity and sunspot cycle.The thermosphere of Venus is cooler than Earth’s, becauseof the greater abundance of carbon dioxide, which is veryefficient at radiating heat to space. Above about 150 km,the temperature is approximately constant with height onthe dayside at about 300K. The terrestrial thermosphere isthe seat of rapid winds, up to 1000m/s or more, and thistends to redistribute energy originally absorbed from theSun over the dark as well as the sunlit hemisphere. Theresult is a modest day–night difference of around 200Kabout a mean temperature of 1000K. On Venus however,the night-time temperature in the thermosphere is very low,around 100K. The transition from the day to nightsidevalues of temperature on Venus also show surprisinglysteep gradients (Keating et al., 1979) and modellers havegreat difficulty in reproducing both the minimum tempera-ture and the short distance across the terminator withwhich it is attained. Somehow the dynamics of Venus’s

thermosphere is such that the flow of air in response to thetemperature gradient is inhibited.

6. Atmospheric dynamics

It was known by the early 1960s that the Y-feature seenin ultraviolet images rotates around the planet in a periodof only 4–5 days. This implied wind velocities of 100m/s atthe cloud tops, surprisingly rapid for such a slowly rotatingplanet. The solid surface of Venus rotates at only about2m/s, or once every 243 days. Comparison of the Y-shapedmarkings in a composite of Earth-based observations in1966, a mosaic of Mariner 10 ultraviolet photographstaken during the period from 5–12 February 1974, and asseen in similar pictures obtained by Galileo, when it flewpast Venus en route to Jupiter in February 1990, all show astriking similarity. Evidently the dynamical processes in theatmosphere of Venus give rise to stable wave modes thatare reflected in the cloud patterns, but when it comes to thedetails, the meteorology of Venus is as mysterious as itssurface geology.Careful measurements of the propagation velocities of

small scale features that move with the winds and that ofthe large Y (representing the phase velocity of the wave orwaves, superimposed on the wind velocity) confirm that thebulk velocity of the atmosphere in the zonal direction(parallel to the equator) at low latitudes is over 100m/s, aresult which is borne out by Earth-based measurements ofthe Doppler shifting of spectral lines. The global-scalewaves propagate upstream at about 20–30m/s. Both ofthese velocities are much larger than the apparent velocityof the Sun with respect to an observer on the surface.Probe and remote sounding measurements show that the

rate at which the atmosphere circulates around theequatorial regions varies considerably with height, itsmaximum speed occurring near the cloud tops where theultraviolet markings apparently originate. Above theclouds, the 100m/s winds are decelerated sharply by thepressure gradient which results from the temperaturedistribution at those levels. The remarkable discovery thatthe air temperature is some 15–20 1C warmer at the polethan the equator from 70–95 km above the surface relatesto this: dynamical models imply that the pressure gradientimplied by this temperature structure is sufficient to arrestthe zonal winds completely by 85–90 km altitude ifcyclostrophic balance is assumed.Below the clouds, the winds fall gradually in velocity as

the atmosphere becomes denser. Doppler tracking of thePioneer probes shows a wind speed of less than 10m/s atthe 10 km altitude level and close to zero at the surface(Fig. 7). All of the zonal winds are westward (in the samedirection as the rotation of the planet), suggesting thatangular momentum is being delivered to the atmosphere bythe solid body of the planet and transported upwards.Alternatively, it has been proposed that the Sun exerts atorque on the atmosphere and so supplies external angularmomentum. This it certainly will do, since the density of

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Fig. 7. Venus wind profiles from the Pioneer Probes. The zonal winds are

retrograde and reach speeds in excess of 100m/s around the equator at

cloud-top heights. Tracking of cloud features in both ultraviolet and near

infrared images has confirmed that there is a small net equator-to-pole

velocity component of a few m/s both at the visible cloud tops at around

60km altitude and in the main cloud layer some 15km lower. The

processes that accelerate the zonal wind to such high values (more than 50

times the rotation rate of the solid planet, at relatively high atmospheric

densities) are still qualitatively unknown.

F.W. Taylor / Planetary and Space Science 54 (2006) 1249–12621258

the atmosphere is non-uniformly distributed with solarlongitude (local time of day) because of thermal tidesinduced by solar heating. In fact, the semidiurnalcomponent of the tide, on which the torque principally isexerted, has been observed to be unexpectedly large,relative to the diurnal component, on Venus, which favoursthis mechanism. Whether the effect is large enough toaccelerate the atmosphere to the speeds observed is asubject of ongoing debate. There is even a possibility thatthe slow retrograde rotation of the planet itself may havebeen established, over geological time, by the torque whichthe atmosphere exerts on the planet—the reverse of theearlier theory.

Less dramatic than the zonal winds, but of even greatersignificance for the general circulation of Venus, is theobserved migration of the ultraviolet markings away fromthe equator towards both poles (i.e., in the meridionaldirection) at speeds of less than 10m/s. Galileo/NIMS alsoobserved poleward motions in the deeper cloud structure,imaged in the near-IR windows. The general impression isof two gigantic circulation cells, one to each hemisphere, inwhich (heated) air rises at the equator and (cooled) airdescends at the poles, travelling more or less horizontally in

between, polewards above the clouds and equatorwardsbelow. Such a flow is the characteristic of a Hadley cell, thesimplest circulation regime which can occur in a planetaryatmosphere. This kind of structure was proposed for theEarth’s atmosphere by Hadley as long ago as 1735. Itseemed logical to him that rising air at the warm equatorialregions and falling air at the cool poles, would lead to pole-to-equator flow near the surface and to motion in theopposite direction at higher levels. This does in facthappen, but in our own atmosphere this simple structureis greatly modified by the development of baroclinicinstabilities in the motion, and the smooth flow of theHadley regime tends to break down under the influence ofthe Earth’s spin. The net result on Earth is that Hadley’scell extends from the equator only to mid-latitudes, withother, smaller, cells taking over the transport nearer thePole. On slowly rotating Venus, it appears that the basicHadley configuration exists in a less modified state,spanning each hemisphere.Dramatic evidence for this type of circulation was

obtained by IR measurements from the Pioneer Venusorbiter, which provided the first observations from abovethe polar regions. Greatly enhanced amounts of IR fluxwere found to be emerging poleward of 801N latitude, in alocalised, elliptical region which evidently is a clearing inthe cloud cover forced by the descending air in the returnbranch of the Hadley cell (Fig. 8). This clarification of thecirculation regime existing near the cloud tops also gives ussome clues to other fundamental questions about Venus,namely its global cloud cover and its high surfacetemperature. Probably, on a long-term average, the air isrising slowly everywhere on Venus and descending onlynear the poles, with the cloud-top patterns seen in theultraviolet pictures and the deep atmosphere cumulusdynamics seen in the near-IR pictures superimposed as theshort-term disturbances or weather. The resulting planet-wide blanket of cloud plays a key role in trapping heatradiation in the lower atmosphere and raising the surfacetemperature. As already noted, sulphuric acid droplets arebetter than water at doing this, because their opticalproperties are such that they scatter radiation conserva-tively at ultraviolet, visible, and near-IR wavelengths,where the Sun emits most of its energy, and absorb in themiddle and far IR, where Venus cools by thermal emissionto space.Near-IR spectroscopic measurements in atmospheric

‘windows’, that is, wavelength regions where the mainatmospheric gases are weakly absorbing, penetrate theclouds, in some windows all the way to the surface. Asshown in Fig. 5, this type of observation reveals the globalcloud morphology at depth, not just in the cloud-topregion, although they are possible only on the nightside ofthe planet, because the emitted signals at these short IRwavelengths are dominated by reflected sunlight on thedayside. They show that the optical thickness of the clouds,far from being largely uniform as had been supposed, isvery variable, by factors of 10 or more. Regions of thick

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Fig. 8. The N polar vortex on Venus, in UV light from Mariner 10 (top left) and at a wavelength of 11.5 mm in the thermal infrared from Pioneer Venus

Orbiter (Taylor et al., 1980). The latter show a ‘snapshot’ of the vortex (top right), a 72-day average, showing the cold polar collar, and a similar average in

a coordinate frame rotating every 2.7 days, showing the dipole structure. The existence of a vortex is not surprising, but its detailed collar-dipole structure

has so far defied explanation and modelling.

F.W. Taylor / Planetary and Space Science 54 (2006) 1249–1262 1259

and (relatively) thin clouds form patterns suggestive oflarge-scale cumulus dynamics, presumably with the cloudmaterial actively condensing and dissipating in rising andfalling air associated with weather systems, although thedetails are lacking because of a shortage of high-resolutiondata in space and time. Galileo, which made only a verybrief encounter with Venus on its way to Jupiter in 1990,has been the only spacecraft equipped for near-IR imageryto visit Venus since ground-based observers in the mid-1980s discovered the existence of the spectral windows.Venus Express, especially the VIRTIS and VMC instru-ments, will provide greatly extended coverage in space,time and wavelength, not only in the equatorial regions butalso over the poles, and in the collar and dipole regimes.

Some of the other important wave phenomena seen onVenus are the circumequatorial belts and the bow-like

waves. The circumequatorial belts are very narrow (lessthan about 50 km in width), very long (lengths of the orderof thousands of kilometres) and transient. As many as fivehave been seen at once, evenly spaced by about 500 km andalways aligned parallel to the equator. They appear in 1 or2 h and propagate, always in a southerly direction, for

about 12� 11

2days at about 20m/s (Belton et al., 1976). The

most satisfactory explanation for the belts is that they aresome form of gravity wave, resonances in the atmospherecaused by density variations propagating as waves underthe influence of gravity as the restoring force. They arecommon in the Earth’s atmosphere, where temperaturefluctuations, associated with the density waves, lead tocondensation in the thermal troughs. Something similarmay be happening on Venus, although it is far from clear inthis case what is exciting the waves. It could be turbulencein the strongly heated subsolar region, or perhaps somelower atmospheric wave propagating upwards. Still, it isdifficult to explain why the waves always seem to travelfrom the north to the south.The bow waves were named for their shape (like that of a

bow, as in archery). However, it turns out that theyprobably have something in common with the bow wavesassociated with the passage of a ship across water, as well.On Venus we see powerful ‘boiling’ of the atmosphere inthe region directly below the Sun, i.e., at local noon. Therising of the heated air in this subsolar disturbance, visibleas convection cells in the images, interferes with the

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smooth, high velocity flow of the upper atmosphere andgenerates ‘ripples’ in the clouds. However, this explanationis even more limited than its obvious oversimplificationwould imply, since the waves travel downstream behind thesubsolar zone, whereas oceanographic bow waves remainfixed with respect to the disturbance producing them.

Two other puzzling Venusian atmospheric phenomenaare observed only in IR images of the planet. These use thethermal emission from the planet as a source, and so coverthe dark or unfavourably illuminated portions of the planetas well as the dayside. They also reveal the temperature andvertical structure of the phenomena that they observe. Thecircumpolar collar is a current of very cold air thatsurrounds the pole at a radial distance of about 2500 km.It is nearly 1000 km across, but only about 10 km thick inthe vertical direction. Temperatures inside the collar areabout 30 1C colder than at the same altitude outside, so thefeature generates pressure differences that would cause it todissipate rapidly were it not continually forced by someunknown mechanism.

Inside the collar lies the region of reduced cloud covercaused by the descending branch of the Hadley cell.Because of the zonal momentum transported from lowerlatitudes, the descending air is also rotating rapidly,forming a polar vortex analogous to the eye of a terrestrialhurricane or whirlpool but much larger and morepermanent. Interestingly, however, the eye of the Venuspolar vortex is not circular but elongated, and withbrightness maxima (presumably corresponding to max-imum downward flow) at either end. This gives the polaratmosphere a ‘dumbbell’ appearance and has led to thename polar dipole for the feature. The dipole rotates aboutthe pole every 2.7 Earth days, i.e. with about twice theangular velocity of the equatorial cloud markings. Ifangular momentum were being conserved by a parcel ofair as it migrated from equator to pole the dipole might beexpected to rotate five or six times faster. In fact, theultraviolet markings are observed to keep a roughlyconstant angular velocity (solid body rotation) from theequator to at least 601 latitude, presumably acceleratingpoleward of this. This assumes, of course, that the rotationof the dipole represents the actual speed of mass motionsaround the pole and not simply the phase speed of awavelike disturbance superimposed on the polar vortex.

Interestingly, the thermal tide on Venus around theequatorial regions also has two maxima and two minima.(The thermal tide is simply the diurnal increase anddecrease of temperature caused by the rising and settingof the Sun.) This does not seem to be connected with thepolar dipole, since the two regions are separated by anarrow latitude band apparently free of planetary-scalewaves, as well as by the predominantly wavenumber-onecollar. The Earth’s atmosphere has a small wavenumber-two component superposed on the familiar early-afternoonmaximum to post-midnight minimum cycle, but thiscomponent dominates on Venus. For once, the dynamicaltheory of atmospheric tides, as developed for Earth, shows

when applied to Venus that the observed state of affairscan be explained as primarily a consequence of the longsolar day on Venus (Fels et al., 1984). It is puzzling, evenalarming, that the same cannot be said of most of the othermajor characteristics of the atmosphere of the closest andmost Earth-like planet (see Fig. 9).

7. Magnetic field, space environment and atmospheric

escape

It is generally thought that Venus accreted anddifferentiated in a manner similar to its near-twin, theEarth, leading in both cases to a molten iron-rich core ofabout half the total radius, overlaid by a crust of mainlysilicate material. The mean density of 5.25 g/cm for Venusis consistent with such a picture. It used to seem reason-able, therefore, to expect that Venus should also have anEarth-like intrinsic magnetic field, until the era of space-craft exploration when it gradually became clear thatVenus has little or none (Luhmann and Russell, 1997). ThePioneer Venus Orbiter made measurements beginning in1979 that included repeated low-altitude passes thatconfirmed that the planet has negligible internal field, withan upper limit of 10�5 times that of the Earth.Such fields as were observed by Pioneer in near—Venus

space were attributed to solar wind interactions directlywith the planet and its atmosphere, which, at high levels,contains layers of charged particles known as the iono-sphere. The upper boundary of this, the ionopause, is thesurface at which the dynamic pressure of the solar wind isin balance with the thermal pressure of the ions andelectrons that make up the ionosphere The height of theionopause varies with solar activity from around 250 km tomore than 800 km from the solid surface of the planet. Itdeflects the solar wind flow around the planet, with theformation of a bow shock several thousand km upstream(Fig. 10).While the detailed physics of the processes outlined

above remain of great interest, the key question for thestudy of the induced magnetosphere around Venus is therole of the deflected solar wind in carrying off atoms andions derived from atmospheric molecules, particularly thelighter elements, and especially hydrogen. Venus may oncehave had a massive ocean that was slowly removed bydissociation of water vapour in the upper atmosphere andsubsequent loss of the hydrogen. This scenario is supportedby the expectation that Venus was initially water-rich, likethe Earth, and also by the strong evidence of fractionationin the isotopes of hydrogen found on Venus, where thedeuterium-to-hydrogen ratio is more than 100 times thatfound on Earth or in meteorites. The implications for thepresent high surface pressure and consequent extremeclimate on Venus are obvious; the mass of the currentatmosphere represents a balance between emissions fromthe crust by volcanism, the chemical recombination ofatmospheric molecules with the surface, and escape tospace. The details and relative proportions of these

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Fig. 10. The solar wind interacts directly with ions and electrons in the

Venusian ionosphere, so no Earth-like magnetotail or trapped radiation

belts are produced. However, Venus does exhibit a bow shock, a

magnetosheath or ionosheath inside the bow shock, and an ionopause

250 km or more above the surface where the solar wind pressure is

balanced by the ionosphere thermal energy (NASA).

Fig. 9. A summary of the main features of the atmospheric circulation on Venus, and some of the principal questions. The latter include the nature of the

polar dipole feature, the extent of the Hadley cell circulation and how it may be balanced by a counter-rotating polar cell and eddy momentum transfer,

and the drive for the zonal super-rotation and its balance via the reverse temperature gradient in the stratosphere.

F.W. Taylor / Planetary and Space Science 54 (2006) 1249–1262 1261

processes and budgets, current and historical, remain quiteunknown.

The Neutral Mass Spectrometer on Pioneer VenusOrbiter observed CO2, CO, O, N2, N, NO, He and H inthe upper atmosphere of Venus. The UV spectrometer onthe same mission established the presence of a corona ofhot atoms, mainly H, O and C, around the planet. Thehomopause on Venus is at an altitude of about 135 km,leading to an increasing preponderance of the lighterspecies above this level. However, the lightest, atomic

hydrogen and helium, are minor constituents in theatmosphere as a whole while atomic oxygen is producedin large quantities in the upper atmosphere by thephotodissociation of carbon dioxide by solar UV, viz.CO2+hn-CO+O. The net effect is that O atoms are thedominant species at altitudes above 170 km, especiallyduring the daytime, followed by atomic hydrogen, helium,and molecular hydrogen (von Zahn et al., 1983).Model calculations by Lammer et al. (2006) compare the

various loss processes for these species. Thermal loss, animportant process for hydrogen loss on Mars, is negligibleon Venus because of the much greater mass of the latter.For the same reason, sputtering (the removal of atmo-spheric atoms or molecules by momentum transfer incollisions with solar wind particles) is important only to theextent that sputtered particles contribute to the hot particlecoronae, where other processes occur. The dominantmechanisms for hydrogen loss are thought to be H+ ionoutflow accelerated along magnetic field lines on thenightside of the planet, and for O+ the same processaugmented by the formation of ionospheric plasma cloudstriggered by the Kelvin–Helmholtz instability.A key question is whether the net loss rates, by all

processes, for hydrogen and oxygen are in the ratio 2:1 aswould be expected if the source molecule is water vapourand there are no large sinks of atmospheric oxygen onVenus’s surface. Within large uncertainties, Lammer et al.(2006) find that this may indeed be the case for Venus,

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although not for Mars where the ratio is about four timeslarger and a surface sink for oxygen almost certainly mustbe invoked. The ASPERA and Magnetometer measure-ments planned for Venus Express are designed to furtherelucidate this ratio and the loss rates for hydrogen,deuterium, oxygen and other species.

8. Conclusion

Despite being the closest planet to the Earth and muchcontemplated and explored, many aspects of Venus as aplanet remain enigmatic. The most urgent problem, andthe primary target for the new generation of Venusmissions, must be to understand the remarkable state ofthe climate, and the processes that are responsible,including volcanism, the atmospheric circulation, cloudchemistry, and atmospheric escape processes. All of these,and a great many other persistent mysteries that unfoldedduring the first great era of Venus exploration in the twodecades from 1965 to 1985, should be much clearer if theVenus Express mission succeeds in making its plannedobservations during the next few years.

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