venus monitoring camera for venus express
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Planetary and Space Science 55 (2007) 1701–1711
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Venus Monitoring Camera for Venus Express
W.J. Markiewicza, D.V. Titova,d,�, N. Ignatievd, H.U. Kellera, D. Crispe, S.S. Limayeg,R. Jaumannc, R. Moissla, N. Thomasi, L. Espositof, S. Watanabeh, B. Fietheb, T. Behnkec,I. Szemereya, H. Michalikb, H. Perpliesa, M. Wedemeiera, I. Sebastiana, W. Boogaertsa,
S.F. Hviida, C. Dierkerb, B. Osterlohb, W. Bokera, M. Kochb, H. Michaelisc, D. Belyaevd,A. Dannenberga, M. Tschimmela, P. Russoa, T. Roatschc, K.D. Matzc
aMax-Planck-Institut fuer Sonnensystemforschung, Max-Planck Strasse 2, 37191 Katlenburg-Lindau, GermanybIDA Technical University Braunschweig, GermanycGerman Aerospace Center (DLR), Berlin, GermanydSpace Research Institute (IKI), Moscow, RussiaeJet Propulsion Laboratory, Pasadena, CA, USA
fLaboratory for Atmospheric and Space Physics, Boulder, CO, USAgUniversity of Wisconsin, Madison, WI, USA
hHokkaido University Sapporo, JapaniUniversity of Bern, Switzerland
Accepted 10 April 2006
Available online 25 January 2007
Abstract
The Venus Express mission will focus on a global investigation of the Venus atmosphere and plasma environment, while additionally
measuring some surface properties from orbit. The instruments PFS and SPICAV inherited from the Mars Express mission and VIRTIS
from Rosetta form a powerful spectrometric and spectro-imaging payload suite. Venus Monitoring Camera (VMC)—a miniature wide-
angle camera with 17.51 field of view—was specifically designed and built to complement these experiments and provide imaging context
for the whole mission. VMC will take images of Venus in four narrow band filters (365, 513, 965, and 1000 nm) all sharing one CCD.
Spatial resolution on the cloud tops will range from 0.2 km/px at pericentre to 45 km/px at apocentre when the full Venus disc will be in
the field of view. VMC will fulfill the following science goals: (1) study of the distribution and nature of the unknown UV absorber; (2)
determination of the wind field at the cloud tops (70 km) by tracking the UV features; (3) thermal mapping of the surface in the 1 mmtransparency ‘‘window’’ on the night side; (4) determination of the global wind field in the main cloud deck (50 km) by tracking near-IR
features; (5) study of the lapse rate and H2O content in the lower 6–10 km; (6) mapping O2 night-glow and its variability.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Venus; Instrument; Atmosphere; Dynamics; Clouds
1. Introduction
Ubiquitously present cloud veil completely hides thesurface of Venus. Early observations demonstrated theefficiency of imaging Venus, especially in the UV-blue
e front matter r 2007 Elsevier Ltd. All rights reserved.
s.2007.01.004
ing author. Max-Planck-Institut fuer Sonnensystem-
-Planck Strasse 2, 37191 Katlenburg-Lindau, Germany.
979 212; fax: +49 5556 979 240.
esses: [email protected] (W.J. Markiewicz),
.de (D.V. Titov).
spectral region in which albedo of the planet has featuresproduced by sulfur dioxide and a still unidentified absorber(Moroz et al., 1985). Spatial and altitude variations of itsabundance in the upper cloud layer produce famous UVmarkings (Rossow et al., 1980). Observations of thesefeatures were extensively used to derive wind speeds at thecloud tops (�67 km) (Rossow et al., 1990; Toigo et al.,1994; Smith and Gierasch, 1996) and to study the nature ofthe mysterious UV absorber (Esposito, 1980; Pollack et al.,1980).
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Fig. 1. The VMC instrument with one side of the housing removed.
W.J. Markiewicz et al. / Planetary and Space Science 55 (2007) 1701–17111702
Water vapour plays an important role in atmosphericchemistry and radiative balance. At the cloud tops it isinvolved in the production of sulfuric acid aerosols.Spectroscopic measurements of water vapour in thisaltitude region indicated significant variability in itsabundance (Schofield et al., 1982; Ignatiev et al., 1999;Koukouli, 2002). Mapping the H2O spatial and temporalvariations could significantly contribute to our under-standing of the dynamics and cloud formation processes atthe cloud tops.
Discovery of the near-infrared transparency ‘‘windows’’by Allen and Crawford (1984), in which weak emissionsfrom the lower atmosphere and the surface can escape tospace, gave a new and very effective tool to sound theatmosphere below the clouds. Radiation in the ‘‘windows’’around 1 mm originates at the hot surface and can be usedfor its thermal mapping. Surface images were capturedduring the Galileo (Carlson et al., 1991) and Cassini(Baines et al., 2000) flybys and from the ground (Lecacheuxet al., 1993; Meadows and Crisp, 1996).
Spectrometry of the Venus night side carried out by theVenera 9 and 10 orbiters showed visible airglows (Krasno-polsky, 1983). They form due to recombination of certainmolecules in the downwelling branch of the solar–antisolarcirculation on the night side. Observations of the airglowsand their variability are indicative of the thermosphericdynamics (Bougher et al., 1997).
Venus Express mission will make a global survey of theatmosphere from orbit (Svedhem et al., 2007). The VenusMonitoring Camera (VMC) was specifically designed tocontinue investigations of the above-mentioned phenom-ena. In comparison to the previous spacecraft experiments,current imaging technology allows one to fulfill the goal bymeans of a relatively simple CCD camera with sufficientspatial and temporal resolution. This paper gives a briefdescription of the VMC design and performance followedby a discussion of the science goals and expected results. Amore detailed technical description of the instrument canbe found in a separate paper (Markiewicz et al., 2007).
2. The VMC experiment
2.1. Description of the instrument
VMC consists of one unit that houses the optics, thedetector, and the electronics (Fig. 1). The camera uses aKODAK KAI-1010 1032� 1024 pixel CCD as a detector.Cooling is provided by a Peltier element connected to thebottom of the CCD by a thermal strap. Four objectivesequipped with narrow band filters simultaneously buildimages in four quadrants, avoiding moving parts in theinstrument. One VMC frame consists of four 411� 411pixel sub-images separated by a ‘‘cross’’ of about 200 darkpixels (Fig. 2). Stray light protection and ghost suppressionis provided by external and internal baffles and by blackobscuration spots located in the center of the visible andnear-IR optical channels. The CCD full well of 30,000
electrons combined with the 14-bit digital converter gives asystem gain of about 5 e�/DN. The readout electronicsprovides a range of exposure times from 0.5ms to 30 sallowing the camera to make both day and night sideobservations. The total field of view is 17.51 (0.3 rad) andangular resolution is 0.7mrad/px. The VMC is rigidlymounted on the +Y wall inside the spacecraft with itsoptical axis approximately co-aligned with the spacecraft+Z-axis and optical axes of VIRTIS, PFS and SPICAV.During observations the pointing is provided by thespacecraft. Table 1 summarizes main design and perfor-mance characteristics of the VMC camera.
2.2. Calibration and performance
This section contains a brief description of the VMCproperties and behavior that have been investigated duringthe on-ground instrument characterization. Completedescription of the laboratory setup, the measurementsand their analysis can be found in the VMC CalibrationReport (VMC-MPAe-RP-SS011-001). The near-Earth ver-ification carried out after launch confirmed the stability ofthe instrument’s properties in flight.
2.2.1. Optical properties
VMC has four narrow band channels centered at 365,513, 965, and 1000 nm with bandwidths of a few tens ofnanometers (Table 1). Fig. 3 shows the spectral responsesof the channels derived from laboratory measurements.The visible channel was used during the assembly of theinstrument to focus the optics on the detector. The focusmeasurements were carried out several times betweenenvironmental tests in order to confirm that the tests hadnot distorted the focus. Full focus tests for all 4 channels
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Table 1
Main performance characteristics of the VMC instrument
Optics
General concept Four objectives sharing a single CCD
Total field of view �17.51 (0.3 rad)
Image scale �0.7mrad/px
Filter bandpasses l/Dl, mmUV 365/40nm
VIS 513/50nm
NIR1 965/40nm
NIR2 1000/40 nm
F-number 7-UV, 5-VIS, NIR1,NIR2
CCD detector and readout electronics
Type Kodak KAI-1010, front illuminated,
interline architecture, antiblooming
Detector size 1032(H)� 1024(V)
Pixel size 9.0� 9.0 mmFull well 30,000 electrons
Gain �5 e�/DN
Total noise �100 e� at +37 1C
Exposure time N�0.504ms, N ¼ 1,2,3 ,y, 64,449
Sensitivity, DN/(erg/s/cm2/
sr/mm)/s
1.8 (UV), 11(VIS), 0.97(NIR1),
0.26(NIR2)
Linearity o1%
Sensitivity variations �20%
DPU
Processor LEON-2
Frequency 20MIPS
Memory 1Gbit
Onboard software features Compression, sub-frame cut, cropping,
binning
Fig. 2. VMC flat field showing the position of four images on the CCD frame.
W.J. Markiewicz et al. / Planetary and Space Science 55 (2007) 1701–1711 1703
were made during laboratory calibration at three tempera-tures: +22, +40, and �20 1C. In all cases the change of thedistance from the optical head to the detector was withinacceptable range of 725 mm. The point-spread function(i.e. the image of the point source) was about 1.5–2 pixelsfor each channel.
2.2.2. Dark signal properties
The VMC dark signal is the sum of the constantelectronic offset (3300–3400DNs) and the dark current,which depends on temperature and exposure time (Fig. 4).For temperatures below +35 1C the dark signal isindependent of exposure time and is almost equal to theoffset. The dark current increase with temperature becomesdominant at TCCD4+40 1C, which is close to the upperlimit of CCD temperatures expected in flight. Fig. 5 showsthe field of dark signal noise as the function of exposuretime and CCD temperature. This plot represents standarddeviation of the difference of two dark images takensequentially with the same exposure time. Except for theregion of high temperatures and high exposures the darknoise of individual pixel is below 25DN. The dark signalscatter over the CCD is greater than this value due to eachpixels’ individuality. A noteworthy CCD feature is thepresence of ‘‘hot columns’’ in which the dark signal is200–300DN higher than that in neighbouring pixels (Fig. 2).However, the dark signal, including the ‘‘hot columns’’, willbe subtracted from the VMC images in a standard calibrationprocedure based on pre-flight laboratory studies.
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Fig. 3. Spectral responses of the VMC channels and quantum efficiency of the Kodak KAI-1010 CCD used in the experiment.
Exposure time, ms
-40
-30
-20
-10
0
10
20
30
40
50
CC
D t
em
pera
ture
, °C
Dark current + Bias, DN
100 101 102 103 1042 4 5 673 2 4 5 673 2 4 5 673 2 24 5 673
Fig. 4. Dark signal as a function of exposure time and CCD temperature averaged over the entire CCD excluding hot pixels and shaded boundary frame.
Measured points are shown with the black circles.
W.J. Markiewicz et al. / Planetary and Space Science 55 (2007) 1701–17111704
2.2.3. Radiometric performance
The laboratory study included radiometric calibrationand characterization of the sensitivity variations over theCCD (flat field) by using a calibrated and uniform source(integrating sphere). Fig. 2 shows an example of the fullframe image of the integrating sphere. Mean sensitivity ofthe channels and its variability and linearity derived fromthe measurements are shown in the Table 1. Greatdifference in the sensitivity between the channels and thevariety of observation conditions make it impossible to getan optimal signal in all four channels with one exposure. Inthis case only good-quality sub-frames will be cut out andtransmitted to the ground.
2.2.4. Near-earth commissioning
Verification of the spacecraft systems and instrumentperformance was carried out during the first month afterlaunch. The campaign included a VMC performancecheck, observations of point sources (Sirius and Venus),Earth–Moon imaging, and stray light characterization. Thein-flight tests showed that VMC had successfully survivedthe launch loads and its performance remains as expected.The dark signal values are well within the range measuredduring calibration. The cooling system performance andthe VMC temperature regime were found to be very similarto those observed in the thermal-vacuum tests. Observa-tions of celestial objects allowed us to check the focus and
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Exposure time, ms
-40
-30
-20
-10
0
10
20
30
40
50
CC
D te
mp
era
ture
, °C
Dark Noise, DN
100 101 102 103 1042 4 5 67 23 2 4 5 673 2 4 5 673 2 4 5 673
Fig. 5. Dark signal noise as a function of exposure time and CCD temperature averaged over the entire CCD excluding hot pixels and shaded boundary
frame. Measured points are shown with the black circles.
Fig. 6. Sketch of the Venus Express orbit and main types of observations.
W.J. Markiewicz et al. / Planetary and Space Science 55 (2007) 1701–1711 1705
to determine the alignment between the VMC channels andwith the +Z-axis of the spacecraft. The camera was foundto be in perfect focus. A small misalignment of 1–1.51between the optical axes of the four channels will notdegrade the instrument performance. The in-flight testsshowed that stray light becomes significant at exposureslonger than 10 s when the VMC optical axis is closer than601 to the sun. This will sometimes affect observations ofthe Venus night side. However, the stray light pattern willbe measured in flight and subtracted from the images.
2.3. Observations at Venus
Venus Express will operate in a polar elliptical orbit withapocentre distance of 66,000 km (Fig. 6). Pericentre islocated at �781N with an altitude of 250–350 km.Revolution period of the satellite is 24 h. The VenusExpress spacecraft will perform several kinds of observa-tions depending on science objectives and conditions. Thebasic observation types are described as ‘‘science cases’’ inthe mission planning system (Titov et al., 2006). Table 2summarizes the main types of VMC observation.
During the pericentre passes (science case #1), VMC willstudy small-scale dynamical phenomena at the cloud topsand cloud structure with high spatial resolution of up to0.2 km/px. The imaging rate will range between 5 s atpericentre and 300 s at a distance of 10,000 km providing acontinuous strip across the Northern hemisphere. Thecamera has the capability to store images collected duringthe pericentre pass in its internal memory and transmitthem to the spacecraft afterwards.
Off-pericentre activity will include observations in theascending arc of the orbit (science case #2) and in the
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Table 2
VMC observations types
Pericenter Off-pericentre and
apocentre
Limb
Distance 250–10,000 km 10,000–66,000 km �2000 km
Total FOV 70–3000km 3000–20,000km �500km
Spatial Resolution 0.2–7 km 7–45 km �1.5 km
Time between Images 5–300 s �10min �5 s
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apocentre vicinity (science case #3) when the distance toVenus ranges from 10,000 to 66,000 km. At a distancegreater than 40,000 km, VMC will have the full Venus discin its field of view. Off-pericentre imaging will be used toinvestigate global atmospheric circulation for a relativelylong time: 8 h in the ascending arc or about 2 h in thevicinity of the apocentre in each orbit dedicated to thesescience cases. Intervals between the images will be10–20min, which roughly corresponds to the displacementof the cloud features by one CCD pixel due to winds at thecloud tops. Spacecraft re-pointing will also be used to trackselected cloud features. During off-pericentre imagingVMC will usually have both the day and night side ofVenus in its field of view. This imposed specific require-ments on suppression of stray light and ghosts in thecamera design (Markiewicz et al., 2007). The verticalstructure of mesospheric aerosols at 60–80 km will bestudied in limb sessions. The imaging rate in this case willbe about one image every 5 s.
3. Science goals and expected results
This section describes in detail the main scientific goalsof the experiment and expected results.
3.1. Distribution and nature of the unknown UV absorber
The UV spectrum of Venus shows a broad absorptionfeature in the 0.25–0.5 mm range (Moroz et al., 1985).Strong absorption below 0.32 mm is explained by thepresence of SO2 at the cloud tops. Spectral dependence ofVenus’ albedo between 0.32 mm and 0.5 mm indicatesanother absorber whose composition and nature has notbeen identified so far. Sulfur allotropes Sx, disulfurmonoxide S2O, ferric chlorine (FeCl3) solution in sulfuricacid and other components were proposed as possiblecandidates (Esposito et al., 1997). Its identification isimportant because this specie absorbs about 50% of thesolar radiation that Venus receives from the Sun that hasimplications for the energy balance and dynamics of theentire atmosphere. Early observations showed that theunknown absorber is located in a few kilometer thick layerwithin the upper cloud deck (Pollack et al., 1980; Esposito,1980; Ekonomov et al., 1984). The UV markings observedon the Venus disc are produced by variations of its amountand of the overlying haze opacity.
The VMC UV filter is centered at 365 nm (Fig. 3)—theregion of specific spectral feature of the unknown absorberand the maximum contrast observed on the Venus disc.The camera will continue Pioneer Venus investigations ofthe distribution and variability of the UV absorber byobserving the contrasts and their dependence on phaseangle (Rossow et al., 1980; Esposito, 1980). Theseobservations will benefit from the VMC capability to takeinstantaneous frames from any distance to the planet andfrom the 3-axes stabilized spacecraft that can provideobservations of a selected region at different phase angles.Combination with the VIRTIS measurements (Drossart etal., 2007) in the near-infrared absorption bands of CO2 willhelp to constrain the altitude of formation of bright anddark features and the location of the absorber within thecloud deck. The combination of VMC imaging andVIRTIS spectroscopy in the UV range will also allow oneto study the correlation of the unknown absorber withsulfur dioxide and other gases at the cloud tops that wouldgive a hint of the origin of the mysterious specie.
3.2. Atmospheric dynamics at the cloud tops
Earlier observations of Venus in the UV-blue spectralrange during the Mariner 10 and Galileo fly-bys, andespecially long term monitoring by the Pioneer Venusorbiter revealed general features of the Venus cloud topmorphology and dynamics (Belton et al., 1976; Beltonet al., 1991; Rossow et al., 1980). The most prominentglobal cloud features seen in the images are the ‘‘Y’’-shapeand bow shape features in the low latitudes, and dark andbright bands in the polar regions. Wave trains andconvection cells were observed at smaller scales. The cloudmorphology changes from patchy in the vicinity of sub-solar point to streaks in the middle and high latitudes.Different techniques were applied to these data sets to
derive wind fields from tracking small scale UV markingsin the images (Limaye and Suomi, 1981; Limaye, 1988;Limaye et al., 1988; Smith and Gierasch, 1996; Toigo et al.,1994). Retrograde zonal flow with wind speed of �100m/sand evidence of mid-latitude jets was found to dominatethe global circulation at the cloud tops. The meridionalpoleward wind component did not exceed 20m/s. Analysisof the brightness patterns and wind filed in the PioneerVenus images revealed several kinds of planetary-scalewaves at the cloud tops. The strongest are diurnal andsemi-diurnal tides and ‘‘Y’’-shape feature (Del Genio andRossow, 1990) imposed on the global circulation flow.Characteristics of both global circulation and planetarywaves vary with time scales from days in case of ‘‘Y’’feature to years for the zonal flow with tentative evidenceof cyclic changes with period of 5–10 years. Analysis of theGalileo images taken with spatial resolution of about15 km and image time separation of about 10min surpris-ingly showed no eddy or wave-like activity (Toigo et al.,1994).
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Investigation of atmospheric dynamics is one of the maingoals of the VMC experiment. The VMC UV channel willcontinue similar studies began by the previous missions.Tracking the motions of the UV markings and analysis ofbrightness variations will be used to derive wind velocitiesand to investigate eddy activity and wave phenomena atthe Venus cloud tops. These observations will cover bothglobal and local spatial scales. VMC will continuemonitoring global scale dynamics in the low latitudes thusextending the Pioneer Venus OCPP long-term series ofobservations. Highly eccentric polar orbit of Venus Expressis ideally suited for global observations of the southernhemisphere. This will provide valuable complement to theearlier missions that had poor coverage of the poles. Theobservations of zonal flow near the poles are expected to beimportant for understanding the momentum balance in theVenus atmosphere (Rossow, 1985).
In the ascending branch of the orbit between 12 and 21 horbital time (Fig. 6), VMC will map full Venus disc with aspatial resolution between 50 and 30 km/px. During thistime the sub-satellite latitude will vary from 701 S to 101 S(see Titov et al., 2006). VMC will monitor global featuressuch as the bright polar band, the dark mid-latitude band,the dark equatorial band, and the southern part of the‘‘Y’’-feature for several hours in each orbit. Images will betaken at a rate of one frame every 30min, whichcorresponds to a displacement of cloud features by severalCCD pixels. These time series will then be used to derivethe wind velocity field in the southern hemisphere andequatorial latitudes by tracking UV markings. The muchhigher rate of the VMC imaging as compared to that of theOCPP experiment on Pioneer Venus (1 image every 4 h)will provide significant improvement in the accuracy ofwind pattern reconstruction.
Later in the orbit (21–23 h) the observed portion of thedisc will decrease to �3600 km and VMC will imageequatorial latitudes with spatial resolution of 10–30 km/px.The goal of these observations is to study mesoscaleprocesses and especially convection in the sub-solar point.Although the cloud feature tracking by Galileo did notshow any evidence of eddy or wavelike activity, this couldbe a result of too short observing sequence (16 h) orobserved local time. VMC will study mesoscale processes inlow latitudes in much more detail.
Observations of the middle and high latitudes in thenorthern hemisphere will be performed from a distance ofless than 10,000 km resulting in a VMC image size of3000–70 km and a spatial resolution from 7 to 0.2 km. Themuch higher spatial resolution and imaging rate ascompared to earlier observations will allow the study ofthe fine structure of dynamic features such as wave trainsand convective cells and their short time scale evolution.
The low pericentre will not permit global observations ofthe Northern middle and high latitudes. In the northernmiddle and high latitudes, VMC observations will consistof relatively narrow strips (Table 2). This will precludedirect comparison of global dynamic phenomena in the
two hemispheres. However, dedicated pole-to-pole imagingcampaigns are included in the mission planning (Titov etal., 2006). These will consist of imaging in both hemi-spheres in 10–15 consecutive orbits. Statistical analysis ofthe data set obtained in the northern hemisphere will resultin reconstruction of global dynamical features that will beused for correlation studies.
3.3. Mesospheric haze structure and aerosol properties
First observations of the Venus limb were carried outduring Mariner 10 fly-by (O’Leary, 1975). Much greaterdata set was obtained during the Pioneer Venus orbitermission. The PV OCPP observations covered northernmid-latitudes (15–451 N) and local times from 9:00 to18:00. Lane and Opstbaum (1983) used these data toretrieve vertical structure and optical properties of theaerosols above the main cloud deck. The upper haze onVenus was found to be remarkably uniform with opticaldepth 0.01 occurring at 80–85 km and haze scale height of1–3 km. Detached haze layers were rare. Fine particles withrefractive index o1.7 and rather narrow range of meanradius between 0.2 and 0.4 mm fit the observations.VMC will take images of the Venus limb to study the
vertical and horizontal structure of the mesospheric haze(Table 2). The observations in the northern middle andhigh latitudes, the region where Venera 15 found strongvariations of the cloud structure (Zasova et al., 2007), willhave the best altitude resolution of 1–2 km. VMC will carryout specific observations of the limb at sunset and sunrise.Since the camera can look close to and even directly at theSun, it will map the brightness distribution on the limb inthe vicinity of the Sun. Such observations performed infour filters and in forward scattering geometry will be usedto study optical and microphysical properties of themesospheric haze. Imaging of the brightness spatialvariation at the terminator will be also used to study thestructure of the upper cloud layer. Additional advantage ofVMC as compared to the PV OCPP limb scanning is thatthe camera will image the limb thus providing two-dimensional structure of mesospheric hazes.
3.4. Water vapour abundance and cloud opacity
Water vapour is involved in the formation of sulfuricacid aerosols in Venus’ upper clouds. Thermal infraredobservations showed high spatial and possibly temporalvariability of its abundance at the cloud tops (Schofield etal., 1982; Ignatiev et al., 1999; Koukouli, 2002). They areprobably caused by convective mixing of the upper cloudlayer in which the vapour vertical gradient is very steep.The VMC NIR1 filter will sound the 0.94 mm H2O band
(Fig. 3). The NIR2 channel will provide a continuumreference point. The absorption expected in the VMCNIR1 channel for an H2O abundance of a few ppm isabout 10%. VMC will map the spatial distribution of watervapour at the cloud tops and its evolution with time. This
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data will be used to constrain the model of sulfuric acidaerosol formation. Correlation of these maps with those ofthe unknown absorber derived from the UV imaging willgive additional clues about convection in the upper cloud.Since the H2O absorption band is formed from multiplescattering, the spectra obtained by other experiments willbe used to constrain the cloud structure.
Measurements in the NIR2 filter on the day side will alsobe used to study variations of the total cloud opacity andatmospheric motions in the main cloud deck, although thebrightness contrasts are expected to be rather low (�1%).
3.5. Surface and lower atmosphere investigation in the 1 mm
transparency ‘‘window’’ on the night side
The discovery of spectral ‘‘windows’’ in the near IRspectrum of Venus by Allen and Crawford (1984), throughwhich thermal radiation from the hot lower atmosphereand even the surface can leak to space, provides a powerfultool to study the atmosphere below the clouds. Since theseemissions are much weaker than solar radiation, they canbe observed only on the night side of the planet. Emissionaround the 1 mm ‘‘window’’ which is covered by the VMCnear-IR filters originates in the lower atmosphere and atthe surface. Its intensity depends on the surface tempera-ture and thus elevation, total cloud opacity, water columndensity and to a smaller extent on the surface emissivityand water vapour vertical distribution. The VMC near-IRfilters have the following specific tasks for night sidesounding. The NIR1 channel is centered on the H2O bandat 965 nm and will focus on the study of water spatialdistribution in the lower 10 km. The NIR2 channel(1000 nm) will perform thermal imaging of the surfaceand monitor the variability of the cloud optical depth andatmospheric dynamics by tracking the motions of the near-IR cloud features.
A dedicated numerical study was carried out to quantifysensitivity of the measurements by the VMC near-IRchannels to various parameters (Ignatiev, 2003). The studyused full radiative transfer model of the Venus atmospherewith accurate account for multiple scattering and absorp-tion in gaseous bands. Vertical profiles of atmospherictemperature and pressure were taken from the Venera 11descent probe measurements. They are very close to theVenus International Reference Atmosphere (VIRA) modelfor low latitudes (Sieff et al., 1985). Aerosol model thatincluded 4 modes of particles was taken from Zasova et al.(1985). This model gives total cloud opacity of �40 at1 mm. Lambert surface with albedo of 0.2 was assumed inthe calculations. The model of the water vapour verticaldistribution was based on recent re-analysis of the Veneraorbiters and descent probes spectra (Ignatiev et al., 1997,1999).
Fig. 7 shows synthetic spectra of the Venus night sidearound the 1 mm ‘‘window’’ with dependence on suchparameters as H2O abundance, surface altitude, and cloudopacity. Fig. 8 presents a summary of the sensitivity study
for both near-IR channels. The NIR1 channel is mainlysensitive to the H2O abundance within lower 20 km. Earlierspectroscopic measurements showed that water vapourvariability is possibly quite limited to 750% around30 ppm in the lower atmosphere (de Bergh et al., 2006).These variations, however, will be well detectable by theVMC NIR1 channel. Intensity in this channel is much lesssensitive to the other parameters.The NIR2 channel covers the spectral range, which is
free from gaseous absorption. For this reason its measure-ments are not sensitive to the water vapour abundance.Intensity in this channel mainly depends on surfacetemperature and cloud opacity. Thus spatial variations ofthe measured brightness are expected to be correlated withthe topography. Surface elevations of �10 km result intemperature contrasts of up to 80 1C that would be clearlyvisible in the images (Lecacheux et al., 1993; Meadows andCrisp, 1996). Since previous in situ measurements indicateda constant temperature structure of the lower atmosphere,the VMC measurements can be used either to study thesurface emissivity and constrain the composition (Hashi-moto and Sugita, 2003) or to derive the vertical tempera-ture profile in the lower scale height under the assumptionthat in a very dense atmosphere the surface temperature isequal to that of the atmosphere (Meadows and Crisp,1996). Also, imaging in the NIR2 filter will be used tosearch for traces of volcanic activity like hot lava outflows.It should be mentioned that the spatial resolution of VMCsurface observations will be limited by scattering in theclouds and will not exceed 50 km. Global thermal imagingof the Venus surface will be performed for the first time andwill significantly complement the Magellan radar investiga-tions.Fig. 8b also demonstrates that the NIR2 measurements
are sensitive to the total cloud opacity. Its variations willproduce up to a factor of 2 contrast that will overlay thesurface brightness distribution. The two patterns can bediscriminated using the fact that we have rather goodknowledge of the surface topography. Also the VIRTISimaging in the other near-IR ‘‘windows’’ can help since theradiation at wavelength longer than 1.2 mm is not sensitiveto the surface temperature. Tracking the near-IR markingson the NIR2 images will be used to derive the generalcirculation at the level of main cloud deck (�50 km).
3.6. Night side observations in the visible filter
Several types of airglow were observed on the Venusnight side. The spectrometer experiment onboard Venera 9and 10 discovered strong visible emission (Krasnopolsky,1983). The measured spectrum led to unambiguousidentification of the Herzberg I and II systems of O2 witha total intensity of �3 kR. Limb observations showed thatthis emission originated in a layer at 90–110 km altitude.The visible VMC filter is positioned roughly in the
middle of the Herzberg system covering a spectral rangefrom 500 to 560 nm. Global mapping the airglow spatial
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Fig. 7. (a) Synthetic spectra of the night side emission around the 1 mm ‘‘window’’ calculated for surface altitude of 0 km, cloud opacity of 40, and variable
H2O mixing ratio top to bottom; 1, 10, 31, 100, 1000 ppm. Positions of the VMC filters are shown with horizontal bars. (b) The same as in Fig. 7a for the
standard H2O model (30 ppm at the surface), cloud opacity of 40, and variable surface elevation top to bottom; 12, 8, 4, 0 km. (c) The same as in Fig. 7a for
standard H2O model (30 ppm at the surface), surface elevation 0 km, and variable cloud opacity at 1 mm top to bottom; 20, 24, 28, 32, 36, 40.
W.J. Markiewicz et al. / Planetary and Space Science 55 (2007) 1701–1711 1709
distribution and its temporal variations with this filter willcontribute to the study of the circulation of the lowerthermosphere (100–130 km). It should be noted, however,that these observations require very long integration time.VMC will be able to take series of images and accumulatethe signal. In addition, observations in the visible filter willalso continue the search for lightning and will measure thevisible albedo of Venus on the day side.
4. Summary
The VMC instrument is a CCD-based camera that willtake images of Venus in four narrow band filters in UV(365 nm), visible (513 nm), and near-IR (965 and 1000 nm)range. The highly elongated orbit of the Venus Expresssatellite will allow VMC to capture images with spatialscales ranging from a global view of the planet with aresolution of �50 km to close-ups with a few hundredmeters resolution. Spectral properties of the VMC filters
were selected to focus on certain science objectives. Thedynamics of the Venus atmosphere will be studied bytracking motions of cloud features in the UV on the dayside and near-IR on the night side. Imaging in the visibleon the night side will map the airglow originating at�100 km. Its structure and variability is indicative ofthermospheric dynamics. Composition investigations willinclude the study of the location, distribution, andvariability of the unknown UV absorber at the cloud tops.The near-IR channel centered on the 0.94 mm H2O bandwill sound the water vapour abundance and distribution atthe cloud tops on the day side and in the lower 20 km atnight. The night side observations in the 1 mm transparency‘‘window’’ will provide almost complete thermal mappingof the Venus surface that would be highly complementaryto the Magellan radar investigation. Structure of the uppercloud and mesospheric hazes will be studied in limbgeometry. The night side observations will also characterizethe total cloud opacity and its variations.
ARTICLE IN PRESS
0
2
4
6
8
10
VM
C F
5 I
nte
nsity,
erg
/ (
s c
m2 s
r µm
)
0.0 0.2 0.4 0.6
Surface Albedo
0 4 10 12
Surface Altitude
20 25 30 35 40
Cloud optical depth at 1 µm
10-7 10-6 10-5 10-4 10-3
H2O near surface abundance
1%
700 720 740 760
Surface Temperature, K
0
20
40
60
80
VM
C F
6 Inte
nsity,
erg
/ (
s c
m2 s
r µm
)
0.0 0.2 0.4 0.6
Surface Albedo
Surface Altitude
20 25 30 35 40
Cloud optical depth at 1 µm
10-7 10-6 10-5 10-4 10-3
H2O near surface abundance
1%
700 720 740 760
Surface Temperature,K
2 86
0 4 10 122 86
a
b
Fig. 8. (a) Sensitivity of measurements in the NIR1 channel to the studied
parameters and (b) sensitivity of measurements in the NIR2 channel to the
studied parameters.
W.J. Markiewicz et al. / Planetary and Space Science 55 (2007) 1701–17111710
The VMC camera will provide imaging context for theother experiments on board the Venus Express spacecraft.In the vicinity of the pericentre, when the satellite velocity
is very high, VMC will be the only imaging instrument andwill provide high-resolution mapping of the northernhemisphere.
References
Allen, D., Crawford, J.W., 1984. Cloud structure on the dark side of
Venus. Nature 307, 222–224.
Baines, K., et al., 2000. Detection of sub-micron radiation from the
surface of Venus by Cassini/VIMS. Icarus 148, 307–311.
Belton, M.J.S., Smith, G.R., Schubert, G., Del Genio, A.D., 1976. Cloud
patterns, waves and convection in the Venus atmosphere. J. Atmos.
Sci. 33, 1394–1417.
Belton, M.J.S., Gierasch, P.J., Smith, M.D., Helfenstein, P., Schinder,
P.J., Pollack, J.B., Rages, K.A., Ingersoll, A.P., Klaasen, K.P.,
Veverka, J., Anger, C.D., Karr, M.H., Chapman, C.R., Davies,
M.E., Fanale, F.P., Greeley, R., Greenberg, R., Head III, J.W.,
Morrison, D., Neukum, G., Pilcher, C., 1991. Images from Galileo of
the Venus cloud deck. Science 253, 1531–1536.
Bougher, S.W., Alexander, M.J., Mayr, H.G., 1997. Upper atmosphere
dynamics: global circulation and gravity waves. In: Venus II,
University of Arizona Press, Tucson, AZ, pp. 259–291.
Carlson, R.W., Baines, K.H., Encrenaz, Th., Taylor, F.W., Drossart, P.,
Kamp, L.W., Pollack, J.B., Lellouch, E., Collard, A.D., Calcutt, S.B.,
Grinspoon, D., Weissman, P.R., Smythe, W.D., Ocampo, A.C.,
Danielson, G.E., Fanale, F.P.O., Johnson, T.V., Kieffer, H.H.,
Matson, D.L., McCord, T.B., Soderblom, L.A., 1991. Galileo infrared
imaging spectroscopy measurements at Venus. Science 253, 1541–1548.
De Bergh, C., Moroz, V., Taylor, F.W., Crisp, D., Bezard, B., Zasova,
L.V., 2006. The composition of the atmosphere of Venus below 100km
altitude: an overview. Planet. Space Sci. 54, 1389–1907.
Del Genio, A.D., Rossow, W.B., 1990. Planetary-scale waves and the
cyclic nature of cloud top dynamics on Venus. J. Atmos. Sci. 47,
293–318.
Drossart, P., Piccioni, G., Adriani, A., Angrilli, F., Arnold, G., Baines,
K., Bellucci, G., Benkhoff, J., Bezard, B., Bibring, J.-P., Blanco, A.,
Blecka, M.I., Carlson, R., Coradini, A., Di Lellis, A., Encrenaz, Th.,
Erard, S., Fonti, S., Formisano, V., Fouchet, T., Garcia, R., Haus, R.,
Helbert, J., Ignatiev, N.I., Irwin, P., Langevin, Y., Lebonnois, S.,
Lopez Valverde, M.A., Luz, D., Marinangeli, L., Orofino, V., Rodin,
A.V., Roos-Serote, M.C., Saggin, B., Sanchez-Lavega, A., Stam,
D.M., Taylor, F.W., Titov, D.V., Visconti, G., Zambelli, M., Hueso,
R., Tsang, C., Afanasenko, T.S., 2007. Scientific goals for the
observation of Venus by VIRTIS on ESA/Venus Express mission.
Planet. Space Sci., this issue, doi:10.1016/j.pss.2007.01.003.
Ekonomov, A.P., Moroz, V.I., Moshkin, B.E., Gnedykh, V.I., Golovin,
Yu.M., Grigoriev, A.V., 1984. Scattered UV solar radiation within the
clouds of Venus. Nature 307, 345–347.
Esposito, L.W., 1980. Ultraviolet contrasts and the absorbers near the
Venus cloud tops. J. Geophys. Res. 85 (nA13), 8151–8157.
Esposito, L.W., Bertaux, J.-L., Krasnopolsky, V.A., Moroz, V.I., Zasova,
L.V., 1997. Chemistry of lower atmosphere and clouds. In: Venus II.
University of Arizona Press, Tucson, AZ, pp. 415–458.
Hashimoto, G.L., Sugita, S., 2003. On observing the compositional
variability of the surface of Venus using nightside near-infrared
thermal radiation. J. Geophys. Res. 108 (nE9), 5109–5116.
Ignatiev, N.I., 2003. Venus Monitoring Camera Sensitivity Study, VMC
internal Report.
Ignatiev, N.I., Moroz, V.I., Moshkin, B.E., Ekonomov, A.P., Gnedykh,
V.I., Grigoriev, A.V., Khatountsev, I.V., 1997. Water vapour in the
lower atmosphere of Venus: a new analysis of optical spectra measured
by entry probes. Planet. Space Sci. 45, 427–438.
Ignatiev, N.I., Moroz, V.I., Zasova, L.V., Khatuntsev, I.V., 1999. Water
vapour in the middle atmosphere of Venus: an improved treatment of
the Venera 15 IR spectra. Planet. Space Sci. 47, 1061–1075.
Koukouli, M.E., 2002. Remote sensing of water vapour in Venus’ middle
atmosphere, Ph.D. thesis, Oxford University.
ARTICLE IN PRESSW.J. Markiewicz et al. / Planetary and Space Science 55 (2007) 1701–1711 1711
Krasnopolsky, V.A., 1983. Venus spectroscopy in the 3000–8000 A region
by Veneras 9 and 10. In: Venus, University of Arizona Press, Tucson,
AZ, pp. 459–483.
Lane, W.A., Opstbaum, R., 1983. High altitude Venus haze from Pioneer
Venus limb scans. Icarus 54, 48–58.
Lecacheux, J., Drossart, P., Laqes, P., Deladerriere, F., Colas, F., 1993.
Detection of the surface of Venus at 1 mm from ground-based
observations. Planet. Space Sci. 41, 543–549.
Limaye, S.S., 1988. Venus: cloud level circulation during 1982 as
determined from the Pioneer Cloud Photopolarimeter images. II.
Solar longitude dependent circulation. Icarus 73, 212–226.
Limaye, S.S., Suomi, V.E., 1981. Cloud motions on Venus: global
structure and organization. J. Atmos. Sci. 38, 1220–1235.
Limaye, S.S., Grassotti, C., Kuetemeyer, M.J., 1988. Venus: cloud level
circulation during 1982 as determined from the Pioneer Cloud
Photopolarimeter images. I. Time and zonally averaged circulation.
Icarus 73, 193–211.
Markiewicz, W.J., Titov, D.V., Fiethe, B., Behnke, T., Szemerey, I.,
Perplies, H., Wedemeier, M., Sebastian, I., Boogaerts, W., Dierker, C.,
Osterloh, B., Ignatiev, N., Boker, W., Koch, M., Keller, H.U.,
Jaumann, R., Michaelis, H., Michalik, H., Crisp, D., Esposito, L.,
Limaye, S.S., Watanabe, S., Thomas, N., Belyaev, D., Dannenberg,
A., Tschimmel, M., Moissl, R., 2007. Venus Monitoring Camera for
Venus Express. ESA-SP in press.
Meadows, V.S., Crisp, D., 1996. Ground-based near-infrared observations
of the Venus nightside: the thermal structure and water abundance
near the surface. J. Geophys. Res. 101 (nE2), 4595–4622.
Moroz, V.I., Ekonomov, A.P., Moshkin, B.E., Revercomb, H.E.,
Sromovsky, L.A., Schofield, J.T., Spaenkuch, D., Taylor, F.W.,
Tomasko, M.G., 1985. Solar and thermal radiation in the Venus
atmosphere. Adv. Space Res. 5, 197–232.
O’Leary, B., 1975. Venus: vertical structure of stratospheric hazes from
Mariner 10 pictures. J. Atmos. Sci. 32, 1091–1100.
Pollack, J.B., Toon, O.B., Whitten, R.C., Boese, R., Ragent, B., Tomasko,
M., Esposito, L.W., Travis, L., Wiedman, D., 1980. Distribution and
source of the UV absorption in Venus’ atmosphere. J. Geophys. Res.
85 (nA13), 8141–8150.
Rossow, W.B., 1985. Atmospheric circulation of Venus. Adv. Geophys.
28A, 347–379.
Rossow, W.B., Del Genio, A.D., Limaye, S.S., Travis, L.D., Stone, P.H.,
1980. Cloud morphology and motions from Pioneer-Venus images. J.
Geophys. Res. 85 (nA13), 8107–8121.
Rossow, W.B., Del Genio, A.D., Eichler, T., 1990. Cloud-tracked winds
from Pioneer Venus OCPP images. J. Atmos. Sci. 47, 2053–2084.
Schofield, J.T., Taylor, F.W., McCleese, D.J., 1982. The global distribu-
tion of water vapour in the middle atmosphere of Venus. Icarus 52,
263–278.
Sieff, A., Schofield, J.T., Kliore, A.J., Taylor, F.W., Limaye, S.S.,
Revercomb, H.E., Sromovsky, L.A., Kerzhanovich, V.V., Moroz, V.I.,
Marov, M.Ya., 1985. Models of the structure of the atmosphere of
Venus from the surface to 100 km altitude. Adv. Space Res. 5, 3–58.
Smith, M.D., Gierasch, P.J., 1996. Global-scale winds at the Venus cloud
top inferred from cloud streak orientations. Icarus 123, 313–323.
Svedhem, H., Titov, D.V., McCoy, D., Lebreton, J.-P., Barabash, S.,
Bertaux, J.-L., Drossart, P., Formisano, V., Hausler, B., Korablev, O.,
Markiewicz, W.J., Nevejans, D., Patzold, M., Piccioni, G., Zhang,
T.L., Taylor, F.W., Lellouch, E., Koschny, D., Witasse, O., Warhaut,
M., Accommazzo, A., Rodriguez-Canabal, J., Fabrega, J., Schirmann,
T., Clochet, A., Coradini, M., 2007. Venus Express—the first
European mission to Venus. Planet. Space Sci. this issue.
Titov, D.V., Svedhem, H., Koschny, D., Hoofs, R., Barabash, S., Bertaux,
J.-L., Drossart, P., Formisano, V., Hausler, B., Korablev, O.,
Markiewicz, W.J., Nevejans, D., Patzold, M., Piccioni, G., Zhang,
T.L., Merritt, D., Witasse, O., Zender, J., Accommazzo, A., Sweeney,
M., Trillard, D., Janvier, M., Clochet, A., 2006. Venus Express science
planning. Planet. Space Sci. 54, 1279–1297.
Toigo, A., Gierasch, P.J., Smith, M.D., 1994. High resolution cloud
feature tracking on Venus by Galileo. Icarus 109, 318–336.
VMC-MPAe-RP-SS011-001, 2006. VMC FS Calibration and Perfor-
mance Report, iss. 1, rev. 1.
Zasova, L.V., Spaenkuch, D., Moroz, V.I., Schaefer, K., Ustinov, E.A.,
Dohler, W., Linkin, V.M., Oertel, D., Dyachkov, A.V., Becker-Ross,
H., 1985. Venera 15 and Venera 16 infrared experiment. III. Some
conclusions about the cloud structure based on the spectrum analysis.
Cosmic Res. 23, 221–235 (in Russian).
Zasova, L.V., Ignatiewv, N.I., Khatountsev, I.A., Linkin, V.M., 2007.
Structure of the Venus atmosphere. Planet. Space Sci., this issue,
doi:10.1016/j.pss.2007.01.011.