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Planetary and Space Science 56 (2008) 1385–1390

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Near infrared imaging spectroscopy of Venus withthe Anglo-Australian Telescope

Jeremy Baileya,�, S. Chamberlaina, D. Crispb, V.S. Meadowsc

aPhysics Department, Macquarie University, NSW 2109, AustraliabJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

cDepartment of Astronomy, University of Washington, Box 351580, Seattle, Washington 98195, USA

Received 15 January 2008; received in revised form 17 March 2008; accepted 26 March 2008

Available online 4 April 2008

Abstract

We have obtained full-disk spatially resolved spectra of the Venus nightside at near-infrared wavelengths during July 2007 using the

Anglo-Australian Telescope and Infrared Imager and Spectrograph 2 (IRIS2). The data have been used to map the intensity and

rotational temperature of the O2 ða1DgÞ airglow band at 1:27mm. The temperatures agree with those obtained in earlier IRIS2

observations and are significantly higher than expected from the Venus International Reference Atmosphere (VIRA) profile. We also

report the detection of the corresponding n ¼ 021 O2 airglow band at 1:58mm with a similar spatial distribution to the 1:27mm n ¼ 020

band. Observations in the 1:18mm thermal window have been used to image surface topography using two different methods of cloud

correction. We have also obtained 2:3mm images that can be used to study cloud motion.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: Venus; Venus atmosphere; Near-infrared spectroscopy; Airglow

1. Introduction

The 3.9m Anglo-Australian Telescope (AAT) and itsinfrared instruments have played an important part in thedevelopment of remote sensing techniques to study theVenus atmosphere. The AAT’s first infrared instrument,the Infrared Photometer Spectrometer (IRPS) was used todiscover the infrared windows at 1.7 and 2:3 mm (Allen andCrawford, 1984; Allen, 1987). The AAT’s Fabry-PerotInfrared Grating Spectrometer (FIGS, Bailey et al., 1988)was used to detect further windows at shorter wavelengths(Crisp et al., 1991). These windows are wavelength regionsin which the atmospheric absorption is sufficiently weakthat thermal emission from the deep atmosphere can beseen. More detailed observations with the Infrared Imagerand Spectrograph (IRIS) showed that the surface of Venuswas detectable in the 1:18 mm and shorter wavelengthwindows, and were used to study the H2O distribution nearthe surface (Meadows and Crisp, 1996). AAT observations

e front matter r 2008 Elsevier Ltd. All rights reserved.

s.2008.03.006

ing author. Tel.: +612 9850 8933; fax: +61 2 9850 8115.

ess: jbailey@els.mq.edu.au (J. Bailey).

at these wavelengths have also been used to study the O2

airglow emission which is a probe of the chemistry anddynamics in the upper atmosphere (Allen et al., 1992; Crispet al., 1996).These near-infrared remote sensing techniques for

studying the surface, deep atmosphere and highaltitude airglows of Venus are now being exploited by theVIRTIS instrument on the ESA Venus Express spacecraft(Drossart et al., 2004, 2007a). While VIRTIS can providea much closer view of Venus than is possible from Earth,ground-based telescopes can still contribute significantlyto such investigations and provide complementary datasets that will enhance the interpretation of VIRTISdata. The AAT’s latest infrared instrument, the InfraredImager Spectrograph 2 (IRIS2, Tinney et al., 2004) canprovide imaging spectroscopy with a spectral resolvingpower of �2400, 12 times that of VIRTIS-M, and doublethat of VIRTIS-H (which does not have imaging cap-ability). Ground-based observations can also provideimaging spectroscopy over the full disk of Venus, includingthe northern hemisphere that cannot be imaged withVIRTIS.

ARTICLE IN PRESSJ. Bailey et al. / Planetary and Space Science 56 (2008) 1385–13901386

Here we report observations of the Venus nightside withIRIS2 made during 2007, the first such observationsobtained during the Venus Express mission. A coordinatedcampaign of ground-based observations in support ofVenus Express was organized for the period 2007 May 23to June 9. Due to an instrument failure it was not possibleto obtain any AAT/IRIS2 observations during this period.However, IRIS2 observations were made during thesomewhat later periods from 2007 July 1 to July 4 andJuly 21 to 26. These dates provided a favourable viewinggeometry for observations of the nightside.

2. Observations

2.1. Narrowband images

The IRIS2 instrument (Tinney et al., 2004) uses a 1024�1024 pixel HgCdTe detector array, and can be used inimaging and spectroscopy modes. We used the imagingmode of IRIS2 to obtain images of Venus in a narrow bandcentred at 2:3mm. These images show the lower cloud ofVenus silhouetted against the hot lower atmosphere. Theycan be used to measure wind speeds, by tracking cloudmotions, particularly when combined with observations atother sites to provide increased time coverage. While windspeeds from cloud tracking can also be obtained fromobservations with VIRTIS and VMC on Venus Express(e.g. Markiewicz et al., 2007), ground-based observationsallow coverage of the northern hemisphere that cannot beimaged with Venus Express. An example of such an imageis shown in Fig. 1.

2.2. Imaging spectroscopy

The bulk of our observations were obtained usingthe spectroscopic mode of IRIS2. Spectral image cubes

Fig. 1. Image of Venus in a narrowband 2:3mm filter on 2007 July 26

showing nightside cloud structure.

covering the full disk of Venus were obtained by scanningthe spectrograph slit (7.5 arc minutes long) across thedisk of Venus while recording a continuous series of fivesecond exposures using the time series mode of IRIS2. Thespectral resolving power ðR ¼ l=DlÞ is �2400, and threewavelength settings were used covering the J-bandð1:0921:33mmÞ, H-band ð1:4721:79mmÞ and K-bandð2:0222:40mmÞ. The data are wavelength calibrated usingexposures of a Xenon lamp, and flat-field correctedusing quartz lamp exposures. Sky background is subtractedusing exposures at the beginning and end of each scan,together with a further subtraction of any residual skyusing regions of the slit on either side of Venus.The resulting spectra are contaminated by scattered light

from the sunlit crescent. This is removed by deriving a pure‘‘scattered crescent’’ spectrum from a region just off thedisk of Venus but close to the sunlit crescent. A scaledversion of this spectrum is then subtracted from eachspatial pixel of the cube with the scaling factor determinedbased on the intensity in wavelength ranges at which Venushas no nightside emission.Table 1 lists the nights on which good quality data were

obtained including the range of UT and the spectral bands(J, H or K) observed. Examples of spectra in the J band areshown in Fig. 2.

3. Results

3.1. O2 airglow at 1:27 mm

Emission in the a1Dg � X 3S�g (0–0) band of molecularoxygen at 1:27mm is a prominent feature of the Venusnightside spectrum. The emission was discovered byConnes et al. (1979). Subsequent ground-based observa-tions (Allen et al., 1992; Crisp et al., 1996) have shown theemission to have complex spatial structure that variesdramatically from night-to-night. The emission is thoughtto arise from oxygen atoms formed by photodissociation ofCO2 on the dayside that are then carried by the upperatmosphere circulation to the nightside. Here they candescend to lower altitudes where three body reactionscan form O2 in excited states that then emit the airglowphotons.The O2 airglow has also been observed using VIRTIS on

Venus Express (Drossart et al., 2007b). VIRTIS can

Table 1

Journal of observations

Date UT Diameter

(arc sec)

Illumination

(%)

Bands

observed

July 1 07:13–08:40 31.4 35.4 J, K

July 4 06:14–08:40 32.8 33.2 J, H, K

July 21 06:52–08:22 42.9 18.8 J, K

July 22 06:16–08:17 43.6 18.0 J, H, K

July 25 07:37–08:08 45.8 15.2 J

July 26 06:34–08:06 46.5 14.3 J, K

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1.24 1.25 1.26

1.24 1.25 1.26

8000

6000

4000

2000

0

Tr = 220 K

Tr = 210 K

Tr = 200 K

Tr =190 K

Tr =187 K (best fit)

Tr =180 K

Tr =170 K

Fit r

esid

uals

Wavelength �m

Fig. 3. Residuals of model fits to an IRIS2 O2 airglow spectrum. The

effects of rotational temperature changes on the spectral fit are illustrated.

The best fit temperature in this case is 187K. Spectrum observed on 2004

July 9.

1.15 1.2 1.25 1.3

Wavelength (�m)

1.15 1.2 1.25 1.3

Wavelength (�m)

O2 Q – branch

1.18 �m window 1.28 �m window1.31 �mwindow

3×104

2×104

104

Inte

nsity

(ra

w d

etec

tor

units

)

3×104

2×104

104

0

0

Inte

nsity

(ra

w d

etec

tor

units

)

Fig. 2. Spectra of Venus in the J band on 2007 July 26. The upper panel is

taken from a region with little airglow emission and shows the thermal

windows, and the lower panel is from a region with strong airglow

emission.

J. Bailey et al. / Planetary and Space Science 56 (2008) 1385–1390 1387

observe the emission in limb viewing geometry and thusmeasure the altitude profile. The emission is found to peakat an altitude of 96� 1 km.

The distribution of rotational line intensities within theO2 band can be used to measure the temperature in theVenus mesosphere. Connes et al. (1979) and Crisp et al.(1996) used high-resolution Fourier Transform Spectro-meter (FTS) observations to measure temperatures of185� 15 and 186� 6K, respectively. While the spectralresolution of IRIS2 is not quite sufficient to fully resolveindividual lines within the band it is nevertheless possible toobtain good quality temperature measurements using aforward modelling approach (Bailey et al., 2008). Thismethod involves generating a high-resolution simulatedspectrum of the O2 emission, correcting this for theDoppler shift of Venus and passing it through a high-resolution model of the Earth atmosphere transmission.The resulting spectrum is then convolved to the observedspectral resolution, added to an empirical model of the1:28mm thermal window spectral shape and compared withthe observed spectrum. Temperatures are derived byadjusting the temperature to fit the spectrum in thewavelength range 1:248021:2672mm, a region chosenbecause the O2 lines are reasonably well separated, andthe underlying thermal emission is weak. An example ofthe sensitivity of the spectrum to temperature in this regionis shown in Fig. 3.

Using this technique with previous IRIS2 observationsof the O2 airglow (taken in July 2004 and December 2005)we have obtained temperature maps for the Venus meso-sphere (Bailey et al., 2008). The resulting intensity weighted

mean temperatures ranged from 181 to 196K. Thesetemperatures are significantly higher than those expectedfrom the Venus International Reference Atmosphere(VIRA, Seiff et al., 1985). We also find that thetemperature falls at low O2 emission rates to values nearerthe VIRA temperature. The results were interpreted asproviding evidence for compressional heating of down-welling gas near the antisolar point, with brightest emissionand highest temperatures associated with the strongestdownflow.High temperatures at these altitudes have also been

reported from stellar occultation observations made withSPICAV on Venus Express (Bertaux et al., 2007). Baileyet al. (2008) concluded that the O2 and SPICAVtemperatures were reasonably consistent. However theobservations were not simultaneous. The new observations

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Fig. 5. Image of the Venus nightside on 2007 July 22 in the 1:58mm O2

(0–1) airglow band (left) and in the 1:27mm (0–0) band (right). Although

the image in the weak 1:58mm band is very noisy it shows spatial structure

near the limb similar to that seen in the brightest part of the 1:27mmemission. Horizontal streaks on the right edge of the left image are

artefacts of the background subtraction process.

J. Bailey et al. / Planetary and Space Science 56 (2008) 1385–13901388

in July 2007 will provide the opportunity to compare O2

temperature measurements to Venus Express observationsat closely similar times.

We have used the techniques described by Bailey et al.(2008) to obtain a temperature map from the IRIS2 J-banddata obtained on July 26 2007. The temperatures rangefrom 171 to 225K, with an intensity weighted mean of191K. The temperatures are similar to those seen in theearlier IRIS2 data from 2004 and 2005. They support thesuggestion that compressional heating of downwelling gaswarms the region where the airglow emission occurs(Bailey et al., 2008).

3.2. Detection of the 1:58mm (0–1) O2 airglow band

Observations with VIRTIS have also been used to detectthe much weaker O2 airglow band at 1:58mm (Zasovaet al., 2007) which is the a1Dg � X 3S�g ðn ¼ 021Þ band. The1:27mm band is the n ¼ 020 vibrational transition for thesame electronic levels.

We have found that this band is also detected in ourIRIS2 H-band data sets. We have extracted the spectrum ofthe Venus nightside from our July 22, 2007 data cube, for arectangular region of 18 by 5 arc seconds near the antisolarlimb, with the short axis of the rectangle along the solarvector, and the centre close to the antisolar point. Thisrectangle covers the region where the 1:27mm airglow isbright in the corresponding J-band data cube. The resultingspectrum is shown in Fig. 4. While the main feature is thestrong thermal window seen at 1:721:75mm, an emissionfeature at 1:58mm corresponding to the Q-branch of the1:58mm airglow feature is also clearly visible.

Fig. 5 shows an image extracted from this data cube inthe 1:58mm feature compared with the 1:27mm image from

106

Inte

nstiy

(ra

w d

etec

tor

units

)

5×105

0

1.55 1.6 1.65 1.7 1.75

Wavelength (�m)

O2 (0–1)

Fig. 4. Spectrum of Venus in the H band on 2007 July 22 extracted from a

rectangular region of 18� 5 arc sec near the antisolar limb. The spectrum

shows the 1:74mm thermal window on the right and the O2 (0–1) airglow

band at 1:58mm on the left.

the J-band data cube on the same night. While the 1:58mmimage is very noisy due to the weakness of the band, itshows a spatial distribution which appears to be the sameas that of the brightest part of the 1:27 mm.We have looked back at earlier IRIS2 data sets taken in

July 2004 and December 2005 and we find the 1:58mm O2

airglow is detected in all of these data sets and alwaysshows the same spatial structure as that seen in the 1:27mmband.

3.3. Observations of the deep atmosphere and surface

Meadows and Crisp (1996) showed that observations inthe 1:18mm and shorter thermal windows detect the surfaceof Venus. Images in these windows, after correctionfor cloud effects, can therefore show the topography ofVenus, since higher altitude features are cooler. Thistechnique is being used by VIRTIS on Venus Express toobserve the surface of Venus and map the surfaceemissivity (Helbert et al., 2006).The IRIS2 J-band observations cover the 1:18mm

thermal window and can therefore also be used to imagesurface topography. Fig. 6 shows surface images obtainedfrom the J-band data on July 26, 2007 by two differentmethods.The left-hand panel of Fig. 6 is derived from an image

of the thermal emission in the 1:18mm window. A direct

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Fig. 6. Images of Venus topography in the 1:18mm thermal window from

the IRIS2 data on 2007 July 26. Left—a thermal image in the 1:18mmwindow, corrected for cloud opacity effects using an image at 1:28mm.

Right—image using the CO2 absorbed edge of the 1:18mm window for

cloud correction. Both images show Venus surface topography, with the

most prominent feature being the dark spot of Beta Regio above centre.

The Phoebe Regio region is also visible just below centre.

J. Bailey et al. / Planetary and Space Science 56 (2008) 1385–1390 1389

image in this window shows structure both due to surfacetopography, and due to cloud opacity effects. To obtain asurface image the data must be corrected for the cloudopacity effects and this can be done by dividing by animage in one of the longer wavelength windows at 1.28 or1:31mm, since these windows are not sensitive to thesurface and show only the cloud effects. In this case wehave used the long wavelength end of the 1:28mm window,as this gives better signal-to-noise than the 1:31mmwindow. The 1:28mm window, does however, containsignificant O2 airglow emission leading to artefacts in thecorrected image near the limb. The resulting image showstopographic features, the most prominent being the high-land region Beta Regio.

The right-hand panel shows an alternative way of cloudcorrecting the topographic image. This image is obtainedby dividing an image over the wavelength range1:1821:184mm (in the centre of the window) by one forthe wavelength range 1:189521:1935mm (on the CO2

absorbed edge of the window). The latter image, becauseof the substantially increased CO2 absorption includesmore emission from the atmosphere and less from thesurface. Division of the two images therefore removes thecloud opacity effects and leaves topographic structure. Thisapproach has the advantage, compared with the use of the1:28mm window, that there is no airglow contamination,but the signal is much stronger than that in the weak1:31mm window.

While the topography of Venus is, of course, now welldetermined from radar observations, such observationscould be used to search for variations in surface emissivity,as is being done with VIRTIS data, and to search forvolcanic plumes or dust clouds in the lower atmosphere.Observations of this type can also be used to study thecomposition of the deep atmosphere by comparing theabsorption spectra over topographies of different heightas was demonstrated by Meadows and Crisp (1996).

The 1:18mm window contains H2O absorption featureswhich are well resolved with IRIS2. The combination ofthe IRIS2 data sets and new and more complete lists of hotH2O absorption lines (e.g. Barber et al., 2006) shouldenable an improved determination of the H2O content anddistribution in the near surface region.

4. Conclusions

We have obtained near infrared spectra of the Venusnightside using the IRIS2 instrument on the 3.9m Anglo-Australian Telescope. The spectra cover the full disk ofVenus at a resolving power of �2400 over three wavelengthbands from 1.09 to 2:4mm.We have used the spectra on 2007 July 26 to derive a

rotational temperature map for the Venus mesosphere atan altitude of about 95 km. The resulting mean tempera-ture of 191K is consistent with earlier IRIS2 data (Baileyet al., 2008) and significantly higher than the temperatureat this altitude expected from the VIRA profile. This resultsupports the suggestion that there is significant adiabaticheating of downwelling gas in the regions around theantisolar point where the airglow is seen. We have alsodetected the n ¼ 021 O2 airglow band at 1:58mm. Theemission in this band has the same spatial distribution asthe much stronger 1:27mm n ¼ 020 band.The data in the 1:18mm thermal window have been used

to obtain images of the Venus surface topography in twodifferent ways. Thermal images in the 1:18mm windowafter correction for cloud opacity effects, show surfacetopography features such as Beta Regio and Phoebe Regio.The spectra in the 1:18mm window can be used to study theH2O content of the lower atmosphere.

Acknowledgements

This work was supported by the NASA AstrobiologyInstitute’s Virtual Planetary Laboratory Lead Team,funded by the National Aeronautics and Space Adminis-tration through the NASA Astrobiology Institute underCooperative Agreement no. CAN-00-OSS-01. We thankthe AAO staff for support of the observations at theAnglo-Australian Telescope.

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