Venus express observations 2006–2011

Venus express observations 2006–2011

Planetary and Space Science 113-114 (2015) 219–225 Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsev...

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Planetary and Space Science 113-114 (2015) 219–225

Contents lists available at ScienceDirect

Planetary and Space Science journal homepage: www.elsevier.com/locate/pss

Water vapor near Venus cloud tops from VIRTIS-H/Venus express observations 2006–2011 V. Cottini a,b,c,n, N.I. Ignatiev d,e, G. Piccioni c, P. Drossart f a

University of Maryland, College Park, MD, USA NASA Goddard Space Flight Center, Building 34, Room s121, Code 693, 8800 Greenbelt Road, Greenbelt, MD 20771, USA c Istituto di Astrofisica e Planetologia Spaziali (INAF IAPS), Rome, Italy d Space Research Institute or Russian Academy of Sciences (IKI RAN), Moscow, Russia e Moscow Institute of Physics and Technology, Dolgoprudny, Russia f LESIA, Observatoire de Paris, Meudon, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 14 March 2014 Received in revised form 24 February 2015 Accepted 12 March 2015 Available online 21 March 2015

This work aims to give a summary of the water vapor at the cloud top of Venus atmosphere using the complete set of observations made using high spectral resolution channel (-H) of Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), on board the ESA Venus Express orbiter, to measure the cloud top altitude and the water vapor abundance near this level. An initial analysis of these measurements by Cottini et al. (2012) was limited to data in 140 orbits in the period 2007–2008. These observations were limited to the Northern hemisphere due to observational geometry in this early part of the mission. In the present paper, the analysis is extended to a larger dataset covering the years 2006–2011, significantly improving the latitudinal coverage. Altitude of the cloud tops, corresponding to unit optical depth at a wavelength of 2.5 mm, is equal to 69 71 km at low latitudes, and decreases toward the pole to 62–64 km. The water vapor abundance is equal to 3 7 1 ppm in low latitudes and it increases reaching a maximum of 5 72 ppm at 70–80° of latitude in both hemispheres, with a sharp drop in the polar regions. This can be explained by the specific dynamics of the atmosphere of Venus affecting the distribution of water vapor such as the transfer of water vapor in the Hadley cell and the dynamic in the polar vortex. The average height of the cloud tops and the H2O near this level are symmetric with respect to the equator. As a function of local solar time, the water vapor shows no particular dependence, and the cloud tops exhibit just a weak maximum around noon. Over 5 years of observations the average values of the cloud top altitude and the water vapor were quite stable in low and middle latitudes, while in high latitudes both quantities in 2009–2011 years are systematically higher than in 2006–2008. Short period variations increasing with latitude are observed, from approximately less than 7 1 km for cloud tops and 7 1 ppm for water vapor in low latitudes to, respectively, 72 km and 7 2 ppm in high latitudes. As a rule there is no correlation between variations of the cloud top altitude, the water vapor content, and the UV brightness. However, numerous examples can be found when UV dark features, with a characteristic size of a few degrees of latitude (several hundred kilometers), coincide with regions of higher cloud tops. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Venus Atmosphere Water vapor Clouds

1. Introduction Water vapor plays an important role in the formation of clouds, in the chemistry and in the thermal balance of the atmosphere of Venus, which makes it one of the main objects of remote sensing measurement methods. The sulfuric acid clouds (aqueous solution of sulfuric acid – H2SO4, e.g. Esposito et al., 1997) surround Venus n Corresponding author at: Building 34, Room s121, Code 693, 8800 Greenbelt Road, Greenbelt, MD 20771, USA. Tel.: þ1 301 286 7932. E-mail address: [email protected] (V. Cottini).

http://dx.doi.org/10.1016/j.pss.2015.03.012 0032-0633/& 2015 Elsevier Ltd. All rights reserved.

completely. They are divided into upper, middle and lower clouds (e.g. Esposito et al., 1983) and have played a key role in the evolution of the planet and its atmosphere. The main cloud deck of Venus spans from about 46 km of altitude (lower cloud) to an upper boundary layer situated around 65 km, with haze layers expanding down to 30 km – while the first 30 km of atmosphere are composed of clear CO2 air – and upwards for a further 10 km (e.g. Esposito et al., 1983). The sulfuric acid in the clouds is produced through photochemical reaction (e.g. Yung and DeMore, 1982; Krasnopolsky and Parshev, 1981) of H2O and SO3 near the cloud tops, at altitudes of

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60 km or higher (upper clouds), where SO2 is oxidized to SO3. Oxygen is made available through photodissociation of carbon dioxide into carbon monoxide and atomic oxygen. In the middle clouds (about 50–57 km) the H2SO4 vapor nucleates then to cloud particles and a downward diffusion and sedimentation of H2SO4–H2O droplets occurs (e.g. Krasnopolsky and Pollack, 1994). At the bottom of the clouds H2SO4 evaporates due to higher atmospheric temperature (e.g. Ragent and Blamont, 1980) and diffuses downward. The vapor then incurs on thermal decomposition into SO2 and H2O at about 38 km of altitude; vertical mixing from regions below the main cloud layer diffuses upward sulfur dioxide and water in the clouds and up to the cloud top, enriching the clouds of these gases and giving origin to a convective cycle (e.g. Krasnopolsky and Pollack, 1994). Therefore the region of the cloud top is a thin, but highly active photochemical layer where the main production of the sulfuric acid droplets from SO2 and H2O occurs; the determination of their abundances and variability over space and time play a key role in understanding clouds properties and dynamics. Numerous observations have demonstrated high variability of water vapor, from fractions to tens and even hundreds of ppm, which have been attributed to real variability, model errors, and different effective altitude ranges of sounding. Mesospheric water vapor abundance has been previously measured with different techniques and at various wavelengths from microwaves to visible. A brief review and references of previous results can be found in Cottini et al. (2012). See also further reviews in Koukouli et al. (2005), Sandor and Clancy (2005), Gurwell et al. (2007), and Krasnopolsky (2010a). More recent ground based spectroscopic measurements of water vapor near the cloud top (Krasnopolsky, 2010b; Krasnoplosky et al., 2013) and measurements from the instrument on the Venus Express spacecraft-VIRTIS (Cottini et al., 2012) and SPICAV (Fedorova et al., 2014)-are generally well consistent with each other but still demonstrate some noteworthy differences. In particular, temporal variability and absolute values of H2O abundance measured from different spectral ranges deserve to be better understood and will be discussed in the Section 3 of this paper. An extensive study of water vapor, its variability and possible correlation with the clouds density and altitude, obtained with measurements acquired by VIRTIS-H over time and space, is hence of particular importance.

from 0.3 to 1 mm made with the moderate spectral resolution mapping channel (-M) of the VIRTIS instrument have been recently calibrated (version 2.0 of the VIRTIS calibrated dataset, released in 2013 on ESA's Planetary Science Archive) and can be used to investigate a possible correlation of the clouds and water vapor with the UV absorber. Due to the polar orbit of the Venus Express satellite (details can be found in Svedhem et al., 2009) a typical track of the VIRTIS-H field of view footprint on the cloud surface during one measurement session (orbit) extends along a meridian from one pole to another or covers just a limited latitude range. This geometry enables building of latitudinal profiles of measured values for different local solar time (Fig. 2). The Venus Express orbit is highly elliptic and the field of view footprint size and the distance between measurements depend on the distance to the planet and the observation angle. Accordingly, the spatial resolution is changing from several kilometers to more than 100 km. First VIRTIS-H observations of Venus, up to orbit 500, were not included in Cottini et al. (2012); they were initially unavailable because of a challenging calibration due to a contamination of the dark signal (used in the computation of the instrument responsivity)

2. Observations and method

Fig. 1. VIRTIS-H day-side part of the spectrum including absorption bands of CO2 and H2O between 2.48 and 2.60 mm that are used to determine the cloud top altitude and water vapor abundance near this level in the measured (blue) and calculated (red) spectra, orbit 1785, order of diffraction 5. Spectral channels used for the retrievals are marked with þ . Dashed lines represent the continuum.

VIRTIS, the Visible and Infrared Thermal Imaging Spectrometer (Drossart et al., 2007) on board the Venus Express mission started to observe Venus in 2006 (Svedhem et al., 2009). It covers a spectral range from UV to thermal IR: 0.3–5 μm. To measure the content of water vapor near the cloud top and the relative altitude we use its VIRTIS high-resolution subsystem (-H), an echelle grating spectrometer with eight diffraction orders focused on a 270  438 pixel array detector (CCD). These eight partially overlapped spectra with variable spectral resolution of 1–3 nm are added to form a VIRTIS-H spectrum, which covers a spectral range from 2 to 5 mm. The day-side spectrum of solar radiation scattered and reflected by the atmosphere of Venus contains absorption bands of atmospheric gases. Their depths depend on the cloud layer altitude and the gas abundances. In particular, the absorption band of CO2 and H2O between 2.48 and 2.60 mm in the VIRTIS-H day side spectrum (Fig. 1) are used to determine the cloud top altitude (Ignatiev et al., 2009; Cottini et al., 2012), defined as the altitude of unity optical depth in the considered spectral range, and water vapor abundance near this level (Cottini et al., 2012). Simultaneous measurements of the Venus spectrum in the range

Fig. 2. Coverage in latitude and local time of Venus' VIRTIS-H spectra used to measure the water vapor content: 250 orbits between Venus Express orbit 42 and 1871 acquired during the years 2006–2011, for a total of about 90,000 spectra. Color coding corresponds to the orbit numbers given on the right scale.

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with some part of Venus' radiation. In later observations the problem was solved (P. Drossart, G. Piccioni et al., personal communication) and the initial data have been successively partly corrected and we are able to include most of them in this work. Nevertheless this had to be done carefully, including only spectra that were checked attentively since the correction in some cases may not be optimal due to possible fluctuation of the average flux after the subtraction of some residual Venus contaminated dark radiation. The new measurements, acquired during the years 2009–2011 are the result of a different approach to the planning of observations of the Venus Express spacecraft. In 2009 the infrared channel VIRTIS-M stopped working, and this event was followed by a decreased frequency in VIRTIS-H measurements in order to extend the lifetime of the instrument over as long a period as possible. The geometry of the observation became different from the measurements acquired during 2006–2008, which were mainly concentrated in the northern hemisphere and in the regions close to the sub-solar point. The observations of 2009–2011, despite being fewer in number (only 38 orbits) – with acquisitions every 10–20 days – are in fact characterized by a more complete latitudinal coverage including both hemispheres. Also, local times of the most recent observations complement the ones of previous data focusing more on local times outside the already wellobserved sub-solar point. A comparison between Fig. 2 and Figure 1 of Cottini et al. (2012) clearly shows the larger number of data available for this work and the wider latitudinal and temporal coverage. In total, 250 orbits from 2006 to 2011 (orbit numbers 42–1871) were processed to derive the water vapor abundance at the cloud tops of Venus and its variations in space and time and the results are presented in this paper. We retrieve cloud top altitude and water vapor abundance from VIRTIS-H spectra by applying radiative transfer calculations to each measurement taking into account the illumination and the geometry of observation; we also include multiple scattering and accurate lineby-line treatment of gaseous absorption. We iteratively compare equivalent widths of respectively the CO2 and H2O bands to the ones of the observed spectra to first retrieve the cloud top altitude and then use this derived value to evaluate the correct atmospheric path of the radiation and finally retrieve the water vapor abundance. For a more detailed description of the VIRTIS-H data, adopted atmospheric model, method of analysis, source of errors and previous results we refer to Cottini et al. (2012).

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3. Results and discussions The analysis of the 2007–2008 measurements (140 orbits in the interval 563–922) led to the conclusions set out in Cottini et al. (2012) and reported below. At low latitudes (7 40°) the average content of water vapor is 3 71 ppm near the top of the cloud at a height of 69.5 72 km, which corresponds to the level of unit optical depth for a wavelength λ ¼2.5 mm. For higher latitudes cloud-top height slowly decreases poleward, reaching a minimum value of about 64 72 km. The average content of water vapor reaches instead a maximum mean value of 57 2 ppm at latitude of 80° with a large scatter in the measurements from 1 to 15 ppm.

Fig. 4. Daily variations in the altitude of the cloud top height for 3 latitude intervals. Average values for 1 h latitude bins are shown with circles and the error bars are equal to the standard deviations of the measurements.

Fig. 3. Cloud tops altitude retrievals (upper panel) used to measure the water vapor content (lower panel) as a function of latitude for all VIRTIS-H observations, acquired during years 2006–2011. Color coding is the same as in Fig. 2. Average values for 10° latitude bins are shown with circles and the error bars are equal to the standard deviations of the measurements.

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Fig. 5. Daily variations of the water vapor abundance for 3 latitude intervals. Black circles and error bars have the same meaning as in Fig. 4.

The mass concentration of sulfuric acid solution in cloud droplets calculated for mode 2 particles (radius E1 μm) and under the assumption of equilibrium between water vapor and cloud, is in the percentage range of 75–83% and in an even narrower range of 80–83% for lower latitudes. The new measurements extend to all latitudes and generally confirm these findings (Figs. 3 and 4). The mean values of the height of the cloud tops and the water vapor content demonstrate a good symmetry with respect to the equator (Fig. 3a and b). A slight apparent asymmetry may be caused by different observing geometries prevailing in the northern and southern hemispheres: in 2006–2008 (up to orbit 922) there were many orbits with inertial orientation of the spacecraft with low observation zenith angles at high latitudes of the northern hemisphere and large ones at low latitudes; in 2009–2011 both hemispheres were observed with nearnadir zenith angles. In fact, although the zenith and azimuth angles of the sun and the observer are taken into account in our radiative transfer computations, limitations of the two-stream plane parallel model with assumed constant water vapor volume mixing ratio vertical profile can cause some small dependence of the results on the geometry of observations. Observations with the incident and emission zenith angles higher than 85° were excluded from our analysis. An absence of any local time dependence of the cloud top altitude and the water vapor abundance (discussed below) in early morning and late evening may be an additional argument in favor of a sufficient precision of the model in the considered interval of zenith angles. We find the same poleward decrease of the cloud-top altitude from an average value of about 69 km at the equator to 64 km in the polar regions, and single values in the range 62–71 km. A very similar latitudinal behavior was found in other similar measurements on Venus Express: by Ignatiev et al. (2009) from VIRTIS-M observations of the CO2 band at 1.6 mm and Fedorova et al. (2014)

Fig. 6. Sensitivity of the radiance to changes of water vapor content as observed in the 2.5 mm (left) and 3.7 mm (right) bands. They are calculated for an atmospheric model which includes a single cloud optical depth at the height of 68 km for a wavelength of 2.5 mm. The colors show the sensitivity function in the various wavelengths within these bands. We can see that different spectral ranges are sensitive to slightly different altitudes in the atmosphere.

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from SPICAV observations at 1.48 mm. Absolute values of the cloud top altitudes reported by Fedorova et al. (2014) are 1–2 km higher, which is naturally explained by the higher extinction of the sulfuric acid clouds at 1.48 mm than that at 2.5 mm. Systematic difference with the cloud top altitudes reported by Ignatiev et al. (2009) are discussed in details in Cottini et al. (2012) and attributed to imperfect quality of the VIRTIS-M spectrum fitting. At longer wavelengths in the thermal IR spectral range at 4.5 mm (Lee et al., 2012; Haus et al., 2013) and 8 mm (e.g. Zasova et al., 2007) cloud tops are located deeper by several kilometers but demonstrate a similar latitudinal trend. The retrieved water vapor mixing ratios at the cloud-top vary for single retrievals in the range 0.5–11.5 and show an average value of about 37 1 ppm at the equatorial regions and increase at higher latitudes reaching its maximum of about 5 72 ppm around

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the latitude of 75° in both hemispheres. Here the error bars mean the root-mean-square deviation of the measured values. Individual measurements at 75° sometimes exceed 10 ppm. At higher latitudes, the water vapor content decreases again toward the poles. More detailed discussion of the results and implications for the cloud particle composition and dynamics can be found in Cottini et al. (2012); we avoid repeating them here and concentrate on the improvements obtained due to much better temporal and latitudinal coverage of the full analyzed dataset. Despite some local variations of the order of a few hundred meters in cloud top altitude and tens of percent in water vapor abundance, average values calculated over the 5 year time span are rather stable and do not show any considerable diurnal variations. To get rid of the influence of the latitudinal trends, we look for the local time dependence of the cloud tops and water vapor

Fig. 7. Cloud top level (left) and water vapor content (right) as a function of time (orbit number) for the same latitude bins as in Figs. 4 and 5. Error bars are equal to the standard deviations drawn around orbit averages.

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abundance separately for three latitude regions: from  30° to 30°, from 30° to 60°, and from 60° to 90° (Figs. 4 and 5). Southern hemisphere has relatively poor coverage (see Fig. 2) and is excluded from these figures. At low and middle latitudes we can notice a weak maximum of the cloud tops near noon, which has anyway no counterpart in the water vapor abundance. At high latitudes the results are highly scattered due to both higher errors and actual variability without any systematic dependence on the local time. Ground-based observations on the HDO band at 3.7 mm, in contrast, show a marked diurnal variation with the water vapor content, which in the morning is more than a factor of 2 higher than in the evening (Krasnopolsky, 2010b). Krasnopolsky (2010b) explained this variation by a photochemical production of sulfuric acid with the consumption of the water vapor during the day; at night this process is terminated, the vapor is brought up from the lower turbulent diffusion layers and its concentration increases. The discrepancy between the two measurements could be explained by the difference in the levels altitudes probed for the two studies (Krasnopolsky, 2010a). Sensitivity functions of the spectrum to the water vapor demonstrate that the effective height sensed at wavelength 3.7 mm is indeed several kilometers higher than this value at 2.5 mm (Fig. 6). Water vapor abundance at altitudes higher than 70 km may be subjected to daily variations related to photochemical processes, while below this level it is more stable. Recent SPICAV measurements at 1.38 mm (Fedorova et al., 2014) show a prominent maximum of the water vapor abundance at low

latitudes attributed to possible injection of the humid air from lower layers. Our average latitudinal profile does not reveal any similar maximum, although it may be seen in some individual measurements (Fig. 3b). These difference can be again explained by different sensitivity ranges: at 1.38 mm the sensitivity interval is equal to 58–66 km (Fedorova et al., 2014), that is lower and wider than 65–70 km at 2.5 mm. We also analyzed the water content at the cloud top over the time spam of the VIRTIS observations of Venus (Fig. 7, same latitude bins as in Figs. 4 and 5). Both the cloud top altitude and the water vapor content are on average quite stable at low latitudes (from  30° to 30°) over the whole period of observations. At higher latitudes variability is definitely much higher, and the water vapor abundance seems to be systematically higher in 2009–2011 than in 2006–2008. Higher southern hemisphere latitudes are not illustrated in Fig. 7 since there are less available data in each latitude bin for good time coverage. SPICAV measurements in the same period demonstrate similar temporal variations (Fedorova et al., 2014). Marcq et al. (2013) observed a maximum of SO2 abundance at the cloud tops in 2007 at low latitudes and a gradual decrease during next years. A similar behavior was observed in the 1980s. These observed variations indicate a temporal variation of the sources and sinks for SO2. Some preliminary modeling by Marcq et al. (2013) suggests that during SO2-rich periods, advection of SO2 originating from the deep atmosphere counter-balances photochemical destruction of SO2, at least in the lower latitudes

Fig. 8. Cloud top altitude (red), H2O abundance (blue) and UV (average 0.375–0.385 mm) brightness (magenta) latitudinal profiles as observed in simultaneous VIRTIS-H and VIRTIS-M measurements. UV dark features often coincide with regions of higher cloud tops, and bright features with lower clouds (some features are indicated with arrows). Orbit and VIRTIS observation session numbers are given above each panel. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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where upwelling is commonplace. We do not observe a similar behavior in water content at low latitudes where it remains constant at the average level of 3 ppm with variations of 71 ppm except for a few orbits with higher uncertainties. Variations at middle and high latitudes are lager but do not seem to correlate with those of the SO2 abundance. Finally, we used also the visible channel of the VIRTIS-M instrument to look for correlations between the UV absorber near the cloud tops of Venus (e.g. Esposito, 1980) and their altitude and water vapor content at the same level. In Cottini et al. (2012) we searched for correlations of our results and absorptions in the UV by looking at the Venus Monitoring Camera (Markiewicz et al., 2007) images at 0.365 mm. We now have available calibrated VIRTIS-M-VIS/UV data, with similar spatial resolution of VIRTIS-H and hence we looked for orbits with simultaneous VIRTIS-H and VIRTIS-M measurements. For the UV absorption we chose an interval between 0.375 and 0.385 mm. Some examples of such measurements are shown in Fig. 8. While we still do not see a systematic correlation between clouds, water vapor, and the UV absorption, we do observe some correlations between the UV absorption and the cloud altitudes. Dark UV features, with a characteristic size of a few degrees of latitude (i.e. several hundred kilometers), tend often to coincide with enhancements in cloud density (or, equivalently, with higher cloud tops) and bright features with less dense (or deeper) clouds.

4. Conclusions In this work we analyzed the complete dataset obtained by the VIRTIS-H spectrometer during the years 2006–2011. We confirmed the conclusions drawn by Cottini et al. (2012) from the analysis of the 2007–2008 measurements. Moreover, the increased coverage obtained with the most recent 2009–2011 observations enabled us to demonstrate hemispheric symmetry of water vapor and cloud tops altitude with respect to the equator. An average value of about 3 ppm is found for water vapor at the cloud-top at the equatorial regions, while we observe an increase at higher latitudes with a maximum average of about 5 ppm reached in both hemispheres around the latitude of 75°. At higher latitudes, the water vapor content decreases again toward the poles. We found no considerable diurnal variations of water vapor and cloud top altitude; long term stability is observed at least for low latitudes, with some increase of both quantities in 2009–2011 years at latitudes over 60°. A comparison of these results with UV spectra measured by the mapping channel of VIRTIS did not show a strict correlation between clouds, water vapor, and the UV absorption, even if dark UV features indeed often coincide with higher cloud tops and vise versa.

Acknowledgments Venus Express is a mission of the European Space Agency. We thank the Agenzia Spaziale Italiana (ASI) – grant ASI-INAF I/050/10/0 – and the Centre National d’Études Spatiales (CNES) for their support to the VIRTIS experiment. N. Ignatiev was supported by the Russian

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Foundation for Basic Research – Grant 12-02-01280-a, the Ministry of Education and Science of Russian Federation of Education – Grant 11.G34.31.0074, and the Presidium of Russian Academy of Sciences Program 22. We thank the editor Colin Wilson and the unknown referees for their suggestions for improving this paper.

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