Water Vapour Abundance and Distribution in the Lower Atmosphere of Venus Sarah Chamberlain – CAAUL / Lisbon Observatory, Portugal. Jeremy Bailey – University of New South Wales, Australia. Vikki Meadows – University of Washington, U.S.A. David Crisp – Jet Propulsion Laboratory, U.S.A. IRIS2, Anglo- Australian Telescope 4 th PHC/Sakura France-Japan Workshop / 3 rd Europlanet Strategic Workshop / 5 th – 8 th March

Venus Near Infrared Wavelengths Venus Crescent λ Alt. 2.3µm – 35km 1.74µm – 24km 1.18µm – 15km

H2O absorptionCO2 absorption

Chemical models suggest the lower atmosphere should have a constant water vapour abundance and even distribution.

(Meadows and Crisp, 1996) AAT/IRIS (This Study) AAT/IRIS2 (De Berg et al, 1995) CFHT/FTS VEX: VIRTIS M VEX: VIRTIS H VEX: SPICAV 2500

The Observations Venus : Near infrared wavelengths Venus : Optical wavelengths –Anglo-Australian Telescope /IRIS2 instrument

Thermal image O 2 Airglow Full-disk spatially resolved spectra - R ~ 2500

Removing the correct terrestrial water vapour contribution Is complex: Standard star observations are usually obtained At a different time, airmass and through a different path Length. Solar spectrum Venus

1: Model the transmittance spectra for the earth 2: Multiply the modelled transmittance spectra to modelled Venus spectra for various Venus water vapour abundances. 3: Find the best fit modelled spectra for each extracted observed spectra from various locations. THE Model

Model sensitivity with altitude for the 1.18 micron window Note for later: that this region is insensitive to water vapour close to the Venus surface

Model Parameters that DO NOT affect the 1.18 µm window shape and therefore the contrast of the water vapour absorption peaks. - Emission angle Zenith angle - Emissivity/Albedo - Lapse Rate

Emission Angle

EmissivityLapse Rate

- Abundance gradient (at sensitive altitudes) - Line list completeness - CO 2 Line shape (at high temperatures and pressures) Model Parameters that DO affect the 1.18 µm window shape and therefore the contrast of the water vapour absorption peaks

Line shape Figure from Meadows and Crisp, 1996

Where and is a modification of that of Perrin and Hartmann (1989) as determined by Meadows and Crisp (1996). The values of, and are given in the table below. Coefficients for the CO 2 χ factor CO 2 Line shape

Venus spectra were then extracted from across the disk of Venus and RMS fitted against various water vapour abundances and for different spectral regions of the 1.18 µm window. The Results Have been checked for spatial variations and for the best fit water vapour abundance.

Match ed to F3 Matched to F1 ~ µmMatched to F2 ~ 1.178µm Matched to F3 ~ µmMatched to – µm Best fit water vapour abundance against x position

Best fit water vapour abundance against y position Matched to F1 ~ µmMatched to F2 ~ µm Matched to F3~ µmMatched to – µm

Matched to F1 ~ µm Matched to F2 ~ µm Matched to F3 ~ µmMatched to µm Vatriations in Water Vapour with altitude

Matched to F3~ µm Matched to F1~ µmMatched to F2~ µm Matched to – µm

f1 f2 f3 Observed Spectra

Higher spectral resolutions will obtain a smaller spread of results.

Observed Spectra multiple unresolved absorption peaks f1

D B A C E F Position F (0 km) / Position A (4km) 4km altitude 0km altitude Water Vapour absorption from the lowest 4 km of the Venus atmosphere is observed in the gradient and individual features. Water vapour in the Lowest 4 km

Conclusions - 32ppmv water vapour at around 16km altitudes This result agrees with previous studies. - Uncertainties connected to : The far wing absorption / continuum The completeness of the CO 2 line list - Higher spectral resolution observations would aid this study by better defining the absorption peak shape and also resolving the multiple bands that contribute to some of the absorption peaks. -There is a possibility that near surface (0 – 4 km) water vapour abundances can be determined from remote observations.

Installation of the Venus GCM at the Lisbon Observatory David Luz CAAUL/Obs. Astronomico de Lisboa Sarah Chamberlain CAAUL/Obs. Astronomico de Lisboa Sebastien Lebonnois Lab. de Meteorologie Dynamique /Lawrence Livermore National Lab.

Venus General Circulation Model (0 – 100km altitude) The dynamical core of the GCM is based on the LMDZ Earth model developed at the Laboratoire de meteorologie Dynamique. (Hourdin et al., 2006) Key Features: - Topography - Diurnal cycle - Dependence of the specific heat on temperature - Consistent radiative transfer module based on net exchange rate matrices (consistent computation of the temperature field as opposed to simple temperature forcing). Consistent with observations: - Superrotation above roughly 40km with comparatively small winds beneath - Meridional circulation consists of equator to pole cells - Temperature structure is globally consistent (with discrepancies in the stability of the lowest layers and equator to pole temperature contrasts within the clouds) - Convective layers at the base of the clouds and the middle of the clouds

Figure from lebonnois et al., 2010

Intended use of the Model: Angular Momentum Budget with respect to circulation components. Mean meridional circulation Transient waves Polar Regions: The dynamical behaviour at polar regions The solar tides in the polar regions The rotational and thermal properties near the poles (the presence of the dipole). S-shaped pattern of the southern polar vortex (polar dipole). The centroid is shown to be displaced by 3 degrees from the geographic south pole. (VIRTIS - 5 µm radiance map from orbit 38.) Luz et al., 2011

Current Status: Sebastien Lebonnois has provided us with two models: - A reference model that has been run for 250 Venus days where the atmosphere was started with the superrotation fully developed. - A second model that has been run for 1050 Venus days from an atmosphere initially at rest. We are currently working on stabilising a zoomed version of the reference GCM that is focussed on the polar regions. Image shows the Zonal flow as Produced by the Reference model After 254 Venus days.