1. Paulina Wolkenberg1, Marek Banaszkiewicz1
Vertical temperature profiles in the Venus mesophere obtained by two retrieval methods from the VIRTIS observations
Paulina Wolkenberg1, Marek Banaszkiewicz1
1Centrum Badań Kosmicznych PAN, ul. Bartycka 18A, Warsaw, Poland
ABSTRACT
Thermal atmospheric fields from around 60 km up to 100 km are possible to retrieve from the nighttime measurements of the VIRTIS (Visual Infrared Thermal Imaging Spectrometer) instrument aboard Venus Express spacecraft [1]. Our purpose is to show the differences and similarities between the vertical temperature profiles retrieved by Chahine’s method [2] adopted by [3] to the Venus atmosphere and by using the statistical approach [4]. The comparison of temperature profiles is going to investigate versus latitude, longitude and local time to present the possibilities of two methods in the study of atmospheric dynamics. The VIRTIS-M spectrometer measures radiation emerging from the atmosphere and provides spectra from the spectral range from 1000 nm to 5100 nm with the sampling step 11 nm. Only a couple of channels at 1.01, 1.10 and 1.18 micron can monitor the surface of Venus. Instantaneous field of view for the VIRTIS-M is 0.25x0.25mrad which corresponds to the horizontal resolution for individual pixels 16.5x16.5 km [1], [5]. The retrieval of venusian atmospheric temperature is based on the absorption band by CO2 at 4.3 micron. In order to model the measured spectra by the VIRTIS, the multiple scattering in the atmosphere is taken into account because of the planet’s cloudy atmosphere and its optical properties. The clouds on Venus are mainly composed of H2SO4 which has significant scattering properties (ssa >0.5). In our study the clouds consist of liquid droplets of a mixture composed of 75% of H2SO4 and 25% H2O with complex refractive indices given by [6]. They can occur between 50 km and 65 km and affect greatly the retrieval of temperature profiles at these altitudes. The temperature profiles are obtained from the measurements over the southern polar vortex using two retrieval methods. Both approaches are the iterative process. The Chahine’s method involves the relaxation equation to derive the temperature profile for the next iteration [2]. This method was successful applied to the measurements from the Venera spacecraft [3], Galileo-NIMS spectra [7], VIRTIS measurements by [8]. The second method is based on the Bayesian approach described by [4] and applied by [9]. The theorem applies the probability density functions to define a function of the state vector and the measurement vector. However, the problem of retrieval on Venus is strongly non-linear, thus it is necessary to use the Gauss-Newton iteration and the Levenberg – Marquardt method.
Venusian southern polar vortex from the VIRTIS
Longitude:
Latitude:
Local time= hr
Emission Angle= deg
Longitude:
Latitude:
Local time= hr
Emission Angle= deg
(5E,64S)
Longitude:
Latitude:
Local time= hr
Emission Angle= deg
Longitude:
Latitude:
Local time= hr
Emission Angle= deg
Longitude:
Latitude:
Local time= hr
Emission Angle= deg
Longitude:
Latitude:
Local time= hr
Emission Angle= deg
Longitude:
Latitude:
Local time= hr
Emission Angle= deg
(8E,71S)
(33E,84S)
(104E,80S)
(116E,81S)
(357E,76S)
(359E,80S)
Conclusions
The vertical temperature profiles obtained for the measurements from the southern polar vortex are in good agreement above 65 km. Below this altitude, discrepancies around 5 K between the profiles are found. For two observations (8E,71S) and (5E, 64S) the temperatures differ about 10K below 65 km. We anticipate that the cloud layer at 50 km extending up to 65 km did not allow us to retrieve correct temperature profiles.
References
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[9] Grassi D. et al. (2014), The Venus nighttime atmosphere as observed by the VIRTIS-M instrument. Average fields from the complete infrared dataset. Journal of Geophys. Res. – Planets,in press.