The jovian anticyclone BA III. Aerosol properties and color change

A study of the vertical cloud structure of oval BA and its red color change is presented in this third part of our complete analysis. A large interest in Jupiter's anticyclone BA was created by its reddening that occurred between 2005 and 2006. In this work we quantify the color change in oval BA by using images taken with the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope (HST) in six filters from the near ultraviolet (F250W) to the deep methane band in the near infrared (F892N). Reflectivity changes are noteworthy in nadir viewing geometry at the ultraviolet and blue wavelengths (F250W, F330W and F435W filters) but almost undetectable or inside error bars in the rest of filters (F550M, F658N and F892N). The observed reflectivity variations are discussed in terms of a commonly accepted vertical cloud structure model for jovian anticyclones in order to explore some causes for the color alteration. Our models of the observed reflectivity variation show that the vortex clouds did not change its vertical extension (top pressure) or its optical depth. We find that a change occurred in the absorbing properties of the particles populating the upper aerosols (single scattering albedo and imaginary refractive index). A discussion on the thermo-physical and dynamical properties of the vortex that could be in the origin of the color change is also presented. (C) 2009 Elsevier Inc. All rights reserved.