A new approach for limb darkening correction on Venus nightside infrared images

We define limb darkening (LD) as the decrease of the radiance that emerges from a planetary atmosphere at increasing emergence angle (i.e. angle between direction of observation and surface normal). The effect is caused by the larger atmospheric column depth (i.e. emergence angle) crossed by radiation (coming from surface or lower atmosphere) observed at larger emergence angle. LD correction consists in retrieving radiance that would be observed at emergence angle equal to zero. In this work a LD correction is applied on Venus nightside infrared images. The reasons that led to choose this planet are the very important role played by its atmosphere (very dense and opaque) and the huge amount of new spectroscopic data furnished by the VIRTIS instrument, mounted on Venus Express spacecraft, that is currently orbiting the planet. Our approach on LD correction is based on statistics of observed data, instead of synthetic models, as in previous works. We obtained linear relations between radiances emerging from atmosphere at five different wavelengths (1.03 μm, 1.31 μm, 1.74 μm, 2.30 μm and 3.71 μm) and emergence angle. Our results agree with previous works. In addition, a link between the parameters of the linear relations and height where emission is originated from has been found. This link can be explained remembering that radiation coming from the upper atmospheric layers is less attenuated than radiation originated in the lower ones.