Possible ground fog detection from SLI imagery of Titan

TL;DR

This study analyzes Huygens probe images to detect potential ground fog on Titan, finding subtle atmospheric features consistent with fog banks near the horizon, indicating surface-atmosphere interactions.

Contribution

First analysis of Huygens SLI images to identify surface-level atmospheric features, specifically fog, using calibrated image processing and statistical confidence methods.

Findings

Detected horizontal radiance features outside 95% confidence limits

Features suggest presence of near-horizon fog bank

Optical depth of features between 0.005 and 0.014

Abstract

Titan, with its thick, nitrogen-dominated atmosphere, has been seen from satellite and terrestrial observations to harbour methane clouds. To investigate whether atmospheric features such as clouds could also be visible from the surface of Titan, data taken with the Side Looking Imager (SLI) on-board the Huygens probe after landing have been analysed to identify any potential atmospheric features. In total, 82 SLI images were calibrated, processed and examined for features. The calibrated images show a smooth vertical radiance gradient across the images, with no other discernible features. After mean-frame subtraction, six images contained an extended, horizontal feature that had a radiance value that lay outside the 95% confidence limit of the predicted radiance when compared to regions higher and lower in the images. The change in optical depth of these features were found to be between 0.005 and 0.014. It is considered that these features most likely originate from the presence of a fog bank close to the horizon that rises and falls during the period of observation.