Possible Detection of Ground Fog on Titan

On 25 December 2004, the Huygens probe was released from the Cassini spacecraft. The probe descended through the atmosphere of Titan and landed on the surface of Titan on 14 January 2005. The probe's descent through Titan's atmosphere took 2.5 hours, and the probe continued to take measurements and relay data for an hour after landing. Smith et al. (2016) present results from the analysis of data taken with the Descent Imager/Spectrometer Radiometer (DISR) on the Huygens probe.

The analysis involved 82 images taken with the Side Looking Imager (SLI), a sub-instrument on the DISR. These images were all taken while the Huygens probe was on the surface of Titan. Out of the 82 images, 6 images appear to show an extended, horizontal feature above the horizon that seems to rise and fall over the course of the observations. The most likely interpretation is that this feature is a fog bank near the horizon that rose and fell over a span of time.

Other than a fog bank near the horizon, a number of other explanations might potentially account for the observed feature. However, none of these alternative explanations appear likely. One alternative is that the observed feature is a low-level cloud. This is unlikely because the shape and position of the feature did not appear to change much throughout the observation duration of over 20 minutes. For comparison, clouds are known to change shape and move on time spans of less than a minute. Nevertheless, the possibility that the observed feature is a low-level cloud cannot be ruled out entirely.

Another explanation is that the observed feature is a mirage. A superior mirage occurs when the image appears above the horizon because the ground is colder than the surrounding air, while an inferior mirage occurs when the image appears below the horizon because the ground is warmer than the surrounding air. If the observed feature is a mirage, it would be a superior mirage as it is above the horizon. However, as the Huygens probe descended though Titan's atmosphere, its measurements indicate increasing temperature with decreasing altitude, and that the air above ground is cooler than the ground itself. As a result, a mirage is an unlikely explanation for the observed feature.

Reference:
Smith et al. (2016), "Possible ground fog detection from SLI imagery of Titan", arXiv:1603.04413 [astro-ph.EP]