A long term study of Mars mesospheric clouds seen at twilight based on Mars Express VMC images

TL;DR

This long-term study analyzes twilight clouds on Mars using VMC images from 2007 to 2020, revealing mesospheric clouds predominantly in southern mid-latitudes during autumn and winter, suggesting unknown regional mechanisms.

Contribution

First systematic analysis of Martian twilight clouds using automated retrieval over 13 years, identifying mesospheric clouds and regional concentration patterns.

Findings

Many clouds are in the Martian mesosphere (40-90 km).

Clouds are concentrated in southern mid-latitudes during autumn and winter.

High altitude clouds are linked to a region with previously observed high-altitude events.

Abstract

We present the first systematic study of clouds observed during twilight on Mars. We analyze images obtained by the Visual Monitoring Camera (VMC) on Mars Express between 2007 and 2020. Using an automated retrieval algorithm we found 407 cases of clouds observed at twilight, in which the geometry of the observations allows to derive the minimum altitude, revealing that many of these clouds are in the mesosphere (above 40km and up to 90km). The majority of these mesospheric clouds were detected in mid-latitudes at local autumn and winter, a new trend only hinted at by previous studies. In particular, we find a massive concentration of clouds in the southern mid-latitudes between Terra Cimmeria and Aonia, a region where high altitude events have been previously observed. We propose that there is an unknown mechanism in these regions that enhances the probability to host high altitude clouds around the southern winter solstice.