Viking Lander 1 and 2 Revisited: the Characterisation and Detection of Martian Dust Devils

Introduction: During the summer of 1976 Viking Lander 1 and 2 touched down at Chryse Planitia and Utopia Planitia, Mars respectively. The primary aim of both Viking Landers was to look for the presence of organic life on the surface of Mars, however each Lander also had a comprehensive meteorological package to monitor the martian atmospheric conditions [1]. The meteorological package consisted of pressure, temperature and wind sensors, enabling the landers to perform the first detailed in-situ investigation of martian weather. Data was logged for the full mission at a variety of sampling rates. High data rates were used early on in the mission (one sample every 8 seconds), but this moved to a lower rate (one sample per minute) after sol 60 for Viking Lander 2. The meteorological results from the Viking Landers highlighted diurnal variations in pressure and temperature. The ambient wind speeds were generally below 10ms. The biggest pressure variation was between the two landing sites; explained by the difference in latitude and altitude. Apart from this variation the long-term pressure was relatively constant at approximately 6 mbar. Large temperature variations were seen to be commonplace on Mars, with a diurnal range of approximately 50K. This diurnal temperature variation is essentially a consequence of the low thermal inertia of the surface and the atmosphere. Dust devils form at the base of convective plumes. These convective plumes are created by surface heating from the Sun. Dust devils, when detected, will produce changes in all of the key meteorological parameters measured by the Viking Landers, namely pressure, temperature, and wind. These vortices may also leave tracks or marks on the surface where they have lofted the surface regolith; these were first seen and identified from Viking orbital photography [2]. The two Viking Lander meteorological instruments did indeed detect possible convective vortices, first reported by Ryan and Lucich, (1983), who gave an indication of annual occurrence statistics. Mars Pathfinder also detected 79 convective vortices inferred from pressure variations [3]. Dust devils have also been detected in some of the Mars Pathfinder wide angle camera images [4]. It is clear therefore that dust devils occur on Mars, and their relevance to the martian global dust cycle is currently a topic of scientific interest [4-6]. Vortex Detection: Convective vortices can be detected by a characteristic behaviour in their meteorological parameters. This signature or change in meteorological parameters could include a change in wind speed and direction, a rise in temperature, or a drop in pressure. The last two parameters will only be truly characteristic if the sensor suite encounters the vortex core. Modelling exists to approximate empirically the wind speed and direction changes characteristic of atmospheric vortices. Wind speed is modelled using the Rankine vortex method [7], where the tangential