21 Oct 2021

The main goal of the field campaign in the Makgadikgadi Salts Pans was to study how dust was lifted into the air using the spare units of the Radiation and Dust Sensor (RDS) instrument from the NASA MArs 2020 mission and the Sun Irradiance Sensor (SIS) from the ExoMars 2022 mission (see Figure 2). These instruments are designed to study dust carried in the atmosphere of Mars using electromagnetic radiation in the visible part of the spectrum.

In addition to giving information (characterising) about the properties of airbourne dust, these instruments are also sensitive to the presence of dust devils, swirling columns of sand and dust that are a common feature of desert areas on Mars and on Earth. The changes over time in the sky brightness produced by a dust devil can be detected by RDS and SIS. This offers a unique opportunity for monitoring and studying such events during the Mars 2020 and ExoMars 2022 missions. However, to be able to characterise and interpret dust devils observed with these instruments on Mars, we first need to evaluate the effect of these events on measured SIS and RDS signals back on Earth. 

To achieve this goal, we planned a field campaign in the southern part of Makgadikgadi Salt Pans (see Figure 3) from 29 Sept to 6 Oct in the Pan near Mopipi town. This location is characterised by frequent dust devils events and conditions that promote the lifting of high levels of aerosols (dust and particles) into the atmosphere.

Each day of the campaign, RDS and SIS were set at two different locations from sunrise to sunset, separated by about 25 m, along with: i) 2 cameras to record panoramic videos during the campaign period; ii) a Vaisala weather station to perform measurements of pressure, wind direction and intensity, temperature and relative humidity; and iii) a ZEN radiometer to measure how much light was absorbed by the dust at different wavelengths. The objective of having the two main instruments at two different locations was observe the dust lifting events from different perspectives.

Dust devils observed in Makgadikgadi Salt Pans (left panel) and on Mars (right panel). A typical dust devil on Mars spans from hundreds of metres to thousands of metres in diameter, with a height 1-8 times as large. Dust devils of Mars are thought to account for the ~50% of the total dust budget, and they represent continuous source of lifted dust, active even outside the dust storms season. For these reasons, they have been proposed as the main mechanism able to sustain the constantly-observed dust haze in the martian atmosphere.

During the campaign, we observed a large number of dust devils (many more than 10) and dust lifting events produced by wind gusts (at least 10). For each dust lifting, we recorded the dust devil distance, the size, duration and direction. For doing this, concentric circles with radius of 25, 50, 75, 100, 125 and 150 m were made on the ground. This information along with the videos made by the cameras, helped us to established the amount of dust lifted by the dust devil as well as their distances to the instruments. All the data collected for each event was key to establish the RDS and SIS capabilities for dust lifting characterization on Mars.

The first two days of the campaign were characterised by high dust-loading conditions and frequent formations of dust lifting events produced by dust devils or wind gusts. During these two days, each dust lifting event registered by the cameras was also detected by RDS and SIS, with signals showing a sharp peak at the time when the event passed within the sensors field of view. Preliminary analysis of RDS and SIS signals suggests that we can infer the differnce between dust lifting events produced by a dust devil or by a wind gust – an important result for the observations on Mars.

The third day of campaign had to be cancelled due to rain. This resulted in a lower dust-loading conditions in the following days, and thus the amount of dust lifted by vortices or wind gusts was smaller compared to the first two days. 

Upon return to BIUST in Palapye, on the 6th of October, we held a seminar for staff and students titled: “Atmospheric Science On Mars: From Earth Analogues To Future Planetary Networks”.

In summary, the campaign was a complete success. Our observations have demonstrated the capability of the RDS and SIS sensors to detect and characterize dust devils on Mars. The analysis of the signals along with the information acquired by the other instruments will allow us to quantitatively establish the sensors limit of detection. In addition, the rainy episode offered us the possibility to study dust lifting events with different aerosol loading.