What did we hear?

Wind is something we hear every day. Now, for the first time, the SuperCam microphone on the Perseverance rover has recorded the whoosh of the wind from a dust devil on Mars. This microphone recording gives us fine details of the wind during the dust devil, demonstrating the Perseverance rover was inside the vortex. The passage was also captured by pressure, temperature, dust and conventional wind sensors along with images to track its path. This was the result of a dedicated observational campaign from the Perseverance mission, involving multiple teams, and leads to a multi sensorial experience which reveals the detailed characteristics of the dust devil. As well as wind, the microphone heard the smattering of dust grains hitting the rover. This lets us examine how many dust grains were in the dust devil. Together these data shed light on turbulence in the Martian atmosphere and the transport and lifting of dust, both crucial components of the modern environment on Mars.

Why is it important?

What is a dust devil and why do they matter?

Dust devils are whirlwinds which occur as warm air from the ground rises and starts to rotate, creating a vortex. They are common on both Mars and the Earth in dry areas such as deserts. A dust devil’s occurrence is an indicator of turbulence in the Martian atmosphere, as they happen when there is a large temperature difference between the ground and air and so the atmosphere is unstable. The air on Mars is extremely thin compared to the Earth. This means that the transfer of heat between the ground and air is difficult which means large air to ground temperature differences occur on a regular basis. The detailed examination of the associated atmospheric conditions and the wind dynamics of a dust devil (as illuminated by this recording) then lets us see how the Martian atmosphere behaves. 

Why is dust in the atmosphere important?

The underlying atmospheric phenomena of a dust devil is called a convective vortex, which describes the associated whirlwind. They are only called dust devils when they carry dust and this is not always the case. As the Martian air is so thin, conditions in which particles can easily be lofted are relatively rare. Convective vortices are responsible for some of the most extreme winds on Mars and so we are interested in determining in what conditions they are able to loft and transport dust. 

Dust plays a vital role in the overall Martian atmosphere. The cycle of dust in the sky on Mars actually forms a similar role to the water cycle on Earth, affecting the overall weather systems. Dust storms can also form, in either local, regional or global scales and understanding how they arise and progress is somewhat an open question. The characterisation of how this dust, and other chemicals are mixed into the atmosphere from the surface is vital for atmospheric models of Mars.

So what exactly did we find?

The recording occurred on 27th September 2021, or Sol 215 (a sol is a Martian day) of the Perseverance mission. Coincidently, this is the same day that, in 1822, Jean-François Champollion announced to the Académie des Inscriptions et Belles Lettres in Paris that he had successfully deciphered the Rosetta Stone. The Rosetta stone contained multiple inscriptions of the same decree in different languages, allowing Champollion and others to begin to uncover the secrets of ancient Egypt. In a similar way, the Perseverance rover employs multiple techniques and measurements to decipher the Martian atmosphere. This includes the SuperCam microphone but also the wind, pressure, temperature and dust sensors carried by the Mars Environmental Dynamics Analyser (MEDA) instrument. Visual/optical information can also be obtained from the Navcam (navigation camera) and SuperCam spectrometers. 

For this dust devil, 199 years later than Champollion down the road in Toulouse, we were able to draw on measurements  from 7 types of sensor across 3 different instruments, providing multiple views for this dust devil. The SuperCam microphone acts as a kind of microscope for the fine scale dynamics (fast wind fluctuations) of the dust devil. This is because the microphone records at a very high sampling rate and is particularly sensitive to changes in wind. In this recording,  the microphone signal hears the winds picking up as the wall of the vortex hits the rover, dying down as the rover is inside the central eye before picking up again as the dust devil completes its passage over the top of the rover.

To fully parameterise the dust devil, however, we needed the help of MEDA and Navcam. The underlying convective vortex of a dust devil can only be confirmed by a distinct drop in pressure. The MEDA wind sensor showed a change in wind direction, another common feature with a dust devil observation, while the temperature data indicated a cooling due to the winds. The dust sensors and Navcam (navigation camera) images track the path, dust content and visual changes brought about by the dust devil. The combination of all these different instruments allowed us to determine its physical size, strength of wind and central pressure drop. This full parameterisation of a dust devil helps us learn about the dynamic conditions during a dust carrying vortex.  

We found an unusual characteristic of this dust devil. It carried most of its dust in the centre, rather than by the high wind speed walls. This was seen in both the imaging and meteorological sensors. The recorded sound of scattered grains within the vortex allows us to quantify the amount of particles carried by this dust devil. To do this we implemented an algorithm to count the number of sound spikes from the impacting grains in the microphone recording. We see three clear sections of grain impacts, two during the vortex walls and a more significant group in the middle of the vortex just after we pass the first wall, which agrees with the images and dust sensors. We can infer the impacts are likely from somewhere close to the microphone. This is because sound on Mars attenuates (the power reduces) very quickly in the carbon dioxide dominated atmosphere and so we would not hear them if they are further away. All together, we now have information on the number of particles carried within a certain area, meaning we can start to quantify the density of grains carried by dust devils.

How did this come about?

This recording is the result of a carefully constructed plan. The commands executed by the Perseverance rover to make such recordings are uploaded in a daily cycle as a result of a careful blend of science, engineering and the overall mission strategy. This particular observation required synchronisation across several teams in order to get simultaneous sound, imaging and meteorological measurements.

Each group of scientists must advocate for their measurement's allotted time and power. Even space missions such as Perseverance do not have unlimited resources and the mission team has to find the best solution. Perhaps the rover is in a location with a particularly special rock to examine or the rover is quickly trying to reach an specific area of the Jezero delta. Atmospheric science on the other hand is typically about background monitoring, with time constraints to capture daily and seasonal variation. 

This particular observation is the result of a fishing expedition. Dust devils are the result of a random process and so their presence cannot be pre-determined. As the sequence of observations must be pre-set, our search is therefore a stab in the dark. Our best chance, however, is during the largest temperature imbalance between the ground and air. This occurs usually at about midday which sets our time constraints. The prevailing wind, current landscape and available surface dust also impact the dust devil and its passage, and so the operator has to choose the best direction in which to look. We had run this command several times and on this occasion got lucky. We calculated that the probability of us observing a dust devil such as this in any one recording was 0.05%. After the full campaign of recordings made over the study period, the probability that one of the recordings would contain a similar event was evaluated to be ~13%. We hope we will be able to capture more recordings of dust devils to build a bigger picture.

Conclusion

Mars is often viewed as a planet frozen in time. There are no plate tectonics and limited recent volcanism, meaning much of the Martian surface is the same as it was billions of years ago. This means one of the main causes of ongoing surface change is due to the atmosphere. Understanding its ability to do so is crucial for the Perseverance rover’s search for signs of life past or present. Another goal of the Perseverance mission is to pave a way for a human presence on Mars and so we must learn about the part of Mars we would be present in. In the distant past, Mars is thought to have had a similar environment to Earth. Understanding the Martian atmosphere’s current dynamics allows us to figure out how it could have changed and, significantly, teach us about how our own atmosphere on Earth works. 

So, as we approach mid-winter, consider the group of elves from across the world gathering each dark Martian night to construct gifts to be brought down from Mars and teach us about our universe.