Twisting dark trails criss-crossing light-coloured terrain on the Martian surface, photographed by NASA's Mars Reconnaissance Orbiter in October 2009. Photo: NASA / HiRISE/ MRO / LPL
Scientists are thrilled to have recorded the sound of a dust devil - a small but violent whirlwind - from planet Mars for the first time.
The recording is crackly and staticky, like hoovering sand from a carpet. This unassuming sound is quite literally otherworldly - it is the first ever recorded dust devil from planet Mars, looming approximately 118 metres above NASA's Perseverance rover, the height of Auckland's Harbour Bridge stacked almost thrice over.
Dust devils are small but violent whirlwinds, which coil across the Earth while whipping up debris. They are common weather on Mars, where Perseverance has spent a year abroad gathering up rock samples.
A dust devil, shown in this image released 14 March, 2000 by NASA, was caught in the act of creating a swirly, dark streak. Dust devils are a common occurrence in dry and desert landscapes on Earth as well as Mars. Photo: NASA
Planetary scientist Dr Naomi Murdoch is part of the team at ISAE-SUPAERO, who helped build Perseverance's microphone. Given that only eight recordings are made a month, lasting a little under three minutes each, the team was "extremely excited" in what Murdoch estimated was a 1-in-200 chance.
"I remember when we received those first recordings and how it gave me shivers right down my spine to be listening to sounds from another planetary world. I still get those shivers when I listen to the recordings. I also really enjoy playing back the Martian sounds at home, but for some reason my two and four year olds don't seem to get quite as excited about it as I do."
Perseverance Mars rover, using its WATSON camera, took this selfie over a rock on 10 September, 2021. Photo: AFP / NASA/JPL-Caltech/MSSS
The team "really hit the jackpot with this encounter", Murdoch said. "The camera was looking in the perfect direction, all of the meteorology sensors were on, and the microphone was also on. We definitely got lucky, but the Mars 2020 rover operations team also did a fantastic job of carefully targeting the instrument observations to increase the chances of success."
Nobody can predict exactly when a dust devil will appear, but they seem to favour midday, when there is the largest temperature difference between the ground and air. Combined with the knowledge that dust devils are usually carried by prevailing winds, Murdoch can figure out a favourable direction to set up Perseverance.
"To observe a dust devil, the rover operations team must ensure that all of the meteorology instruments, the cameras and the microphone are recording at the same time, and then we cross our fingers."
Dust devils typically manifest themselves as a distinctive drop in the atmospheric pressure data, often coincident with an abrupt change in wind direction.
Murdoch knew that they had successfully recorded a dust devil when she saw the pressure, wind and sound recordings aligned just right. Using this, they could construct an environmental story.
"The sound data was incredibly useful as it allowed us to determine almost immediately that the dust devil had passed directly over the rover.
"In the audio we can hear two periods of the low frequency wind as the dust devil leading and trailing walls pass over the rover, with a very calm period in between when the rover is right in the eye of the vortex. The images from the rover's navigation cameras quickly confirmed this first interpretation.
Murdoch said the acoustic data was useful for studying dust devils. The microphone has such a rapid sampling rate that the team could hear each individual grain as it whacked Perseverance, then counted the number of particles inside the vortex.
They have invented a completely new kind of measurement to study dust lifting on Mars. This was important both for understanding Martian atmosphere and climate, and also for space exploration.
"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. The characterisation of how this dust, and other chemicals are mixed into the atmosphere from the surface is vital for correctly understanding the meteorology and climate of Mars.
"However, current models of global dust storms fail to correctly capture dust storm activity. With this microphone recording we have demonstrated that sound recordings can be used to directly observe dust transport, a key element of improving our understanding of dust lifting and dust storms.
"Understanding dust lifting is also critical for space missions on Mars - if there is too much dust lifting the particles can cause damage to hardware. The wind sensors on Perseverance have actually been damaged in this way."
Murdoch and the team were thrilled about their achievement.
"For the first time we are hearing the soundtrack to accompany images coming from Mars, and it is wonderfully rich," she said.
"This encounter clearly demonstrates the unique observations that can be made using a microphone. However, for me the true beauty of this study is the multi-sensorial aspect. Due to careful planning and luck we have simultaneous recordings from multiple weather sensors, we have images and we have the sound.
"This provides a very complete understanding of this dust devil and highlights how important multi-instrument synergies are for science. It has been a pleasure to work with fantastic colleagues within the Mars 2020 team who have helped to provide detailed analyses, interpretations and modelling using data from the multiple instruments."