Acoustics Reveals Short-Term Air Temperature Fluctuations Near Mars’ Surface

Abstract

Acoustics is new on Mars: it allows the characterization of turbulence at smaller scales than previously possible within the lowest part of the Planetary Boundary Layer. Sound speed measurements, by the SuperCam instrument and its microphone onboard the NASA Perseverance rover, allow the retrieval of atmospheric temperatures at 0.77 m above the ground, at 3 Hz, with a ∼10 ms response time that is 20–100 times shorter than for typical thermocouple sensors used on Mars. Here we report on the first measurements of the sound speed-derived temperature and its fluctuations near the surface. Data highlight large and rapid fluctuations up to ±7 K/s, whose amplitude over such a timescale has never been reported, nor predicted by atmospheric models. These fluctuations follow the daytime pattern of the turbulence and highlight occasional high amplitude events that are likely due to the conjunction of low thermal inertia and strong winds.Plain Language SummaryThe atmospheric surface layer of Mars, is prone to various interactions between the surface and the atmosphere, which control most of the climate and the weather of the red planet. There, large temperature gradients generate an intense turbulence during the daytime. Hence, the measurement of the air temperature variations close to the surface is important to understand the spatial and temporal scales of this turbulence. The SuperCam instrument onboard the NASA Perseverance rover enables the retrieval of the near-surface atmospheric temperatures, and their fluctuations, at an unprecedented short timescale. Sound speed-derived temperatures, also call sonic temperatures, collected over the Northern spring and summer of Martian Year 36 reveal large and rapid thermal fluctuations up to ±7 K/s, whose amplitude over such a timescale is not reported by any weather station sensors, nor predicted by models that simulate small-scale eddies. These fluctuations follow the daytime pattern of the turbulence with a maximum amplitude early afternoon. Some occasional high temperature fluctuation events are observed, suggesting a complex effect of ground properties and local meteorological conditions on the turbulence. Overall, acoustics is a new and promising technique that records a unique view of atmospheric temperature variations near the surface of Mars.Key PointsSound speed derived temperature is used to study the microscale turbulence at an unprecedented short response timeAir temperature experiences fluctuations as high as ±7 K/s, which has never been reported in situ, nor resolved by mesoscale atmospheric modelsSonic temperature fluctuations follow the daytime turbulence pattern and find their origin in complex surface-atmosphere interactions