Acoustics Reveals Short‐Term Air Temperature Fluctuations Near Mars' Surface

Abstract

The Planetary Boundary Layer (PBL) includes the lower atmospheric layers directly in contact with the surface (Petrosyan et al., 2011). On Mars, it plays a critical role, strongly impacting the climate system, as it controls the dust lifting, the volatile fluxes and the exchange of heat and momentum between the surface and the atmosphere 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.

Wind and Turbulence Observations With the Mars Microphone on Perseverance

“…This is consistent with Chide et al. (2022) who demonstrate that sound speed derived thermal fluctuations are greatest at this time. The wind earlier on appears more episodic where there is a mix of small and large signal power recordings.…”

Near Surface Atmospheric Temperatures at Jezero From Mars 2020 MEDA Measurements

“…Hence the nighttime RTG‐induced perturbations are most evident when the rover heading is to the S/SE/E, except between 01 and 05 LTST when winds are weak with very variable directions. Excluding those events, temperature fluctuations observed by MEDA at 1.45 m are comparable but smaller than those derived from sonic temperatures using the SuperCam Microphone during Laser Induced Breakdown Spectroscopy experiments (Maurice et al., 2021), which sample the atmosphere at an effective height of 0.77 m, and at a faster rate than MEDA (Chide et al., 2022; Maurice et al., 2022).…”

Nocturnal turbulence at Jezero crater, as determined from MEDA measurements and modeling

“…A strong dependence of nocturnal turbulence on the ambient wind was also observed at InSight's location (Chatain et al 2021). Turbulent fluctuations in temperature in Jezero crater are explored further in other papers in this issue: using measurements from ATS (de la Torre-Juarez et al, 2022 this issue; Munguira et al, 2022 this issue) and the SuperCam microphone (Chide et al, 2022;this issue). More examples of the simultaneous fluctuations of surface and air temperature (at 0.85m, 1.45 m and ~40 m height) and wind speed and direction (at 1.5 m height) as observed by MEDA are shown in the Supplementary Material (Figures S1-S6).…”