Chide Baptiste, Blanc-Benon Philippe, Bertrand Tanguy, Jacob Xavier, Lasue Jérémie, Lorenz Ralph D, Montmessin Franck, Murdoch Naomi, Pla-Garcia Jorge, Seel Fabian, Schröder Susanne, Stott Alexander E, de la Torre Juarez Manuel, Wiens Roger C
Space and Planetary Exploration Team, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Laboratoire de Mécanique des Fluides et d'Acoustique, Centre National de la Recherche Scientifique, Ecole Centrale de Lyon, Institut National des sciences appliquées Lyon, Université Lyon 1, Ecully, France.
J Acoust Soc Am. 2024 Jan 1;155(1):420-435. doi: 10.1121/10.0024347.
The Perseverance rover is carrying out an original acoustic experiment on Mars: the SuperCam microphone records the spherical acoustic waves generated by laser sparks at distances from 2 m to more than 8 m. These N-shaped acoustic waves scatter from the multiple local heterogeneities of the turbulent atmosphere. Therefore, large and random fluctuations of sound travel time and intensity develop as the waves cross the medium. The variances of the travel times and the scintillation index (normalized variance of the sound intensity) are studied within the mathematical formalism of the propagation of spherical acoustic waves through thermal turbulence to infer statistical properties of the Mars atmospheric temperature fluctuation field. The comparison with the theory is made by simplifying assumptions that do not include wind fluctuations and diffraction effects. Two Earth years (about one Martian year) of observations acquired during the maximum convective period (10:00-14:00 Mars local time) show a good agreement between the dataset and the formalism: the travel time variance diverges from the linear Chernov solution exactly where the density of occurrence of the first caustic reaches its maximum. Moreover, on average, waves travel faster than the mean speed of sound due to a fast path effect, which is also observed on Earth. To account for the distribution of turbulent eddies, several power spectra are tested and the best match to observation is obtained with a generalized von Karman spectrum with a shallower slope than the Kolmogorov cascade, ϕ(k)∝(1+k2L2)-4/3. It is associated with an outer scale of turbulence, L, of 11 cm at 2 m above the surface and a standard deviation of 6 K over 9 s for the temperature. These near-surface atmospheric properties are consistent with a weak to moderate wave scattering regime around noon with little saturation. Overall, this study presents an innovative and promising methodology to probe the near-surface atmospheric turbulence on Mars.
“毅力号”火星车正在火星上开展一项原创声学实验：“超级相机”麦克风记录了激光火花在2米至8米多距离处产生的球面声波。这些N形声波会从湍流大气的多个局部不均匀性中散射。因此，当声波穿过介质时，声传播时间和强度会出现大的随机波动。在球面声波通过热湍流传播的数学形式体系内，研究了传播时间的方差和闪烁指数（声强的归一化方差），以推断火星大气温度波动场的统计特性。通过简化假设（不包括风的波动和衍射效应）与理论进行了比较。在最大对流期（火星当地时间10:00 - 14:00）获取的两个地球年（约一个火星年）的观测结果表明，数据集与该形式体系吻合良好：传播时间方差恰好在第一个焦散出现密度达到最大值的地方偏离线性切尔诺夫解。此外，平均而言，由于快速路径效应，波的传播速度比平均声速快，这种现象在地球上也有观测到。为了考虑湍流涡旋的分布，测试了几种功率谱，与观测结果最佳匹配的是一种广义冯·卡门谱，其斜率比科尔莫戈罗夫级联谱更平缓，即ϕ(k)∝(1 + k²L²)-4/3。它与表面上方2米处11厘米的湍流外尺度L以及温度在9秒内6K的标准差相关。这些近地表大气特性与中午时分弱至中等程度的波散射状态且几乎不饱和的情况一致。总体而言，这项研究提出了一种创新且有前景的方法来探测火星近地表大气湍流。
