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There are two messages in our title, one is infrasound and the other is Mars. What kind of relationship is there? This is the story we need to tell today. The main content of this article comes from the article: Modeling Seismic Recordings of High-Frequency Guided Infrasound on Mars.

First of all, we have to figure out, what exactly is infrasound? Infrasound generally refers to a sound whose frequency is lower than 20Hz and is generally inaudible to the human ear. Infrasound has a long wavelength, long propagation distance, little energy loss and strong penetration. There are also many sources, such as volcanic eruptions, earthquakes, tsunamis and so on. From the characteristics of infrasound, we can know that the propagation distance and energy loss of infrasound make it can be detected by some instruments far away from the event. What does that sound have to do with Mars? It was a very accidental meteorite impact.

Mars (Picture Source: NASA) Insight on NASA, we will not repeat it here. The information it brings to Mars has made great progress in the study of Mars. Here we only discuss the data of meteorite impact events. Because Insight carries a seismograph, it can monitor fire tremors and related activities at any time. Among them, the location of the S0986c meteorite impact event shows that it is located in the southeast direction of Insight, which is 85.1 kilometers apart.

The location of Insight and the location of the meteorite impact event (Xu et al., 2022) interact with the Martian surface and atmosphere, producing seismic waves and infrasound. Seismic waves are visible between 0 and about 200 seconds, while infrasonic waves are visible after 200 seconds. In the following figure, you can see the data and diagrams of the S0986c event, including the vertical component data of the seismograph and its frequency spectrum. We can see that in the blue box, the dispersion signal is very obvious, and it is also very obvious in the waveform, even stronger than the previous direct impact signal. Therefore, the author gives such a model to explain the signal process of infrasound.

Infrasound propagation model and seismograph data (Xu et al., 2022) meteorite impact will produce two kinds of signal sources, one of which propagates directly underground and then reaches the seismograph, and the two propagate in the atmosphere at the bottom of the atmosphere to form infrasonic guided waves, and finally reach the seismograph. Because the propagation speed of the vibration signal in the underground medium is much faster than that in the air, the underground vibration signal is much earlier than the infrasound signal. We also have to talk about why such infrasonic guided waves are formed. Sound velocity distribution is the key. What is the approximate speed of sound in the Martian atmosphere? Because the Martian surface cools rapidly at night and its high-altitude wind speed is higher, it may produce a low-speed layer of sound near the surface, which is equivalent to a pipeline that constrains infrasound in this low-speed layer to form infrasonic guided waves. We can simply understand the propagation of light in optical fiber.

Optical fiber schematic diagram (source: Bing) now that we know the source of the infrasound signal, why does it have dispersion? That is, the wave velocity is different under different frequency infrasound. In fact, we can get our more reliable atmospheric sound velocity model by simulating the sound velocity model of the atmosphere to fit the real data of infrasound. In addition, because the seismograph measures the signal characteristics of sound-ground coupling, the infrasound signal can be used to analyze the simple structure of the shallow surface. The most important of these is "compliance", which is called flexibility in Chinese. This is because the change of atmospheric pressure will cause surface deformation, and its strength mainly depends on the intensity of underground structure and disturbance, propagation speed and so on. Therefore, we can establish an underground structure model to fit the actual observation data, so as to update the velocity structure and obtain a more accurate velocity structure. We will not repeat the derivation of specific mathematical and physical formulas here.

Therefore, if a more detailed analysis of signals like this can constrain the velocity model of the atmosphere and the shallow surface at the same time, it can be said to kill two birds with one stone.

reference

Xu, Zongbo & Froment, Marouchka & Garcia, Rapha ë l & Beucler, E. & Onodera, Keisuke & Kawamura, Taichi & Lognonn é, Philippe & Banerdt, William. (2022). Modeling Seismic Recordings of High-Frequency Guided Infrasound on Mars. The Journal of Geophysical Research Planets. 10.1029/2022JE007483.

This article is from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Xingyu

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