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NASA's Insight lander was launched on May 5, 2018 and successfully landed on Mars on November 26, 2018, where it performed the first human exploration of Mars's "inner secrets", that is, the study of Mars's internal structure. As the most critical part of this project, a very sensitive seismograph, The SEIS (Seismic Experiment for Interior Structure), needs to be placed on the surface of Mars, which is also the first time that humans have detected the ground motion of other planets.

Mars seismograph (source: NASA Chinese website) on March 18, 2019, Insight finally completed the installation of the fire seismograph and detected Mars microtremors a few weeks later. The cause of this Martian microshock is the Martian wind, natural noise can cause Rayleigh surface waves propagating on the Martian surface, and this microshock is mainly low-frequency surface waves. In the follow-up observation cycle, the fire seismograph has observed more than 1313 fire tremors, which is of great help to understand the internal structure of Mars and the activity of Martian ground motion. According to the current research, the internal structure of Mars has been constrained by clear evidence.

Insight has detected the largest fire shock so far, with a magnitude of 5 (source: NASA / JPL-Caltech). We are already familiar with the earth, and those who have studied high school geography should also know the structure of the earth's interior, crust, mantle, core, Moho, Gutenberg and other interfaces. The initial discovery and exploration of these structures also benefited from the study of earthquakes. Therefore, in the face of the earthquake as a natural disaster, human beings have carried out in-depth research from the other side. Combined with the earth's seismic research, then naturally the fire shock study of Mars can also provide us with more information about the internal structure of Mars. In a new GRL paper, Dur á n et al analyzed a fire shock event (S1000a) which was the second farther away from Insight. The fire shock was mainly due to the impact of a distant meteorite with a moment magnitude of about 4.1. Why analyze this incident separately? This is because the distance (epicenter distance) is relatively far in the current event, so seismic waves can travel deeper inside Mars to get more information about the interior of Mars. This is the first time that researchers have observed P-diff (hereinafter labeled Pdiff), also known as nuclear diffraction wave, in a fire earthquake, which propagates directly along the core-mantle boundary and then reaches the seismograph. This fire shock signal is a wide-band signal (from 10s to 5Hz). Its frequency range can be seen from the A subgraph of the following figure, which is a power spectral density graph calculated by continuous wavelet transform. A very obvious reflection in the frequency domain can be seen. In the process of seismic phase identification, the author mainly identifies three kinds of seismic phases: PP and SS, so as to carry out the follow-up research work.

The PSD and amplitude characteristics of the S1000a event and the seismic phase identification location (Dur á n et al., 2022) are simply explained as the different characteristics of seismic waves. Because of the complexity of the propagation path of seismic wave underground, the final seismic waveforms are quite different in different propagation paths. Therefore, seismic phases can be used to identify different propagation paths and their corresponding characteristics. The PP wave is the seismic phase recorded by the seismograph after the seismic P wave passes through a reflection, and the SS wave is the seismic phase recorded by the seismograph after the seismic S wave is reflected once. The P-diff wave discussed in this paper propagates directly at the core-mantle boundary and is recorded by the seismograph. Therefore, the P-diff wave arrives fastest, followed by PP and SS waves. Because of the different propagation paths, the amplitudes of P-diff waves are also very different, and the amplitudes of P-diff waves are smaller as a whole, with a difference of one order of magnitude. The following picture is a schematic diagram of the three seismic phases.

Three kinds of seismic phase propagation diagrams (source: AGU) through the corresponding inversion method, according to the Martian underground seismic wave propagation velocity structure, combined with the corresponding petrophysics and other information, the author successfully located the location of the earthquake source, which has a higher relationship with the meteorite impact. Therefore, this event is also a meteorite impact event. S1000a in the image below is the location of the meteorite impact event. By analyzing the path of P-diff, we can clearly see that it passes through the whole fire mantle, so the P-and S-wave velocity gradient (one-dimensional) of the fire mantle on this path can also be well described. Through this impact event, the mantle velocity gradient can be effectively obtained, providing more and more effective information for the internal research of Mars.

Location of S1000a (Dur á n et al., 2022) Why should we study the internal structure of Mars? Some people say, what is the use of this? It is useless at present, but human curiosity drives human progress. It's like a bad saying: why do people climb the mountain, because the mountain is there. The study of Mars can also enable us to better understand the earth, understand the solar system, and deepen our understanding of our home!

References:

Dur á n, C., Khan, A.A., Ceylan, S.S., Charalambous, C., Kim, D., Drilleau, M., et al. (2022). Observation of a core-diffracted P-wave from a farside impact with implications for the lower-mantle structure of Mars. Geophysical Research Letters, 49, e2022GL100887. Https://doi. Org/10.1029/2022GL100887

Friends who are interested in the relevant technical details can read the original text directly, the original link:

Https://doi.org/10.1029/2022GL100887

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

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