On Mars, two exceptional earthquakes recorded in the “shadow zone”

The Mars probe InSight recently recorded two rather exceptional earthquakes thanks to which researchers were able to examine the bowels of the red planet.

Mars may have been the subject of thousands of different studies, but it continues to raise a host of questions that researchers are trying to answer with various devices, including some that have been deployed directly on site. The most fashionable of them is certainly the latest arrival, namely the Perseverance rover, whose exploits never cease to amaze planetary scientists.

But they are not the only ones to make great discoveries. At the other end of the red planet, it is the InSight probe which has just distinguished itself with a fine observation; its onboard seismometer was recently rocked by two rather impressive earthquakes.

These two events, called S1000a and S0976a and, were detected in September 2021. Since then, the probe operators have had plenty of time to dissect the data. And the least we can say is that many surprises awaited them at the turn.

Biggest “Marsquakes” Ever Recorded

They were able to determine that these events had reached respective magnitudes of 4.1 and 4.2 on the Richter scale. These are therefore tremors which, on Earth, would have been qualified as relatively moderate; but on Mars, these numbers are exceptional. It is even a record in all categories. Indeed, these two earthquakes were about 5 times stronger than any other Martian earthquake recorded to date.

These tremors came from the Valles Marineris, the “Grand Canyon” of Mars. © NASA/JPL/USGS

And the power of these tremors is not the only element that has intrigued the researchers. “It is not only the strongest and most distant events with a considerable margin, S1000a also presented an unmatched seismic profile and duration to our knowledge.”, explains Anna Horleston, seismologist at the University of Bristol and lead author of the study in a press release. In effect,

The location of the epicenter is equally remarkable. These earthquakes originate in the Valles Marineris, an area sometimes referred to as the “Grand Canyon” Of March. And for good reason ; it is the largest canyon complex in the entire solar system. Space.com recalls that researchers had already identified traces that pointed to past earthquakes in the area. But this is the first time that seismic activity has been recorded there. And the mere fact of having been able to record these readings is already disturbing in itself.

Indeed, seismographs are mainly based on the detection of two types of seismic waves: P and S waves. By cross-checking the data from several sensors that have captured these waves, we can then determine the exact position of the epicenter and a lot of other interesting data.

A map of the seismic events in question. © Horleston et al.

An earthquake located in the “seismic shadow zone”

The problem is that waves, including light, are capricious; they do not spread uniformly in all environments. We can cite the well-known example of the laser which changes trajectory when it passes from air to another medium such as glass or water. This is called refraction, and this phenomenon also exists for seismic waves.

Consequently, the speed of the wave and its trajectory change radically depending on the media they pass through. This is an interesting element for geologists. It makes it possible to identify the different structures of the subsoil by carefully studying the behavior of the wave.

But this also has a more disabling consequence: the spherical shape of the planets means that no matter where the seismograph is positioned, there is bound to be a “zone d’ombre” in which it is theoretically impossible to detect these P and S waves. (see the video below). However, they managed to get around these limits by using two other types of waves that are much more difficult to analyze, PP and SS waves.

A window into the bowels of Mars

Recording events through this gray area is real progress in our understanding of Mars.”, rejoices Savas Ceylan, seismologist at ETH Zurich and co-author of the study. “Since these waves also pass through the central zone which is usually responsible for the shadow zone, we can study parts of Mars that we have never been able to probe seismologically before.”, he specifies.

From now on, they will continue to dissect this data, hoping to discover some interesting clues about the dynamics and constraints of the crust, mantle and core of the red planet. But these measures will not be sufficient on their own; it is therefore to be hoped that other shocks of this kind will make it possible to complete the vast puzzle of Martian geology, with all that this implies for our global knowledge of Mars and for the search for traces of past life.

The text of the study is available here.

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