Scientists use seismic noise to image first hundred meters of Mars

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NASA’s InSight lander installed a seismograph on Mars, and the earthquakes it discovered helped us map the planet’s interior. This data provides the big picture of Mars’ interior – how big is the core, is there anything molten, and so on. But it doesn’t capture small details, like what the land just below InSight looks like.

This week, researchers describe how they have managed to find quiet periods on Mars that gives them a closer image of the surface. The results, combined with some nearby surface features, suggest that InSight is above two large lava flows, separated by layers of sediment.

be very quiet

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Marsquakes are not useful for sorting local features. If their seismic waves come from far enough away, their behavior is mostly influenced by the material they spent most of their time traveling through. If an earthquake strikes nearby, things get too energetic to detect the fine details that are caused by local features. So, to see the local geology, you have to look at the background seismic noise that is being continuously picked up by InSight.

On Earth, most seismic noise is generated either by human activities or by the oceans. But Mars lacks both of these noise sources, and its background is dominated by air interacting with features on Mars.

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But when the data were examined during the daytime, when winds were generally strong, the noise dominated the frequencies generated by the wind interacting with the lander. So the researchers focused on what was the beginning of evening, the time of Mars, when the winds died down. At that point, most of the seismic noise is generated by weak winds that interact with the nearby geology rather than with the lander.

Geologists use seismic noise to reconstruct features on Earth by comparing the horizontal and vertical components of noise. This is a process that may correspond to a large collection of possible structures near the surface of Mars. To constrain the list of possibilities, the researchers focused on characteristics that appeared in most possible solutions. They also looked at exposed rocks in nearby craters to look for visible features that might be related to what their models were suggesting.

what’s down

Closest to the surface, Mars’ regolith is formed from fragments resulting from impacts of dust and rocks. It appears to be only 1.5 meters thick, although the researchers caution that data on the uppermost 20 meters of material is very uncertain. Three meters below the surface, there appears to be a layer of volcanic rock formed from major eruptions in Mars’ distant past.

Below that, about 30 meters by 80 meters (these figures are very accurate), there is another layer of material where the seismic signals move slowly. Researchers have concluded that it is likely to be a layer of sedimentary rock. Beneath it are more volcanic deposits.

The researchers concluded that the deepest volcanic deposits are those of the Hesperian, a period of widespread volcanic activity that ended 3 billion years ago. Descending sediment accumulated when Mars experienced cold, dry conditions similar to its current state. After it consolidated, and sometimes during the Amazonian period of Mars, additional eruptions covered the sediment. Since then, impacts and Martian winds have deposited a layer of loose material on top of the volcanic layers.

Obviously, all this is in line with what can be seen in nearby craters. Still, it’s impressive how much information the researchers were able to extract from a little noise.

Nature Communications, 2021. DOI: 10.1038/s41467-021-26957-7 (About DOI).

image listed by NASA/JPL-Caltech

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