Mon. Dec 6th, 2021

InSight places a windshield over its seismometer.
Enlarge / InSight places a windshield over its seismometer.

NASA’s InSight lander installed a seismograph on Mars, and the Martian earthquakes it discovered have helped us map the planet’s interior. These data give the big picture of Mars’ interior – how big the core is, whether something has melted and so on. But it does not capture the small details, such as what the ground immediately below InSight looks like.

This week, scientists described how they have managed to find quiet periods on Mars that let them take pictures closer to the surface. The results, combined with some nearby surface features, reveal that InSight is likely above two large lava flows, separated by layers of sediment.

Be very quiet

Marsquakes are not useful for sorting local functions. If their seismic waves arrive far enough away, then their behavior is mostly influenced by the materials they spent most of their time traveling through. If the March quake happens nearby, things are too energetic to see the fine details caused by local features. So to look at the local geology, you have to look at the seismic background noise that is constantly being captured by InSight.

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

However, when the data were examined at times of the day when the wind was generally high, the noise was found to be dominated by frequencies produced by the wind’s interaction with the lander itself. So the researchers focused on what was early evening, Mars time, when the winds tended to subside. At that time, most of the seismic noise is generated by weak winds that interact with nearby geology rather than with the lander itself.

Geologists have used seismic noise to reconstruct features of the Earth by comparing the horizontal and vertical components of the noise. This is a process that may be consistent with a large collection of potential structures near the surface of Mars. To narrow the list of possibilities, the researchers focused on features that appeared in most potential solutions. They also looked at the rocks exposed in craters nearby to search for visible features that correlated with the things their models suggested might exist.

What’s downstairs

Closest to the surface, Mars’ regolith is formed by dust and rock fragments produced by shocks. It appears to be only 1.5 meters thick, although researchers warn that the data on the top 20 meters of material is very uncertain. Three meters below the surface, there appears to be a layer of volcanic rock, formed by large eruptions in Mars’ distant past.

Below that, from about 30 meters to 80 meters (these numbers are quite inaccurate), is another layer of material where seismic signals are moving slowly. The researchers conclude that this is probably a layer of sedimentary rock. Below that are additional volcanic deposits.

Scientists conclude that the deepest volcanic deposits date back to the Hesperian, a period of widespread volcanic activity that ended over 3 billion years ago. The overlying sediment deposit was formed while Mars was experiencing cold, dry conditions similar to its current state. After it consolidated, and sometime during Mars’ Amazon period, further eruptions covered the sediments. Since then, impacts and Mars’ winds have deposited a layer of loose material on top of the volcanic layers.

Clearly, all of this is consistent with what can be observed in nearby craters. Still, it’s impressive how much information the researchers were able to extract from just a bit of noise.

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

List image by NASA / JPL-Caltech

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