Scientists Image First Hundred Metres of Mars Using Seismic Noise
Scientists are using noise created by Mars’s wind to determine what lies beneath the InSight lander.
Marsquakes detected by the seismograph installed on NASA’s InSight lander have helped to map out the interior of Mars. The data collected is able to provide the big picture of what the planet's interior looks like. While it doesn’t show small details like what the surface looks like, we are now able to determine how big the core is, whether it is molten and much more.
Researchers have worked to find the quiet time periods on Mars which allows them to use seismic signals to image closer to the surface. They have found that InSight is sitting above two sizable lava flows, separated by layered sediment.
Don’t make a sound
Usually marquakes hinder the collection of data as the great amount of energy makes it difficult to interpret the minor details created by nearby features. But when marsquakes occur from a distance their seismic waves travel from far away, and therefore their behaviour is often influenced by materials they’ve travelled through. Consequently, in order to understand local geology, scientists must look at the background seismic noise which is being constantly picked up by InSight.
On our own planet, most seismic noise is produced by human activities or our oceans. Because Mars lacks both of these, Mars’s winds and how the wind interacts with its geology, makes up the majority of the noise produced on the red planet.
At times of day when winds were higher, it was found that dominant frequencies were made up of the wind interacting with the InSight itself, making it difficult to interpret any other frequencies. To combat this, scientists opted to measure winds in the early evening when winds weren’t as strong, seismic noise generated at this time tended to reflect local geology rather than the InSight lander.
This technique is not new however, for years geologists have used seismic noise to map geological structures on earth by comparing vertical and horizontal components of noise. This same process can be used to gather data on the existence of many structures on the surface of the red planet. To minimise the list of potential structures, scientists only looked at geological features which consistently showed up in their tests. They also cross checked seismic noise data with any visible features which correlated to their hypothetical models of Mars’ surface, to get the most accurate representation possible.
What lies below
The surface layer of Mars is formed by a 1.5m thick layer of fragmented rock and dust, produced by impacts to the surface over time. Underneath this layer is 20m of undetermined materials. Scientists have reason to believe that 3m below the surface there are layers of volcanic rock, which were formed by eruptions on Mars long ago.
Roughly 30 to 80 metres below the surface is another layer of which seismic signals move slowly though. Researchers hypothesise that this is likely sedimentary rock. Below this layer are more volcanic deposits.
Researchers estimate the deepest volcanic deposits to be around 3 billion years old, and come from the Hesperian, a period which consisted of widespread volcanic activity. Mars’ mid most sediment layer likely formed as Mars experienced a cool period, boasting dry conditions which mimic the planet's present state. Following this was the planet's Amazonian period, in which volcanic eruptions covered the sediment layer. Since then regular impacts from asteroids, meteors and other space objects have created the layer of loose dust and rock which form its red surface today.
Although most of what scientists have found can be seen from nearby craters, it is impressive how much information they have been able to extract and cross examine using just a little noise.