Authors: Simon C. Stähler, Amir Khan, W. Bruce Banerdt, et al.
First Author’s Institution: Institute of Geophysics, ETH Zürich, Zürich, Switzerland
Status: Published in Science [closed access]
The cores of planets have always been elusive to scientists, even on our own planet. Predictions were made in the 1700s that Earth might have a core with different composition and density from surface rock, but it wasn’t until 1929 that Inge Lehmann, noticing some odd features in earthquake data, detected the core itself.
In today’s paper, scientists use NASA’s InSight lander (Figure 1) to detect the Martian core for the first time. This is super exciting, because it’s the first core we’ve ever been able to directly detect aside from those of the Earth and Moon!
Secrets beneath the surface
So what do we want to know about Mars’ core? The short answer is almost everything. We know that Mars is separated into layers just like the Earth (a core, mantle, and crust) but we’re not sure how big the core is, what it’s made of, or whether it has both inner and outer portions. Figuring all of this out is important because cores can affect a lot of their planet’s properties, causing volcanoes or tectonic activity, and even affecting the climate.
The goal of this study is to figure out how big Mars’ core is. If the core is small compared to the planet’s overall size (~3400 km), then the core is pretty dense, and therefore it’s likely that Mars has a lower mantle layer similar to Earth’s. On the other hand, if the core is larger than ~1800 km, then a lower mantle probably doesn’t exist on Mars; thus the core could be less dense, and contain lots of light elements in addition to the heavier iron generally found in cores. Determining which one of these scenarios is correct can help us learn more about how the red planet formed, and why it evolved so differently than the Earth.
We observe cores using seismographs, which sense motions of the ground. The idea of seismology has been around for a long time (check out this ancient Chinese earthquake detector here!), but in astronomy, seismology is more commonly used to study stars from afar. Opportunities to use seismographs on planet surfaces other than the Earth are few and far between, which is why today’s study is so unique.
To “see” the core of Mars, the authors use data from the InSight lander, which has collected about a Martian year’s worth of data on marsquakes. Just like waves of light or sound, seismic waves from quakes will travel at different speeds through different types of material and can reflect off of surfaces. There are two main types of seismic waves – P and S. P waves are really fast and can travel through any material. S waves are the ones that are key to this study, because they move slowly and can only travel through solids. This is a very useful trait, because one of the only things we do know about Mars’ core is that it’s liquid. So if an S wave from a marsquake reaches the core, it will bounce off it and arrive at the seismograph slightly later than the rest of the S waves. Therefore, waves that interact with the core can be picked out by their distinctive pattern in the seismograph.
A cartoon of what InSight could be seeing is shown in Figure 2. The blue lines are the fast moving P waves, and the red lines are the S waves. One of the interesting things that InSight noticed was that there’s a really large region (the “shadow” by Tharsis) where the lander cannot detect waves. Instead, waves from marsquakes in this region will be redirected to other points on the surface, so there could be a lot more marsquakes happening that we just can’t see from InSight’s position.
One of the marsquakes that InSight was able to detect is shown in Figure 3. The waves at the beginning of the plot are the P waves, which reach the detector first. The S waves that didn’t interact with the core are next to arrive. Finally, the shaded region shows the S waves that interacted with the core before hitting the detector.
The authors collected these core-bouncing S waves from multiple quakes. By comparing the timing and strength of those signals with geological models, the authors concluded that Mars’ core has a radius of about 1830 km, roughly half the total radius of the planet, putting it in the “large core” classification (by comparison, Earth’s core is only about 1/5 of the planet’s total radius). This means that Mars doesn’t seem to have a dense lower mantle like Earth does. Because the core is so large, it also means that it is probably less dense and could hold a fairly large amount of light elements, like carbon, oxygen, or hydrogen — a finding that’s exciting for scientists that study how different elements separate into specific layers as planets form. For now, that’s about as much detail as we can get from InSight, but the lander is still taking new data every day so stay tuned for updates!
Edited by: Aldo Panfichi, Ishan Mishra
Featured image credit: NASA/JPL-Caltech