Marsquakes and some smart instruments help scientists confirm the size of the Red Planet’s core

Australian geophysicists have been able to confirm the size of Mars core by monitoring its frequent earthquakes (sorry, marsquakes) – with just one instrument.

Since 2018, NASA’s Mars InSight lander has been beaming down information about the interior of the planet.

In the latest bevy of discoveries, two Australian researchers have developed a neater method to scan the interior.

With just one seismograph, they’ve been able to confirm the size of Mars’ core (around 3,620 kilometres in diameter) – a trick which would work equally well on other planets or, importantly, the Moon.

“It is a new, innovative method to scan any planetary core,” says Professor Hrvoje Tkalčić, from the Australian National University’s Research School of Earth Sciences, and co-author on a paper describing the research, published in Nature Astronomy.

“In the near future, we probably won’t be able to come up with the technology to carry more than a single instrument or a few instruments to future planetary missions. So we need to be creative with what we have.”

Tkalčić, along with lead author Dr Sheng Wang, who did the research as part of his PhD, developed a mathematical trick to glean more from the quakes which rumble across the planet’s surface.

“We basically used the so-called reciprocity principle, and according to that principle, you can swap the locations of receivers with sources,” says Tkalčić.

“So that all of the recorded marsquakes become virtual receivers, and our receiver itself becomes a virtual source.”

Computer-generated image of mars cut in half, with core labelled inside and marsquakes zigzagging between surface and core but not passing through core
An illustration of the Martian interior. Quakes at the surface are felt at specific points – and the physics works out if you treat those quakes like stations, and the station like a quake. Credit: Credit: Dr Sheng Wang and Professor Hrvoje Tkalčić/ANU

Next, the researchers examined the similarities between weak waveforms that followed the bigger shakes from marsquakes.

Tkalčić says that this part of the analysis is “equal to classical interferometric methods that are used to scan the Earth’s interior” – like the planetary equivalent of a CT scan.

Between these two ideas, the researchers were able to build a picture of the Martian interior – confirming the 2021 findings from a completely different source.

“It’s a different type of data and it’s a completely innovative method,” says Tkalčić.

It will also work for any celestial body that has seismic activity. Next in Tkalčić’s sights is the Moon, where both China and the US are planning to send seismometers in the coming decade. (The Apollo missions left five seismometers on the Moon in the 1960s and 70s, but they weren’t sensitive enough to generate the information needed in this study.)

But the trick could also work on asteroids, rocky planets, or moons of other planets too.

“Although there are many studies on planetary cores, the images we have of planetary interiors are still very blurry,” says Wang.

“But with new instruments and methods like ours we’ll be able to get sharper images which will help us answer questions such as how big the cores are and whether they take a solid or liquid form.”

This is important, because the nature of a planet’s core plays a role in its magnetic field – something which shields Earth from dangerous cosmic rays, and something which Mars conspicuously lacks.

Read more: Rejuvenated Insight ‘hears’ three strong marsquakes

Accompanying Wang and Tkalčić’s paper are three other studies using InSight’s data.

Two, both published in Science by international teams of researchers, examined the seismic data that came from two major meteorite crashes on Mars in 2021.

Aside from generating interesting data themselves, Tkalčić says that these impacts were a useful way to verify his and Wang’s method of studying the planet.

“In our method, we rely on the known locations of the events – and with these two new impacts, we can pinpoint exactly where they happen,” he says.

Another study, also published in Nature Astronomy by a team of Swiss researchers, compiles much of what we know about marsquakes to draw a bigger picture of Mars’ geological activity.

Coloured top-down view of martian craters
Colour-coded of the Cerberus Fossae: reds and whites are relatively higher than blues and purples. The image is based on a digital terrain model of the region. Credit: ©ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO,

Many of the quakes originate in one place: the Cerberus Fossae region – and the behaviour suggests that there is liquid magma afoot on Mars.

This correlates with other recent work from Tkalčić’s lab.

“It’s a good indication that that particular area is volcanically active, but also that the entire Martian interior is mobile, which is something that we perhaps didn’t anticipate – that Mars is so active as a planet,” says Tkalčić.

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