Mars' core composition confirmed

For the first time scientists have managed to measure what is inside our cosmic neighbour. By using a seismometer - a tool that measures tremors - aboard NASA’s InSight lander, and relying on Mars quakes and even a meteor impact to shake the planet, the University of Bristol’s Jessica Irving and her team were able to use the passage of the vibrations through the Martian ground to confirm the predicted models of what Mars is made of.

Jessica - So the seismometer is similar to the seismometers that we would use on Earth. There are actually two in one big instrument. So doing seismology on Mars was always a part of the InSight plan, if Mars cooperated.

Will - Listeners to the show might remember a programme we did a short time ago based on working out the Earth's core by using vibrations of tectonic plates to work out composition of the inner Earth. Is that the same as what you did on Mars?

Jessica - They are the same techniques and actually many of the seismologists that have been working on Mars also work on Earth data. But on Earth we have seismometers on every continent, but on Mars we just have one seismometer for the whole planet and we also have many, many fewer quakes on Mars than we do on Earth.

Will - How does a Mars quake happen then?

Jessica - A Marsquake, so far as we've seen them, is just a small quake of the sort you'd get inside a tectonic plate. They're not very big, but over the lifetime of the InSight mission, we've managed to detect more than a thousand mask quakes. There's another sort of seismic source that we're also able to use for Mars. And again, this is one that we hoped would work and we couldn't guarantee it. And that's the meteorite impact on the surface of the planet.

Will - That sounds like a pretty big meteorite then if it's going to be able to reveal data about the entirety of the core of Mars.

Jessica - Yes, and we needed a really big meteorite impact to make that work. And we got one! We were really lucky. On the far side of Mars from where we have our seismometer, we had a very large meteorite impact. The impact made a crater that was about 130 metres across and the dust and fallout around the crater was so big it could be seen from space.

Will - And what did it reveal about Mars' core?

Jessica - So Mars' core has a radius that's about 1800 kilometres, and this is similar to, but a little bit smaller, than previous estimates made using different sorts of techniques, but we've not actually managed to get waves that have been into Mars' core before, so we were really happy to have these new signals to work with.

Will - Inevitably, when we talk about Mars, we end up comparing it to Earth. Is there a difference in the composition of the two planet's cores?

Jessica - What we see is that Mars's core is about half of the radius of the planet, and Earth's core is about half the radius of the planet. But Mars's core has a slightly different composition. They're both mostly iron, but Mars's core has about twice as many light elements in it as Earth's core does. So light elements are things like carbon, oxygen, it's probably a lot of sulphur in Mars' core and we think maybe a sprinkling of hydrogen as well. So different compositions that probably reflect both how the cores were formed and the building blocks they were formed from. As our planets grew.

Will - If they are both iron and yet Earth has a magnetosphere and Mars does not, do you think that the slight difference in composition is responsible for that?

Jessica - There are probably multiple different things which have meant that Mars doesn't have a magnetic field today, but Earth does. Certainly we know that Mars used to have a magnetic field. So billions of years ago there was a magnetic field not too dissimilar to Earth's and that would've been good at protecting the atmosphere of Mars. But now that magnetic field has gone, Mars is relatively unprotected and certainly we know that the magnetic fields are generated inside planetary cores for planets like Earth and Mars. So there are definitely reasons to think that what's happened to Mars' core over its history and what happens to Earth's core over its history have had different roots and different evolutions to give us these different setups today.

Will - What is the overall use then of understanding these differences between the two planet's core compositions?

Jessica - If we want to think about planetary habitability, then understanding if we have a planetary magnetic field that is present and likely to stay there is a really important long-term goal. But as well as that we actually get to look back in time and think about how planets grew and how planets formed. And we can look outwards into our solar system and see some moons of other planets and think how their internal setups work and how might we understand how they've evolved and changed through history?

Will - Is that the plan then? Are we headed out to other bodies in the solar system to do the same?

Jessica - I think it would be the dream of many seismologists to do the same across the solar system. So the seismometer on Mars has unfortunately died. It ran out of power because its solar panels got completely covered in dust. But there are hopes that there will be seismology missions to other planets and especially moons in the near future. So there is a mission which NASA is looking at to run as part of the Artemis missions. That's the far side seismic suite, which should put a seismometer onto the Moon. That hasn't been done since the Apollo missions. And there's also a mission which is hoping to put a seismometer onto the moon Titan, way out at the far end of the solar system. So that will have some seismology results if everything goes to plan as well.

Will - Very exciting then. Good time to be a seismologist in space.

Jessica - It's a wonderful time to be a seismologist anywhere with any of this data available.