Martian sounds reveal the secrets of the red planet's core

Geoff Marsh

In November 2018, the InSight lander arrived on Mars equipped with a seismometer with the ultimate goal of determining the interior structure of the red planet.

Amir Khan

Of all the geophysical techniques, seismology is the most important simply because it has the highest resolution.

Geoff Marsh

This is seismologist Amir Khan, from ETH Zurich.

Amir Khan

There are other methods to sound the interior structure of the planet. But all of these other methods, they don't have the same resolution. As I said, we don't have the same detail that we have with seismology. And that's certainly what we've seen on the Earth from you know, more than 100 years of seismic monitoring of the Earth. And also, the Moon right, the Apollo missions landed on the Moon and seismology was also a very important tool for looking inside the Moon.

<rumbling sounds>

Geoff Marsh

For the past four years, Amir has been analysing the planetary rumblings of Mars trying to paint a picture of the density and composition of its insides. But initially, hunting for marsquakes came with several challenges.

Amir Khan

You know, when you come to a new planet, that's kind of a difficult thing, because you don't know what to look for. You know about earthquakes and we also know about moonquakes but what does a marsquake look like?

<rumbling sounds>

Amir Khan

The Martian seismometer sitting on the surface of the planet has to first of all be covered by a so-called wind and thermal shield, because the variations in temperature are extreme.

<wind sounds>

Amir Khan

And then there is of course the wind, you want to shield the seismometer from the wind because if not, that would shake your seismometer, and it will be picking up that shaking rather than the shaking of the planet.

<wind sounds>

Amir Khan

For only about one third of the day, i.e. six to eight hours, is Mars quiet enough that we could actually look for marsquakes the rest of the day — that is during the day itself, the Martian day — there's too much noise produced by the wind that simply buries any marsquakes in the noise.

<wind sounds>

Geoff Marsh

It's not just the time of the Martian day muffling marsquakes, the time of Martian year also turns out to be important.

Amir Khan

So, we landed in November, and that was at the end of the winter season on Mars. And we sort of slowly moved into the spring and summer. Remember that a Martian year is about two Earth years. But this noise this wind activity is strongly correlated with the seasons on Mars, so that during the autumn and the winter, we have these strong storms, and actually during the entire day, you cannot see any marsquakes because it's too noisy.

<sounds>

Geoff Marsh

Eventually, Amir and the rest of the marsquake service found their sweet spot, the quiet evening periods in the Mars' spring and summer, when the noise was low enough to get nice clear marsquakes.

<rumbling sound>

Amir Khan

Over the four years, almost, that this instrument has been in action we have discovered more than 1300 marsquakes.

<rumbling sound>

Geoff Marsh

Within each marsquake, there are many different waves that have taken different paths through the Martian interior. But together they start to paint a picture of what lies beneath the surface.

<rumbling sound>

Amir Khan

So, um, seismic waves travelling the interior of Mars or the Earth or the Moon, for that matter, in terms of the different waves, they're the pressure waves travelled just like sound waves, but they travel faster than the other waves which are called S waves or shear waves, which have a vibrational pattern that is perpendicular to that of in the sense of travel, and they come after the pressure wave or the P wave. So, I can see my P wave coming in. And then a little later, depending on the distance, I can see my S wave coming in. The second thing I can look for is to look at what we seismologists call the polarisation which is essentially the particle motion, you know as in moving purely up/down, is it moving in circles, is it moving east, west or north, south? I can use that to infer the direction. Now, you can have another seismic wave that travels from your marsquake. And instead of travelling the straightest path to the inside lander, you'll have another wave that sort of travels upwards, gets reflected underneath the surface of the planet, and then bounces back into the interior. And there are many seismic waves like that, that will bounce not just a single time underneath the surface of the planet, but twice, three times, the PP and the Triple P and a bunch of other phases that have interacted with the interior of the planet.

<rumbling sound>

Geoff Marsh

What other information do you need to be able to make educated guesses then about Mars' core, for example, because I guess you could interpret that seismic data in different ways if you had different assumptions about what Mars was made of?

Amir Khan

In order to go from seismic velocities to the composition of the core, for example, to say whether it's liquid or solid, and what is it made of. For that you would like to try to compare with analogues of those potential materials that you measure in the laboratory. What you want to do is to take metal, iron metal and squeeze it to high temperature and high pressure and then measure say the P wave velocity, and that P wave velocity, we can then compare with the P wave velocity that we have measured with InSight.

Geoff Marsh

Using these techniques, Amir and his colleagues published a paper a while back, which raised some questions about the Martian core.

Amir Khan

About two years ago, we published a paper in Science, saying that the Martian core was about 50% radius, you know, roughly the same order as on Earth. However, the density turned out to be so low, the mean Martian core density was six grammes per cubic centimetre. Now, you compare that to pure liquid iron at the same pressure and temperatures, that's about eight grammes per cubic centimetre, it would have to contain more than 20 weight percent of light elements, oxygen and sulphur and hydrogen and carbon, which is such a significant amount that it was just very difficult to understand what exactly was going on here in the interior of Mars. Did we do something wrong? Or is there something we're missing out on? But, given the data that we had back in those days, i.e. two years ago. You know, we just kept wondering about this number. Why is it so low?

Geoff Marsh

But luck was on the marsquakes service’s side.

<meteorite sound>

Amir Khan

All of a sudden, this meteorite impact happened and produced a lot of seismic energy that travelled through the interior of the planet, the deepest part, the central part, the core. With a couple of colleagues here at ETH, we got together and said, oh, well, now we can reanalyse this problem based on data about the interior of the planet, which we didn't have two years ago, because there we only had the size of the core, there was no energy that was transiting the core itself. So, with my colleagues, Dongyang Huang and Paolo Sossi we said well, what we need is to simulate those properties, i.e. seismic wave velocities, and the density of these alloys: iron, nickel, metal alloys on a computer. And, um, through that, we systematically calculated all of these different properties for iron, nickel, and you know, all of these carbons, sulphur, oxygen, and hydrogen, all these light alloying elements. And what we observed was that it was very difficult in the sense of impossible to simultaneously fit the density and the P wave velocity at the core-mantle boundary that we had determined two years ago. It just didn't make any sense. It took us a while to figure out that actually, what we were thinking was the, you know, the outermost liquid metal core, does not represent the outermost core, but it represents the lowermost mantle, the bottom of the mantle.

Geoff Marsh

And it's not just semantics, it's a completely different thing. It's–

Amir Khan

–exactly–

Geoff Marsh

–it's, it's, a rocky–

Amir Khan

–it's a rocky layer instead of a metal layer consisting, of course, of silicone liquids. That's why the size of the core has decreased in this new study.

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Geoff Marsh

Are you saying that that meteorite strike on the other side of the planet was like a real stroke of luck? Could you have done this paper without that meteorite strike?

Amir Khan

No, there is no way because there is another far-side marsquake that we have seen, but it's not as clear, it's not as strong as this meteorite impact.

Geoff Marsh

So, you've got this new idea that Mars has a smaller core that reaches about halfway to its surface, and then that is surrounded by this 150 kilometre thick layer of hot molten rock. What does that current picture of Mars tell you about its evolution? What does it tell you about its past?

Amir Khan

Well, first of all, what it did is, of course, if you decrease the size of the core, you also increase the density. So, in terms of that composition, you know, that ridiculous composition with this low mean density that we had seen two years ago, we sort of solved that problem, right, because the mean density of the planet has now gone up. And you therefore need less light elements. And we sort of have the same kind of about 90% of iron, nickel and 10% of light elements making up Mars’ core, which is very Earth-like, if you want. The other difference, if I can, talking about the past, or the history of Mars is, you know, the Earth has a liquid outer core and a solid inner core. And the freezing of the outer core is what is believed to provide the energy to drive the geodynamo, which is what provides us with a magnetic field around the Earth, right. Mars today does not have a magnetic field. It is believed to have had one maybe 4 billion years ago, but that shut down because the conditions for maintaining the dynamo ceased to exist. However, at some point, Mars is cooling down at the moment, right, and the liquid iron metal will start to freeze and as in the Earth produce a solid inner core. At which point, you know, the Martian dynamo may pick up again and produce a Martian magnetic field around Mars.

<music>

Geoff Marsh

So, the InSight lander with its ear to the ground has diligently listened to hundreds of marsquakes and a couple of lucky meteorite impacts which researchers will doubtless pour over for decades to come. But this data set ends here. The InSight seismometer is no longer listening.

Amir Khan

Well, it's actually, it ceased operation already. What happened was that the solar panels providing the power source for seismometer, to dry the seismometer, got more and more dusty. As a result of which we simply ran out of energy. You know, you have to keep the seismometer in particular the electronics you have to keep them warm. And once you don't have this ability to heat the electronics, it just dies.

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