While the Mars rover Perseverance the US space agency Nasa prepared to take superficial rock samples Insight into the deep. The device landed near the Martian equator in November 2018. Since then, the earthquake sensor from Insight his ear on the Martian floor. Similar to a doctor with a stethoscope into a patient, the Seismic Experiment for Interior Structure (Seis) listens into the interior of Mars. The dome-shaped detector has now registered more than 1000 marsquakes.
Several international research teams are now reporting what these signals tell about the interior of Mars. Put simply, Mars has an unexpectedly thin crust, a rather thick solid upper part of the mantle, and a much larger and lighter core than expected. Three in the trade magazine Science published articles explain the details.
The Seis instrument of the Mars rover “Insight” rests on the surface of Mars and measures vibrations.
(Photo: NASA / JPL-Caltech)
“Although Mars and Earth were very similar celestial bodies when they formed, Mars went through a completely different development than the Earth,” says Domenico Giardini, Professor of Seismology and Geodynamics at ETH Zurich, who leads the studies and is involved in all three publications is. “Ultimately, we want to better understand why this is the case and whether Earth could face a similar fate as Mars.”
According to Giardini, the researchers proceeded like a playful treasure hunt: they first had to find the first station. This contained knowledge about the mantle of Mars and information on the way to the second station: the planet’s core. And so forth. The treasure to be recovered is the understanding of the origin and development of the red planet.
The first big step was to find out more about the underlying structures of Mars. Of the stronger earthquakes, the researchers were only able to detect so-called sky waves that travel into the interior of Mars. Where the structure changes, the space waves are reflected and partly reach the surface with Seis. However, these strong earthquakes did not trigger so-called surface waves. The researchers suspect the reason is that the earthquakes are too weak and a little too deep. A crucial source of information was therefore missing: only by combining both types of waves can more precise information be given about the propagation speed of the waves and the properties of the inner layers of Mars.
“It took us more than a year to solve this problem,” says Giardini. It consisted of examining almost a dozen of the 43 strong deep Marsquakes registered to date with the clearest signals. Because these were reflected back and forth like an echo several times between the inner structures of Mars and the Martian surface. “From these echoes we were able to derive information about the speed of propagation and thus about the chemistry and temperature of the mantle,” says Amir Khan, lead author of the corresponding study.
That was the first stop on the treasure hunt. It provided information about the structure of the mantle down to a depth of around 800 kilometers. The mantle is therefore very rigid down to a depth of 500 kilometers and thus belongs to the so-called lithosphere of the planet. “Overall, the mantle of Mars is a simpler version of the mantle of the earth,” says Khan. “But we still know little about the deeper mantle down to a depth of around 1500 kilometers.”
Once the propagation speed of the waves in the mantle was known, the researchers were able to search specifically for waves that were reflected by the core. To do this, they looked for special space waves, the so-called shear waves. These cannot spread in liquids. Therefore, they do not penetrate into a liquid planetary core. It was expected that the core of Mars would consist of liquid iron and nickel. “Now we know for sure,” says Giardini.
Why did light elements like sulfur and hydrogen pollute the core?
The radius of the core is around 1830 kilometers, as the researchers in Science to report. That is a little more than half the distance from the surface to the center of the planet – a similar size ratio between the core and the whole planet as with the earth. This means that the core of Mars is, firstly, larger and, secondly, much lighter than expected. This means: “In addition to the heavy elements such as iron and nickel, lighter elements such as sulfur, carbon, oxygen and hydrogen must also be present in the core,” says Simon Stähler from the Institute for Geophysics at ETH Zurich, lead author of the core study. A big question now is: How did these light elements get to the core? According to Stähler, part of the explanation is that Mars was formed earlier than Earth and is made of different material.
The liquid planetary core poses another mystery to the scientists: it was supposed to generate a magnetic field. “The magnetic field on Mars was very active for around a billion years,” says Giardini. “Then it disappeared. We don’t know why.”
In the Cerberus Fossae region, researchers have measured some of the strongest Martian quakes to date.
(Photo: NASA / JPL-Caltech / University of Arizona)
Now the researchers are looking for signals from quakes on the other side of the planet, whose waves penetrate the core, in order to learn more about it. Because the lack of a magnetic field is a decisive factor that distinguishes Mars from Earth: Without a magnetic field, there is no protection from the solar wind. This has gradually blown the once existing atmosphere of Mars out into space and thinned it out considerably.
The third study is about the crust of Mars. To this end, researchers working with Brigitte Knapmeyer-Endrun from the Bensberg earthquake station at the University of Cologne examined rather weak earthquakes with high-frequency signals. Their origin lies at a shallow depth in the crust. These tremors are literally trapped in the crust and propagate almost undamped.
The studies help to better understand other planets and exoplanets
“It has been shown that the crust is much thinner than expected,” says Giardini. It is between 24 and 45 kilometers thick. Models had suggested a thickness between 30 and 90 kilometers.
“These three studies severely limit the possible internal structures of Mars today,” write Sanne Cottaar and Paula Koelemeijer from the University of Cambridge in one also in Science published itemsFiling the three studies. “This will improve our understanding of how the planet was formed billions of years ago and how it evolved over time.” According to Stähler, the current studies are very useful for validating the models of planet formation. “This helps us to better understand other Earth-like planets or exoplanets, on which we can never land and operate a seismometer.” Another station to be found, according to Giardini, is the exact distribution of the water. Occurrences are known at the poles. However, it is unclear at what depths and in what quantities water is present in the interior of Mars.