For the first time, the internal structure of a planet other than Earth is being studied using earthquakes. The target is Mars, and the success is that of NASA’s InSight spacecraft, which reached the red planet in November 2018.
The lander installed a sensitive seismometer on the surface of the red planet using a robotic arm. It was the second attempt to discover “Martemotos”, the nickname for Mars earthquakes. The first occurred with the Viking probes in the 1970s but ended in an unfortunate fate as one of them failed to properly expose the instrument to exposure and the second was insufficiently sensitive to truly identify seismic events.
All that then came out was that Mars was seismically less active than Earth, which was perhaps the most disappointing of the results from these probes.
With InSight, NASA was ready to rise to the challenge decades later. Developed at the Institute for Terrestrial Physics in Paris, the Six Seismometer (acronym for Seismic Experiment for the Internal Structure) would be installed directly on the surface of Mars by a robotic arm and then protected by a cap to reduce the effects of vibrations from the atmosphere the measurements.
It started operating in February 2019 and the instrument has been collecting data ever since. There is still a lot of “noise” created by vibrations from the atmosphere itself, but researchers have finally been able to identify the first Martemotos.
As the Vikings have indicated, Mars is seismically quieter than Earth. There are significant numbers of earthquakes, but all of them are generally quite modest. None of those already discovered has passed level 4 on the Richter scale, and if anyone were there they would only feel the bottom when they were a few miles from the epicenter.
Earthquakes are excellent tools for understanding the internal structure of a world. That’s because it’s impossible to travel to the center of a planet, but the shock waves created by the tremors move there with much less difficulty. Most importantly, as they traverse regions with different characteristics, they experience changes in speed and frequency. Based on “tell me how you are and I’ll tell you where you’ve been” scientists can use seismic waves to take an “x-ray” (note the quotation marks, nothing has x-rays) of the planet .
Here on Earth we can currently do this with many seismometers around the world, which not only gives us an excellent sense of our planet from within, but also gives us important practical applications such as understanding the distribution of earthquakes around the world and the generation of tsunami warnings.
At least on Mars, the work has to be done with just one device for the time being, which makes the challenge more complex and the uncertainties greater. Still, there’s a lot to learn, as shown by three scientific articles published in this week’s issue of Science.
“These studies provide the first direct observations of the crust, mantle and core structure of another rocky planet, results and effects that can be compared and contrasted with the properties of the earth,” comment Sanne Cottaar and Paula Koelemeijer of Cambridge University in the UK the results in the same issue of Science.
One of the works that, as first author Simon Stähler from the Swiss Federal Institute of Technology in Zurich, Switzerland, focused on the study of the Martian core is based on seismic waves that traveled there and reached the InSight seismometer.
The researchers found that Mars has a metallic-liquid iron-nickel core, similar to Earth, but proportionally much larger. With a radius of approx. 1,830 km, it extends almost halfway (Mars has a radius of 3,390 km). On the other hand, despite its size, it is far less dense than terrestrial, which leads the researchers to suspect that there must be a relatively higher proportion of lighter elements like sulfur.
They are important pieces of the puzzle trying to figure out why the red planet has lost its global magnetic field. It is known that Mars once had one through magnetization in surface rocks and that the core, like that of Earth, once acted like a dynamo, creating a protective magnetosphere. But it is currently “over”.
The second article, first written by Amir Khan, also from ETH Zurich, dealt with seismic waves that could reveal details about the structure of the Martian mantle. Instead of ricocheting off the core, they traveled directly from the epicenter of the Martemotos to the seismometer, and the researchers found that between 400 and 600 km in depth they gradually slowed down, reducing the possible boundary between the lithosphere (upper layer) and the mantle ( where there is convection of material that appears to be moving very slowly).
The third study, first authored by Brigitte Knapmeyer-Endrun from the University of Cologne, focused on examining the Martian crust, the upper part of the lithosphere. According to him, InSight’s data is consistent with two models, one would indicate a local crustal depth of around 20 km and the other 39 km. They extrapolate local data to the global scale and estimate the depth of the Martian crust to be between 24 km and 72 km.
In addition, the modeling seems to indicate that the crust should be 13 to 21 times more enriched in radioactive heat-generating elements than the mantle, an estimate far higher than that based on measurements of surface materials. (Unfortunately, InSight was unable to properly install its thermometer on Mars, which would help to get a clearer picture of these results.)
The work offers the first concrete opportunity to compare two rocky planets in our solar system, Earth and Mars. Its internal structure is a direct result of the processes that led to its formation and the history it has had over the 4.5 billion years since it emerged from the solar nebula.
And there is more to come. The InSight mission has been extended by NASA to 2022, and “the number of high-quality observations is expected to double, which offers plenty of opportunities to add detail and improve Mars models,” say Cottaar and Koelemeijer.