For the first time, a planet other than Earth has its internal structure probed from the record of 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 used a robotic arm to install a sensitive seismometer on the surface of the red planet. It was the second attempt to detect “martemotos”, the nickname given to Martian earthquakes. The first occurred with the Viking probes, in the 1970s, but ended up ill-fated because, in one of them, the proper exposure of the instrument failed and, in the second, the sensitivity was insufficient to identify genuinely seismic events.
All that came out then was that Mars was less seismically active than Earth, in what was perhaps the most disappointing of the results from those probes.
With InSight, decades later, NASA was ready to return to the challenge. The Seismometer Six (English acronym for Seismic Experiment for the Internal Structure), developed at the Institute of Terrestrial Physics in Paris, would be installed by a robotic arm directly on the surface of Mars and then protected by a cap to reduce the impact of vibrations from the atmosphere about the measurements.
Its operations began in February 2019 and, since then, the instrument has been collecting data. There is still a lot of “noise” produced by vibrations from the atmosphere itself, but researchers were able to finally detect the first martemotos.
As the Vikings already suggested, Mars is much more seismically quiet than Earth. There are a significant number of earthquakes, but all are generally quite modest. None of those already detected have surpassed magnitude 4 on the Richter scale, and if anyone were there, they would only be able to feel the ground shake if they were a few miles from the epicenter.
Earthquakes are excellent tools for identifying the inner structure of a world. That’s because it’s impossible to travel to the center of any planet, but the shock waves generated by the tremors travel there with much less difficulty. And most importantly, as they pass through regions with different properties, they experience changes in speed and frequency. So, on the basis of “tell me how you’re doing, and I’ll tell you where you’ve been”, scientists can use seismic waves to construct an “radiograph” (notice the quotes, nothing to do with X-rays) of the planet.
Here on Earth, at this point, we can do this with many seismometers around the world, which allows us not only to have an excellent sense of what our planet is like from the inside, but we can also have important practical applications, such as understanding the distribution of earthquakes. across the globe and the generation of tsunami alerts.
On Mars, for now at least, the work needs to be done with just one piece of equipment, which makes the challenge more complex and the uncertainties greater. Still, you can learn a lot, as shown by three scientific articles published in this week’s issue of the journal Science.
“These studies provide the first direct observations of the crust, mantle and core structure of another rocky planet, results and implications that can be compared and contrasted with the Earth’s characteristics,” say Sanne Cottaar and Paula Koelemeijer of Cambridge University in United Kingdom, commenting on the results in the same issue of Science.
One of the works, which has as its first author Simon Stähler, from the Federal Institute of Technology in Zurich, Switzerland, focused on the investigation of the Martian nucleus, based on seismic waves that traveled there, were bounced and reached the InSight seismometer.
The researchers determined that Mars has a metallic liquid iron-nickel core, similar to Earth, but that it is proportionately much larger. With a radius of approximately 1,830 km, it goes almost halfway to the surface (Mars has a radius of 3,390 km). On the other hand, despite its size, it is much less dense than terrestrial, which makes researchers suggest that there must be a relatively higher proportion of lighter elements, such as sulfur.
They are important pieces of the puzzle that tries to portray why the red planet lost its global magnetic field. It is known, through magnetization in surface rocks, that Mars once had one, and the core, like the Earth’s, once acted like a dynamo, producing a protective magnetosphere. But currently it is “off”.
The second article, first authored by Amir Khan, also from the Federal Institute of Technology in Zurich, focused on seismic waves that could reveal details about the structure of the mantle of Mars. Instead of bouncing off the core, they traveled directly from the epicenter of the martemotos to the seismometer, and the researchers determined that they gradually slowed down between 400 and 600 km in depth, revealing the possible boundary between the lithosphere (the upper layer) and the mantle (where there is convection of material, moving very slowly, it seems).
The third study, first authored by Brigitte Knapmeyer-Endrun, from the University of Cologne, Germany, focused on investigating the Martian crust, the upper part of the lithosphere. According to him, InSight’s data is consistent with two models, one that would indicate a local crust depth of about 20 km, and the other, 39 km. Extrapolating local data to the global scale, they estimate the Martian crust to be between 24 km and 72 km deep.
What’s more, the modeling seems to suggest that the crust should be 13 to 21 times more enriched in radioactive heat-generating elements than the mantle, an estimate far greater than that based on measurements of surface materials. (Unfortunately, InSight was unable to properly install its thermometer on Mars, which would help create a clearer picture of these results.)
The works offer the first concrete chance 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 in the last 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, creating many opportunities to add detail and enhance Mars models,” say Cottaar and Koelemeijer.