InSight Lander makes the best Martian depth maps to date

What is hiding on the red planet? Despite being only a tenth of the mass of Earth, Mars appears to have once been habitable like our own world, leading scientists to wonder if such a similarity affects the cores of both planets. In its bowels, is Mars still a shrunken mirror of the Earth, or is the interplanetary resemblance only to the depth of the crust?

Enticing clues have been gleaned from the gravitational data provided by past missions. But now the interior of Mars has been revealed like never before, thanks to unprecedented measurements from NASA’s InSight lander. Shortly after reaching the Martian surface in late 2018, InSight monitors seismic waves propagating across the planet and uses the echoing reflections from these “marsquakes” to map the subsoil. Only the Earth and its moon have already been subjected to such a thorough examination. The results show a world both similar to and different from ours, and offer an exciting second data point in a vast universe of rocky orbs. “InSight is like the first telescope to observe the interior of the planet,” says Michael Meyer, senior scientist in NASA’s Mars Exploration Program at the agency’s headquarters.

InSight (Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport) is not your typical mission to Mars. While others, such as the recently landed Perseverance, were sent to scientifically rich destinations that may have once supported life, InSight’s landing zone at Elysium Planitia was decidedly unremarkable, described by some as a parking “. Flat and smooth, almost without relief except for the scattered rocks and impact craters, the site was the perfect place for the stationary lander to study the Martian interior. The Seismic Experiment Instrument for the Interior Structure (SEIS), provided by the French space agency and gently placed on the surface by InSight’s robotic arm in December 2018, was encased in a dome-shaped shield, it allowing the detection of waves moving through Mars without interference from wind or sandstorm. storms. SEIS “can see movements on the order of atomic-sized vibrations,” says Andrew Lazarewicz, who participated in a 1976 attempt to detect seismic waves with a seismometer on NASA’s Viking 2 lander.

In a series of articles published today in the journal Science, the researchers describe how they used this instrument to trace seismic waves caused by dozens of earthquakes detected across the Martian interior. These events may have been caused by meteorites hitting the planet’s surface or even by movements of magma (some have been located near Cerberus Fossae, a geological formation showing signs of recent volcanic activity). At less than magnitude 4 on the current magnitude scale, all of these earthquakes were so small that they would barely be noticeable on Earth. But SEIS recorded them clearly, allowing researchers to track their reverberations inside Mars, all the way to its core, revealing what was going on inside.

Simon Stähler of the Institute of Geophysics at the Swiss Federal Institute of Technology Zurich and his colleagues measured the reflections of the waves on the nucleus to calculate its size and bulk composition. They discovered that it is probably 1,830 kilometers in radius, several hundred kilometers longer than expected. And the strength of the reflected waves suggested that they were bouncing off a core made mostly of molten iron and nickel. The size of the core was a “surprise,” Stähler says. “People thought it must be around 1,500 or 1,600 kilometers,” based on the fact that, kilogram for kilogram, Mars is a little less dense than Earth, and the core should be mostly iron. and nickel, which is heavier than rock. Instead, the results show that the ratio of Mars’ core radius to its planetary radius is similar to that of Earth, which counterintuitively means that the relatively low density Martian core must be enriched with other elements, such as sulfur and oxygen, which are comparatively less abundant. at the heart of our planet. Why the nucleus of Mars would have a different composition than ours is not clear. “If you assume that Mars was made from the same building blocks as Earth, then it’s not that easy to explain,” Stähler says.

Going outward, Amir Khan of the Institute of Geophysics and his colleagues used seismic waves to probe the mantle of Mars, the region between the planet’s core and the surface crust. Although the Earth has a lower layer of insulating liquid mantle that sits above its core, there is no such feature on our neighboring world. “This lower mantle does not exist on Mars,” says Khan. Instead, above the core, Mars’ lower mantle resembles Earth’s upper mantle, which then gives way to a cooler, more brittle upper layer called the lithosphere. The study shows that Mars’ lithosphere is about 500 kilometers thick, compared to about 250 kilometers thick for Earth’s lithosphere. Such a thick lithosphere, according to Khan, could explain why Mars today lacks plate tectonics. This supernatural configuration of underground layers could also explain how the Red Planet lost its heat because, unlike Earth, it lacks an insulating liquid mantle layer above its core.

On the surface, Brigitte Knapmeyer-Endrun from the University of Cologne in Germany and her colleagues measured the thickness of the Martian crust. They found two possibilities for the crust under InSight: One interpretation of the data suggests a two-layered crust like that of the Earth with a thickness of 20 kilometers. The other alludes to the presence of three layers totaling 39 kilometers thick. For the entire planet, the researchers estimate a crust thickness of up to 72 kilometers, or several tens of kilometers less than expected. If correct, this estimate could be an important window into the fundamental differences between the initial formation of Earth and Mars. “Most of the crust is very old and dates very early on the planet, whereas on Earth we have a lot of recycling due to plate tectonics,” says Knapmeyer-Endrun.

The results taken together reveal intriguing differences between Earth and Mars. “What they have done with this unique instrument is remarkable,” says Lazarewicz. Although these are rocky worlds that appeared in relatively close proximity to the sun, these two planets may not have formed the same way. They could have, say, merged from different mixtures of materials circulating in the disk of gas and dust that surrounded the young sun. Additionally, if InSight is able to seismically probe Mars’ inner core during its mission, it could help solve the long-standing mystery of how the planet lost its protective magnetic field, an event that allegedly occurred there. perhaps four billion years ago and which could have allowed solar winds to sweep much of the world’s atmosphere.

It wasn’t until 1889 that we made our first measurements of seismic waves passing through the Earth’s mantle, gaining a glimpse of the interior of our own world. Today, more than a century later, we have our first comparative measurements for another planet in the universe, although this may only be a glimpse of what is yet to come then. as scientists dig deeper into InSight’s data. “Now that we know the size of the nucleus and that we know more about the crust and mantle, we can reinterpret the events that we have detected so far in light of the interior pattern we have now,” says Stähler.

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