A meteorite that hit the surface of Mars in September 2021 has provided scientists with a rare snapshot of the Red Planet’s internal structure. The impact sent seismic waves through the planet’s core that were detected by the NASA Insight lander’s sensitive seismometer. The waves suggest the presence of a molten layer of rock at the bottom of the Martian mantle, right on top of the liquid-metal core. Such a layer does not exist on Earth.
The finding, described in two separate studies in Nature, helps scientists make sense of previous measurements that pointed to an unexpectedly large, liquid-metal Martian core that extended more than halfway to the planet’s surface. The core was measured thanks to seismic shear waves, which can travel only through solid rock and bounce back to the surface when they encounter a liquid layer.
“These measurements that we took 2 years ago, they still stand,” said Amir Khan, a geophysicist at ETH Zürich who led one of the two new papers. “There is still a reflection, but it’s not from the top of the core: It’s from the top of this [liquid] silicate layer.”
The newly discovered layer is 150 kilometers thick, comprising almost a third of the volume previously attributed to the core. It was revealed by compressional seismic waves known as P waves, which, unlike shear waves, do travel through liquids. The meteorite impact generated P waves powerful enough to travel through the molten layer of rock before being reflected back to the surface when they encountered the denser metallic core. Their slow travel time revealed that the layer must be made up of molten silicate rock. Whereas the study led by Khan points to a fully molten layer, the second study, led by Henri Samuel, a geodynamicist at the Institut de Physique du Globe de Paris, points to a gradual transition between the solid and liquid mantle, with a mushy upper zone where the rocks aren’t entirely molten.
Less Dense? Makes Sense
A smaller core makes more sense to planetary scientists, who were bewildered by earlier measurements of a core that appeared to be less dense than theoretically possible.
A smaller core makes more sense to planetary scientists, who were bewildered by earlier measurements of a core that appeared to be less dense than theoretically possible. To achieve such a low density, the core would have had to contain lots of elements lighter than iron—mainly sulfur, carbon, hydrogen, and oxygen. However, Khan explained, the pressure and temperature of the Martian core wouldn’t allow for high quantities of these elements, and cosmochemists don’t think they were abundant during the formation of the terrestrial planets anyway.
“We always wondered what was going on,” Khan said. “Did we miss something?”
Although researchers are happy to dispel the question of the core’s density, the interpretation of the seismic waves might still be subject to scrutiny in future studies. “It’s just a situation we don’t have on Earth, and therefore, we are not used to dealing with such multiple liquid layers with very different wave speeds one on top of another,” said Simon Stähler, a seismologist also at ETH Zürich who wasn’t involved with either study. “I think that is something that future seismologists will have to work on more carefully.”
Additional Evidence
Insight’s seismometer, which operated from 2019 to 2022, also detected other clues that pointed to a partially molten layer somewhere inside the planet. For instance, it showed that the Martian mantle was very conducive to seismic waves, allowing the seismometer to detect even faint quakes in far-off areas of the planet. Conversely, the tides caused by Phobos, Mars’s larger moon, attenuated rapidly.
“It was difficult to reconcile that Mars was highly attenuating for Phobos’s tide but poorly attenuating at seismic frequencies.”
“It was difficult to reconcile that Mars was highly attenuating for Phobos’s tide but poorly attenuating at seismic frequencies,” Samuel said. This now makes sense, because a partially molten layer somewhere inside the planet can accommodate the deformation induced by the gravitational interaction with Phobos without affecting seismic waves traveling through the upper layers.
Furthermore, scientists already suspected that the planet must have gone through a magma ocean stage early in its history, during which it was fully molten. When such an ocean cools down, heterogeneities are expected to appear in the mantle as its elements partition and solidify at different stages. At the same time, highly radioactive elements such as thorium and uranium also sink and accumulate in this layer, contributing to keeping it molten over geologic time.
On the basis of these clues, Samuel authored a paper in early 2021 suggesting that a molten layer like the one that has now been detected could exist within Mars. “All the models before our work assumed that the mantle of Mars is homogeneous, so it’s basically assuming that its early heterogeneity has been deleted but without further explanation,” Samuel said. “At the time, there [were] no data to distinguish between a homogeneous mantle and a heterogeneous mantle, so people went for the simplest case.”
New Answers, More Questions
The presence of the newfound molten layer influenced every aspect of Mars’s thermal and chemical evolution. For instance, the layer insulates the core, increasing its temperature. This might impede the formation of a magnetic dynamo, like the one that drives Earth’s global magnetic field. Because we know that a similar field once existed on Mars—it was recorded in parts of the crust that formed while it was still active—researchers may now need to find another explanation for how such a global magnetic field was produced on early Mars.
As to why Mars has this molten layer and Earth doesn’t, the answer leads to the bigger question of how the two planets have evolved since their formation. “In the past, Mars and the Earth were much more alike: There was a magnetic field, there was liquid water, there was a thick atmosphere, there was even perhaps plate tectonics on Mars, even if it was transient,” Samuel said. Maybe, he pointed out, Earth once had a similar layer but, unlike Mars, had the ability to get rid of it. “This is a possibility, not the only one, but this one can be investigated.”
—Javier Barbuzano (@javibarbuzano), Science Writer