Seismic waves reveal complexities in Mars’ mantle

14 Nov 2023

Complex interior: illustration showing the two different layers of the Martian mantle between the core and crust. Also shown is how the layers affect seismic waves detected by InSight. (Courtesy: IPGP-CNES)

Mars’ mantle is divided into a partially molten outer layer and a fully molten, silicon-rich layer that lies closer to the planet’s core. This discovery was made by two independent teams and challenges the previous view that the mantle – which lies between the Martian crust and core – has a uniform composition and structure. The new analyses used seismic data from NASA’s InSight Mars lander and could help shape our understanding of how the red planet formed and evolved.

Some of the seismic waves studied were created by meteorites impacting the planet. The waves will have traveled deep within Mars before they reach InSight’s seismometer, and studying them provides important information about the Martian interior.

“Such large epicentral distances allowed for the propagation of compressional waves that travelled in the lowermost Martian mantle as a diffracted wave,” explains Henri Samuel at CNRS in Paris, who headed one of the studies. “It was found that the propagation of these waves was too slow to be explained by a homogeneous mantle.”

Surprising abundance

The research has also provided further clues about the elemental composition of the Martian core. Previously, this had been calculated to contain a surprisingly high abundance of lighter elements, including carbon, oxygen, and hydrogen. However, these latest studies suggest that these lighter elements are not as common as had been predicted and the core is smaller and denser than previously thought.

The other study was led by Amir Khan at ETH Zurich, who explains, “This need for a large complement of these [lighter] elements posed serious cosmochemical problems, since it is difficult to imagine how Mars would have accreted such a large proportion of light elements, and sequestered them into its core”.

In their respective studies, Samuel and Khan’s teams both performed inversions of InSight’s seismic data – a mathematical technique that transforms the information into subsurface models of a planetary interior.

Afterward, each team took a slightly different approach to interpreting their inversions. For Khan and colleagues, this involved building up their calculations from first principles. “We computed the seismic wave speeds and density of iron–nickel light element alloys using quantum mechanics, which is completely novel for the conditions equivalent to Mars’s core,” Khan explains.

Attenuating structures

Samuel’s team went beyond considerations of density, composition, and seismic velocity and looked at how the interior structure of Mars attenuated seismic waves. “From this, we were able to infer the first attenuation structure model of Mars’s mantle based on seismological and other geophysical data,” he explains.

Yet even with these different methods, both teams came to a surprising conclusion. “Unlike the Earth, Mars appears to have a strongly stratified mantle with this enriched silicate layer above its core,” Samuel says. “The lower part of the layer is fully molten, while the thinner upper part is partially molten.”

Khan explains that his team reached a very similar conclusion. “The composition of the molten layer in our calculations is very close to that of the silicate mantle, which helps explain our finding of a slightly denser silicate layer relative to the mantle. The fact that the silicate is found to be slightly denser also explains why the layer remains stable at the bottom of the mantle.”

Despite the similarities in their results, the teams’ differing approaches allowed them to explore different implications of their discovery. For Samuel’s team, revealing the mantle’s structure in terms of attenuation allowed them to better explain the orbital path of Mars’ closest moon, Phobos.

Gravitational field

According to the team, a molten silicon layer could deform more easily under the moon’s tidal forces than would the colder, partially molten layer above. This would better explain the relationship between Mars’ gravitational field and Phobos’ orbit while staying consistent with InSight’s measurements.

Through their own examination of Mars’ core, Khan’s team calculated that about 9–15% of its mass is made up of light elements. In terms of models of how Mars formed, this lower abundance seems more reasonable than the estimates of over 20% made in previous studies based on the assumption of a homogeneous mantle.READ MORE

NASA craft provides an insight into Mars’ interior

For both teams, the discovery marks a turning point in our understanding of how Mars first formed and evolved over the past 4.5 billion years. “With the presence of the stratification in the Martian mantle, we need to go back to re-analyze and re-interpret the roughly four-year-long seismic record and all other geophysical observables in the light of this new paradigm,” Samuel says. “This could lead to additional discoveries on the deep structure of the Martian mantle and its core.”

Beyond improving our knowledge of Mars, the result could also help astronomers gain a better understanding of rocky planets beyond the solar system. “Through the acquisition of new data and new methods of analysis, we make new discoveries and keep refining and updating our current understanding of what the terrestrial planets are made of,” adds Khan. “Ultimately, this will be needed for understanding the origin and evolution of extrasolar planetary systems.”

Both teams report their studies in Nature. The Samuel paper is here and the Khan paper here.

Sam Jarman is a science writer based in the UK

FROM PHYSICSWORLD.COM    15/11/2023