Scientists have found evidence of strange mass anomalies buried deep underground on Mars, according to a study based on observations captured by NASA’s InSight lander. The new research also reports a slight acceleration in the planet’s rotation and bolsters the theory that Mars lacks a solid inner core, which has implications for its past habitability.
These latest results from InSight, which ended its mission in December 2022, open a new window into Mars’ subterranean layers, including its core and mantle, and suggest that its rotational rate may shift due to unexplained dynamics in its interior, atmosphere, or ice caps. In addition to probing the modern structure of Mars, InSight’s observations yield details about the formation and evolution of the planet over the past 4.5 billion years, which could shed light on the tantalizing question of whether it was ever inhabited by alien life.
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InSight—which stands for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport—landed in the equatorial Elysium Planitia region of Mars on November 26, 2018. Over the course of the next four years, the lander detected more than 1,300 “marsquakes,” which are the Martian version of earthquakes, allowing it to peer into Mars by capturing seismic waves that provided an unprecedented look at the innards of an alien planet.
While many of InSight’s discoveries emerged from its seismic recordings, the mission was also equipped with a fascinating system called the Rotation and Interior Structure Experiment (RISE) that bounced radio signals across interplanetary space between the lander and giant radio antennas on Earth.
Now, researchers led by Sébastien Le Maistre, a planetary scientist at the Royal Observatory of Belgium, have used RISE “to determine fundamental properties of the core, mantle, and atmosphere of Mars,” leading to a host of astonishing discoveries, according to a study published on Wednesday in Nature.
“Basically all [of it] was surprising somehow!” Le Maistre told Motherboard in an email. “The slow acceleration in rotation is a big surprise that no theory has predicted so far. The almost hydrostatic shape of the core (i.e. the core shape of a fluid rotating planet) was also unexpected since the surface and crust of Mars are not in hydrostatic equilibrium! We thought long and hard to identify plausible processes that could explain that unexpected flattening of the core.”
“Despite the fact that the RISE data is more accurate than I had expected before the spacecraft arrived on Mars, the level of effort it took us to extract the target signal from it was surprisingly high,” he added.
The new RISE results hint that internal “mass anomalies,” which are regions with distinct densities and compositions from their surroundings, are buried in Mars’ mantle, which sits about 12 miles under the planet’s surface, and extends down for another 969 miles until it hits the core.
“The shape and gravity field of Mars are mainly determined by its rotation rate and to a lesser extent by surface and internal mass anomalies,” Le Maistre explained. “It has been shown in previous studies that the observed shape and gravity field can be explained by the load induced by the surface topography and from a relief (or mass anomaly) along the crust–mantle interface (the MOHO).”
“The implied core shape is, however, not in agreement with the results of our study that either require a mass anomaly several hundredth of kilometers deeper—at the bottom of the lithosphere—or at the MOHO and bottom of the mantle,” he continued. “A present-day thick lithosphere provides constraints about the cooling rate of the planet and repartition of heat-producing radioactive elements within the mantle and crust. Mass anomalies within the lithosphere are likely of chemical origin and from density anomalies locked inside the stiff lithosphere resulting from lateral temperature variations.”
RISE allowed scientists to track InSight’s location to within a foot, even across the hundreds of millions of miles that separate Mars and Earth. This sensitivity produced incredibly accurate measurements of the rotation of Mars, as well as its wobbling axis, which are both influenced by the interior composition of the planet. So while InSight’s seismometer glimpsed Mars’ interior using marsquakes, RISE could explore subterranean structures with its keen sense of the planet’s rotational and axial movements.
Le Maistre is part of a team headed by Véronique Dehant, a geophysicist at the Royal Observatory of Belgium and co-author of the study, that is now leading the analysis of RISE data. The new study contains observations collected between November 27, 2018—literally the day after InSight landed on Mars—all the way through to May 31, 2021. This large dataset spanned 890 Martian days, known as sols, and included 544 tracking passes with about 25,000 individual data-points.
For decades, scientists have debated whether Mars has a solid inner core, similar to the center of Earth, or if its core is made wholly of molten liquid. By studying the minute details of Mars’ rotation with RISE, Le Maistre’s team was able to separately characterize the core and mantle layers of Mars, revealing that the core is likely entirely made of molten iron-sulfur alloy that extends across a radius of about 1,170 miles.
“The absence of an inner core informs about the present-day temperature inside the core if complemented with our knowledge about its chemical composition,” Le Maistre said.
These findings have indirect implications for assessing whether Mars once hosted life billions of years ago, during an ancient era when the planet had a thicker atmosphere, a protective magnetic field, and liquid water on its surface. Whereas Earth’s solid inner core still powers our planet’s magnetic field, Mars may have become inhospitable to life in part due to its lack of a solid core, resulting in the collapse of its nascent magnetic field.
“The thermal state and composition of the core are important for the history of the magnetic dynamo, which in turn could have important consequences for the retention of the atmosphere and the possible habitability of the surface early in Mars’ history,” Le Maistre said.
The absence of a solid core is also “in agreement with the early cessation of the core-generated magnetic field about [three billion years] ago,” he noted.
Last but not least, the researchers identified a slight increase in the rotational spin of Mars in the RISE data, which remains unexplained. The team speculated that this slow acceleration could be driven by long-term shifts in the interior of Mars, or potentially by a small decrease in atmospheric pressure linked to the gradual accumulation of ice at the polar caps.
“The reasons why Mars has accelerated slightly over the past 40 years can be multiple (i.e. several reasons at once),” Le Maistre said. “It could even be an observational artifact, although we find this in independent datasets, processed with different software and teams.”
“Future observations could definitely help solve this mystery first by confirming the signal and second by constraining its nature (for instance if it is seen in pressure data, one could conclude that it is related to the atmosphere of the planet),” he continued, noting that his team plans to publish new research on the topic in the future.
InSight died at the end of last year after its solar panels became so dusty that it could no longer generate power. But the new study shows that the mission continues to live on through its unrivaled observations of the Martian interior, which have resolved fundamental mysteries about our neighboring world, even as they also raise evocative new questions for future missions.
“There is actually more to do with RISE data than has been already done, despite the fact that InSight already retired,” Le Maistre concluded. “For instance, we hope that soon, by analyzing the complete RISE dataset, we will be able to further constrain not only the interior, but also the dynamics of the Martian atmosphere and its interaction with the planet’s ice caps.”
