A deep pocket of hot, low-density material hiding in the Martian mantle may be responsible for gradually speeding up the planet’s spin, according to a peer-reviewed study that connects orbital gravity measurements to recent radio-tracking data from NASA’s InSight lander. The finding ties two previously separate lines of evidence together: a large negative mass anomaly beneath the Tharsis volcanic region and an observed acceleration of Mars’ rotation by roughly 4 milliarcseconds per year squared. If the connection holds, it would mean the Red Planet’s interior is far more dynamic than standard models have assumed, with consequences for how scientists interpret Mars’ rotational history and plan future missions.
A Hidden Mass Deficit Beneath Tharsis
The Tharsis Rise is the largest volcanic province in the solar system, a swollen dome on Mars’ western hemisphere crowned by enormous shield volcanoes and extensive lava plains. Gravity maps built from decades of orbiter tracking have long shown a strong positive gravity signal over Tharsis, ringed by a surrounding negative anomaly. But after accounting for the mass of surface topography and crustal thickness, researchers found that a significant mismatch remained at long wavelengths, one that surface geology alone could not explain.
A study in the Journal of Geophysical Research: Planets concluded that a substantial negative mass anomaly exists in the mid-mantle beneath the Tharsis Rise. The researchers modeled this deficit as hot, buoyant, or compositionally depleted mantle material sitting well below the crust, rather than as a shallow crustal feature. By systematically varying parameters such as crustal density, lithospheric elastic thickness, and the depth extent of the anomaly, they showed that only models including a deep, low-density region could reproduce the long-wavelength gravity residuals that persisted after standard lithospheric flexure corrections.
The gravity data underpinning the study comes from the GMM-3 gravity field, maintained by NASA’s Goddard Space Flight Center. GMM-3 provides spherical harmonic coefficients derived from years of Doppler and range tracking of Mars orbiters, allowing scientists to invert for subsurface density structure. The negative ring surrounding Tharsis’ gravity high, clearly visible in GMM-3 maps, was a key input for isolating the deep anomaly from shallower crustal signals. By filtering out shorter-wavelength contributions, the authors highlighted a broad deficit that they interpret as a region of unusually light mantle.
In physical terms, such a deficit could represent a long-lived mantle plume or a zone of partial melt that has remained buoyant over geological timescales. Either way, the feature implies that Mars has retained enough internal heat to sustain large-scale mantle heterogeneity, even though the planet is much smaller and cooler than Earth. That heterogeneity, in turn, affects how mass is distributed relative to the spin axis, an essential quantity for understanding rotation.
InSight Measured Mars Spinning Faster
Separately from the gravity work, scientists using the Rotation and Interior Structure Experiment (RISE) on NASA’s InSight lander discovered that Mars’ rotation is accelerating. A 2023 paper in Nature reports that the RISE instrument used precise Doppler tracking between the lander and NASA’s Deep Space Network to monitor the tiny shifts in radio frequency caused by the planet’s spin. Over several years of data, the team detected a steady increase in rotation rate corresponding to about 4 milliarcseconds per year squared.
NASA summarized the result by noting that Mars’ day is shortening by a fraction of a millisecond annually. The sources describe the unit slightly differently, with some reporting the figure as milliarcseconds per year and others as milliarcseconds per year squared, but both converge on an acceleration of roughly that magnitude. For comparison, the effect is far too small to matter for daily operations but large enough to be measurable with modern radio science.
The RISE data also yielded constraints on Mars’ deep interior, including properties of the liquid core inferred from subtle wobbles in the planet’s rotation known as nutations. A conference abstract presented at the Europlanet Science Congress described the nutation-based analysis that the InSight team used to infer core size and light-element content. Those constraints narrow the range of possible interior density profiles, providing independent context for interpreting what a mantle mass anomaly might mean for rotational dynamics. If the core radius and density are pinned down, any additional changes to the planet’s moment of inertia must come from the mantle and crust.
How a Mass Deficit Could Speed Up a Planet
The physics linking a negative mass anomaly to faster rotation rests on angular momentum and the distribution of mass around a planet’s spin axis. A planet’s spin rate depends on its moment of inertia, which is determined by how mass is arranged with respect to the axis of rotation. When mass is removed from or reduced at a given radius, the moment of inertia decreases. If angular momentum is conserved, a lower moment of inertia means a higher spin rate, the same principle that makes a figure skater spin faster by pulling their arms inward.
A large pocket of anomalously light material in the mid-mantle beneath Tharsis would reduce Mars’ effective moment of inertia compared to a uniform-density interior. In simplified terms, replacing dense mantle with lighter material at similar depths makes the planet slightly easier to spin. If the anomaly has evolved over time (for example, if the region has been heating, melting, or chemically changing), it could gradually alter the inertia tensor and contribute to the observed spin-up.
This reasoning has deep roots in planetary dynamics. A NASA technical report on Mars’ long-term dynamics explains how internal mass redistribution is linked to spin-axis evolution and rotation changes over geological timescales, while clarifying which processes can and cannot alter a planet’s rotation state. The report emphasizes that internal density changes, driven by mantle convection, phase transitions, or volcanic loading, can modify the inertia tensor even in the absence of significant external torques from the Sun or other bodies.
The Tharsis region itself has long been implicated in large-scale rotational shifts. Research on true polar wander driven by Tharsis volcanism showed that the massive volcanic and mantle-load anomalies associated with the rise can reconfigure Mars’ inertia tensor so dramatically that the entire planet reorients, moving the bulge toward the equator. If Tharsis-related mass anomalies were large enough to drive such wholesale reorientation over billions of years, it is plausible that a residual deep-mantle deficit could also produce a smaller, ongoing effect on spin rate that is now being captured by InSight.
What Remains Uncertain
The connection between the Tharsis mass deficit and the RISE spin-up measurement is suggestive but not yet sealed by a single unified model. The gravity study identifies the anomaly and demonstrates that it explains long-wavelength residual gravity patterns after standard corrections. The InSight data independently shows that Mars is spinning faster at a rate of about 4 milliarcseconds per year squared. However, no peer-reviewed paper has yet combined both datasets into a quantitative dynamical framework that predicts the observed acceleration directly from the inferred mass deficit.
Several uncertainties complicate the picture. First, the depth, shape, and exact density contrast of the Tharsis anomaly are not uniquely determined by gravity alone; alternative models with different internal structures can sometimes fit the data nearly as well. Second, the RISE measurement covers only a few Martian years, so it is not yet clear whether the observed acceleration is strictly steady, part of a longer-period oscillation, or influenced by seasonal exchanges of mass between the atmosphere and polar caps. Third, other internal processes (such as gradual core solidification or mantle phase changes far from Tharsis) could also modify the moment of inertia over time.
To test the proposed link, researchers would need to construct time-dependent interior models that track how mantle anomalies evolve thermally and chemically, then compute the resulting changes in inertia and compare them directly with the RISE-derived acceleration. Such models would have to respect the constraints from GMM-3 gravity, InSight nutation data, and broader geological evidence for the history of Tharsis volcanism and tectonism. They would also need to explore whether plausible rates of mantle evolution can produce an effect as large as the one InSight recorded.
For now, the emerging picture is that Mars is not the geologically dead world it was once thought to be. A persistent, low-density mantle region beneath Tharsis hints at ongoing internal dynamics, while the subtle but measurable spin-up captured by InSight reveals that the planet’s rotation is still changing. As future missions refine Mars’ gravity field, extend rotation measurements, and perhaps deploy new seismic or geodetic instruments, scientists will be better positioned to decide whether a hidden hot spot in the mantle is truly nudging the Red Planet into a slightly faster spin, or whether an even more complex story lies beneath the surface.
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*This article was researched with the help of AI, with human editors creating the final content.
