Mars Has a Warm Region Deep Inside — And It's Reshaping Our Understanding of the Red Planet
Something extraordinary is stirring deep beneath the rusty surface of Mars. A team of researchers at the University of Arizona and Brown University, led by planetary scientist Alexander Byrne, has discovered a striking thermal anomaly lurking within the Martian mantle beneath the planet's southern highlands. Using a cutting-edge geophysical technique known as tidal tomography, the scientists have identified a subsurface region that measures approximately 200 to 400 degrees Celsius warmer than the surrounding mantle — a discovery that could fundamentally alter our understanding of how Mars formed, evolved, and whether it remains geologically active today.
The finding raises profound questions about the internal life of Mars — a world long assumed to be a cold, geologically dead planet. If confirmed and better characterized, this warm anomaly may rewrite the story of Martian geodynamics and even shed new light on the history of water and volcanism on the Red Planet.
Reading Mars From the Outside In: The Science of Tidal Tomography
To understand how scientists detected a warm region hundreds of kilometers beneath the Martian surface without ever drilling into it, one must first appreciate the elegance of tidal tomography. This scientific technique leverages the gravitational interactions between a planet and its host star — in this case, Mars and the Sun — to probe the interior of a world from the outside.
As Mars travels along its elliptical orbit, the Sun's gravitational pull exerts a constantly varying force on the planet. This force gently stretches and compresses the Martian body, producing what scientists call a tidal bulge. Because Mars's orbit is not a perfect circle, the intensity of this gravitational pull fluctuates throughout the Martian year, causing the planet's shape to subtly deform over time.
"As Mars travels around the Sun, the Sun's gravity gently stretches and squeezes the planet. Because Mars' orbit is not a perfect circle, the strength of this pull changes over the course of a Martian year, so the amount that Mars deforms changes as well. This deformation is called a tidal bulge. This tidal bulge causes mass inside the planet to shift slightly, producing tiny changes in its gravitational pull. An orbiter passing over a region with slightly stronger gravity speeds up a little, while one passing over slightly weaker gravity slows down. So, we can use the measured speed changes of orbiters to estimate the amount Mars is squishing."
— Nick Wagner, Postdoctoral Researcher, Brown University
These minute velocity changes in orbiting spacecraft — measured with extraordinary precision using Doppler tracking — serve as a kind of seismograph for the entire planet. By mapping how Mars deforms differently in different regions, scientists can infer the composition, temperature, and physical properties of the underlying mantle and crust. Think of it as using gravity itself as a medical imaging tool, performing an MRI on an entire planet from orbit.
This technique is remarkably powerful because it is non-invasive and leverages existing spacecraft assets. Unlike traditional seismology, which requires instruments placed directly on or within a planetary body, tidal tomography can be applied remotely to any world orbited by spacecraft with precise tracking capabilities.
Sixteen Years of Orbital Data Reveal a Hidden Heat Source
The discovery did not emerge overnight. Byrne and his colleagues painstakingly analyzed 16 years of tracking data collected from three veteran NASA spacecraft:
- Mars Global Surveyor (MGS) — Operated from 1997 to 2006, providing the foundational dataset for Martian gravitational mapping.
- Mars Odyssey — Launched in 2001 and still operational, offering decades of continuous orbital monitoring.
- Mars Reconnaissance Orbiter (MRO) — In service since 2006, equipped with advanced telecommunications allowing highly precise Doppler tracking.
Temporal variations in the gravitational field data revealed a clear and consistent signal: a thermal anomaly within Mars's mantle concentrated beneath the planet's southern highlands. This region, already notable for its greater crustal thickness compared to the northern lowlands, appears to harbor a warmer-than-expected mantle that may be causing partial melting at depth.
Crucially, the researchers believe this upwelling of hot material is stalled before it reaches the surface, blocked by the comparatively thick southern crust. This could explain several puzzling features of the Martian south, including enhanced magnetization of the ancient southern crust and localized thickening of crustal material — both of which are consistent with a history of magmatic underplating driven by this subsurface heat source.
Mars's Crustal Dichotomy: A Planet of Two Faces
One of the most enduring mysteries in planetary science is the so-called crustal dichotomy of Mars — the dramatic difference in elevation, crustal thickness, and geological character between the planet's northern and southern hemispheres. The northern lowlands are geologically younger, smoother, and underlain by a relatively thin crust, while the southern highlands are ancient, heavily cratered, and topped by crust that can be twice as thick.
The new findings from Byrne's team add a crucial thermal dimension to this long-studied divide. The mantle beneath the northern hemisphere appears comparatively cool and thin, while the southern mantle harbors this newly identified warm anomaly. This thermal asymmetry could be both a cause and a consequence of the crustal dichotomy, suggesting that the two hemispheres have had fundamentally different geological histories since the earliest epochs of Martian evolution.
Several competing hypotheses exist to explain the crustal dichotomy, including a single giant impact in the north, degree-one mantle convection, or a cluster of multiple impacts in the southern hemisphere. The newly identified thermal anomaly adds new constraints to all of these models. As the researchers note, the extra insulation provided by the thick southern crust may have preserved the warm mantle region for billions of years, while the thinner northern crust allowed heat to escape more efficiently into space over geologic time.
The Origin of the Warm Spot: Ancient History or Cosmic Collision?
Perhaps the most tantalizing aspect of this discovery is the unresolved question of how the thermal anomaly came to exist in the first place. The researchers propose several competing explanations, each with profound implications for Martian history:
- Primordial origin: The warm region may be a remnant of Mars's original formation approximately 4.5 billion years ago. If the planet accreted asymmetrically, with greater concentrations of heat-producing radioactive elements — such as uranium, thorium, and potassium — in the south, this could have established a long-lasting thermal anomaly that persists to the present day.
- Ancient giant impact: A massive impactor striking early Mars could have deposited enormous amounts of thermal energy into the southern mantle, kick-starting a localized heating episode that still echoes billions of years later. Such an impact scenario has already been proposed to explain the crustal dichotomy itself.
- Compositional heterogeneity: Like the Large Low Shear Velocity Provinces (LLSVPs) discovered beneath Earth's Pacific Ocean and African continent, the Martian anomaly may involve not just a temperature difference but also a distinct chemical composition in the mantle. Wagner explicitly raises this possibility, suggesting that a compositional component — perhaps an enrichment in incompatible heat-producing elements — could be sustaining the anomaly over cosmic timescales.
- Deep mantle plume activity: A thermochemical plume rising from the deep mantle or even the core-mantle boundary could be delivering heat to the southern hemisphere, analogous to mantle plume systems responsible for hotspot volcanism on Earth and potentially linked to the massive Tharsis volcanic province on Mars.
"What's interesting is that there is also a compositional component, meaning it's not just a temperature difference. I think this Martian anomaly may also have a compositional component, but a follow-up study will need to be done."
— Nick Wagner, Brown University
Tidal Forcing Across the Solar System: Mars in Planetary Context
Mars is far from alone in experiencing tidally induced internal deformation. Across the solar system, tidal forcing plays a critical role in shaping the geology, thermal evolution, and even the potential habitability of planetary bodies.
The most dramatic example is Io, the innermost of Jupiter's four Galilean moons. Caught in a gravitational tug-of-war between Jupiter and its neighboring moon Europa, Io is subjected to relentless tidal stretching and squeezing that generates enough internal heat to make it the most volcanically active body in the solar system. Hundreds of active volcanoes constantly resurface Io, producing a world utterly unlike any other.
On Earth, similar phenomena have been studied using dense networks of GPS stations. Byrne's colleague Harriet Lau, a postdoctoral supervisor at Brown University, used comparable techniques to constrain the buoyancy and physical properties of the LLSVPs — enormous structures in the deep Earth that influence mantle circulation and may be linked to major episodes of flood basalt volcanism and even mass extinctions at the surface.
Looking beyond Mars, scientists are eagerly anticipating the application of tidal tomography to other compelling targets in the outer solar system. The Jupiter Icy Moons Explorer (JUICE), a flagship mission of the European Space Agency, will carry a dedicated radio science experiment designed to precisely map the gravitational field of Ganymede, Jupiter's largest moon. By measuring Ganymede's tidal deformation, scientists hope to determine whether a deep liquid water ocean lies beneath its icy shell — a finding that would have enormous implications for the search for life beyond Earth.
Other worlds ripe for tidal tomographic investigation include:
- Enceladus — Saturn's small but spectacular moon, already known to harbor a global subsurface ocean and active water-vapor plumes erupting from its south pole.
- Mercury — The innermost planet, whose anomalously large iron core and surprisingly active geology make it a fascinating target for tidal studies.
- Europa — Jupiter's ice-covered ocean moon, arguably the most promising candidate for extraterrestrial life in the inner solar system.
Implications for Mars's Geological Present and Future
The discovery of a warm mantle anomaly challenges the prevailing view of Mars as a geologically inert world. While Mars lacks the plate tectonics that continually recycle and refresh Earth's crust, growing evidence — including marsquakes detected by NASA's InSight lander and observations of geologically recent volcanic features in the Elysium Planitia region — suggests that Mars may still be geologically active in localized ways.
If the southern mantle anomaly is indeed driving partial melting beneath the crust, it raises the possibility that magmatic activity could still be occurring at depth in the southern hemisphere, even if it never breaches the surface. Such activity could have sustained subsurface hydrothermal systems in the geologically recent past — environments that, on Earth, are known to support diverse microbial ecosystems. This lends new relevance to Mars exploration strategies focused on the search for past or present life.
Continued orbital measurements by existing and future spacecraft will be essential to further characterize this anomaly. As the researchers note, forthcoming studies could help scientists:
- Refine models of crustal thickness variations and the origins of Mars's hemispheric dichotomy.
- Constrain the thermal evolution of the Martian interior over geological time.
- Assess the likelihood and scale of ongoing or recent deep volcanic activity and magma production.
- Better understand the ancient Martian magnetic field, whose remnant signatures are concentrated in the magnetized southern highlands.
A New Window Into Planetary Interiors
Ultimately, the work of Byrne, Wagner, and their colleagues represents a landmark application of tidal tomography to Mars — and a powerful demonstration of what can be achieved by creatively mining data from existing planetary missions. By combining 16 years of precise orbital tracking data with sophisticated geophysical modeling, the team has opened a new window into the deep interior of the Red Planet, revealing a surprising and scientifically rich thermal anomaly that demands further investigation.
As planetary scientists continue to refine these techniques and apply them to worlds across the solar system, tidal tomography is poised to become one of the most powerful tools in the exploration of planetary interiors — allowing us to peer into the hidden hearts of worlds without ever setting foot — or drill bit — upon them.
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