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  • Researchers used sixteen years of orbital gravity data to model Mars' interior, revealing a sharp thermal boundary between hemispheres.
  • The southern mantle appears hundreds of degrees hotter than the north, potentially explaining magnetic anomalies and seismic wave behaviors.
  • While the findings clarify structural differences, the specific mechanism causing this heat distribution remains uncertain among scientists.

A significant new study published in Nature on August 26 proposes that Mars possesses a profound internal thermal asymmetry, with its southern hemisphere running hundreds of degrees hotter than the north. This discovery addresses long-standing questions regarding why the planet’s two halves look so drastically different on the surface. The northern region is defined by smooth, low-lying plains, while the south is rugged, mountainous, and heavily cratered. The new findings suggest that this visible dichotomy extends deep into the mantle, potentially reshaping our understanding of Martian geological history.

Previous models of Mars’ interior relied heavily on data from NASA’s InSight lander, which operated near the equator from late 2018 until late 2022. Because InSight represented a single stationary point, earlier assumptions often treated the planet’s internal structure as relatively uniform around its circumference. However, the latest research team adopted a different methodology, analyzing variations in the Red Planet’s gravitational field over time. These gravitational shifts occur seasonally due to Mars’ elliptical orbit and tilted axis, which alter the sun’s tidal influence on the planet’s shape throughout the year.

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By examining sixteen years of velocity data from three NASA orbiters—the Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter—scientists constructed a detailed model of the subsurface. The analysis indicates that the southern hemisphere is approximately 400 to 750 degrees Fahrenheit hotter than the northern half. Furthermore, the modeling suggests that this warmer region is partially molten. The boundary separating these thermal zones aligns closely with the geographical divide seen on the surface, reinforcing the idea that large-scale north-south variations dominate Martian geology.

The implications of this thermal anomaly extend beyond simple temperature differences. Researchers believe it may explain several puzzling phenomena observed on Mars. For instance, magnetic anomalies have been detected in iron minerals within the southern hemisphere. The intense internal heat could indicate a past global magnetic field strong enough to create lasting magnetic distinctions between the two halves. Additionally, data from InSight showed that seismic waves dissipate more rapidly in the south, a behavior consistent with higher temperatures and a partially molten mantle.

Understanding this internal structure is critical for assessing Mars’ hydrological history. Water plays a central role in the search for past life, as it is essential for biological processes as we know them. The thermal dichotomy may have influenced how basins formed and whether they could have retained water over geological timescales. If the southern crust’s thickness and heat retention prevented certain geological processes, it could have altered the planet’s ability to sustain liquid water on its surface or near its subsurface.

Despite the clarity of the thermal model, the cause of this uneven heat distribution remains a subject of debate. Several hypotheses have been proposed to explain why the south is so much warmer. One theory suggests that a massive impact event early in Mars’ history caused the northern hemisphere to lose its internal heat more efficiently. Another possibility involves spontaneous upwelling from the core, which might have transported heat specifically to the southern mantle. A third explanation points to the physical properties of the crust itself; the southern crust is roughly 15.5 miles thicker on average than the north, which could trap heat and prevent it from dissipating into space.

Not all experts are convinced that the current model accounts for every geological feature. Paul Byrne, a planetary geologist at Washington University in St. Louis who was not involved in the study, noted lingering uncertainties. He pointed out that Mars’ largest volcanic province is located near the equator, on the edge of the identified thermal anomaly, rather than directly above the hottest region. This discrepancy suggests that while the thermal model explains many features, it may not fully capture the complexity of Martian volcanism and tectonic activity.

The study builds on a growing body of research into Mars’ interior. Last year, separate teams using InSight data reported that the mantle contains remnants of smaller planetary bodies that collided to form Mars, while another group suggested the existence of a solid inner core. These findings, combined with the new thermal map, paint a picture of a planet with a complex and layered history. As scientists continue to refine these models, the focus remains on how internal dynamics shaped the surface conditions that might have once supported life.

Future missions will likely need to account for this thermal asymmetry when planning landing sites or drilling operations. The partial molten state of the southern mantle could affect seismic readings and subsurface exploration strategies. While the exact mechanism driving the heat difference is still unknown, the confirmation of a sharp internal divide provides a crucial framework for interpreting past data and guiding future investigations into the Red Planet’s deep structure.

The research underscores the importance of multi-source data in planetary science. By combining gravitational measurements from orbiters with seismic data from landers, scientists can overcome the limitations of single-point observations. This integrated approach not only reveals hidden thermal structures but also helps resolve contradictions in previous models. As more data becomes available, the picture of Mars’ interior will continue to evolve, offering deeper insights into its formation and potential for habitability.

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  • Smithsonian Magazine↗Mars' Southern Half Is Weirdly Warm Underground. The 'Thermal Anomaly' Might Explain Why the Planet's Hemispheres Look So Different