Mapping the Martian Interior Through Gravity
Scientists have uncovered a major temperature imbalance deep inside Mars. The planet's southern interior appears significantly hotter than its northern half. Gravity measurements indicate the southern mantle is 200 to 400 degrees Celsius warmer than the north. This region likely contains partially molten material. The findings provide a new window into the geologic history of the Red Planet. They clarify periods when Mars might have held environments capable of supporting life.
The research was led by Alexander Berne, a Caltech alumnus now at the University of Arizona. His team published their work on August 27 in Nature. Berne created a model while at Caltech that uses subtle variations in gravity to map internal planetary structures. He applied this logic to decades of data from the Mars Global Surveyor, Mars Odyssey, and the Mars Reconnaissance Orbiter. By tracking small changes in spacecraft velocity, the team reconstructed the gravitational field surrounding the planet.
Mars moves along an elliptical orbit and rotates on a tilted axis. This movement causes the Sun's gravitational pull to fluctuate across the planet's seasons. The researchers employed a method called tidal tomography to analyze how these gravitational signatures change over time. This allowed them to construct a three-dimensional model of the planet's internal structure. Berne notes that while scientists often assume planetary interiors are spherically symmetric, that assumption is frequently incorrect. As more data arrives, these three-dimensional models offer a blueprint for future mission design and scientific exploration. Understanding these internal processes helps reveal how planetary bodies form and evolve.
The Significance of the Thermal Divide
Mars exhibits a visual contrast between its two halves. The southern surface features tall mountains and thick craters. The northern hemisphere consists of broad, low-lying plains. This new data proves that the contrast extends far below the surface. Researchers were surprised to find the southern interior holds heat hundreds of degrees higher than the north. This internal heat explains several features that have long puzzled planetary scientists.
Specific iron-rich minerals in the southern hemisphere show unusual magnetic signatures. A hotter southern mantle suggests Mars once possessed a magnetic field strong enough to drive these magnetic differences between the two hemispheres. Furthermore, seismic data from NASA's InSight mission previously showed that seismic waves lose energy more quickly in the south. The higher temperatures discovered in this study account for that specific seismic behavior. This link between temperature and wave propagation confirms the accuracy of the gravity-based model.
Implications for Martian Evolution and Water
Amirhossein Bagheri, a Caltech postdoctoral scholar and co-author of the study, explains the importance of this dichotomy. The north-south difference provides information about processes that influenced Martian hydrology. These processes likely included the formation of basins capable of holding surface water. Understanding this temperature difference offers new clues about how the landscape developed during eras when liquid water possibly covered parts of the surface.
Researchers do not yet have a definitive cause for this thermal anomaly. Several theories remain under investigation. One hypothesis suggests a giant impact stripped heat away from the northern hemisphere. Another possibility posits that spontaneous convection occurred within the southern Martian mantle long ago. A third theory suggests that dense geological structures in the south trapped heat and prevented it from escaping. These factors indicate that the planet's internal heat regulation is more complex than previously estimated.
This paper, titled "Tidal Tomography Reveals a Thermal Anomaly Beneath Mars's Crustal Dichotomy," includes contributions from a wide international team. Researchers from Brown University, NASA Goddard Space Flight Center, and the University of Rome all provided data and analysis. UCLA, UC Santa Cruz, and Delft University of Technology also supported the project. The research received funding from NASA. This work demonstrates how gravitational data from existing missions continues to yield new discoveries about planetary bodies without the need for new landings.