A Marsquake Reveals Why Mars has Two Very Different Hemispheres

By Evan Gough - January 24, 2025

Elevation map of Mars — Elevation data of Mars featuring the lower elevations of the northern lowlands primarily in blue and the much higher elevations of the southern highlands primarily in orange and red. (Credit: MOLA Science Team)

Even with all we have learned about Mars in recent decades, the planet still holds significant mysteries, particularly surrounding the existence of potential life and the foundational geological characteristics of the Martian surface. One of the most intriguing questions that have scientists baffled is the Martian dichotomy—why are the northern and southern hemispheres of Mars so fundamentally different in composition and elevation?

Understanding the Martian Dichotomy

The southern hemisphere is predominantly highlands and is approximately 5 kilometers (3 miles) higher in elevation than its northern counterpart. In comparison, it has a thicker crust, is older, and is significantly more cratered. The northern hemisphere, on the other hand, is characterized by vast, smooth plains, which exhibit a thinner crust, relatively fewer craters, and lower levels of magnetization compared to the southern hemisphere.

Elevation map of Mars from Mars Global Surveyor's MOLA instrument — Elevation map of Mars, based on data obtained by the Mars Global Surveyor's MOLA instrument. The northern hemisphere is a smooth plain with a lower elevation compared to the southern hemisphere. Image Credit: NASA/GSFC

Theories Behind the Dichotomy

Scientists have proposed several theories to explain this dichotomy. One leading hypothesis includes the theory of a massive celestial impact in Mars' early history. Some researchers using geophysical modeling propose that a Pluto-sized body collided with Mars, potentially forming the northern lowlands as a massive impact basin. This theory suggests a catastrophic event significantly altering the planet's geological structure.

In contrast, other researchers advocate for an internal geological explanation, indicating that the dichotomy may be attributed to Martian endogenic processes such as plate tectonics or mantle convection. This would imply that the planet's internal activities contributed to the observed differences in geological features.

Exploring Mars with InSight

To investigate these theories and understand the Martian internal structure, NASA, in collaboration with the German Aerospace Center (DLR), launched the InSight lander, which successfully reached the Martian surface in November 2018. The name "InSight" stands for **Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport**. Among its instruments was SEIS (Seismic Experiment for Interior Structure), which allowed scientists to observe and analyze hundreds of marsquakes that yielded valuable data on Mars' crustal thickness, mantle composition, and core size.

Research Findings from InSight Data

Researchers have continued to analyze the wealth of data collected by InSight, leading to recent findings published in the AGU's Geophysical Research Letters. The research letter is titled “Constraints on the Origin of the Martian Dichotomy From Southern Highlands Marsquakes,” authored by Weijia Sun and Professor Hrvoje Tkalcic. Their analysis suggests that mantle convection is a primary factor in forming the Martian hemispheric dichotomy.

The Martian dichotomy is characterized by notable differences in elevation and crustal thickness between the Northern Lowlands and the Southern Highlands. The elevation difference corresponds to the height of Earth's tallest mountains, indicating a fundamental geological imbalance that has persisted for billions of years.

Importance of Marsquakes

This particular study focused on a newly located cluster of marsquakes detected in the Terra Cimmeria region, giving the researchers vital seismic information about previously unexplored regions. Professor Tkalcic noted the importance of this data, stating, “By analyzing waveform data from low-frequency marsquakes recorded by InSight's seismograph, we located and analyzed a cluster of six previously detected but unlocated marsquakes in the southern highlands.”

A MOLA map showing the boundaries of Terra Cimmeria and other nearby regions. Image Credit: By Jim Secosky modified NASA image. Public Domain.

Terra Cimmeria is situated in Mars’ southern highlands, characterized by its rough terrain and geological diversity, contrasting sharply with the smoother northern hemisphere. It plays a critical role in understanding the hemispheric differences.

The image shows Cerberus Fossae in context — The image on the left is a vertical plan view of Cerberus Fossae. The pair of trenches are very young and formed from volcanic activity only a few million years ago. Image Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. The image on the right shows Cerberus Fossae in context. Image Credit: NASA MGS MOLA Science Team.

Comparative Seismic Data

The six marsquakes detected offer new insights, allowing for comparison with data collected from the Cerberus Fossae region in the northern lowlands, which is a significant feature characterized by a series of near-parallel fissures associated with volcanic activity.

Using refined signal-to-noise ratios, researchers pinpointed the precise locations of the marsquakes. They employed the spectral ratio method to assess the quality of seismic waves through the Martian crust. The quality of these waves—indicated by the 'Q' value—showed variations between the northern and southern regions, providing insights into the subsurface characteristics of Mars.

Key Findings on Crust and Mantle Differences

Mars Region	Q Value	Observations
Terra Cimmeria	481–543	Higher attenuation of seismic waves, indicating potentially higher temperatures and more vigorous mantle convection.
Cerberus Fossae	800–2000	Lower attenuation, suggesting cooler temperatures and less active convection processes.

The Implications of the Findings

The findings reveal significant differences in subsurface conditions based on temperature and convection patterns between the northern and southern hemispheres of Mars. As stated by the researchers, “The attenuation difference might be linked to the temperature differences between the two hemispheres, along with more vigorous convection beneath the Southern Highlands.” This finding could have profound implications for understanding Martian geology and its evolutionary history.

Furthermore, these observations suggest that planetary processes such as mantle dynamics could have played a critical role in shaping the Martian surface. "Understanding whether convection is taking place offers clues into how Mars has evolved into its current state over billions of years," observed Professor Tkalcic.

Moving Forward: Understanding Mars’ Geological History

The primary goal for researchers studying the Martian dichotomy is to determine if the observed features result from endogenic (internal) or exogenic (external) processes. While the impact hypothesis remains popular among some researchers, geochronological constraints show significant discrepancies between crater data, mineral distributions, and the presence of river channels on Mars. These contradictions lend weight to the notion that processes occurring within the planet itself are responsible for its current state.

This figure illustrates some of the results of research on Martian dichotomy. — This figure from the research letter illustrates some of the results. It shows the endogenic origin of the Martian dichotomy from seismological observations. Image Credit: Sun and Tkalcic 2025.

Conclusions and Future Research

Understanding the geological history of Mars is crucial, not only for gaining insights into the planet itself but also for implications regarding the potential for past life. As researchers continue to analyze data from the InSight lander, they are deepening their comprehension of how Mars evolved during its formative years. The ongoing investigation into the dichotomy will remain a foundational aspect of Martian research, aiding in understanding this enigmatic planet.

For More Information

For detailed insights, consider checking out the following:

For comprehensive reporting on planetary science and astronomy, visit Universe Today.

Marsquake Insights on Martian Dichotomy