Audio recordings of seismic activity on Mars, or “marsquakes,” gathered by a robot on the Martian surface may have finally elucidated a 50-year-old enigma: why one hemisphere of Mars varies so significantly from the other.
Since the 1970s, scientists have understood that Mars is divided into two predominant regions. The northern lowlands encompass roughly two-thirds of the planet’s northern hemisphere, while the southern highlands make up the remainder and have an average altitude approximately 3 miles (5 kilometers) greater than that of the northern lowlands. Mars’ crust, which floats atop a mantle of molten rock akin to that found within Earth, is also denser in the southern highlands. This planetary discrepancy is referred to as the “Martian dichotomy.”
Two primary hypotheses exist regarding the origins of the Martian dichotomy. One suggests the separation was initiated by some unexplored mechanism within the planet’s core. The alternative proposes that a colossal impact from a moon-sized body or several smaller celestial objects altered the planet’s surface. Nevertheless, the ages of the rocks found on Mars’ surface indicate that whatever caused the dissimilarity transpired in the very early epochs of the solar system, complicating the identification of the precise cause.
However, a recent investigation published on Dec. 27, 2024, in the journal Geophysical Research Letters, involved researchers examining data from NASA’s InSight lander, which captures how seismic waves from marsquakes bounce around the planet, to ascertain if any indication of an internal source for the Martian dichotomy could be found.
InSight is positioned close to the boundary between the northern lowlands and the southern highlands, permitting the team to analyze how seismic waves propagated through the mantle beneath two locations: one on either side of the divide.
Related: 10 extraordinary findings on Mars in 2024
“Analyzing these two [locations] indicated that the waves dissipated energy more swiftly in the southern highlands,” the authors of the study noted in The Conversation. “The most plausible explanation is that the [molten] rock underlying the southern highlands is at a higher temperature than that in the north.”
“This temperature variation between the two sections of the dichotomy reinforces the notion that the separation was triggered by internal dynamics on Mars, rather than some exterior force,” they remarked.
Internal origin versus external impact
The research team proposes that this thermal discrepancy could be attributed to ancient tectonic movements that have since vanished from Mars.
“At a certain time, Mars had shifting tectonic plates like Earth,” the investigators wrote. “The activity of these plates along with the molten rock beneath could have formed something analogous to the dichotomy, which was subsequently solidified when the tectonic plates ceased their movements, resulting in what scientists term a ‘stagnant lid’ on the planet’s molten interior.”
In this scenario, the magma beneath the southern region is continuously forced against the crust, while the magma under the northern region descends toward the planet’s core. This would further clarify why the crust is more robust in the south, according to the researchers.
Nonetheless, it might be premature to dismiss the external impact theory entirely, which several recent investigations have demonstrated to be potentially viable.
“To conclusively determine what caused the Martian dichotomy, further marsquake data, along with comprehensive models depicting how Mars originated, are required,” the researchers stated. “However, our investigation reveals an essential new component of the puzzle.”