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Mars is home to perhaps the greatest mystery of the solar system: the so-called Martian dichotomy, which has puzzled scientists since its discovery in the 1970s. The southern highlands of Mars (which cover about two-thirds of the planet's surface) rise five to six kilometers higher than the northern lowlands. Nowhere else in the solar system do we see such a large, stark contrast on such a scale.
What caused this dramatic difference? Scientists have been divided on whether it was the result of external factors, such as a collision with a huge moon-sized asteroid, or internal ones, such as the flow of heat through the planet's molten interior. In a new study published in Geophysical Research Letters, we analyzed earthquakes detected by NASA's Insight lander, located near the boundary separating the two sides of the dichotomy. Studying how earthquake vibrations propagate has revealed evidence that the origin of the Martian dichotomy lies deep within the Red Planet.
Martian dichotomy
Elevation is not the only difference between the two sides of the Martian dichotomy. The southern highlands are dotted with craters and streaks of solidified volcanic lava flows. In contrast, the surface of the northern lowlands is smooth and flat, with almost no visible scars or other prominent features. We also know from geophysical and astronomical measurements that the Martian crust is significantly thicker beneath the southern highlands. Moreover, the southern rocks are magnetized (indicating that they date back to an ancient era when Mars had a global magnetic field), while the northern lowlands are not.
The Martian dichotomy was discovered in the 1970s, when images taken by the Viking probes showed differences in the height and density of impact craters. The surface density of craters (the number of craters per unit area) can be used to calculate the age of surface rocks - the older the surface, the more craters. Thus, the southern highlands appear older than the northern lowlands. Scientists also believe that Mars once had a vast ocean of liquid water, probably in the same region as the northern lowlands.
This is a subject of much debate, as the existence or absence of sedimentary deposits, landforms, and certain minerals that form when land is covered by ocean are used as the main evidence for and against. The existence of liquid water is a prerequisite for life, so it is not difficult to understand the interest of the scientific community and space agencies in this problem.
Outer space or internal forces?
The origin of the Martian dichotomy has been a long-standing mystery in planetary science. What kind of gradual or violent natural process, phenomenon, cosmic force, or catastrophe on early Mars (given the age of the rocks on the surface) could have answered this question?
Two main hypotheses emerged.
First, there is the so-called endogenous hypothesis. It suggests that the difference in heat transfer due to the rise of warmer and sinking of cooler material in the Martian mantle led to the apparent dichotomy on its surface.
Second, there is the exogenous hypothesis, which states that the cause of the dichotomy comes from space. This would imply a catastrophic impact, either by a single moon-sized body or by several smaller bodies, that alters the shape of the planet's surface.
Marsupials
On Earth, we can use data from hundreds or even thousands of seismometers to triangulate the location of an earthquake. On Mars, we have data from just one instrument on the Insight lander. To determine the location of an earthquake, we must rely on measuring the difference in arrival times between different types of vibrations (called P and S waves). This allows us to calculate the distance to the earthquake. We can also determine the direction of the earthquake by observing the movement of particles in the ground.
When we built an earthquake detection system based on Insight data, we compared it to known events, such as meteorite impacts, that had been detected by satellite cameras. We found that our methods reliably pinpointed a cluster of earthquakes in the Terra Cimmeria region in the southern highlands. We then examined how S-waves lost energy as they passed through the rocks of the southern highlands. We also performed similar calculations for previously observed earthquakes in the Cerberus Pit region of the northern lowlands. Comparing the two showed that the waves lost energy faster in the southern highlands. The most likely explanation is that the rocks beneath the southern highlands are hotter than in the north.
What earthquakes tell us about dichotomy
This temperature difference between the two halves of the dichotomy supports the idea that the split was caused by internal forces on Mars, rather than an external influence. The full explanation for why is complex. To simplify, scientists have created models of how the dichotomy could have formed based on an initial unevenness in Mars’ crust far in the past. At one point, Mars had moving tectonic plates, just like Earth. The movement of these plates and the molten rock beneath them could have created something like a dichotomy, which then froze in place when the tectonic plates stopped moving, forming what scientists call a “stagnation cap” in the planet’s molten interior.
These events may have enabled convection patterns in molten rocks that could explain the dichotomy we see today, with upwelling beneath the southern highlands and subsidence beneath the northern lowlands. Our data on the temperature difference in this dichotomy are consistent with these models. To definitively answer the question of what caused the Martian dichotomy, we will need more earthquake data, as well as detailed models of how Mars formed and comparisons with Earth and other planets. However, our study reveals an important new piece of the puzzle.
