Why is one half of Mars so different from the other? ‘Marsquakes’ may reveal the answer

A map showing the “Mars dichotomy”: southern highlands are yellow and orange, northern lowlands are blue and green. Credit: NASA / JPL / USGS
Mars is home to perhaps the solar system’s greatest mystery, the so-called “Martian dichotomy,” which has baffled scientists ever since it was discovered in the 1970s.
Mars’ southern highlands (which make up about two-thirds of the planet’s surface) rise 5 to 6 kilometers higher than the northern lowlands. Nowhere else in the solar system do we see such large and sharp contrasts at this scale.
What causes this dramatic difference? Scientists wonder whether it is due to external factors, such as a collision with a giant moon-sized asteroid, or whether it is due to heat flow through the planet’s molten interior. Opinions are divided as to whether this is due to internal factors.
A new study published in Geophysical Research Letters analyzed earthquakes detected by NASA’s InSight lander located near the boundary separating the two sides of the binary opposition. A study of how the vibrations of Mars earthquakes are transmitted has revealed evidence that the origin of Mars’ dichotomy lies deep within the Red Planet.
Mars dichotomy
Altitude is not the only difference between the two sides of the Mars binary.
The southern highlands are dotted with craters and streaked with frozen flows of volcanic lava. In contrast, the surface of the northern lowlands is smooth and flat, with few visible scars or other significant features.
Geophysical and astronomical measurements also show that Mars’ crust is significantly thicker beneath the southern highlands. Furthermore, while the southern rocks are magnetized (suggesting that they date from an ancient time when Mars had a global magnetic field), the northern lowland rocks are not.
Mars’ dichotomy was discovered in the 1970s, when images from the Viking spacecraft showed differences in the height and density of impact craters.
The surface density of craters (number of craters per unit area) can be used to calculate the age of surface rocks. The older the surface, the more craters it has. Therefore, the southern highlands appear to be older than the northern lowlands.
Scientists also believe that Mars once had a vast ocean of liquid water, perhaps in the same region as the northern lowlands.
There is a lot of debate about this, as the sediments that form when land is covered by ocean, the topography, and the presence or absence of certain minerals are used as the main evidence for and against. Since the presence of liquid water is a necessary condition for life, it is not difficult to understand the interest of the scientific community and space agencies in this problem.
Outer space or inner power?
The origin of the Mars dichotomy has been a long-standing mystery in planetary science. What gradual or violent natural processes, phenomena, cosmic forces, or catastrophes during Mars’ early stages (given the age of the surface rocks) could provide an answer to this question?
Two main hypotheses emerged.
The first is the so-called endogenous hypothesis. This argues that differences in heat transfer due to the upwelling of warm material and subduction of cold material within Mars’ mantle have led to the visible dichotomy on its surface.
The second is the extrinsic hypothesis, which states that the cause of the binary opposition comes from the universe. This means that either a single moon-sized object, or several smaller objects, can have a devastating effect and reshape the planet’s surface.
mars earthquake
On Earth, data from hundreds or thousands of seismometers can be used to triangulate the location of earthquakes.
On Mars, we only have data from one instrument aboard the InSight lander. To locate large earthquakes, we need to measure the difference in arrival times of different types of vibrations (called P waves and S waves).
This allows us to calculate the distance to a large earthquake. By observing the movement of particles on the ground, we can also determine the direction of an earthquake.
Once they created a system to accurately locate earthquakes from Insight data, they matched it to known events, such as meteor strikes, detected by satellite cameras. We find that our method reliably indicates a firequake swarm originating in the Terra Cimmerian region of the southern highlands.
They then studied how S waves lose energy as they pass through rocks in the southern highlands. They also performed similar calculations for similar earthquakes previously observed in the Cerberosfosse region of the northern lowlands.
Comparing the two, they found that waves lose 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 can teach us about binary oppositions
The temperature difference between the two halves of this binary supports the idea that the split was caused by Mars’ internal forces, rather than external influences.
The full explanation of why is quite complex. To simplify things, scientists created a model that shows how the dichotomy formed based on early heterogeneities in Mars’ crust dating back to the distant past.
At one point, Mars had moving plates just like Earth. The movement of these plates and the molten rock beneath them may have created something like a binary opposition. When the tectonic plates stopped moving, the dichotomy froze in place, forming what scientists call a “stagnant lid” in the planet’s molten interior.
These events may have allowed for the convective patterns in the lava that result in the upwelling below the southern highlands and the downwelling below the northern lowlands that we see today. I can explain the dichotomy.
Our Mars earthquake evidence for temperature differences across the dichotomy is consistent with these models.
To finally answer the question of what caused the Mars dichotomy, we need more than just detailed models showing how Mars formed and how it compares to Earth and other planets. fire and earthquake data will be needed. However, our research reveals an important new piece of the puzzle.
Presented by The Conversation
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