Why is one half of Mars so different from the other? The ‘Marspotres’ may have just discovered the answer

Mars is home to perhaps the biggest mystery of the solar system: the so-called Martian dichotomywhich has puzzled scientists since it was discovered in the 1970s.

The southern highlands of Mars (which cover about two-thirds of the planet’s surface) rise as much as 5 or 6 kilometers higher than the northern lowlands. Nowhere else in the solar system do we see such a large, sharp contrast at this level.

What caused this dramatic difference? Scientists are divided on whether this is the result of external factors – such as a collision with a giant asteroid the size of the moon – or internal factors, such as the flow of heat through the planet’s molten interior.

In a new study published in Geophysical Research Letterswe analyzed earthquakes detected by NASA’s Insight lander, located near the boundary that separates the two sides of the dichotomy. Studying the movement of earthquake vibrations has revealed evidence that the origin of the Martian dichotomy lies deep within the red planet.

Martian dichotomy

Altitude isn’t the only difference between the two sides of the Martian dichotomy.

The southern highlands are cratered and dotted with frozen volcanic lava flows. In contrast, the surface of the northern lowland is smooth and flat, with almost no visible scars or other significant 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 (suggesting that they date from an ancient era when Mars had a global magnetic field), while those in the northern lowlands are not.

The Martian dichotomy was discovered in the 1970s, when images from the Viking probes showed a difference in the height and density of impact craters.

The areal 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 there are. Therefore, the southern highlands appear older than the northern lowlands.

Scientists also believe that there was once a vast ocean of liquid water on Mars, possibly in the same region as the northern lowlands.

This is much debated because the presence or absence of sediments, landforms, and certain minerals that form when land is covered by ocean is used as the primary 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.

Space or internal forces?

The origin of the Martian dichotomy has long been a puzzle in planetary science. What gradual or violent natural process, phenomenon, cosmic force, or catastrophe on early Mars (given the age of the surface rocks) might offer an answer to this question?

Two main hypotheses emerged.

The first is the so-called endogenous hypothesis. This argues that the difference in heat transfer through the upwelling of warmer and sinking of cooler material within Mars’ mantle has led to the visible dichotomy on its surface.

It’s another exogenous hypothesis, according to which the cause of the dichotomy comes from the universe. This would mean a catastrophic impact of either a single moon-sized body or several smaller bodies, reshaping the planet’s surface.

earthquakes

On Earth, we can use data from hundreds and even thousands of seismometers to triangulate the location of an earthquake.

On Mars, we only have data from one instrument on the Insight lander. To find the location of an earthquake, we must rely on measuring the difference in arrival time 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 an earthquake by looking at the movement of particles on the ground.

After building a system for pinpointing earthquakes from Insight data, we compared it to known events such as meteoroid impacts that have been spotted by satellite cameras. We found that our methods reliably indicate a cluster of earthquakes in the Terra Cimmeria region of the Southern Highlands.

We then studied how S waves lose energy as they travel through the rocks of the southern highlands. We also made similar calculations for previously observed earthquakes in the Cerberus Fossae region of the northern lowlands.

A comparison of the two showed that the waves lost energy faster in the southern highlands. The most likely explanation is that the rock under the southern mountains is warmer than in the north.

What earthquakes tell us about the dichotomy

This temperature difference between the two halves of the dichotomy supports the idea that the rift was caused by internal forces on Mars, rather than some external influence.

A full explanation of why is quite complex. To simplify, scientists have modeled how the dichotomy could have formed based on initial bumps in the Martian crust far back in time.

At one point, Mars had moving tectonic plates like Earth. The movement of these plates and the molten rock beneath them could have created something of a dichotomy—which was then frozen in place when tectonic plates it stopped moving and formed what scientists call a “stale cap” on the planet’s molten interior.

These events may have then enabled convection patterns in the molten rock that may explain the dichotomy we see today, with upwelling under the southern highlands and downwelling under the northern lowlands.

Our earthquake evidence for the temperature difference in the dichotomy is consistent with these models.

To finally answer the question of what caused the Martian dichotomy, we will need more earthquake in March 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.

This article was republished from Conversation under Creative Commons license. Read it original article.