The planet Mars is very asymmetrical … and we now know why

For decades, scientists have puzzled over the origin of one of the most amazing features of Mars: the planet is asymmetrical, and the northern hemisphere of Mars is a plain that is five to ten kilometers lower than the southern hemisphere.

One of the hypotheses is that early in the planet’s history, 4.5 billion years ago, a fireball nearly 2000 kilometers in diameter hit its surface obliquely, nearly destroying the planet… but producing a giant impact basin in the north: Borealis Basin.

Until now this hypothesis was theoretical, it is now supported by observations, recently published by our international team.

Mars is asymmetrical: its average altitude north of the “dichotomy” is lower than to the south. We also see the places studied by the research team inside the system of canyons of the Valles Marineris and marked with stars – Daniel Mège / The Conversation

These new elements also provide information about the geological evolution of the Red Planet, which could have protected a hydrothermal “cocoon” after this giant impact – these hydrothermal sites are considered to be conducive to the appearance of life. And by the way, our observations can shed new light on our knowledge of Mars’ metallic resources.

​The “rings” of deformation point to the hypothesis of an impact

Our investigation started somewhat by accident at the Polish Space Research Center, where we examined some very high-resolution images of the two deepest areas of Mars’ great trench, the Valles Marineris – images that had not yet been interpreted. These images were supplemented by a metric precision topographic model calculated in Grenoble.

On this data from the HiRISE orbital telescope, from NASA’s Mars Reconnaissance Orbiter mission, the tectonic eye has seen over a few tens of square kilometers a complex system of deformation of faults that can only develop at levels of the Earth’s crust several kilometers deep, where the temperature, several hundreds of degrees, is high enough to start making the rock malleable.

It is the first time that this type of deformation, called a “ductile brittle shear zone” (pieces break and move like honey), has been identified on Mars. On Earth, the vertical movements of tectonic plates are actually associated with erosion, which allows us to see this type of structure, for example in Canada, Australia or Finland, on the border of old tectonic plates.

On Mars, there is no plate tectonics, and only the extraordinary conditions in the Valles Marineris are favorable. Although these shears are important structures in the Martian crust, hundreds or even thousands of kilometers long, in widths of one to several kilometers, they are mostly “hidden” in the crust.

The shear movements are parallel to the edge of the Borealis Basin. The scissors would be part of the “ring of failure” generated by the disaster. This interpretation of a growing body of observations makes it possible to solve various puzzles about the ancient past of the Martian crust.

Long fracture bundles of unexplained origin north of Valles Marineris could thus be a “shear reactivation”. Three very old mountain ranges near the Valles Marineris on the Ophir Plateau, equally enigmatic, also line up parallel to the boundary of the Borealis Basin and would naturally be explained as remnants of rings, for example the rings formed by the impact around the Mare Oriental Basin on the Moon.

The pieces of the Mars puzzle are falling into place

Near the scissor, we also identified a long strip of crushed rock, or “megabreak”, some blocks of which have dimensions approaching a hundred meters. Only very large meteorite impacts can release the energy sufficient for such fragmentation of the rock.

Hebes Chasma, in the Valles Marineris Rift on Mars.  The distance between the two rock peaks at the ends of the picture is 65 kilometers, the maximum height difference is 8700 meters
Hebes Chasma, in the Valles Marineris Rift on Mars. The distance between the two rock peaks at the ends of the image is 65 kilometers, the maximum height is 8700 meters – Adapted by Daniel Mège, ESA / DLR / FU Berlin (G. Neukum)

We know this fracture is very old because it is covered in lava dated to 3.4 billion years. Additionally, meteorite falls that can create an impact of this size are extremely rare today, but were more common when the solar system formed.

A final aspect concerns recent observations from the seismometer of the InSight mission (NASA/CNES/DLR). On August 25, 2021, this instrument observed an earthquake whose location is consistent with the identified shear zone. It is therefore possible that the Martian crust at the edge of the Borealis Basin continues to deform at present.

Thus, the existence of the Borealis impact goes from hypothetical to probable, and this has implications for the search for traces of Martian life.

A large hydrothermal laboratory for Martian biology?

To determine the composition of the deformed rock, we used data from the Mars Reconnaissance Orbiter spectrometer and a new method for identifying mineral mixtures developed at Orsay. The mineralogical studies carried out in Wroclaw, Poland using this method show that the rock is a gabbro-type dark igneous rock in which fluids (water, CO2) have circulated. These liquids in gaseous and liquid state, called “hydrothermal fluids”, move in the cracks of the rock under the influence of pressure and temperature.

To supply Martian hydrothermal systems at the time of impact, about 4.5 billion years ago, there was no shortage of water: it is well established that the Martian crust must have contained water in abundance. Under the influence of the heat released by the impact, the start of a hydrothermal circulation was inevitable.

Existing models show that this hydrothermalism could have persisted for millions of years. In fact, the long-term stability of hydrothermal environments is now considered to be the key to the origin of life. Hydrothermal systems developed in the Martian crust cracked by the Borealis impact would promote the creation and protection of organic molecules under permanently favorable microclimatic conditions, in contrast to the hostile and fluctuating climatic conditions on the surface, where they would be at the mercy of extreme temperature variations, hygrometry, wind, sunshine or solar and galactic radiation.

Currently, no traces of life have been found on Mars, but the return of samples with the Mars Sample Return mission on the horizon of 2031 could well reveal surprises for us. Looking for traces of ancient biological activity on Mars in the Valles Marineris might be another good idea.

Gold and silver

On Earth, 30% of gold in mined deposits comes from hydrothermal geological sites of the type identified here in Valles Marineris. There are associated base metals and rare metals: copper, tin, bismuth, molybdenum, tungsten, silver, gold, platinum and others.

Such deposits are unlikely to be found elsewhere on the surface of Mars, in current knowledge. If it seems unreasonable to bring them back to Earth, for obvious reasons of cost and pollution, one can imagine in the very distant future exploiting them on site.

These interpretations have a great internal coherence and open up unexpected perspectives. But does this internal consistency guarantee accuracy? The “dustbins” of the history of science are full of irrefutable demonstrations, staggering discoveries and startling revelations. We make deductions from what we know and what we see, and not from the extent of what we don’t know: what would the Martian crust reveal if it were eroded as intensely as in Valles Marineris?

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This analysis was written by Frédéric Schmidt, Professor of Planetary Surface Geology at the University of Paris-Saclay, and Daniel Mège, Professor of Earth and Universe Sciences at the Space Research Center of the Polish Academy of Sciences (Warsaw).

The original article was published on the website of The conversation.

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