Mars: Mega impact in the south instead of in the north?

Daring scenario contradicts previous theories on the origin of the Mars dichotomy

The northern and southern hemisphere of Mars are very different, as this topographical map shows. © NASA / JPL / USGS
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According to popular theory, a huge impact could be to blame that the north of Mars is so much shallower and deeper than the south. But Swiss geophysicists are now reversing this scenario: According to their model, it was not the north but the southern hemisphere that was hit by a moon-sized iron protoplanet. As they report in the journal "Geophysical Research Letters", this could explain the present-day landforms of Mars even better.

Mars almost seems to have two parts: the terrain of its northern hemisphere is between one and three kilometers lower than the south and the crust is thinner below the shallow lowlands of the north. Why the two hemispheres differ so clearly, there are several hypotheses. In addition to early tectonic processes, one or more impacts of large asteroids are discussed as the cause.

Mega impact in the south?

Most researchers assume that Mars was hit shortly after its formation in its northern hemisphere by a very large chunk. The northern lowlands would therefore be the relics of a huge impact basin whose elliptical shape was subsequently changed by volcanic eruptions. However, geophysicists at ETH Zurich around Giovanni Leone now propose an alternative scenario: an impact of a moon-sized celestial body in the southern hemisphere of Mars.

At first this sounds absurd, because typically an impact leaves a depression, not an elevation. On Mars, however, the south is higher and more mountainous than the flat north. But the computer model of the geophysicists shows that a rock impactor could not have produced today's landscape forms - but rather a giant car of predominantly iron of about one-tenth of the Marsmasse.

First magma sea, then highland

In their model, the researchers had a celestial body of 80 percent iron with a radius of 1, 600 kilometers and a speed of about five kilometers per second hit Mars. Time of the incident: About 4 to 15 million years after the formation of the red planet. At that time, the Martian crust was only very thin, under which was concealed a still-liquid interior. display

Their simulation shows that this impact could have generated so much energy that it broke through the crust of the southern hemisphere. It spread a magma ocean that covered the whole hemisphere. When this molten rock solidified about 3.5 billion years ago, it formed the mountainous highlands that make up today's southern hemisphere of Mars. The model has perfectly represented the extent and shape of the highlands and the two hemispheres, the researchers report.

The Olympus Mons, the largest volcano of Mars and the solar system NASA / JPL

Unequal volcanic distribution explained

The impact could also have triggered a strong volcanic activity lasting hundreds of millions of years, as the model shows. Especially around the equator there would be numerous mantle plumes, which migrated to the S dpol and united there. According to the researchers, this might explain why most of the Mars volcanoes are located in the southern hemisphere. "Our model mirrors the actual distribution of volcanism almost congruently, " emphasizes Leone. No other model has been able to explain it yet.

According to the researchers, the extinction of the volcanoes could also be explained by the impact scenario: With increasing cooling of the magma ocean and the upper layers of the planet, the heat flux also increased from. After one billion years, the heat flux was one-tenth of its initial value too little to sustain volcanism itself. Even recent models and mineralogical studies suggest an end to the volcanic phase around this time.

Some contradictions

"Our scenarios are more in tune with a multitude of observations of Mars than the theory of an impact in the Northern Hemisphere, " Leone emphasizes. However, where in the early solar system a moon-sized celestial body should have come, which consists almost entirely of iron, is open. Until now, planetary researchers have assumed that even the largest chunks, which did not later become planets, had roughly the same material distribution as the earth and its "siblings".

In addition, Leone's scenario would suggest that Mars was earlier warmer but not really life-friendly. "Before this planet became today's cold and dry place, it was marked by great heat and volcanic activity, " says the researcher. It seems almost impossible that oceans or water courses ever appeared on the Red Planet. However, he stands against increasing indications of a water-rich past of the Red Planet. In 2013, for example, the Mars rover Curiosity discovered evidence of a freshwater lake and living conditions.

Whether the provocative theory of the researchers will prevail in the long term, or whether this is rather a bizarre outlier, will have to show. (Geophysical Research Letters, 2015; doi: 10.1002 / 2014GL062261)

(Swiss Federal Institute of Technology Zurich, 30.01.2015 - NPO)