The northern hemisphere of Mars is the largest impact crater in the solar system

Three groups of scientists who used gravity data and altitudes on Mars from spacecraft orbiting it and a computer simulation concluded that the Martian anomaly in which the northern hemisphere is flat and low and the southern hemisphere is rugged and high is due to the impact of a giant asteroid at the beginning of the days of the solar system

New analyzes of the Martian surface taken by the MRO and Mars Global Surveyor spacecraft have revealed what appears to be the largest crater, by orders of magnitude, ever discovered in the Solar System. This is what three groups of researchers published on Thursday in the journal Nature. According to one of the groups of researchers, the force of the impact that created the huge crater was equivalent to one million billion atomic bombs of the type dropped on Nagasaki in 1945.

The two spacecraft orbiting and photographing the surface of the red planet with a selection of cameras at different wavelengths have provided detailed information about altitude and gravity in the northern and southern hemispheres of Mars. Researchers have taken these data and according to them they may solve one of the biggest puzzles in the solar system: why does Mars have two completely different types of surface in the different hemispheres - a more or less smooth surface in the north compared to mountains and valleys in the south? This huge crater, which covers half of the planet, has always fascinated the scientific community.

The huge northern basin, which covers about 40% of the surface of Mars, is sometimes called the Borealis basin and is a remnant of a giant impact in the early days of the solar system, according to the new study. The fact that it does not have craters is not a direct result of the impact because in the 4.5 billion years that have passed since the impact, several small objects crashed on Mars and created craters, however due to the height difference, materials tended to sink in the northern hemisphere (whether these were sediments from an ancient ocean or sediments volcanic). Either way, the result is a smooth surface these days.

The diameter of the crater - about 8,500 kilometers is 4 times wider than the largest craters in the solar system - the Lass crater, in the southern hemisphere of Mars and the South Pole/Atkin crater on the moon, which have a more or less similar diameter, about 2,500 km. In the report accompanying the study, the scientists calculated that the object that created the crater must have been at least 1,900 km in diameter - larger than Pluto, or slightly smaller than the width of the continent of Australia.

The mystery of the two faces of Mars has baffled scientists since the comprehensive series of images of the surface of Mars by NASA spacecraft ended in the 1984s (mainly the Viking spacecraft). The main hypothesis was that an ancient asteroid impact or some internal process caused the melting of the surface and the layers below in the northern hemisphere. The idea of ​​the impact crater proposed in XNUMX fell out of favor due to the fact that the shape of the basin did not seem to fit the expectations of a round crater. The new data convinces some experts who doubted the impact scenario.

"We may not have been able to prove XNUMX percent the impact theory, but we were able to turn the tide," said Jeffrey Anrados-Hannah, a postdoctoral researcher at MIT. Andrews-Hannah and his collaborators Maria Zuber of MIT and Bruce Bandart of NASA's JPL in California reported the findings in this week's issue of the journal Nature.

A research group led by Francis Nimmo from the University of California at Stana Cruz, performed a computer simulation and showed that damage to certain conditions in the past could have created the current appearance of Mars. These conditions indicate that a space rock with a diameter of one-third to one-half of the Earth's moon hit Mars at an angle of 30-60 degrees. Falling at such an angle creates an elliptical crater.

"It's an old idea, but no one has yet done the numerical calculations to examine what might happen when a large asteroid hits Mars," said Nimmo, an Earth and planetary science researcher at the University of California, Santa Cruz.

This study is reinforced by another study led by Prof. Oded Aharonson, professor of planetary sciences at the California Institute of Technology (Caltech). "The dichotomy is that this is the oldest formation on Mars," explains Dr. Aaronson in an interview with the BBC website. This shape formed about 4 billion years ago, before the rest of the geological history of Mars took place and got in the way. It was around the same time that a larger object hit Earth, throwing materials from the young planet's crust into space. These materials recrystallized and formed the moon. According to him, apparently this is not just a coincidence. "This happened right at the end of the process of creating the four terrestrial planets - Mercury, Venus, Earth and Mars" says Craig Agnor, Francis Nimmo's research partner.

"We believe that the planets were formed from a disk of rocks. When the rocks collide you get bigger rocks and so on, evolutionarily speaking we ended up with four planets and lots of rocks in the growing selection. As far as the planets were constantly snapping rock fragments, these could have been the last big fragments. Shock waves from the impact traveled through the planet and disturbed the crust on the other side as well, thus causing changes in the magnetic field that were also measured on the southern side.

"This is an important result because it has implications not only for the early development of Mars but also for the formation of the Earth," says Michael Meier, the chief researcher of Mars at NASA headquarters in Washington. "The northern hemisphere of Mars is one of the smoothest surfaces to be found in the solar system. The southern half is high, rugged, full of craters. The average elevation of the surface in the south is between 4 and 8 kilometers higher than the crater floor.

Other giant impact craters that have been discovered are also closer in shape to an ellipse than a circle, but a complex analysis of the surface of Mars from two different spacecraft was required to reveal the elliptical shape of the Borylis crater, and this feature is also consistent with the discovery that it is an impact crater.

One of the factors complicating the discovery was the discovery of the elliptical shape of the crater after the time of the impact, which occurred approximately in the first hundred million years of the formation of the Martian crust. Huge volcanoes formed on one side of the basin and created a large area of ​​mountains and rocks that blurred the line of the basin. It took combinations of gravity data, which reveal the subsurface structure, along with data on the current surface elevation to reconstruct the elevation map of Mars as it existed before the volcanoes erupted.

"In addition to the elliptical borders of the crater, there are also signs of a second, outer ring - a typical feature of large impact craters," Bandert said.

Simulation on supercomputers

Prof. Oded Aharonson, born in Israel (1973), stayed in the USA from the age of 13, and returned to study there for his academic studies. In the 2002s, he completed his bachelor's and master's degrees at Cornell University, as a student of Steve Squires - director of the Spirit and Opportunity robotic vehicle program on Mars. He then moved to MIT, where he received his PhD in XNUMX, and then joined Caltech as a professor. In this framework, he is a researcher in the Department of Geology and Planetary Sciences.

In a conversation with the Hidan website, Prof. Aharonson explains that the research has a personal side and a professional side: "When I was a student of Steve Squires at Cornell (head of the Mars rover project), he proposed to me the problem whose results have now been published in Nature. At that time computers were less fast than today, I tried to solve the problem, but in the end we had no conclusive results. At Caltech, a student named Margarita Marinova came to my department. I proposed this problem to her and because we had access to a supercomputer including a 4,000-core cluster, she was able to solve the problem and is the first signatory of the paper. It turns out that at the same time, a group at MIT and a group in Santa Cruz also studied the subject, and yesterday in Nature the results of the three articles were published together."

How is the dichotomy of Mars explained?

Prof. Aharonson: "Already in telescopic observations you can see that one side of Mars is smooth and does not have many craters, while the other side is rough and has many craters on it. When we examined the smooth hemisphere, we saw that it is lower, and also that there are differences in the attraction field between it and the other hemisphere. The combination of these two factors shows that the thickness of the crust in the northern hemisphere is thinner than in the southern hemisphere. Steve Squires already suggested twenty years ago that the answer to this puzzle is that there was a big explosion, that a big body hit Mars."

We were able to simulate the event and check if such an impact really could a.) not melt most of the crust of Mars to such an extent that there would be evidence of this explosion, b.) that the result of the impact would be an elliptical crater like we actually see - and this was determined by the second article in Nature of Andrews-Hannah. and c.) that this crater will not have a raised rim like the high basin rim of the smaller craters."

We called the necessary conditions "The Sweet Spot". A term in tennis that refers to a point on the racket that, if the ball hits, it bounces to the right place. In any case, our simulations show that under very reasonable conditions when the planets grew 4.6-4.5 billion years ago, these conditions are very reasonable and the result of such an explosion will be as we see."

"We started the research from a situation where someone proposed that the difference between the north and the south on Mars - the dichotomy - was created by the big bang and there were alternatives to this hypothesis, such as an internal process inside Mars. We started from a situation where there was a theory that this is how it was created, but there was no possibility to test it, and now what we were able to do is to check and see that two parameters exist: firstly, that the required conditions are reasonable conditions that existed at the beginning of the days of the solar system, and secondly, that they lead to results that are as we observe on Mars today .”

Around the same time a large object hit the Earth and caused the formation of the Moon?

Prof. Aaronson: "The explosion that hit Mars was 100 times smaller than the one that caused the moon to form from the Earth. It is similar but two orders of magnitude smaller. Earth was hit by an object the size of Mars and Mars was hit by an object the size of Australia or a little smaller.

Is it because of the difference in size that Mars did not have a moon as significant as Earth's?

Probably because of the intensity of the impact, no moon was formed. If size is an important figure, but not only it, the angle of impact is also important.

Is there no such danger today?

"The bodies of the relevant size, Mars, Venus, Earth and Ceres, are in fixed orbits and there is no danger of collision between them."

For information on the NASA website