A massive impact with an asteroid that measured around 400 miles across is the reason that Mars is a planet of two distinct halves, where the northern and southern hemispheres look different.
This strange feature was first observed by Nasa’s Viking missions to Mars in the 1970s and ever since scientists have puzzled over why there are relatively young, smooth, low-lying plains in the north and relatively old, heavily cratered highlands in the south.
The mystery deepened 20 years later, when the Mars Global Surveyor probe showed that the crust of the planet is much thicker in the south and also revealed magnetic anomalies in the southern hemisphere but not in the north.
Now the theory that it was subject to a cataclysmic collision between 3.9 billion and 4.5 billion years ago has received strong support from computer simulations carried out by two groups.
A third study has identified what appears to be by far the largest impact scar found in our Solar System, four times bigger than the closest rival.
The latter analysis by researchers at Massachusetts Institute of Technology and Nasa suggests that a giant basin that covers about 40 percent of the surface of Mars, sometimes called the Borealis Basin, is actually the remains of a colossal impact very early in the solar system’s formation, measuring about the size of the combined area of Asia, Europe and Australia.
Planetary scientists at the University of California, Santa Cruz, were involved in both computer simulation studies - which also appear in the journal Nature - and come to the same conclusion: that Mars once collided with an object about one-half to two-thirds the size of the Moon, striking at an angle of 30 to 60 degrees to leave a massive oval shaped basin.
The predicted changes are consistent with observations of magnetic anomalies in the southern hemisphere. In addition, new crust that formed in the northern lowlands would be derived from deep mantle rock melted by the impact and should have significantly different characteristics from the southern hemisphere crust.
The basin that resulted from the Mars impact, 5,300 miles across and 6,600 miles long, is about four times wider than the next-biggest impact basins known, the Hellas basin on Mars and the South Pole-Aitken basin on the moon.
"A key finding of our study is that the northern lowlands is actually elliptical in shape, and thus resembles other smaller impact basins such as Hellas and the South Pole-Aitken basin."
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