Miťrcoles 2 de Julio de 2008, Ip nļ 236

Asteroid impact made Mars two-faced
Por Roger Highfield

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 Nasaís 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.

The culprit was either some internal process related to the planetís internal structure, or an ancient impact.

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 systemís 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.

"Itís a very old idea, but nobody had done the numerical calculations to see what would happen when a big asteroid hits Mars," said Dr Francis Nimmo, one of the authors. "The impact would have to be big enough to blast the crust off half of the planet, but not so big that it melts everything."

The impact would have been a million-billion times more energetic than the atom bomb that exploded over Nagasaki, said another group member, Dr Craig Agnor of Queen Mary University, London.

"Whereas an atom bomb might destroy an area a few tens of miles wide, this impact overturned the surface of half the planet [an area thousands of miles wide]."

Dr Nimmoís group worked out the impacts in two dimensions. The effects in three dimensions, but at a lower resolution, were tackled in the second paper is by Drs Margarita Marinova and Oded Aharonson of the California Institute of Technology, working with colleagues in California.

"The two approaches are very complementary; putting them together gives you a complete picture," Dr Nimmo said. "The two-dimensional model provides high resolution, but you can only look at vertical impacts. The three-dimensional model allows nonvertical impacts, but the resolution is lower so you canít track what happens to the crust."

Shock waves from the impact would travel through the planet and disrupt the crust on the other side, causing changes in the magnetic field recorded there.

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.

Certain Martian meteorites may have originated from the northern crust, Nimmo said. The study also suggests that the impact occurred around the same time as the impact on Earth that created the Moon.

There are one or two even larger impacts thought to have occurred in the early solar system - a controversial theory that there was one on the innermost planet Mercury, and a widely accepted one that the Earth was struck by a planet as big as Mars, melting the crust and ejecting it into space where some of it clumped together to form our moon.

But in both of those cases, the impacts were so enormous that they completely obliterated all visible signs of the event.

It is only through indirect analysis, including study of rocks brought back from the moon by the Apollo astronauts, that these giant ancient impacts have been reconstructed.

The new finding adds yet another major event to the growing list of large impacts that have been recognized over the last few decades as having the shaped the planets and moons of the solar system as we know them today.

"The early solar system was a very dangerous place to be a planet," said one of the MIT team, Dr Jeffrey Andrews-Hanna, who did his study with Prof Maria Zuber and Bruce Banerdt of NASA-Jet Propulsion Laboratory. "But without those impacts, we wouldnít have the planets as we know them today."

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."

  25/06/2008. Telegraph.