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Epic crash may explain two faces of Mars
June 25, 2008
World Science staff
Scientists have been hard-pressed to explain why the two halves of Mars look very different—low-lying plains in the north and cratered highlands in the south. Artists' concept of a giant
impact proposed to have formed the Martian dichotomy, based simulations by
Marinova and colleagues (Courtesy Jeff Andrews-Hanna). Click
here for animation in mpeg format (Courtesy S. Lombeyda/ M.
Marinova/ O. Aharonson)
It now turns out a huge asteroid or comet impact long ago can explain the peculiarity, say researchers working with computer simulations. “It’s a very old idea, but nobody had done the numerical calculations,” said Francis Nimmo, associate professor of Earth and planetary sciences at University of California, Santa Cruz.
(See animation).
The dichotomy, as the two-faced characteristic of Mars is called, “is arguably the oldest feature on Mars,” said California Institute of Technology planetary scientist Oded Aharonson. It’s believed to have arisen more than four billion years ago, about when Earth
took a bombardment that led a piece of it to fly off and form the Moon.
On Mars, myriad later events in the planet’s complex geologic history changed
its surface, but beneath all these blips the ancient dichotomy looms
large.
Scientists had questioned whether a single impact could have created it, though, said Margarita Marinova, a graduate student in planetary sciences at Caltech.
Marinova is lead author of a paper co-authored with Aharonson and published in the June 26 issue of the research journal
Nature, proposing the impact scenario. A second paper in the same issue, by Nimmo and others, reaches the same conclusion based on different simulations. Yet a third paper offers high-resolution mapping of the boundary between the contrasting zones.
Scientists had doubted the single-collision idea for several reasons, Marinova said. For one, it was thought one impact would make a circular mark, but the northern lowlands are rather elliptical. There’s also no crater rim. Some scientists also thought a mega-projectile would erase signs of its own impact by temporarily melting much
of Mars.
“We set out to show that it’s possible to make a big hole without melting the majority of the surface,” Aharonson said. His group modeled a range of projectile parameters that could yield a cavity
about the size and shape of the Mars lowlands without melting everything or making a rim.
After cranking out 500 simulations through a large network of computers combining various energies, velocities, and impact angles, the researchers narrowed in on a “sweet spot”—a range of parameters that would make craters just like that on Mars. The crash would have had an energy right between the one thought to have led to the extinction of dinosaurs on Earth 65 million years ago, and the one believed to have created our planet’s moon four billion years ago, according to Marinova’s group.
Nimmo’s group did simulations in two dimensions rather than three, but this allowed more detail, said Nimmo, adding, “The two approaches are very complementary.”
That the Mars and Moon events occurred around the same time is no coincidence, Marinova said: “this size range of impacts only occurred early in solar system history.” The findings could also shed light on other large impact events, like the Aitken Basin on the moon and the Caloris Basin on Mercury, she added.
NASA’s Viking missions to Mars in the 1970s revealed Mars’ split personality. Some 20 years later, the Mars Global Surveyor mission additionally found the Martian south has a much thicker crust and magnetic anomalies not found up north.
“Two main explanations have been proposed… either some kind of internal process that changed one half of the planet, or a big impact hitting one side of it,” Nimmo said. “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. We showed that you really can form the dichotomy that way.”
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Scientists have been hard-pressed to explain why the two halves of Mars look very different—low-lying plains in the north and cratered highlands in the south. It now turns out a huge asteroid or comet impact long ago can explain the peculiarity, say researchers working with computer simulations. “It’s a very old idea, but nobody had done the numerical calculations,” said Francis Nimmo, associate professor of Earth and planetary sciences at University of California, Santa Cruz. The dichotomy, as the two-faced characteristic of Mars is called, “is arguably the oldest feature on Mars,” said California Institute of Technology planetary scientist Oded Aharonson. It’s believed to have arisen more than four billion years ago, about when Earth underwent a bombardment that caused the Moon to form. Myriad later events in the Mars’ complex geologic history changed the red planet’s surface further, but beneath all these blips the ancient dichotomy looms prominently. Scientists had questioned whether a single impact could have created it, though, said Margarita Marinova, a graduate student in planetary sciences at Caltech. Marinova is lead author of a paper co-authored with Aharonson and published in the June 26 issue of the research journal Nature, proposing the impact scenario. A second paper in the same issue, by Nimmo and others, reaches the same conclusion based on different simulations. Yet a third paper offers high-resolution mapping of the boundary between the contrasting zones. Scientists had doubted the single-collision idea for several reasons, Marinova said. For one, it was thought one impact would make a circular mark, but the northern lowlands are rather elliptical. There’s also no crater rim. Some scientists also thought a mega-projectile would erase signs of its own impact by temporarily melting much of the planet. “We set out to show that it’s possible to make a big hole without melting the majority of the surface,” Aharonson said. His group modeled a range of projectile parameters that could yield a cavity the size and ellipticity of the Mars lowlands without melting everything or making a rim. After cranking out 500 simulations through a large network of computers combining various energies, velocities, and impact angles, the researchers narrowed in on a “sweet spot”—a range of parameters that would make craters just like that on Mars. The crash would have had an energy right between the one thought to have led to the extinction of dinosaurs on Earth 65 million years ago, and the one believed to have created our planet’s moon four billion years ago, according to Marinova’s group. Nimmo’s group did simulations in two dimensions rather than three, but this allowed more detail, said Nimmo, adding, “The two approaches are very complementary.” That the Mars and Moon events occurred around the same time is no coincidence, Marinova said: “this size range of impacts only occurred early in solar system history.” The findings could also shed light on other large impact events, like the Aitken Basin on the moon and the Caloris Basin on Mercury, she added. NASA’s Viking missions to Mars in the 1970s revealed Mars’ split personality. Some 20 years later, the Mars Global Surveyor mission additionally found the Martian south has a much thicker crust and magnetic anomalies not found up north. “Two main explanations have been proposed… either some kind of internal process that changed one half of the planet, or a big impact hitting one side of it,” Nimmo said. “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. We showed that you really can form the dichotomy that way.”