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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. 

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).

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)


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.”