mars_meteorite_writethru_000831 CHICAGO -- Analysis of new Martian
meteorites is confounding planetary scientists with clues that simply don't add up.Rather than clearing up existing questions about the
Red Planet, results from the new meteorites seem to be opening up a Pandora's box of questions about Mars. Evidence from the rocks doesn't seem compatible with one of the most trusted scientific conclusions about the planet: that the vast majority of the Martian surface is billions of years old.The puzzle came into sharp focus here this week at the annual meeting of the Meteoritical Society, as scientists announced their findings from three recently discovered
Mars rocks. The Los Angeles meteorite, which was discovered last fall by a Los Angeles rock hunter, was revealed to be only 175 million years old -- contemporary in geologic terms. It is a volcanic rock that crystallized from magma near the Martian surface. Larry Nyquist and a research team at NASA's Johnson Space Center and Lockheed Martin in Houston determined the age by measuring the state of certain weakly radioactive isotopes within the
meteorite.
New meteorites seem to be opening up a Pandora's box of questions about Mars.
When a rock solidifies, a sort of internal stopwatch is turned on. Certain unstable isotopes are locked into the crystals of the rock, and they begin to decay. By carefully measuring just how much of various unstable isotopes have decayed away, scientists can determine how long it has been since the material crystallized to become rock. This age is called a rocks crystallization age, and is very important to scientists who study meteorites.
While surprisingly young, the meteorite is not unique in its youth.
"[The Los Angeles meteorite] is one of now seven meteorites with ages of about 175 million years old," Nyquist said.
This applies to nearly half of the 15 Martian meteorites with known ages. A 16th Martian meteorite, known as Dhofar 019, was discovered by a Russian expedition traveling in the desert of Oman earlier this year. It has yet to be dated conclusively, but some think it will end up being the eighth 175 million-year-old Mars rock. There is, of course, no agreement about that forecast.
"The Los Angeles meteorite's young age -- indicating that Martian volcanism has been active until very recently -- supports recent interpretations of [Mars Global Surveyor] imagery," Nyquist said.
The Mars Global Surveyor, which has been in orbit around Mars since 1997, has recently sent back images that seem to suggest some areas of Mars have young volcanic deposits. But photo interpretation is a subjective business, and scientists have been searching for other evidence in the matter.
The speculation that Mars may remain volcanically active stands in dramatic contrast to older, more accepted theories about Mars, which hold that geological activity on the Red Planet ended billions of years ago.
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In addition to being of similar age, the newest Martian rocks that have turned up on Earth are all of the same type -- Shergotite -- the most abundant of the four classes of Martian meteorites.
"Suddenly a bunch of new Mars meteorites have been found in the desert, and they're all of the same class," said Tim Swindle, a meteorite expert at the University of Arizona's Lunar and Planetary Laboratory. The apparent boom in discoveries is exciting, but the similarity of the samples is disappointing, Swindle said. "I'd sure like to see some new Nakhlites, or a new Allan Hills 84001," he said, referring to two other classes of
Mars meteorites whose members are much older. 
"We're only going to sort that out by getting sample returns from Mars."
Nakhlites are named after Nakhla, Egypt, where the first meteorite of its type was found in 1911. There are now three known Nakhlites. These pieces formed on Mars about 1.32 billion years ago. Allan Hills 84001 is the name of the only meteorite of its type -- the rock made famous in 1996 when a NASA team announced that it appeared to contain evidence of past Martian life. Most scientists found that evidence didnt hold up, but the meteorite is unique in other ways. It is the only rock of its type and is by far the oldest of the Martian rocks, at about 4.4 billion years old.
The predominance of young meteorites in the Mars collection is not only a disappointment to scientists who yearn for more clues about the planet's geology, it is starting to get downright puzzling.
If the vast majority of Mars' surface is billions of years old, then it seems reasonable to expect that a similarly large proportion of meteorites from the planet would reflect this age.
"We're starting to wonder why we aren't seeing many older meteorites," said Lars Borg, a planetary geochemist who specializes in dating meteorites at the University of New Mexicos Institute of Meteoritics.
The simplest explanation, Borg said, is that all the 180 million-year-old Martian meteorites originate from the same Martian location. The seven similar rocks may have been ejected at one time by a catastrophic asteroid impact: the asteroid just happened to slam into one of the rare areas of young volcanic rock on an otherwise old Mars.
A tidy explanation, Borg said, if it didnt conflict with information about the length of time that each of the meteorite samples spent in space.
Work by several other researchers shows that explaining the origin of these rocks is not so easy. This is work involves the cosmic-ray-exposure age of the meteorites. That is a measure of how long the rocks spent in transit between Mars and Earth.
Cosmic rays radiate from the sun. They are extremely energetic and highly destructive. When they hit material in space, they cause nuclear interactions that leave behind radioactive products, called radionuclides.
Both rock and thick atmospheres provide shields against cosmic rays. All the known Mars meteorites originated more than a few yards (meters) below the planets surface, so they were protected from cosmic rays while on their home planet. Once they reached Earth they were protected again. By counting the amount of radionuclides in a given meteorite, scientists can determine how long the rock spent in space.
Scientists can then measure the strength of those radionuclides to find out how long those radioactive products have been decaying. That tells them how long the rocks have been on Earth, protected from the damage of cosmic rays. Adding together the time that a meteorite has been on Earth and the time it spent in space, reveals just how long ago rocks were thrown off Mars.
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Kunihiko Nishiizumi is a cosmo-chemist who specializes in cosmic-ray studies at the University of California at Berkeley. He has calculated that the seven young Shergotites found on Earth came from four different impact events that ejected material from Mars. In total, the 16 Martian meteorites came from no less than nine different ejection events, he said.
Because they left Mars at four different times, there is no way that all the young rocks came from one place on Mars, Nishiizumi said.
"I think that the most important thing that is coming out of the combined cosmic-ray-exposure ages and the crystallization ages of these meteorites is that we may be missing something fundamental about Mars," said David Mittlefehldt, a geochemist and meteorite expert at Lockheed Martin in Houston, Texas.
In 1993, Mittlefehldt proved that the Allan Hills 84001 meteorite was of Martian origin. Previously it had been misidentified as part of an asteroid.
The age of the Martian surface has been calculated by examining the number of craters on Mars, Mittlefehldt explained. The oldest surfaces would have been exposed to meteorite impacts from space for the longest time, and thus would have the most craters on them. Young surfaces would be relatively free of craters.
Using the current understanding of crater density, a maximum of 15 percent of the Martian surface could be as young as 175 million years old, Mittlefehldt said. Even getting that high of a percentage is really stretching the model, he said. It simply doesn't make sense that half the meteorites from Mars and half the big impact events that sent them to Earth just happened to hit the 15 percent of the surface that is young, he said.
What that means is that
Mars may be much younger than most scientists have thought, he said. The accepted models for how fast craters build up on Mars could be entirely wrong, which would throw into question a whole host of conclusions about Martian surface history.A final answer to the question could be many years away, though. Clues from meteorites alone don't seem to be settling the debate. "If anything, I think the problem may be getting worse instead of better," Mittlefehldt said.
"We're only going to sort that out by getting sample returns from Mars," he said. On this point, Nishiizumi, Borg and the others agree. The need to send a spacecraft to the planet to bring back fresh samples from known locations is one proposal about which there is no debate.
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