|Close-up of famous shapes measuring 20 to 200 nanometers across in Allen Hills meteorite [ALH84001], found at Allen Hills, Antarctica, showing what has generated debate and controversy around claims of ancient fossilized microbial life. "Several lines of evidence suggest that the volume of a sphere about 200 nanometers across is needed to house the chemistry of a cell that has a biology familiar to us." --A. Knoll Around 28 Mars meteorites have been identified so far.
Image Credit: NASA
The city of Hammerfest lies at the northern tip of Norway, well above the Arctic Circle. If you board a ship heading north from there, just before you reach the polar ice cap you run into a group of islands known as the Svalbard archipelago.
For the past two summers, a group of scientists has traveled to the largest of these islands to study an environment that sheds light on a notorious meteorite, discovered at the opposite end of the Earth, in Antarctica.
The meteorite, ALH84001, began as a rather unremarkable piece of volcanic rock that formed about 4.5 billion years ago, on Mars. About a billion years later, its interior was chemically altered through interaction with water. After that, it remained on the martian surface until about 16 million years ago, when a massive impactor - a comet or asteroid - slammed into Mars, spewing material into space at such tremendous velocity that some of it, including ALH84001, was able to escape Mars's gravity. After drifting through interplanetary space for millions of years, eventually the meteorite collided with Earth. That was about 13 thousand years ago. In 1984 it was discovered by meteorite hunters in the Allen Hills region of Antarctica.
ALH84001 gained international fame when scientists at NASA Johnson Space Center (JSC) announced in 1996 that it contained evidence of life - martian life. Miniscule structures within the meteorite looked similar to fossilized bacteria seen on Earth. A form of magnetite (iron oxide) was detected in the meteorite that, on Earth, is produced only within the bodies of certain bacteria. And researchers found unusual microscopic carbonate globules, which they believed were formed by living organisms.
Carbonates are common on Earth. England's famous White Cliffs of Dover, for example, are made from calcium carbonate, or limestone. But the carbonates in ALH84001 were far from being common limestone. They were unique. Scientists had no way to tell where on Mars the meteorite had come from, or what its history had been prior to being thrown into interplanetary space. There was no way to know for certain how the carbonate globules had formed. But their unusual appearance, one of several distinctive features of the rock from another world, led the JSC researchers to conclude that living organisms had once made their home there.
Members of the scientific team who made the original announcement still believe that ALH84001 contains evidence of martian life. Most researchers, however, now think that its various microscopic features can be explained purely by geologic and chemical processes. Recent discoveries made in Svalbard bolster the majority opinion.
|The ALH Meteorite, about the size of a softball and one of more than two dozen Mars samples available for study on Earth today. [ALH84001] was found at Allen Hills, Antarctica.
Image Credit: NASA/ Johnson Space Center
Although the ALH84001 carbonate globules were novel at the time the meteorite was discovered, scientists have since discovered that rocks in Svalbard contain carbonate globules remarkably similar to those found in ALH84001. They were anxious to learn what they could about how the Svalbard carbonate globules formed. Researchers can only speculate about how the carbonate globules in ALH84001 formed, billions of years ago and millions of miles away. But in Svalbard, says Andrew Steele, "the geology's in context."
Steele, who is with the Carnegie Institution of Washington, is a member of AMASE (Arctic Mars Analog Svalbard Expedition), an international team of scientists who for the past three years have been studying the Svalbard environment. The major aspect of their work is to test out life-detection instruments that will be used on future missions to Mars. But it was the discovery of Svalbard's carbonate globules that first caught their attention.
"Originally we didn't set out to try and confirm or refute" whether the carbonate globules in ALH84001 were "formed by biology or not. We basically went up there to look at the context and find out just how these things are formed on Earth, and then try to draw some conclusions about their formation mechanisms on Mars," said Steele.
The context is a volcano, Sverrefjell, that erupted about a million years ago, forcing magma up through an overlying glacier. The carbonate globules in the Svalbard rocks were found embedded inside material that was spewed out when the volcano erupted. An analysis of the material surrounding the globules - a mineral known as olivine, for its dull green color - showed that it came from the Earth's mantle, some 40 to 50 kilometers (25 to 30 miles) beneath the surface. Before the eruption, it was in a molten state, deep underground. Within a few days of being ejected onto the surface, it had cooled and hardened in the freezing glacial environment aboveground. During this cooling process, the carbonate globules became deeply embedded within the surrounding rock.
"This is an abiotic production method," said Steele. No living organisms could have been present in the molten subterranean depths. Nor could microbes have colonized the molten material in the short span of a few days during which the rocks cooled and hardened, sealing the globules deep within.
|Major investigated regions of Antarctica where meteors have been successfully identified. At any given moment, the interplanetary sample transit works out to about one Martian meteorite landing on Earth each month. Scientists had thought it took a serious wallop to instigate these interplanetary exchanges. Impacts of this size and larger occur every 200,000 years or so on Mars. Yet research now finds that craters as small as 1.9 miles (3 kilometers) wide on Mars could have been the starting points for meteorite launches towards Earth. Credit: JSC/NASA Meteor Program
Armed with the knowledge that the Svalbard globules were formed abiotically, Steele and his colleagues performed a painstaking comparison between them and the ALH84001 globules. Using one of the most sophisticated instruments of its type in the world, a Raman spectrometer, the AMASE team examined thousands of tiny spots both within samples of ALH84001 and within rocks collected in Svalbard. The Raman spectrometer enabled them to catalog in detail the mineral components in the carbonate globules within the two rocks. They found a high degree of similarity.
"That doesn't mean to say that [the Svalbard globules] are exactly the same as the martian globules and are formed in exactly the same conditions," Steele said, "but it gives us a window into that formation process. There is a formation mechanism for them that doesn't rely on biology."
The ALH84001 saga is not over. There will undoubtedly be discussion about its various unusual features for many years to come. But by showing how carbonate globules, similar to those in the martian meteorite, formed without the involvement of living organisms, Steele and his colleagues have made less compelling the argument that the visiting rock from our planetary neighbor contains evidence of life.
Related Web Pages
New Martian Meteorite
Life in Tiny Tunnels?
Life without Volcanic Heat
Diving for Life Under Antarctic Ice
New Signs of Polar Life
Tricorder Going to Mars
Proof of Life?
Life in Ice
AMASE expedition TV feature