Studies of Meteorite Give Boost to Theory of Fossil Life From Mars
In a major boost for scientists trying to prove that forms on an Antarctic Martian meteorite could be evidence of ancient extraterrestrial life, two separate groups of researchers have pulled the rug out from under one of the main arguments against the fossil life hypothesis.
Working with mere specks of the celebrated Mars rock, groups at Caltech and the University of Wisconsin demonstrated that pancake-shaped globules in the rock grew at temperatures as balmy as boiling water, and never got hotter than 350 Celsius, a tolerable environment for life forms that like it hot.
The results, reported today in the journal Science, dramatically contradict what some scientists have been saying since last summer’s historic NASA announcement of possible life on Mars: that the structures formed on Mars at temperatures so searing that any potential life would have been charred to toast.
The studies also ruled out the possibility that the carbonate globules seeped into the rock after it landed in Antarctica about 13,000 years ago. In addition, the Caltech researchers discovered that a protective magnetic field surrounded Mars 4.5 billion years ago when the rock was formed--making life even more plausible.
Scientists believe that the forms were created early in Martian history, when the red planet was warmer, wetter and more amenable to life. The rock was knocked off Mars by a crashing meteorite 16 million years ago, researchers say, then wandered the solar system before landing on the Antarctic ice, where it was discovered in 1984.
While the new findings hearten scientists discouraged by recent reports raising doubts that the forms are fossils, they still fall short of proving that the structures are evidence of life.
“It would be wonderful to look in there [and] see a live, squirming bacterium,” said Wisconsin geochemist John Valley. But that is not possible.
One of the main doubters, Jeffrey Bada of Scripps Institute of Oceanography, said the authors of today’s Science papers had “impeccable credentials. I have to believe that they’re calling it as they see it.” On the other hand, he said, his reaction to the finding that the structures formed at life-friendly temperatures was: “So what?” Other places in the solar system have the same kinds of temperatures and do not necessarily support life, he said.
Supporters of the life on Mars theory found the studies quite convincing. “These papers are quite significant,” said Stanford chemist Richard Zare, a member of the original team that discovered evidence of ancient life. “They show that life as we know it is consistent with the data that was found.”
The minute, tube-shaped forms that grabbed international headlines in August are far too small to be analyzed. Instead, the researchers focused on the larger carbonate “droppings” these uncannily bacteria-like beings purportedly left behind. (The suspected fossils were found in the dark outer rims of the Martian “pancakes.”)
These mineral secretions are similar to those produced by bacteria on Earth and include needle-like magnetite crystals that change direction in response to a magnetic field. In the speck they studied, the Caltech researchers, led by magneto-geobiologist Joseph Kirschvink, were able to detect the traces of two distinct ancient magnetic fields rotated almost at right angles to each other.
Because magnetic fields get “frozen” into rocks at cool temperatures, and melt away at high temperatures, the findings suggest that while it was on Mars, the rock never got hotter than the 350 degrees Celsius required to realign the magnetic fields--and probably not above 100 degrees Celsius.
“There’s no way [the minerals] could have been heated” after the structures were formed, said Kirschvink. “It’s an amazing result, that from a sand-grain-sized speck of meteorite, we know not only that Mars had a magnetic field 4 billion years ago, but also that the rock has not been heated in its subsequent history.”
Just as significant, the failure of high temperatures to scorch the center of the rock during its fiery trip through Earth’s atmosphere suggests that getting life from Mars to Earth “is not that hard,” Kirschvink said.
The fact that Mars had an Earth-like magnetic field 4 billion years ago would have allowed it to retain an atmosphere and shield the planet from the damaging electrically charged wind that blows continually off the sun. This could have permitted the emergence of life.
To detect such a tiny remnant of magnetism, Kirschvink used a supersensitive superconducting detector that picks up tiny electric currents generated by almost imperceptible magnetic fields.
In another feat of technological wizardry, the team at Wisconsin bombarded its Mars speck with a highly focused beam of electrically charged cesium atoms, punching a hole in it a quarter the diameter of a human hair. With a second instrument, they shot high-energy electron beams at the rock and analyzed the X-rays that scattered backward. Both techniques allowed them to look deep inside the carbonates at extremely sharp resolution.
What they found were sharply defined concentric rings of various minerals, “pancakes within pancakes,” Valley called them. From layer to layer, clearly separated zones contained unique chemical signatures and mineral content.
At high temperatures, such zones would have fused into each other, in effect, melding together, Valley said. “It couldn’t have stayed at high temperature for more than a few days,” he said.
Moreover, the ratio of two different kinds of oxygen isotopes and two different carbon isotopes found in the sample were typically Martian, and unlike anything measured on Earth, he said. Still, none of this new evidence is conclusive and most scientists are still waiting impatiently to receive their own specks of Mars rock from NASA in order to conduct experiments. NASA halted distribution of samples shortly after August’s announcement, when it became clear that there would be an avalanche of requests that needed to be evaluated.
Bada still doesn’t have a sample to work on. Like many of his colleagues, he worries that chips of the rock are being handed down from researcher to researcher without any clear trail of where they have been or who has handled them.
“If somebody was not careful and laid a sample on a table and picked it up with their hands, you’d be a dead duck,” he said, meaning that the sample would be hopelessly contaminated and useless for research.
However, Bada’s analysis of similar Martian meteorites suggests carbon compounds known as PAHs that are “identical” to those found in the globules. He found similar compounds in water melted from Antarctic ice.
Several meetings over the next two weeks are expected to produce dozens of research findings on the Mars rock, but no one expects the matter to be resolved any time soon. “What we’ve got is a four-pound rock that was randomly plucked from the surface of Mars,” said Valley, “and it’s going to be hard to prove that there was life there. If you got a four-pound rock that was randomly plucked from the surface of Earth, you might have trouble convincing people there was life here, too--even though life is abundant.”
Proof that the carbonates came from Mars, Bada said, would be if they contained amino acids that spiraled to the right. All biologically produced amino acids on Earth spiral to the left. He plans to conduct experiments to find out as soon as he gets his own Mars speck, “hopefully” within a couple of months.
