Microbial Iron Miners On Mars?
Tiny Tunnels In Meteorite Suggest The Possibility Of Ancient Life On The Red Planet
By Jennifer Glass
A meteorite that shattered over the Egyptian village of El-Nakhla in 1911 has Oregon geologists and microbiologists intrigued.
The 1.3-billion-year-old Martian meteorite had a violent descent to Earth. Despite its meager dimensions, the basketball-sized flying ball of rock allegedly vaporized a dog before punching three-foot-deep holes into the desert sand.
Now, it's sparking scientific controversy about the possibility of ancient Martian life.
The meteorite, christened "Nakhla," (Arabic for "date palm," the tree of life) in honor of its impact site, has been studied extensively for nearly a century.
In the February issue of the journal Astrobiology, researchers from Oregon State University, Portland State University, the Kinohi Institute in Pasadena, Calif., and the Smithsonian Institution in Washington D.C. reported that the meteorite Nakhla contains minute tunnels possibly carved by microorganisms living beneath the Martian surface millions of years ago.
Study leader Martin Fisk, professor of marine geology and geophysics at Oregon State University in Corvallis, and colleagues have identified microscopic tunnels in Nakhla, the widest about half the diameter of the finest human hair, that closely resemble similar tunnels thought to be produced by bacteria in terrestrial rock samples.
But does that mean the tunnels are definitely biological in origin? That question is the center of current scientific debate.
Investigation of the tunnels using a microscope designed specifically for studying slivers of rock revealed that tunnels originate from clay-filled fractures in the meteorite. This is significant because tunnels thought to be produced by bacteria in Earth rocks also originate from clay-filled fractures, and because clay in the Martian meteorite is evidence that water once percolated through the rock.
Water is vital for all life forms. Microorganisms in rocks rely on water to supply them with energy and nutrients.
Nakhla contains minerals rich in ferrous iron, that is, iron isolated from exposure to oxygen. Ferrous iron, when exposed to oxygen, can be an energy source for microbes living in water.
Previous studies of terrestrial samples have shown that microbes can live at the boundary between water and minerals containing iron, like those in Nakhla. "The microbes may have been using chemical compounds to facilitate excavation of iron from the rock," says Fisk. "This may be the source of the tunnels."
Radu Popa, an associate professor of biology at Portland State University and one of the study's authors, says that microbes must continuously mine iron from minerals: "Because they need energy for respiration, if they stop digging, they die."
The scientists found similar tunnels in iron-rich terrestrial samples infiltrated by water.
These rocks included a seawater-bathed lava rock drilled from more than 4,000 ft beneath Hilo, Hawaii, and other iron-rich rocks collected from Oregon, northern California and from a depth of more than 26,000 ft in the Tonga Trench north of New Zealand. A previous study found tunnels in 3.5 billion-year-old basaltic rocks from South Africa.
Tunnels found in Earth rocks are believed to form by biological processes instead of weathering by water due to their unusual morphologies.
"These are irregular structures," says Olivia Mason, an OSU oceanography Ph.D. student and author on the study. "Non-biologic weathering generally creates a linear, smooth alteration front. These tunnels come in multiple sizes and shapes," says Mason.
Since tunnels have never been created in a laboratory experiment, they cannot be assigned a clear biological or non-biological origin. "Until we have a clear demonstration of how to make tunnels, either with bacteria or without them, we can't be sure whether or not they are associated with biologic activity," says Fisk.
Evidence of tunnels' biological origin in Earth rocks is the presence of DNA, the biochemical backbone of life, located in the entrances to tunnels. The presence of DNA suggests that living organisms may have inhabited fractures in the rock and left behind their biochemical signatures.
No DNA was found in the Nahkla meteorite. Like in an abandoned mine shaft, "there's been lots of excavation, but no miners remain," says Popa.
Claims of organic matter and biologically-formed minerals in another Martian meteorite, ALH 84001, launched a new field of science in 1996: astrobiology, or the study of life and habitable environments in the universe.
Most of the claims for microbial traces in ALH 84001 were subsequently refuted, based largely on the likelihood of contamination from terrestrial meltwater after ALH 84001 landed on Antarctic ice fields.
The aridity of the Egyptian desert and Nakhla's near-immediate collection indicate that the tunnels in Nakhla were present before the meteorite fell. Some scientists, however, are unconvinced that the tunnels are evidence of life.
Anthony Irving, an associate professor in the Department of Earth and Space Sciences at the University of Washington in Seattle, is skeptical of such morphological evidence of ancient Martian life, as well as the presence of liquid water on Mars as recently as 600 million years ago.
"Most evidence points toward near-surface water on Mars having only existed for about 500 million years, back around 3.5 billion years ago," says Irving.
The surface of Mars now averages 80 degrees below 0 F.
Nakhla‘s clay veins have been dated to have formed billions of years after the alleged warm, wet period on Mars. How could water still be percolating through the Martian sub-surface so recently?
Fisk suggests that if there is in fact ice close to the surface of Mars, as many scientists now believe, there is likely to be liquid water beneath the ice due to increasing temperatures with increasing depth. "If this water circulates in a way that would allow gas exchange with the Martian atmosphere, then microbes could still be living on Mars today." The microbes would be relying completely on chemical energy derived from rock weathering, Fisk adds, and thus they would have very slow growth rates.
"It's an interesting idea, that the erosion of glassy minerals could be interpreted as evidence that biology can move into rock veins after water has passed through," says John Baross, professor of biological oceanography at the University of Washington School of Oceanography in Seattle. Research on microbial tunnels in low-oxygen environments on Earth, such as those in deep-sea hydrothermal vents, might shed light on these processes.
Jennifer Glass is a 2006 University of Washington graduate holding double degrees in earth and space sciences and oceanography.
Image at Top:
Microscopic view into a thin slice of Martian meteorite Nakhla. A fracture in Nakhla (tan) with tunnels (in boxes) are similar in size and shape to tunnels associated with DNA in terrestrial rocks. The tunnels are about 10 micrometer long. Typical bacteria are about 1 micrometer across. Photo: Martin Fisk/OSU
|