The Allan Hills meteorite from Mars is peppered with tiny magnetic crystals that on our planet are made only by bacteria.

by Patrick L.Barry

The case for ancient life on Mars looks better than ever after scientists announced that they had discovered magnetic crystals inside a Martian meteorite -- crystals that, here on Earth, are produced only by microscopic life forms.

The magnetic compound, called magnetite or Fe₃O₄, is common enough on our planet. It is present, for example, in household video and audio tapes. But only certain types of terrestrial bacteria, which can assemble the crystals atom by atom, produce magnetite structures that are chemically pure and free from defects.

Scientists studying the Allan Hills meteorite, a 4-billion-year-old rock from Mars that landed in Antarctica about 13,000 years ago, found just such crystals deep inside the space rock.

"Finding this type of magnetic crystal in any material from another planet is an amazing and important finding," said Dr. Dennis Bazylinski, a geobiologist at Iowa State University. Bazylinski leads one of the few labs capable of culturing these magnet-producing bacteria, which are common in many freshwater and marine environments on Earth.

Bazylinski was one of nine researchers conducting the four-year investigation, which was funded by NASA's Astrobiology Institute. A report of their research is in the December 2000 issue of the journal Geochimica et Cosmochimica Acta.

"We're not claiming that this is proof of life on Mars," said Dr. Everett Gibson, an astrobiologist at NASA's Johnson Space Center in Houston, Texas, who also participated in the study. 

"What we're claiming is that these magnetites (from the meteorite) are basically indistinguishable from certain biogenic (i.e., biologically-produced) magnetites on Earth. And furthermore, we know of no other mechanism to make them, either on Earth or Mars," Gibson said.

An electron microscope image of magnetite crystals from the meteorite.

The scientists believe that these crystals traveled from Mars in the meteorite, rather than being produced on Earth by bacteria that contaminated the meteorite after it arrived in Antarctica.

"That was a real concern -- whether (the magnetite crystals) could be terrestrial contamination," Gibson said. But several facts support a Martian origin, including the deep embedding of the crystals in the carbonate material of the meteorite and the preference of the magnetite-producing bacteria for low-oxygen environments, making it unlikely that such bacteria would live where the meteorite was found.

This meteorite -- called the Allan Hills meteorite after the Antarctic ice sheet where it was found -- is the same one that caused a stir in 1996 by providing the first potential evidence of bacteria-like life on Mars. These magnetite crystals were one of the four pieces of evidence from the meteorite that supported the '96 announcement. But little was known about the specific traits of bacteria-produced magnetite then.

"At that point, we just knew that there were tiny magnetite crystals made by bacteria, and we didn't know much about them," Gibson said. "And we now have studied (the crystals) in detail, and ones known to be made by bacteria have the same properties (as those from the meteorite)."

Crystals made by magnetite-producing bacteria are chemically pure and free from defects in the crystalline structure. They are slightly elongated along a particular crystalline axis, and they range in size from 35 to 120 nanometers (a nanometer is one-billionth of a meter). They also show a particular pattern of faceting -- like a cut diamond. These properties are so unusual that they have only been seen in magnetite crystals produced by biological processes.

The Allan Hills Meteorite

The researchers discovered that about one-fourth of the magnetite crystals in the meteorite have these exact properties. The other three-fourths of the crystals are assumed to have formed geologically, researchers said.

Bacteria are able to make such precise crystals because they control the construction of the crystal at an atomic level.

"The magnetites are grown atom by atom inside the bacteria. The bacteria form a little membrane around the crystal that controls the growth of the magnetite, and then they pump iron atoms into that membrane and form these crystals (which consist of iron and oxygen atoms). By carefully controlling crystal growth with the membrane, the bacteria keep the crystals from growing in one direction and allow them to grow in another," Gibson said.

The direction in which the bacteria elongate the crystals maximizes the magnetic strength of the magnetite. The bacteria, which are mostly from the Magnetospirillum genus, then line up several of these crystals to collectively act as a bar magnet, which allows the bacteria to align itself with Earth's magnetic field.

An example of a magnetite-producing bacterium Image courtesy of Dr. Dennis Bazylinski of Iowa State University.

Why would a bacterium want to line up with our planet's magnetic field? It turns out that such behavior can help an aqueous microbe find water with the right mix of oxygen. Generally, differing concentrations of oxygen in a body of water are arranged in horizontal layers, like the floors of a building. Earth's magnetic field lines, in addition to pointing toward the pole, also make a vertical angle with the ground. These lines provide a sort of slanted "elevator shaft" that help the bacteria search the "building's floors," which can be more efficient than an aimless search.

But such an internal compass would be of no use to a Martian bacterium unless Mars had a natural magnetic field like Earth does.

An artist's concept comparing the present day magnetic fields on Mars (A) and Earth (B).

In contrast, the earliest well-documented life on Earth dates back to between 3.6 and 3.7 billion years ago, Gibson said. Both planets formed about 4.5 billion years ago.

"Now we are trying to answer the question of whether (magnetite-producing) bacteria could have actually lived on Mars," Bazylinski said. "And we have found certain aspects of their metabolism which suggest that they might have been able to do so."

The journal Science has published research showing evidence of widespread sediment layers on Mars, which the researchers interpreted to be the product of ancient lakes that once dotted Mars's surface. Because these lakes may have provided a habitat for bacteria, this finding supports the possibility that the bacteria may have existed on Mars, Bazylinski said.

Though the new evidence from the Allan Hills meteorite does not prove that life once existed on Mars, Gibson said that, "We think it's evidence that is hard to explain by any other hypothesis."