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.
Right: A transmission electron microscope (TEM) image of magnetite crystals from the meteorite. Click on the image for aof these crystals with magnetite produced by the MV-1 bacteria strain.
"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.
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"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.
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.
Above: The Allen Hills meteorite (ALH84001), which scientists believe comes from Mars. The black cube is 1 cm. [more information]
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 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.
Above: One example of a magnetotactic (magnetite-producing) bacterium. Note the line of slightly-elongated magnetite crystals down the bacterium's center. These crystals act as a compass, aligning the bacterium with the Earth's magnetic field. Image courtesy of Dr. Dennis Bazylinski of Iowa State University.
"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.
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.
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."
Above: An artist's concept comparing the present day magnetic fields on Earth and Mars. Earth's magnetic field is generated by an active dynamo - a hot core of molten metal. The magnetic field surrounds Earth and is considered global (image B). The various Martian magnetic fields (image A) do not encompass the entire planet and are local. The Martian dynamo is extinct, and its magnetic fields are "fossil" remnants of its ancient, global magnetic field. [more information]
The journal Science recently 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."
In addition to Bazylinski and Gibson, the scientists involved with this investigation are Kathie Thomas-Keprta, Simon Clemett, and Susan Wentworth from Lockheed Martin at Johnson Space Center; David McKay at JSC; Joseph Kirschvink at the California Institute of Technology; H. Vali at McGill University, Montreal; and Christopher Romanek at Savannah River Ecology Laboratory.
NASA's Astrobiology Institute -- Home page
Mars Global Surveyor -- Home page
Fossil Life in ALH84001? -- Summary of the 1996 Science article by Dr. David McKay et al. that first announced possible evidence of Martian life in the Allan Hills meteorite
What is ALH84001? -- Information on the Allan Hills meteorite, including how scientists know it came from Mars and how it got to Earth
Animation -- A 980 kb mpeg movie simulating a meteor impact that could have launched the Allan Hills meteorite from the face of Mars
Science@NASA Stories about Mars:
Sedimentary Mars -- New Mars Global Surveyor images reveal sedimentary rock layers on the Red Planet that may have formed underwater in the distant martian past.
Making a Splash on Mars -- On a planet that's colder than Antarctica and where water boils at ten degrees above freezing, how could liquid water ever exist? Scientists say a dash of salt might help.
Unearthing Clues to Martian Fossils -- The hunt for signs of ancient life on Mars is leading scientists to an otherworldly lake on Earth.
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|For lesson plans and educational activities related to breaking science news, please visit Thursday's Classroom||Author: Patrick L. Barry
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