The case for ancient life on Mars looks better than ever after scientists announced last week 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 have found just such crystals deep inside the Allan Hills meteorite (also known as ALH84001). The meteorite is a 4-billion-year-old rock from Mars that landed in Antarctica about 13,000 years ago, the space rock.
|A transmission electron microscope (TEM) image of magnetite crystals from the meteorite. Click on the image for a side-by-side comparison of 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.
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 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.
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.
The Allan Hills meteorite (ALH84001), which scientists believe comes from Mars. The black cube is 1 cm wide. [more information]
"We looked at it very carefully and convinced ourselves that the magnetite had to be from Mars," Gibson said. "No one (in the scientific community) is really questioning that."
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.
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.
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.
Evidence of Life from the Allan Hills Meteorite: Latest Findings
New Report Offers Evidence of Primitive Life on Mars
Original NASA press release describing studies of micromagnets in the Allan Hills meteorite.
Elongated Prismatic Magnetite Crystals in ALH84001 Carbonate Globules: Potential Martian Magnetofossils
Abstract of the research team's report in the journal Geochimica et Cosmochimica Acta.
Evidence of Life from the Allan Hills Meteorite: Earlier Findings
Exploring Mars: Ancient Life Studies
Web page from NASA Johnson Space Center describing research into signs of ancient life in the Allan Hills meteorite.
Fossil Life in ALH84001?
A 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.
Background on 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.
Technical Discussion: Martian Origin
Reasons the Allan Hills meteorite is believed to come from Mars.
Animation: Meteorite from Mars
A movie simulating a meteor impact that could have launched the Allan Hills meteorite from the face of Mars.
Allan Hills Far Western Icefield Explanatory Text and Meteorite Listing
Description and maps of the ice sheet in Antarctica where the Allan Hills meteorite was found.
Water on Mars
Evidence of Martian Land of Lakes Discovered
Layers of sedimentary rock paint a portrait of an ancient Mars that may have featured numerous lakes and shallow seas.
Magnetic Fields on Mars
Plate Tectonics on Mars?
Magnetic stripes on the surface of Mars are similar to fields in the sea floors of Earth and may indicate ancient crustal movements on the Red Planet.