by R. Cowen
If Martians could look down on Earth, they might be amused. Ever since a startling report last August, Earthlings have hotly debated whether a potato-shaped meteorite contains fossil evidence of primitive life on the Red Planet.
Some of the controversy focuses on whether carbonates found in the meteorite were formed at low temperatures, in an environment hospitable to life, or at much higher temperatures. Speaking to a standing-room-only crowd last week at NASA's Goddard Space Flight Center in Greenbelt, Md., David S. McKay, a leader of last August's discovery team, disputed a recent report arguing for a high-temperature origin for material in the meteorite.
He also presented new evidence that the rock, dubbed ALH84001, contains fossils of primitive bacteria from Mars.
McKay, of NASA's Johnson Space Center in Houston, countered criticisms that the tiny, wormlike structures thought to be microfossils in the meteorite could not, in fact, have been formed by a living organism (SN: 12/14/96, p. 380). John P. Bradley of MVA in Norcross, Ga., and the Georgia Institute of Technology in Atlanta and his colleagues had reported evidence that the apparent microfossils in ALH84001 are elongated, whiskerlike crystals of magnetite.
The whiskerlike shape has been associated with high-temperature activity, which cannot support water-based life, near volcanic vents. Bradley and his colleagues also found that some of the magnetite crystals contain structural defects that are not known to be produced in a biological environment. One of them, a spiral defect, typically occurs when atoms crystallize from a high-temperature vapor. In the other defect, known as twinning, atomic lattices that are mirror images of each other join at a common boundary.
Although McKay continues to assert that his team has not seen the whiskerlike shapes or the defects in microfossils from ALH84001, he also cites two older reports, previously overlooked, demonstrating that tiny bacteria can produce both whiskers and twinning.
"Two out of three of [Bradley's] arguments are completely out the window," asserts Joseph L. Kirschvink of the California Institute of Technology in Pasadena. Bradley notes, however, that McKay's team had originally claimed that the apparent microfossils were and should be flawless if they were produced by living material. No one has yet demonstrated that bacteria can produce the spiral defect, he adds.
During his talk, McKay displayed intriguing micrographs that show what appear to be filmy coatings around carbonate globules in the meteorite. McKay suggests that the coatings, which survived even after the carbonates were etched away by acid or ions, are either a clay mineral or a protective, biological layer known as a biofilm. Modern colonies of bacteria can produce biofilms.
In a report to be given in March at the annual Lunar and Planetary Science Conference in Houston and already posted on the Internet, Kirschvink's team argues that ALH84001 hasn't been heated to more than 110 degrees C since 4 billion years ago, long before any bacteria could have infiltrated the rock. The researchers found that two tiny, adjacent pieces of the meteorite showed dramatically different responses to an applied magnetic field. Exposure to high temperatures would have erased any such differences, they assert.
These results also provide a hint that ancient Mars had a magnetic field. That's good news for proponents of life on Mars, because the field would have steered away charged particles, belonging to the solar wind, that would otherwise have eroded the planet's atmosphere. An eroding atmosphere, with its large climate variations, could have proved catastrophic for life on the Red Planet.References:
Kirschvink, J.L., A. Maine, and H. Vali. 1997. Abstract of report to be presented at the annual Lunar and Planetary Science Conference. Houston.
McKay, D.S. 1997. Lecture at NASA's Goddard Space Flight Center. Greenbelt, Md.Further Readings:
Bradley, J.P., R.P. Harvey, and H.Y. McSween. 1996. Magnetite whiskers and platelets in the ALH84001 Martian meteorite: Evidence of vapor phase growth. Geochimica et Cosmochimica Acta 60(December).
Cowen, R. 1996. Searching for life in a Martian meteorite. Science News 150(Dec. 14):380.Sources:
John P. Bradley
5500/200 Oakbrook Parkway
Norcross, GA 30093
David S. McKay
NASA Johnson Space Center
Houston, TX 77058