The case for ancient life on Mars looks betterthan ever after scientists announced that they had discovered magnetic crystalsinside
a Martian
meteorite -- crystals that, here on Earth, are produced onlyby microscopic life forms.The magnetic compound, called
magnetite or Fe3O4,is common enough on our planet. It is present, for example, in household videoand audio tapes. But only certain types of terrestrial bacteria, which canassemble the crystals atom by atom, produce magnetite structures that arechemically pure and free from defects.Scientists studying the Allan Hills meteorite,found just such crystals deep inside the space rock.These magnetites (from the meteorite) arebasically indistinguishable from certain biogenic magnetites on Earth. Andfurthermore, we know of no other mechanism to make them, either on Earth orMars. .The scientists believe that these crystalstraveled from Mars in the meteorite, rather than being produced on Earth bybacteria that contaminated the meteorite after it arrived in Antarctica.This meteorite - called the Allan Hillsmeteorite after the Antarctic ice sheet where it was found like life on Mars.These magnetite crystals were one of the four pieces of evidence from themeteorite that supported the '96 announcement. But little was known about thespecific traits of bacteria-produced magnetite then.At that point, we just knew that there were tinymagnetite crystals made by bacteria, and we didn't know much about them And wenow have studied (the crystals) in detail, and ones known to be made bybacteria have the same properties (as those from the meteorite).Crystals made by magnetite-producing bacteriaare chemically pure and free from defects in the crystalline structure. Theyare slightly elongated along a particular crystalline axis, and they range insize from 35 to 120 nanometers (a nanometer is one-billionth of a meter). Theyalso show a particular pattern of faceting - like a cut diamond. Theseproperties are so unusual that they have only been seen in magnetite crystalsproduced by biological processes.The researchers discovered that about one-fourthof the magnetite crystals in the meteorite have these exact ormed geologically.Bacteria are able to make such precise crystalsbecause they control the construction of the crystal at an atomic level.The magnetites are grown atom by atom inside thebacteria. The bacteria form a little membrane around the crystal that controlsthe growth of the magnetite, and then they pump iron atoms into that membraneand form these crystals (which consist of iron and oxygen atoms). The direction in which the bacteria elongate thecrystals maximizes the magnetic strength of the magnetite. The bacteria, whichare mostly from the Magnetospirillum genus, then line up several ofthese crystals to collectively act as a bar magnet, which allows the bacteriato align itself with Earth's magnetic field.Why would a bacterium want to line up with ourplanet's magnetic field? It turns out that such behavior can help an aqueousmicrobe find water with the right mix of oxygen. Generally, differingconcentrations 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 topointing toward the pole, also make a vertical angle with the ground. Theselines provide a sort of slanted "elevator shaft" that help thebacteria search the "building's floors," which can be more efficientthan an aimless search.But such an internal compass would be of no useto a Martian bacterium unless Mars had a natural magnetic field like Earthdoes.It's clear thatearly on, Mars had a strong magnetic field, and that's about the time we thinkthese magnetites were formed: about 3.9 billion years ago. In contrast, the earliest well-documented lifeon Earth dates back to between 3.6 and 3.7 billion years ago. Both planetsformed about 4.5 billion years ago."Now we are trying to answer the questionof whether (magnetite-producing) bacteria could have actuad on Mars, Andwe have found certain aspects of their metabolism which suggest that they mighthave been able to do so.The journal Science has publishedresearch showing evidence of widespread sediment layers on Mars, which theresearchers interpreted to be the product of ancient lakes that once dottedMars's surface. Because these lakes may have provided a habitat for bacteria,this finding supports the possibility that the bacteria may have existed onMars.Though the new evidence from the Allan Hillsmeteorite does not prove that life once existed on Mars. We think it's evidencethat is hard to explain by any other hypothesis. Have wefound past life on Mars? The answer is neither yes or no, leaving a strongmaybe. We still argue that past life on Mars is a reasonable interpretation ofthe data on hand. the question of pastlife on Mars may never be completely resolved by additional detailed studies ofthis meteorite, although such studies are clearly necessary. Each of the other12 meteorites from Mars should also be carefully studied. Perhaps they mightcontain additional or alternative evidence of life on Mars. The question oflife on Mars, whether fossil or existing, will never be completely.solveduntil we can bring back the right samples from that planet.