Life on Mars?

"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 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.