Life on Mars?

An example of a magnetite-producing bacterium Image courtesy of Dr. Dennis Bazylinski of Iowa State University.

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 Mars (A) and Earth (B).

"When we first wrote the original paper in '96, it was thought that Mars had never had much of a magnetic field," Gibson said. "But then the Mars Global Surveyer detected a very strong remnant magnetism in some of the rocks in the crust of Mars. ... So it's clear that early on, Mars had a strong magnetic field, and that's about the time we think these magnetites were formed: about 3.9 billion years ago."

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."

The journal Science has 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."