The Martian Meteorites

Scientists believe that about 16 million years ago, a huge comet or an asteroid struck Mars, ejecting a piece of the surface. For millions of years, this rock floated through space, falling onto the Earth about 13,000 years ago as a meteorite.

Possible source crater for the impact event

Most of the Martian meteorites are less than 1.5 billion years old, and were formed after the time when Mars had liquid water. The sole exception is the Martian meteorite ALH84001, found in Antarctica. It is about 4.5 billion years old, or nearly as old as Mars itself. This meteorite must have already been there when the surface of Mars had liquid water and, perhaps, life. For that reason, NASA scientists studied the meteorite carefully for signs of life.  For a good slide show about the Martian meteorites, click here.

Evidence for fossil life in this meteorite was found in tiny carbonate globules deposited in cracks in the rock. Inside the globules, scientists found three indicators of possible life: complex organic molecules (polycyclic aromatic hydrocarbons), microscopic grains of magnetic iron (magnetite) similar to those made on Earth by certain bacteria, and tiny rod-shaped objects that resemble fossilized bacteria.

Possible cellular structures

Possible fossilized bacteria

Each of these features might have a non-biological explanation, but the combination of all of them persuaded these scientists that ancient life on Mars might be the simplest explanation. There is now a spirited debate among scientists about the evidence, and the verdict may not come in until we receive definitive samples from the planet.