The most convincing evidence for life on the red planet comes from a Martian meteorite that landed on Earth around 13,000 years ago. This meteorite contained microscopic structures that could have been formed by living organisms.
The general consensus now is that the original rock formed 4 billion years ago on Mars. It was eventually catapulted into space by an impact and wandered the solar system for millions of years before landing on Earth 13,000 years ago.
Over 50 other meteorites have been identified as coming from Mars, but ALH84001 is by far the oldest, with the next in age being just 1.3 billion years old. “That alone makes ALH84001 a very important sample,” says Allan Treiman of the Lunar and Planetary Institute. “It’s our only hope to understand what Mars was like at this time period.”
The first thing that struck researchers examining the meteorite was the presence of 300-micron-wide carbonate globules that make up 1% of the rock. Dave McKay from NASA’s Johnson Space Center and his colleagues determined that the carbonate most likely formed in the presence of water.
Although evidence for a wet ancient Mars has accumulated in the subsequent years, the claim that ALH84001 once sat in water was pretty revolutionary at the time, says Kathie Thomas-Keprta, also from the Johnson Space Center.
Inside the ALH84001 carbonates, McKay spotted odd features that resembled very small worm-like fossils, so he asked Thomas-Keprta to look at them more closely with electron microscopy. “I kind of thought he was crazy,” she says. “I thought I would join the group and straighten them out.”
In the end, she helped the team characterize the biomorphic features, as well as unusual grains of the mineral magnetite found in the meteorite. In a 1996 Science paper, these two phenomena – along with the chemical distribution in the globules and the detection of large organic molecules – were taken collectively as signatures of biological activity occurring long ago on Mars.
