Still searching for life on Mars

At first ALH84001 was misclassified, so it wasn’t until 1993 that researchers even realized the rock came from Mars. That was interesting enough, because at the time fewer than a dozen Martian meteorites were known to science.

But ALH84001 also turned out to be much more ancient than the other known Martian meteorites. At 4.5 billion years old, it dates from a period of Martian history when liquid water — a requirement for the presence of life — probably existed at the now barren planet’s surface.

It made sense to ask: Could there be fossils of ancient Martian microbes, or maybe traces of them, preserved in the cracks and pore spaces of ALH84001?

The NASA scientists proffered four reasons to support their view that the answer to that question is “Yes.”

First, chemical analysis showed that the meteorite contained a variety of organic molecules known as polycyclic aromatic hydrocarbons, or PAHs. PAHs can be produced by biological processes, and that’s what McKay and his colleagues argued. But they are also commonly found in asteroids, comets and meteorites, not to mention the Antarctic ice where ALH84001 is estimated to have lain for 13,000 years. For that reason, skeptics immediately dismissed the importance of PAHs in the Martian meteorite.

A second line of evidence — that the elongated blobs in the electron microscope images could be fossils of ancient Martian bacteria — was also rejected pretty quickly by most scientists.

The problem was, those blobs were much smaller than any bacteria that have ever been observed on Earth. A National Research Council panel concluded in 1998 that the blobs were 100 to 1,000 times too small to be free-living organisms because they couldn’t have held all the proteins, DNA and other molecules necessary for even the simplest metabolic processes.

You could argue that perhaps Martian life evolved a more compact biochemistry, or that the blobs shriveled as they fossilized. At one point McKay and the other NASA scientists suggested the blobs might be pieces of larger organisms.

“That was only mentioned once or twice and never brought up again,” said Allan Treiman, a geologist at the Lunar and Planetary Institute in Houston.

The two other lines of evidence survived longer. Both revolved around minerals sprinkled through the meteorite that could have been produced by microbes.

The first mineral, carbonate, is typically formed on earth by the remains of living organisms that make shells and other skeletal parts out of minerals they extract from seawater. Some of those organisms can be quite tiny. So finding carbonate in ALH84001 could indicate the presence of ancient microbes in the rock.

The story is similar for magnetite, the other mineral of interest in ALH84001. Some bacteria produce extraordinarily small and pure magnetite crystals, then align the magnetic grains to make a microscopic compass needle that helps them navigate.

The bacteria don’t use their internal compasses to find north; they use them to tell up from down. Earth’s spherical shape means that a compass needle in either hemisphere points at least somewhat downward, so the magnetite grains help the microbes sense where they are with respect to the planet’s surface.

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