Evidence of Ancient Martian Life in Meteorite ALH84001?

ALH84001 was discovered in Antarctica, where rocks that fall from the heavens are easy to spot on the icy glacial plains. Its name indicates that it was the first meteorite found during the 1984 research season in the Allan Hills, an especially meteorite-rich area in the Trans-Antarctic Mountains. ALH84001 is a 1.9 kg meteorite from Mars.

The original igneous rock solidified within Mars about 4.5 billion years ago, about 100 million years after the formation of the planet, based on isotope ages of the igneous component of the meteorite. Between 3.6 and 4 billion years ago the rock was fractured by meteorite impacts. Water then permeated the cracks, depositing carbonate minerals, perhaps allowing primitive bacteria to live in the fractures. About 3.6 billion years ago, the bacteria and their by-products became fossilized in the fractures. 16 million years ago, a large meteorite struck Mars, dislodging a large chunk of this rock and ejecting it into space, based on the cosmic ray exposure age of the meteorite. 13,000 years ago, the meteorite landed in Antarctica.

How do we know ALH84001 is from Mars?

ALH84001 is one of twelve meteorites discovered on Earth thought to be from Mars. Most meteorites formed early in the history of the solar system, some 4.6 billion years ago. Eleven of the twelve martian meteorites have ages less than 1.3 billion years, ALH84001 at 4.5 billion years old being the only exception. All twelve are igneous rocks crystallized from molten magma in a way which suggests they formed in a planetary-sized body, not an asteroid. They have similar oxygen isotope characteristics to each other and higher concentrations of ferric iron, water, and other volatiles than other meteorites. All twelve also show evidence of shock heating, presumably as a result of the impact which ejected them into space. Gas bubbles trapped in one meteorite, EETA79001, have a composition which matches the current martian atmosphere as measured by the Viking Landers, compelling evidence that this meteorite and by association the others, including ALH84001, came from Mars.

An examination of ALH 84001 believed to be from Mars shows: 1) hydrocarbons which are the same as breakdown products of dead micro-organisms on Earth, 2) mineral phases consistent with by-products of bacterial activity, and 3) tiny carbonate globules which may be microfossils of the primitive bacteria, all within a few hundred-thousandths of an inch of each other.

The suggestion for life hinges on three important pieces of evidence, all discovered within mineralized fractures in the meteorite in close proximity to each other. One is the discovery of abundant polycyclic aromatic hydrocarbons (PAHs) on the fracture surfaces. These are a family of complex organic molecules which are commonly found on dust grains and certain types of meteorites in outer space, presumably formed by non-biological chemical reactions. However, when micro-organisms die they break down into PAHs as well. The mixture of PAHs found on ALH84001 is very different from that found on dust grains and other meteorites, suggesting the possibility of a biological origin. Thousands of different types of PAHs are found all over the Earth, but those in ALH84001 do not appear to be contaminants which have leaked into the meteorite.

Another line of evidence involves unusual mineral phases found beside the PAHs. These carbonate minerals form "globules" about 50 micrometers across, some of which have cores containing manganese and rings of iron carbonate and iron sulfides, and also contain magnetite and pyrrhotite. These minerals bear strong resemblance to mineral alterations caused by primitive bacteria on Earth. This diversity of minerals in such a small area, formed under the presumed conditions, seem to make a non-biological origin unlikely. Their ages are uncertain, estimates ranging from 1.2 to 4 billion years.

High-resolution scanning electron microscopy has revealed the presence of tiny "ovoids" which may actually be fossil remnants of tiny (20 to 100 nanometer) bacteria. If so, they are 100 times smaller than any bacteria microfossils found on Earth, except for some supposed "nanofossils" recently discovered in very young terrestrial rocks, a finding currently not generally accepted as fossil organisms.

Caveat. 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.
Caveat. A second caveat is 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.

Another line of evidence that has survived longer is one that revolves around minerals sprinkled through the meteorite that could have been produced by microbes, magnetite and carbonate. The second 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. Some of the most evolutionarily ancient bacteria on Earth produce magnetite, McKay and his colleagues pointed out. Perhaps ancient Martian microbes did as well; at least some of the magnetite grains in ALH84001 share the shape, small size and remarkable purity of those produced by bacteria on Earth.

Of all the lines of evidence presented by the NASA scientists, magnetite grains are the most provocative. They were embedded in the carbonate along with other iron-containing minerals in such an unusual arrangement that something out of the ordinary must have put them there could it have been alive?