The Curious Case of ALH84001

About 4 billion years ago, a meteorite slammed into Mars. The location where it hit contained some carbonates and some iron. The pressure of the impact pushed the shocked Martian soil well below the surface. There it was under pressure and cooled only slowly. Then a few million years ago, another impact event on Mars liberated a rock containing this material and gave it a speed greater than the Martian escape velocity. This rock traveled the solar system between Earth and Mars until about 17,000 years ago. It then entered the Earth’s atmosphere and landed in an area of Antartica that we now call Allan Hills. Only recently have humans deliberately scoured Antartica looking for meteorites and this rock was found in 1984 and given the name ALH84001. Meteorites are fun but not all that special and that was the fate of ALH84001 until someone decided to compare the gases trapped inside of it to the Martian gases as measured by the 1976 Viking landers. Suddenly, the dull rock ALH84001 was a rock star (pun intended). This happened in the early 1990’s and in 1996, a famous paper by Dave McKay et. al. claimed that ALH84001 showed possible signs of biological activity. What followed was a debate between the biotic proponents and the abiotic proponents. The debate continues. Here are some highlights.

As soon as it was determined that ALH84001 came from Mars, the odd shapes on the meteorite started to look suspicious. Some of the shapes looked like bacteria. Could these be fossils of bacteria? Several alternative explanations were advanced.

* The bacteria are nothing but terrestrial contamination.
* The procedures for preparing samples for an electron microscope scan might have created these kinds of structures.
* The structures seem to be too small to be from bacteria.
* Pictures taken with an electron microscope show apparent structure that isn’t really functional.
* Many abiotic mechanisms could create similar structures.

Although all of these explanations were refuted by the biotic proponents, it became clear that arguing based on shapes was not convincing.

PAH stands for Polycyclic Aromatic Hydrocarbons. An example of a PAH is Napthalene, C10H8. This is the substance that gives mothballs their particular odor (they aren’t called “aromatic” for nothing). These chemicals can be either complex or simple. PAHs tend to be created by biological processes on Earth but it is not the only way that they can be created. However, without some kind of process generating PAHs, they will eventually break down into simpler substances (mothballs don’t last forever).

Some abiotic proponents say that the PAHs are the result of terrestrial contamination but that has been shown to be unlikely. The PAHs found in ALH84001 are mostly of the simple variety while Earth-bound PAHs tend to the more complex. Also, more PAHs were found in the inner sections of the meteorite than on the outside.

The magnetite found in ALH84001 was found to have characteristics that are only found in magnetotactic bacteria – a kind of bacteria that produces magnetite. This bacteria was only discovered on Earth in 1975. But magnetite can be made in both biotic and abiotic processes. It was demonstrated by the abiotic group that magnetite can be formed by shocking another mineral, siderite. The characteristics of the magnetite found in ALH84001 that are not compatible with nonbiological magnetite are:

* Uniform crystal size,
* Gaps between crystals,
* Orientation of elongated crystals and
* Flexibility of magnetite chains,

This continues to be the strongest point for the biotic proponents. All abiotic attempts to create magnitite do not create the types of magnitite created biologically. The debate remains open with both sides hoping that further Martian exploration, hopefully to someday include sample returns, will declare the victor. In the meantime, it is interesting how the more we search for life on Earth the more places we find it, but the more we look for life on Mars, the more alternative explanations come forth. And that’s how it should be.