On August 7, 1996, an historic press conference was held at NASA Headquarters in Washington DC. News that scientists had found evidence of life in a Mars meteorite had leaked out, and NASA had to make an announcement. A few minutes before, President Clinton made these remarks at the White House before heading out on a trip to California:

"This is the product of years of exploration and months of intensive study by some of the world's most distinguished scientists. Like all discoveries, this one will and should continue to be reviewed, examined and scrutinized. It must be confirmed by other scientists... I am determined that the American space program will put its full intellectual power and technological prowess behind the search for further evidence of life on Mars."

At the press conference, several scientists from NASA and Stanford University announced their findings -- they confirmed that they had found evidence of ancient, fossilized, microscopic life from a Martian meteorite, known as ALH84001. The meteorite was catapulted away from Mars fifteen million years ago when a huge comet or asteroid impacted the surface. The meteorite travelled through space for millions of years and then encountered the Earth. It entered Earth's atmosphere about thirteen thousand years ago and landed at Antarctica. The meteorite lay there until 1984, when a team from the NASA Johnson Space Centre found it while exploring the Allan Hills ice field, and brought it back to Houston. It was initially classified as a lunar meteorite, but in 1993 was correctly identified as from Mars. It is one of only twelve "SNC" meteorites, which match the unique chemical signature of Mars.

The scientists talked their reasoning for the discovery -- they had four independent lines of evidence which, when taken together as a whole, ancient life on Mars is the logical conclusion.

 

Do these orange carbonate globs contain the remains of millions of ancient Martian life forms? (Courtesy Science Magazine)

Carbonate globules
The carbonate patterns form a unique signature of life, and the density and composition of the carbonate patterns is consistent with how terrestrial bacteria operate. Carbonate by itself, however, is abundant in non-living materials.

Polycyclic aromatic hydrocarbons (PAHs)
Organic compounds usually created by bacteria were present in the meteorite. There has been much controversy about whether or not the PAHs are contaminants from the Antarctic environment, or are present from Mars.

Magnetite globules
These globules are created by bacteria on Earth as well as some chemical processes. However, the distinctive tear-shape in the Martian evidence is only created by bacteria. Some scientists are still not convinced.

Microscopic fossil-like structures
The most dramatic evidence of all, pictures were shown at the press conference of worm-like structures present in the meteorite. While they are much smaller than terrestrial bacteria, they look very similar, but could also just be mineral structures. More pictures have been created since then. The initial scepticism about them has partially been silenced by recent discoveries of similar terrestrial fossils of similar (nanobacterial) scale.

Are these worm-like structures fossilised Martian bacteria? (Courtesy NASA/JSC)

Implications of Mars Life
While the life they talked about was only microscopic, it has several implications for us macroscopic creatures. If life on Mars is ever proven to to exist (or have existed at some point in time), it would mean that the creation of life is not something that happens because of freak chance or divine influence, but is in fact a probable occurrence given the right conditions. Even further, if all that life requires is an aqueous solution like liquid water to grow and thrive (which is the current theory), then the universe is literally teeming with life. The suspected liquid water oceans on some of Jupiter's moons (Europa and Callisto) could be filled with life, and life could still be present underneath the Martian surface, where liquid water and thermal energy are still present.

The Viking Experiments: More Evidence for Life?
Apparent evidence for life on Mars has been found before, and it's worth a second look now. The Viking Landers, which were sent to Mars in the 1970s, carried several experiments designed to detect organic materials and organisms on the Martian surface. However, the experiments were designed with limited knowledge of Mars because at that point, no spacecraft had ever successfully landed on the Red Planet. When the Viking Landers arrived on Mars, the results were ambiguous. While one experiment detected no organic compounds in the soil, another -- the Labelled Release experiment (LR) -- found positive results. The LR was designed to drop a nutrient solution into a soil sample from Mars, and then measure the changes in the gaseous sample container to determine if the changes were organically induced (if bacteria were multiplying because of the nutrients they'd been given). When the experiment was conducted on both Viking Landers, it gave positive results almost immediately.

Most scientists on the Viking mission came to the conclusion that the positive results were attributed to the discovery of oxides in the soil, and that a chemical reaction occurred when the nutrient solution was mixed with the oxides. However, the LR's designer and principal investigator, Dr. Gilbert Levin, was convinced that his experiment found life. He remains convinced to this day and has recently published a paper with additional findings. However, his conclusions have been rejected by mainstream scientists (which is not surprising considering their implications!)

Levin also says that the experiment which did not find organic materials in the soil were not sensitive enough to detect it in small amounts. This has been confirmed by NASA as possible, since ALH84001 clearly contains organic materials (which may not necessarily have been created by living organisms). The experiment in question was tested in Antarctica and found negative results, which is definitely wrong because there are organic materials there!

Solving the Search for Life Question
The debate continues. Future missions to Mars will address the question of Life on Mars. In 2005, a Mars Sample Return mission will attempt to bring back likely candidate samples of minerals in which these kind of fossils would occur. If successful, and similar evidence is found, the critics will be silenced. However, it's a long-shot that a suitable sample can be found by a remote-controlled rover. The best thing would be to have a trained field geologist with a rock hammer. A human explorer would offer on-site experience, better mobility (presumably he'd have his own rover to drive around in), and a much better chance of finding evidence for life. So, in the end, the best way to settle the debate is to send people to Mars. In the opinion of the Mars Society and many experts, this question alone is enough to justify a Humans to Mars program.