Tiny egg-shaped or tubular features were found in Mars meteorite ALH84001. These features resemble many bacteria on Earth and were thus interpreted as fossils of ancient Martian life. Most of the fossils are between 20 and 100 nanometers in diameter, smaller than any normal bacteria on Earth. Recent evidence, however, suggests that there do exist on Earth organisms of this size dubbed "nanobacteria". There is still debate as to whether or not Earth nanobacteria have been proven to be life. If nanobacteria are in fact living organisms, the finding of these features in ALH84001 is good evidence for past life on Mars. For more information on nanobacteria, please see the University of Queensland.

Carbonate is a type of rock formed from liquid H₂O (water) and CO₂ (carbon dioxide) gas. Carbonate can be formed inorganically, without the help of life. On Earth, however, life often helps in the formation of carbonate. The carbonate found in the Martian meteorite ALH84001 was surrounded by microscopic features interpreted as fossils by the scientists studying the meteorite. Though this finding in itself is inconclusive as to the presence of life, it does indicate that liquid water was once present on Mars.

Magnetite is another type of rock found in meteorite ALH84001. The specific form of this rock found in the meteorite is chemically pure with no defects and is present in long, straight chains. On Earth, magnetite in this form found is only known to be produced by bacteria. It is reasonable to conclude, therefore, that this form of magnetite could only have been formed on Mars by Martian bacteria.

Complex molecules made of carbon (C) and hydrogen (H) forming many rings were found in ALH84001. These molecules, known as Polycyclic Aromatic Hydrocarbons (PAHs), are created when living organisms decay. (They are also found in the dust between the stars - the interstellar medium (ISM) - where no life is known to exist.) Those PAHs found in the Martian meteorite are the same types of PAHs associated with the decay of life. (These specific PAHs are not necessarily in the same form as those created in the ISM.) In addition, the PAHs in ALH84001 were found near the carbonate, suggesting that the rock was indeed formed with the assistance of living organisms. The PAHs found in ALH84001 were not found in other, non-Martian meteorites found in the same location (Allyn Hills, Antarctica). This indicates that the PAHs found in ALH84001 were not formed through the decay of Earth organisms that found their way into the meteorite.