Arguments Against Life on Mars
Many scientists were skeptical of the evidence presented for ancient life on Mars. They argued that many of the observed features said to indicate life could be attributed to purely inorganic processes: life was not necessary for these features to be formed. The main arguments against the evidence for life on Mars are summarized below.
The model for the theory that life formed the features seen in ALH84001 is an equilibrium model. This means that the meteorite was only subjected to conditions of equilibrium - thermal, barometric, etc. The meteorite was obviously not always in equilibrium. The impact needed to remove the rock from the surface of Mars subjected it to intense pressure and temperature. Most of the features said to be microscopic fossils could have formed inorganically by these shock events and heating.
Any evidence of microscopic fossils in the Martian meteorite came from contamination during its 11,000 years on Earth before its discovery. This contamination came either in the form of Earth life making its way into the meteorite, or from weathering and other effects and formed features that look to the human eye to be of biological origin.
Skeptics claim that the special form of magnetite rock found in ALH84001 could have formed by heating iron-rich carbonate, though they do not say what temperatures are needed for this formation to occur. CO and CO2 are by-products of making magnetite in this manner.
The PAHs found in ALH84001 are not necessarily the products of decaying organic material. They could have been formed inorganically from CO (carbon monoxide), CO2 (carbon dioxide) and H2 (molecular hydrogen). The CO and CO2 would have been by-products of the formation of magnetite described above.