Possible Life on Mars
When and Where?
Whole rock, ALH84001. The oldest martian meteorite.
Since life requires liquid water, the history of water on Mars controls when and where life may have originated and flourished. The denser atmosphere early in Mars history contributed to a warmer, wetter environment. These are conditions more favorable for life. Traditional wisdom also directs the search for evidence of life to regions of water-lain sediments. Do the martian meteorites support these ideas? Can characteristics of possible martian life or habitat be inferred from meteorite evidence?
Close view of central region of carbonate (away from rim areas) showing textured surface and nanometer ovoids and elongated forms (arrows). They resemble nanobacteria found in some terrestrial rocks.
This electron microscope image shows tubular structures of martian origin that may be fossils of organisms.
The recent findings of evidence for possible fossil life in the oldest martian meteorite ALH84001, the only martian meteorite dating back to this early period when conditions for life were more hospitable, do reinforce the idea of searching for life in Mars' ancient regions. Within the distinct, layered carbonates of ALH84001, lie four lines of evidence suggesting the possibility of life on early Mars. 1) Pancake-shaped features, up to 250 µm across with rims of layered Ca-Mg-Fe carbonates, are found along fractures and in pore spaces in the meteorite through which a fluid penetrated. They resemble some terrestrial bio-deposition features. 2) Martian-origin polycyclic aromatic hydrocarbons (PAHs), detected at very low levels by laser-excitation mass spectrometry, are associated with the fracture zones. The PAH compositions are few in number and relatively simple ring structures, consistent with remains of in-situ chemical decay biological cyclic compounds. 3) Very high resolution transmission electron microscopy (TEM) reveals the coexistence of tiny grains of magnetite and iron sulfides with a morphology and composition similar to grains deposited biogenically. A complex oxidation-reduction environment would be needed to account for this co-precipitation inorganically. 4) Finally, high resolution scanning electron microscopy has revealed within the carbonate fracture zones ovoid features, 20 to 100 nanometers across, that resemble nanobacteria recently detected on terrestrial rocks. Although individually these four lines of evidence have alternative, inorganic process interpretations, taken as a whole, biogenic activity is the simplest explanation to describe the observed features.
Indeed, if this evidence for fossil life is confirmed, it is an indication that hydrothermal environments in ancient igneous rocks are additional niches in which to search for martian life.
