considers some generic features that could not be generated abiologically and that would be the foundation of a sound approach to the recognition of nonterran life.
Abiotic chemistry, both organic and inorganic, provides important information about the pathways that might have led toward an origin of life. Unfortunately, there is in origin-of-life scenarios no consensus about the synthesis of organics on early Earth or elsewhere, and so astrobiologists cannot search for a specific chemistry. Among the models suggested as possibly relevant for the origin of life are atmospheric electric discharges, as proposed by Miller and Urey,4 which have been shown to synthesize a range of organic compounds, including amino acids, from mixtures of methane, ammonia, and water. Discharge experiments yield few organic compounds when carried out in the kinds of oxidized gas mixtures of carbon dioxide thought to have predominated on early Mars. Additional processes that might have contributed to the inventory of organic compounds on early Mars include those associated with the transient effects of bolide impacts5 and, more importantly, a variety of mineral-catalyzed chemical reactions including water-rock reactions (e.g., serpentinization) and Strecker, Fischer-Tropsch, and FeS-driven organic synthesis.6 Water-rock reactions produce copious amounts of hydrogen that could lead to the subsurface formation of hydrocarbons from carbon dioxide and have also been shown to reduce nitrogen to ammonia,7 both of which could make their way to planetary surfaces. Strecker synthesis is the reaction of ammonia, hydrogen cyanide, and aldehydes to give amino acids and related products. Fischer-Tropsch chemistry is the mineral-catalyzed high-temperature reaction of carbon monoxide and hydrogen to give hydrocarbons. FeS-driven organic synthesis, first proposed by Wächtershäuser,8,9 has been experimentally demonstrated for only a relatively limited set of syntheses.
It is safe to assume that organic compounds that might have contributed to the prebiotic potential of the planet could have been synthesized elsewhere in the solar system or in interstellar space and then carried to the surface of Mars via carbonaceous chondrites and interplanetary dust particles. Since there is no consensus about the past history of prebiotic processes on Mars, it is more constructive to first consider the availability of the elements that constitute organic matter.
Carbon. C is found as gaseous carbon dioxide in the martian atmosphere, as carbon dioxide ice, and as carbonate minerals. Carbonates have been found in small amounts in martian meteorites but have not been detected in significant quantities by orbital remote sensing techniques or in chemical analyses of the martian regolith by landers.
Hydrogen. H is present as water ice and vapor and in hydrated minerals, and may be present within the crust as liquid water. The high D/H ratios of martian water show that Mars has lost a fraction of its water to space from the upper atmosphere. Because of the low atmospheric pressure, liquid water is not stable at the surface of modern Mars. The polar ice caps are thought to contain significant quantities of water ice, and the Gamma Ray Spectrometer on the Mars Odyssey spacecraft has detected significant quantities of subsurface hydrogen, presumably in the form of water ice.10 Thus, the abundance of hydrogen would not have hindered life on Mars at any time in its history.
Nitrogen. N is poorly retained by the inner planets owing to its volatility and stability as N2 and also to the relative instability and solubility of its involatile forms. Currently, 2.7 percent of the martian atmosphere is nitrogen. Although nitrogen is crucial for life, it may be rare on Mars.11 The observed ratio of 15N/14N suggests that a large fraction of the planet’s nitrogen inventory has been lost to space. No measurements have yet identified nitrogen stored in surface or subsurface minerals.
Oxygen. O is present in H2O and CO2, in oxides and sulfate minerals on the highly oxidized surface, and in silicates and other minerals within the crust.
Phosphorus. Phosphate minerals are actually more abundant in meteorites than in most igneous rocks on Earth. Volatile compounds of phosphorus (phosphorus pentoxide and phosphine) are rare, making phosphate minerals more valuable as sources of phosphorus for organisms than other biotic elements with common volatile forms.