Part of the argument used to support the notion that life may have arisen on Mars early in its history depends on the presence of carbonates in the notorious meteorite ALH84001 found in Antarctica. Supposedly having been ejected by an impact on the Martian surface (based on its oxygen isotope composition and the blend of noble gases trapped within it), ALH84001 also contains the minute structures that were prematurely announced in a blaze of publicity as fossilized alien life forms by US and British meteorite specialists in 1996. The discoverers claimed that carbonate minerals within it clearly evidenced the rotting of silicates by liquid water containing dissolved CO2; so they do in terrestrial rocks. However, carbonates also occur in meteorites that by no shred of the imagination can have formed within sizeable planets. Many probably accreted in a near vacuum from dusts that occur in clouds within our galaxy, while the solar system was forming.
Using infrared spectra to assess the mineral composition of dust clouds surrounding stars, a team of European and American cosmochemists has found that in two cases such dust contains calcite and perhaps dolomite (Kemper, F. et al. 2002. Detection of carbonates in dust shells around evolved stars. Nature, v. 415, p. 295-297). Because liquid water cannot exist in a near vacuum, production of these carbonates cannot have taken place by the familiar silicate-rotting process. More likely, they formed on the surfaces of silicate dust or ice grains by reactions between calcium and magnesium ions and those in which carbon and oxygen were combined.