On Earth, biology, hydrology, and geology are interlinked such that certain types of life are often associated with specific conditions, including rock type, pressure, temperature, and chemistry. Life on Earth has established itself in diverse and extreme niches, presenting the possibility that Mars, too, may hold records of fossilized and/or extant life in diverse environments. Geologic, paleohydrologic, and climatic conditions through the evolution of Mars are similar in many respects to conditions occurring during the evolution of Earth and, as such, may point to environments on Mars with potential to have supported living systems. Here, we discuss examples of those Martian settings. Such extraterrestrial environments should be targeted by international robotic and/or manned missions to explore potential fossilized or extant life on Mars.

Abstract This contribution summarizes the considerations that are of major importance in inorganic mineral formation before we look at specific biological minerals. Some factors which have to be taken into account (other than those that are well-known from inorganic (abiological) precipitations) are: the nature of the biological organic matrix; the restricted volume, outside or inside the cytoplasm, which can cause differences in impurity content (Mg); crystal morphology; and isotopic fractionation. Cases such as those of corals, foraminifera and coccoliths are taken as examples. The formation of a mineral from a solution is under a number of different controlling factors (Crick 1986; Frauâ´sto da Silva Williams 1991). The simplest case we can consider is an equilibrium between a particular form of a mineral, say calcite, and the activities, not concentrations, of the ions, for example The activities depend upon the salt concentration and the equilibrium constant depends upon the temperature and the pressure. (Pressure becomes especially important in the oceandepths.) This equation is made complicated by the interaction in solution of ions with certainanions and the interaction of with cations (especially the proton) so that at least one, a second, equilibrium is very important