There is also a net input of carbon to the Earth from extraterrestrial material, around 5×10 5 kg yr −1 (some 2×10 15 kg over the Earth's lifetime). Superimposed on these, and acting throughout, is the biosphere. On Earth today, there are three major carbon-bearing reservoirs that are interrelated: the atmosphere the hydrosphere and the lithosphere. If that is the case, identification of how and where carbon occurs within an environment is an important signal to know whether the setting is, or is capable of, hosting life. Thus, the presence of life (whether it be extinct, extant or dormant) must be evinced by the presence of carbon (the reverse is not true, because there are many non-biological sources of organic molecules). Thus, the presence (or absence) of water is not necessarily diagnostic for the occurrence (or lack) of life.Īll life on Earth is based on carbon the variety of organic and organo-metallic compounds formed cannot be paralleled by any other element. Admittedly, water is the fluid with greatest versatility, in terms of the temperature range over which it is liquid, and to consider the origin of life in the absence of water is probably fantastic, but we know that life can survive in severe conditions on Earth, and remain viable in the absence of water, even if it originally arose when water was relatively plentiful. Given appropriate ambient environmental parameters, such as temperature and pressure, other fluids could perform these functions, negating the requirement for liquid water as a necessity for life. However, the importance of water is in its functions as a solvent and a transport medium, as well as its role in providing support to cell structure. The search for life beyond Earth is frequently (and not unrealistically) considered in terms of the presence, or otherwise, of liquid water. By looking at how Earth's carbon cycles have changed with time, as both the Earth's tectonic structure and a more sophisticated biology have evolved, and also by constructing a carbon cycle for Mars based on the carbon chemistry of Martian meteorites, we investigate whether or not there is evidence for a Martian biosphere. However, these observations might not necessarily have held in the past. In contrast to Earth, Mars presently has no active tectonic system neither does it possess a significant biosphere. The overall carbon balance is kept at equilibrium on the surface by a combination of tectonic processes (which bury carbon), volcanism (which releases it) and biology (which mediates it). ![]() Superimposed on this is the biosphere, and its presence influences the fixing and release of carbon in these reservoirs over different time-scales. The carbon cycle (or, more properly, cycles) on Earth is a complex interaction among three reservoirs: the atmosphere the hydrosphere and the lithosphere. This could be redefined as a search for carbon. One of the goals of the present Martian exploration is to search for evidence of extinct (or even extant) life.
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