The carbon cycle on early Earth--and on Mars?

Abstract

One of the goals of the present Martian exploration is to search for evidence of extinct (or even extant) life. This could be redefined as a search for carbon. The carbon cycle (or, more properly, cycles) on Earth is a complex interaction among three reservoirs: the atmosphere; the hydrosphere; and the lithosphere. Superimposed on this is the biosphere, and its presence influences the fixing and release of carbon in these reservoirs over different time-scales. 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). In contrast to Earth, Mars presently has no active tectonic system; neither does it possess a significant biosphere. However, these observations might not necessarily have held in the past. 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.

Figure 1

Schematic representation of the pathways through which the carbon cycles operate on Earth today.

Figure 2

Abundance and isotopic composition of magmatic carbon in chassignites and shergottites. The component is defined as that liberated on combustion between 600 and 1000°C. The symbols represent the different shergottite subgroups: circles, basaltic; triangles, lherzholitic; squares, olivine-phyric. A77, Allan Hills 77005; C, Chassigny; DaG 476, Dar al Gani 476; L88, Lewis Cliff 88156; LA, Los Angeles; Q94, Queen Alexandra Range 94201; SAU 005, Sayh al Huaymir 005; S, Shergotty; Z, Zagami.

Figure 3

Abundance and isotopic composition of carbon in carbonates in the nakhlites and ALH 84001. The data were acquired using dissolution in 100% H₃PO₄ and are taken to represent carbon in the Martian hydrosphere. A84, Allan Hills 84001; GV, Governador Valadares; L, Lafayette; N, Nakhla; Y, Yamato 000593.