PARALLELS BETWEEN THE EVOLUTION OF THE CLIMATES OF EARTH AND MARS?

A vision of the evolution of the atmospheres and climates of Mars, Venus and Earth from a common early climate has developed during the past three decades. This vision, encapsulated in such concepts as the Goldilocks Paradox and the Faint Young Sun Paradox, is based on the carbonate geochemical cycle. On Earth this cycle is thought to have maintained Earth within the habitable zone throughout geologic history. On Mars it is envisioned to have maintained an equitable climate through the first 500 million years of Mars history, but then slowly failed as carbonate recycling ceased due to lack of geologic reprocessing of the crust of Mars. On Venus the runaway greenhouse broke the carbonate cycle through the escape of hydrogen, and loss of liquid water. While these concepts are appealing since they link the histories of three planets in a simple way, they are increasingly under scrutiny. On Earth, methane is being considered as an important greenhouse gas returning us to the concept of an atmosphere that is more reducing, as the atmosphere was envisioned during the 1960s. On Mars numerous investigators have challenged the greenhouse models which suggest early warm, moist climates. Instead the martian river valleys may be partly wind eroded, or created in very short episodes following large asteroid or comet impacts.