On the habitability of Earth and Mars

The habitable zone (HZ) around the Sun is defined as the region within which an Earth-like planet might enjoy moderate surface temperatures needed for advanced life forms. Usually, this definition is equivalent to the existence of liquid water at the planet's surface. Our Earth system model consists of the components solid Earth, hydrosphere, atmosphere, and biosphere and couples the increasing solar luminosity, the silicate-rock weathering rate, and the global energy balance to estimate the partial pressure of atmospheric and soil carbon dioxide, mean global surface temperature, and the biological productivity as a function of time. We find that our Earth will leave the HZ in about 500 million years. The position of Venus was never in the HZ, but the Martian position was within the HZ up to about 500 million years ago. Furthermore, we have investigated an Earth model scaled down to Mars size with respect to ocean mass, continental area, oceanic area, plate tectonics etc. We find that an early Mars with plate tectonics and an ocean may have been habitable for some billion years.