Caldeira Lab Research:Paleoclimate and geochemical cycles

The Goldilocks problem: climatic evolution and long-term habitability of terrestrial planets

M.R. Rampino & K. Caldeira

Of all the planets in our solar system, only Earth supports life. The reasons why Earth came to be the planet with an environment that is "just right" (the Goldilocks planet) are not known. However, there are many different theories regarding each aspect of Earth's development. This paper outlines several hypotheses regarding why Earth was able to develop the environment to support life for 4 billion years instead of becoming too hot like Venus or too cold like Mars.

Rampino, M.R. and K. Caldeira, The Goldilocks problem: climatic evolution and long-term habitability of terrestrial planets, Annual Review of Astronomy and Astrophysics 32, 83–114, 1994.

Temperature curves for three different planets: The surface temperature of Mars, Earth, and Venus, graphed as a function of water's vapor pressure. One theory regarding Venus's high temperature states that as H2O vapor accumulated on the planet, the temperature rose due to its greenhouse effect. The rising temperatures were curtailed on Earth and Mars due to the saturation of vapor pressure and eventual freezing or condensation. On Venus, saturation was never reached due to its proximity to the sun. This suggested scenario could have caused a "runaway greenhouse" effect, producing the incredibly high temperatures seen on its surface today.

The faint young sun paradox: Surface temperature, graphed here as the dashed line Ts against solar luminosity and as a function of time, has been calculated to have been below the boiling point of water only 2 billion years ago. This low temperature would have been caused by the "faint young sun" -- the sun was much less powerful when it was first developing than it is now. However, fossil records show that Earth was supporting life as long as 4 billion years ago. Hence, the paradox: how could Earth have supported life in an environment with such low solar luminosity?


Why is Venus too hot, Mars too cold, and Earth "just right" for life? (The allusion to the fairy tale involves the three bowls of porridge belonging to Papa Bear, Mama Bear, and Baby Bear--one too hot, one too cold, and one just right--tested by a hungry Goldilocks.) A simplistic answer might be that planet’s surface temperature is to a large extent a function of its distance from the Sun, and Earth just happens to be at the "right" distance for comfortable temperatures and liquid water. However this is far from the whole story. The Goldilocks Problem involves the early history of the planets and the evolution of their atmospheres. Its solution must also take into consideration the long term evolution of the Sun, and hence the so-called faint young Sun problem, that is, the fact that the early Earth was apparently warm enough for liquid water despite the 25-30% lower luminosity of the early Sun (Newman & Rood1 977; Gough1 981). Had Earth been too cold initially for liquid water to exist on its surface, the resulting icy planet would have had a high albedo or reflectivity, lowering temperatures further, and might have become irreversibly ice-covered the "white Earth catastrophe" (Caldeira & Kasting 1992a). Yet evidence exists that liquid water has been abundant on Earth for at least the last 3.8 billion years.