Faint Young Sun Paradox

Theories of stellar evolution indicate that as stars mature on the main sequence, they grow steadily hotter and brighter. About the time of the formation of Earth (~4.56 Ga), the Sun’s luminosity was roughly two-thirds its present value; the surface temperature of early Earth would have had a mean temperature of -15 °C (258 K). However, there is no geological evidence on Earth for a cooler Sun. For example, there is evidence of liquid water on Earth as early as ~4 Ga. To resolve this disparity, it is often proposed that Earth’s greenhouse effect was greater early in its history (probably enhanced by high concentrations of methane, CH4). Indeed the Gaia hypothesis argues that the Earth’s temperature has been regulated by life throughout its existence.

The paradox less easily resolved in the case of Mars, which has features like those produced by running water, but would have had a mean temperature of -77 °C (196 K). It is possible the fluid eroding martian channels was not water but instead fluidized avalanche debris (a mixture of dust, rocks, and ice) supported by CO2 vapor and no unusual past climate needs to be invoked.

Some or all content above used with permission from J. H. Wittke.