The Faint Young Sun Problem Revisited

Climate Change, Anthropocene, & Associated Issues

Spencer opines that Earth and Mars should have been frozen worlds in their early history because of lower solar luminosity but were not, which challenges our understanding of early atmospheres and surface conditions and/or our understanding of solar evolution. This is known as the “faint young Sun problem.” One resolution to the problem is that the Sun was more massive and luminous in its youth before blowing off mass. Astrophysical studies of stellar evolution and behavior, however, including recent analysis of Kepler space-telescope data, indicate that mass loss is both insufficient and occurs too early to allow for a more luminous Sun after ca. 4 Ga.

Alternatively, greenhouse gases were surprisingly effective at warming young Earth and Mars. High concentrations of CO₂ with the possible addition of biogenic CH₄
are likely dominant factors promoting open-water conditions on Archean Earth. Evidence of precipitation and flowing water on young Mars, including river valleys
thousands of kilometers long, is more problematic. Recent studies indicate that 3–4 Ga river valleys and delta deposits in crater lakes could have been produced in < ~107 years. Highly transient warm periods during times of favorable orbital parameters possibly led to brief melting under otherwise icy conditions. Seasonal melting and runoff would be more likely with ~1%–10% atmospheric H₂ and CH₄, perhaps derived from serpentinization of olivine in the martian crust and released from frozen ground by impacts and volcanism, and/or derived directly from volcanic outgassing. The recently recognized effectiveness of hydrogen and methane at absorbing infrared radiation
in a thick CO₂ -dominated atmosphere, in a process known as “collision-induced absorption,” is probably essential to the solution to the faint young Sun problem for Mars.

Spencer concluded that:

1. Total solar-energy production is increasing gradually due to well-understood physics controlling rates of thermonuclear fusion in the solar core (Bahcall et al., 2001). A more massive and luminous young Sun is not supported by recent astrophysical studies.

2. Archean sedimentary rocks on Earth include many indicators of liquid water at Earth’s surface, including sedimentary rocks containing evidence of microbial life that in turn indicate open water with sunlight. The warm Archean Earth resulted from high atmospheric concentrations of CO2 , with possible additional warming from methane and hydrogen, lower cloud albedo, a low ratio of land to water at Earth’s surface, and other factors. It is not clear which additional factors were dominant or if we are missing something fundamental.

3. Abundant evidence of martian river channels and crater lakes at ca. 3.3–3.9 Ga indicates warm conditions in otherwise icy highlands of equatorial and southern Mars. Although transient melting might occur under favorable orbital parameters, augmentation of such warming by a few percent atmospheric H₂ and CH₄ released from crustal or mantle reservoirs may be a viable solution to the faint young Sun problem for Mars.