The Faint Young Sun Problem Revisited

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. INTRODUCTION The basic concepts involved in stellarenergy generation were known by the 1950s and include the insight that stellar luminosity gradually increases over time because of increasing density in stellar cores resulting directly from thermonuclear fusion (e.g., Burbidge et al., 1957) (Fig. 1). Solar luminosity at birth was calculated to be ~70% of modern luminosity. The idea that Earth should have geologic evidence of its presumably frozen youth was gradually determined to be inconsistent with growing evidence for liquid water at the surface of Archean Earth. The problem was first addressed by Sagan and Mullen (1972), who proposed that atmospheric ammonia was crucial to early warming. More recent robotic exploration of Mars similarly indicates surprisingly warm and wet conditions during its early geologic history. The discrepancy between low solar-energy production and warm early Earth and Mars is known as the “faint young Sun problem” (Ulrich, 1975; Feulner, 2012). This article is a brief review of solar evolution and the faint young Sun problem for Earth and Mars that highlights recent developments. STELLAR ENERGY PRODUCTION Stars form by gravitational contraction of clouds of interstellar gas dominated by hydrogen. During contraction and adiabatic heating, increasing stellar energy production by nuclear fusion of hydrogen into helium eventually terminates gravitational contraction (e.g., Haxton et al., 2013). Over millions of years, helium produced by fusion of hydrogen accumulates in the cores of stars and increases core density, causing gravitational contraction and adiabatic heating which, in turn, raise fusion rates and energy generation. This process occurs gradually and continuously, resulting in increasing core temperature and total luminosity (Fig. 1) (Bahcall et al., 2001). The Sun began with ~71% hydrogen Jon Spencer, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, spencer7@email.arizona.edu GSA Today, v. 29, https://doi.org/10.1130/GSATG403A.1. Copyright 2019, The Geological Society of America. CC-BY-NC. Figure 1. Evolution of solar properties (from Bahcall et al., 2001). A simple approximation of solarluminosity evolution (Equation 1 of Gough, 1981) is also shown. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4