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The Faint Young Sun Problem Revisited

Jon Spencer, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, spencer7@email.arizona.edu

ABSTRACT INTRODUCTION known as the “faint young Sun problem”
Earth and Mars should have been frozen The basic concepts involved in stellar- (Ulrich, 1975; Feulner, 2012). This article
worlds in their early history because of energy generation were known by the is a brief review of solar evolution and the
lower solar luminosity but were not, 1950s and include the insight that stellar faint young Sun problem for Earth and
which challenges our understanding of luminosity gradually increases over time Mars that highlights recent developments.
early atmospheres and surface conditions because of increasing density in stellar
and/or our understanding of solar evolu- cores resulting directly from thermonu- STELLAR ENERGY PRODUCTION
tion. This is known as the “faint young clear fusion (e.g., Burbidge et al., 1957) Stars form by gravitational contraction
Sun problem.” One resolution to the (Fig. 1). Solar luminosity at birth was cal- of clouds of interstellar gas dominated by
problem is that the Sun was more mas- culated to be ~70% of modern luminosity. hydrogen. During contraction and adia-
sive and luminous in its youth before The idea that Earth should have geologic batic heating, increasing stellar energy
blowing off mass. Astrophysical studies evidence of its presumably frozen youth production by nuclear fusion of hydrogen
of stellar evolution and behavior, how- was gradually determined to be inconsis- into helium eventually terminates gravita-
ever, including recent analysis of Kepler tent with growing evidence for liquid tional contraction (e.g., Haxton et al.,
space-telescope data, indicate that mass water at the surface of Archean Earth. 2013). Over millions of years, helium pro-
loss is both insufficient and occurs too The problem was first addressed by duced by fusion of hydrogen accumulates
early to allow for a more luminous Sun Sagan and Mullen (1972), who proposed in the cores of stars and increases core
after ca. 4 Ga. Alternatively, greenhouse that atmospheric ammonia was crucial density, causing gravitational contraction
gases were surprisingly effective at to early warming. More recent robotic and adiabatic heating which, in turn, raise
warming young Earth and Mars. High exploration of Mars similarly indicates fusion rates and energy generation. This
concentrations of CO with the possible surprisingly warm and wet conditions process occurs gradually and continuously,
2
addition of biogenic CH are likely during its early geologic history. The dis- resulting in increasing core temperature
4
dominant factors promoting open-water crepancy between low solar-energy pro- and total luminosity (Fig. 1) (Bahcall et al.,
conditions on Archean Earth. Evidence duction and warm early Earth and Mars is 2001). The Sun began with ~71% hydrogen
of precipitation and flowing water on
young Mars, including river valleys
thousands of kilometers long, is more
problematic. Recent studies indicate that 1.4 Gough (1981) (luminosity)
Gough (1981) (luminosity)
3–4 Ga river valleys and delta deposits surface temperatureface temperature
sur
in crater lakes could have been produced 1.2
in <~10 years. Highly transient warm radius (r)
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periods during times of favorable orbital 1.0
parameters possibly led to brief melting
under otherwise icy conditions. Seasonal
melting and runoff would be more likely 0.8
area (r ))
with ~1%–10% atmospheric H and CH , y (value relative to present value) area (r 2 2
4
2
perhaps derived from serpentinization of 0.6 luminosity
luminosity
olivine in the martian crust and released
from frozen ground by impacts and 0.4 runaway H 2 O greenhouse runaway H 2 O greenhouse
volcanism, and/or derived directly from
volcanic outgassing. The recently recog- 0.2
nized effectiveness of hydrogen and on Earth on Earth
methane at absorbing infrared radiation
in a thick CO -dominated atmosphere, 0.0 -4 -3 -2 -1 0 1 2 3
2
in a process known as “collision-induced
absorption,” is probably essential to the x (gigayears from present)
solution to the faint young Sun problem Figure 1. Evolution of solar properties (from Bahcall et al., 2001). A simple approximation of solar-
for Mars. luminosity evolution (Equation 1 of Gough, 1981) is also shown.

4 GSA Today | December 2019

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