Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars

The relatively warm temperatures required on early Earth and Mars have beendifficult to account for via warming from greenhouse gases. We tested whetherthis problem can be resolved for both Earth and Mars by a young Sun that isbrighter than predicted by the standard solar model. We computed high-precisionsolar evolutionary models with slightly increased initial masses of M_i = 1.01to 1.07 M_sun; for each mass, we considered three different mass lossscenarios. We then tested whether these models were consistent with the currenthigh-precision helioseismic observations. The relatively modest mass loss ratesin these models are consistent with observational limits from young stars andestimates of the past solar wind obtained from lunar rocks, and do notsignificantly affect the solar lithium depletion. For appropriate initialmasses, all three mass loss scenarios are capable of yielding a solar flux 3.8Gyr ago high enough to be consistent with water on ancient Mars. We find thatall of our mass-losing solar models are consistent with the helioseismicobservations. The early solar mass loss of a few percent does indeed leave asmall fingerprint on the Sun's internal structure. However, for helioseismologyto significantly constrain early solar mass loss would require higher accuracyin the observed solar parameters and input physics, namely, by a factor ofabout 3 for the observed solar surface composition, and a factor of 2 for thesolar interior opacities, the pp nuclear reaction rate, and the diffusionconstants for gravitational settling.