The Faint Young Sun problem

5.3 The Faint Young Sun problem

Evolutionary models of the solar interior lead to the prediction that the Sun should have been about 25% fainter 3.8 Gyr ago (Gough, 1981 ; Bahcall et al., 2001 ). This is a fairly robust result that has been known for a long time. However, this creates significant difficulties for those who study the climate history of planets in our solar system. With the young Sun this faint, the planets should have been significantly colder than they are now. On Earth and Mars, surface water should have been frozen, but this contradicts geologic evidence that abundant water existed and flowed on the surfaces of both of these planets. This is the so-called “Faint Young Sun” paradox (Sagan and Mullen, 1972 ; Kasting, 1991

). Most attempts to solve this problem have appealed to increased amounts of greenhouse gases in the early atmospheres of Earth and Mars, which allowed the surface temperatures to remain similar to present-day temperatures despite a fainter Sun (see Walker, 1985 ; Kasting and Ackerman, 1986 ; Kasting, 1991 ).

However, a couple theories have been advanced that involve the solar wind. One of these is simply that the young solar wind was strong enough to significantly decrease the mass of the Sun, meaning that the young Sun was more massive and thus more luminous than standard solar evolutionary models predict (Guzik et al., 1987 ; Sackmann and Boothroyd, 2003 ). If the Sun was only about 2% more massive 3.8 Gyr ago, then the Sun would still have been bright enough to maintain sufficiently warm temperatures on Earth and Mars. The mass loss evolution law derived from stellar wind measurements in Equation (4 ) does indeed suggest that the solar wind was stronger in the past. Unfortunately, it was not strong enough. If Equation (4 ) is correct, the Sun could not have been more than 0.2% more massive 3.8 Gyr ago. Thus, this theory ultimately is not supported by the stellar wind measurements (Minton and Malhotra, 2007 ). Update

However, there is a less direct method that has been proposed by which a stronger young solar wind could contribute to a solution of the Faint Young Sun problem. There have been several claims that cosmic rays have important effects on the Earth’s climate by stimulating cloud formation, which is believed to generally cool the Earth’s atmosphere (see Svensmark, 1988 ). However, cosmic rays are modulated by the solar wind, and a stronger solar wind would reduce the flux of cosmic rays into the Earth’s atmosphere. Thus, the idea is that the stronger wind of the young Sun led to lower cosmic ray fluxes, helping the Earth (and perhaps Mars) to maintain warm temperatures (Shaviv, 2003 ). However, the whole idea of cosmic rays influencing the terrestrial climate to this degree remains very controversial (see Carslaw et al., 2002 ).