First articulated in the early 1970s, the faint Sun paradox (also known as the faint young Sun paradox) reveals that the Sun today is about 30 percent brighter than it was when life originated on Earth 3.8 billion years ago. The question then is how did life survive the radical change in the Sun’s luminosity?
In part 1, I defined the faint Sun paradox and described astronomers’ and physicists’ initial attempts in the 1970s and 1980s to explain how early Earth compensated for the lower solar luminosity. Here, in part 2, I will show how research teams discovered complications that made the faint Sun paradox seem unresolvable.
Greenhouse Gas Complications
For 20 years, prevailing scientific thought held that volcanic gas emissions pumped enough extra carbon dioxide into the early Earth’s atmosphere to sufficiently enhance the greenhouse effect of Earth’s atmosphere so as to perfectly compensate for the lower solar luminosity. But in 2003–2004, origin-of-life researcher James Kasting and others recognized there was no way this scenario could work.1 In order to offset a 25–30 percent decreased solar luminosity at the time of life’s origin, carbon dioxide in Earth’s atmosphere would have needed to exist at a steady-state concentration of 0.3 bars (present-day atmospheric pressure at sea level = 1.013 bars).2 At such a high concentration the carbon dioxide would have condensed at the planet’s poles, thereby bringing the atmospheric concentration below the needed 0.3 bars.3 Furthermore, the lack of siderite (iron carbonate, FeCO3) in paleosols, or fossil soils, proves that atmospheric carbon dioxide concentrations were at least a factor of five times below what would be needed to compensate for the fainter Sun.4
Acknowledging the failure of the carbon dioxide explanation, Kasting and his colleagues then proposed methane as the solution.5 However, methane has a short half-life in Earth’s atmosphere due to photo-oxidation and, to a lesser degree, gravitational escape. Getting the atmospheric methane abundance up to the necessary levels would have required a steady flux of methane from methanogens (methane-producing bacteria) at a level that is at least 7–10 times greater than what takes place on Earth today.
Kasting’s team claimed that life’s emergence 3.8 billion years ago spawned an enormous proliferation of methanogens. However, in order for Kasting’s model to work life would have had to originate under conditions far too cold for life to even have survived.
There are two additional known problems with the methanogens proposal. First, such high concentrations of atmospheric methane (7–10 higher than today) are inconsistent with the “mineralogy of Archaean sediments, particularly the ubiquitous presence of mixed-valence Fe(II-III) oxides (magnetite) in banded iron formations.”6 Second, such high atmospheric methane concentrations are also inconsistent with the known metabolic constraints of methanogens.7
More Nitrogen? Lower Albedo?
With the elimination of ammonia, carbon dioxide, methane, and combinations of these three as possible candidates for resolving the faint Sun paradox, atmospheric chemists and physicists sought for other ways to enhance or complement the impact of the maximum available greenhouse gases. A team led by British geophysicist Colin Goldblatt hypothesized that all or nearly all the nitrogen currently trapped in Earth’s crust and mantle was somehow released into the atmosphere during the faint Sun epoch.8 While nitrogen is not a greenhouse gas, its presence in the atmosphere has the effect of broadening the absorption spectral lines of both methane and carbon dioxide, causing both compounds to absorb more of the Sun’s heat than normal.
Yet the nitrogen-enhancement hypothesis faces several unsolved problems. Releasing trapped nitrogen into the atmosphere is difficult. And it isn’t easy keeping any released nitrogen from returning to the crust and mantle. Atmospheric physicists and chemists note that measurements show the balances of nitrogen in Earth’s atmosphere, crust, and mantle to be extraordinarily stable. It is hard to conceive of any disturbance that would place all the planet’s nitrogen reserves into the atmosphere. Moreover, nitrogen is a crucial nutrient for life. Without adequate nitrogen reserves in the ocean and crust, life would not survive.
A team led by Danish geologist Minik Rosing proposed yet another solution to the faint Sun paradox. They suggested that a much lower albedo on the early Earth played a major role in compensating for the dimmer Sun.9 Albedo is a measure of reflectivity. With less of the Sun’s light and heat reflected into outer space, Earth could have retained enough heat and light to compensate for the Sun’s fainter luminosity. Rosing’s team claims that, during the faint Sun epochs, early Earth’s lower continental landmass coverage (see figure 1) combined with lower levels of biologically induced cloud condensation nuclei would have lowered Earth’s albedo sufficiently and made up for whatever compensation the greenhouses gases at that time were unable to provide.
One challenge that the Rosing model faced was that water’s albedo is not low. Water reflects light better than virtually all continental land areas except those covered by snow and ice. And even then, frozen ocean water reflects light just as well as, if not better than, frozen landmasses.
Most clouds do reflect light better than either surface water or land. However, it did not necessarily follow that there was less cloud cover during the faint Sun epochs. (Most atmospheric chemists are persuaded that the greater quantity of greenhouse gases in early Earth’s atmosphere generated more clouds.) Nor was it by any means settled that biologically induced cloud condensation nuclei were less during the faint Sun epochs. They could even have been greater, especially in light of the fact that, at that time, Earth was rotating much more rapidly and the Moon was exerting much greater tidal forces.
But not all hope for resolving the faint young Sun paradox was lost. While increasingly frustrated geophysicists tried to find some set of circumstances on the early Earth that would adequately compensate for the fainter Sun, teams of astronomers researched the Sun’s history and observed the behavior of young solar analog stars that revealed weaknesses in the Sagan-Mullen model. I will unfold the rest of this story in part 3 of this series.
1. A. A. Pavlov et al., “High Methane Abundance throughout the Precambrian,” American Geophysical Union, Spring Meeting (May 2004): abstract #U22A-04.
2. A. A. Pavlov et al., “High Methane Abundance in the Archean and Proterozoic Atmosphere. Why CO2 Is Not Enough,” American Geophysical Union, Fall Meeting (December 2003): abstract #PP21B-1168.
3. Michael T. Mellon, “Limits on the CO2 Content of the Martian Polar Deposits,” Icarus 124 (November 1996): 268–79.
4. Rob Rye, Phillip H. Kuo, and Heinrich D. Holland, “Atmospheric Carbon Dioxide Concentrations before 2.2 Billion Years Ago,” Nature 378 (December 7, 1995): 603–05.
5. Alexander A. Pavlov, “Methane-Rich Proterozoic Atmosphere?” Geology 31 (January 2003): 87–90; A. A. Pavlov et al., “Why CO2 Is Not Enough,” abstract #PP21B-1168; A. A. Pavlov et al., “High Methane Abundance throughout the Precambrian,” abstract #U22A-04.
6. Minik, T. Rosing et al., “No Climate Paradox under the Faint Early Sun,” Nature 464 (April 1, 2010): 744–47.
7. Ibid., 744–47.
8. Colin Goldblatt et al., “Nitrogen-Enhanced Greenhouse Warming on Early Earth,” Nature Geoscience 2 (December 2009): 891–96.
9. Minik T. Rosing et al., “No Climate Paradox,” 744–47.