Resolving Faint Sun Paradoxes, Part 1

A “paradox” is not two articles residing on your computer. Nor is it two PhDs. Rather, it is an apparent contradiction for which known facts must provide some resolution. In science, resolving a paradox almost always results in a much deeper understanding of the record of nature and often produces additional evidence for supernatural design. New and developing discoveries are yielding both for the faint Sun paradox that has intrigued astronomers—and should have plagued evolutionary biologists—for the past 40 years.

Throughout this series, I will explore the complexities of this paradox, how researchers resolved it, and how it provides spectacular evidence for the supernatural design of the solar system.

Faint Sun Paradox
In 1972, Carl Sagan and his colleague George Mullen published a famous paper in Science in which they articulated the faint Sun paradox (also known as the faint young Sun paradox).1 They pointed out that as the Sun fuses hydrogen into helium in its nuclear furnace, that fusion gradually increases the Sun’s core density. A higher core density increases the fusion efficiency of the Sun’s furnace with the net result that as the Sun ages it becomes progressively brighter. Sagan and Mullen calculated that the Sun today is about 30 percent brighter (see figure 1) than it was when life originated on Earth 3.8 billion years ago.

The fainter youthful Sun poses a problem because the existence of life on Earth requires a relatively fixed surface temperature. A solar luminosity change of as little as 2 percent would make the planet uninhabitable.2 The paradox is that despite the Sun’s increasing brightness, somehow, life persisted on Earth’s surface for nearly 4 billion years.

Sagan and Mullen also hinted at a solution. They suggested that the early Earth’s atmosphere contained sufficient greenhouse gases to compensate perfectly for the dimmer Sun. That is, they claimed Earth’s primordial atmosphere was so richly endowed with greenhouse gases that it trapped enough of the Sun’s heat and light to make up for the fact that the Sun was about 30 percent less luminous.

Figure 1: Sun’s Luminosity or Brightness throughout Earth’s History (according to Sagan and Mullen’s model) Image credit: Hugh Ross

Proposed Greenhouse Gas Solutions
Sagan and Mullen presumed that ammonia, a powerful greenhouse gas, at several parts per million in Earth’s ancient atmosphere would resolve the faint Sun paradox. However, it did not take atmospheric chemists very long to discern that ammonia is not only unstable in Earth’s  atmosphere today, it’s been unstable throughout the past 4 billion years. Moreover, they could not uncover any conceivable source for that much ammonia.

The forced abandonment of the ammonia solution resulted in a team, headed up by origin-of-life researcher James Kasting in the 1980s, proposing the less-potent-but-more-abundant carbon dioxide as a resolution to the paradox. The team calculated that for life to survive on Earth 3.8 billion years ago the planet’s primordial atmosphere would have needed to contain from 100–1,000 times as much carbon dioxide as it does today.3

As to where all the extra carbon dioxide came from, the team suggested volcanic gas emissions as the source. Until the end of the twentieth century, scientists presumed that this carbon dioxide contribution provided an adequate resolution to the faint Sun paradox. In part 2 of this series, I will explain how recent discoveries completely unraveled this proposed resolution and alerted scientists to the great complexity of the faint Sun paradox.

1. Carl Sagan and George Mullen, “Earth and Mars: Evolution of Atmospheres and Surface Temperatures,” Science 177 (July 7, 1972): 52–56.
2. A brighter Sun without a compensating factor would generate a runaway evaporation of Earth’s surface liquid water, turning it into vapor. As a greenhouse gas, the extra water vapor would trap more of the Sun’s heat, causing yet more evaporation and so on until all the planet’s surface liquid water would be converted into water vapor. On the other hand, a dimmer Sun without any compensating factor would generate a runaway freezing of the surface water. A dimmer Sun would also cause greater snowfalls, which is much more reflective compared to soil and rock. Thus, extra snow would reflect more of the Sun’s light away from Earth, thereby cooling the planet’s surface. This extra cooling would cause even more snow to fall and even more of the Sun’s light to be reflected away from Earth. Eventually, Earth’s entire surface would be covered in snow or ice.
3. W. R. Kuhn and James F. Kasting, “Effects of Increased CO2 Concentrations on Surface Temperature of the Early Earth,” Nature 301 (January 6, 1983): 53–55; James F. Kasting, James B. Pollack, and David Crisp, “Effects of High CO2 Levels on Surface Temperature and Atmospheric Oxidation State of the Early Earth,” Journal of Atmospheric Chemistry 1, no. 4 (December 1984): 403–28; James F. Kasting, “Earth’s Early Atmosphere,” Science 259 (February 12, 1993): 920–26.


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