Resolving the Faint Sun Paradox

The world changed dramatically in 1972, as the U.S. forged diplomatic relations with the People’s Republic of China and NASA inaugurated the space shuttle program. Meanwhile, in a quiet corner of the scientific community, astronomers Carl Sagan and George Mullen first brought to light a previously unrecognized, world-changing puzzle called the faint Sun paradox.¹ After nearly 40 years of intense research, this enigma continues to defy natural explanation.

Sagan and Mullen noted that as the Sun’s nuclear furnace continues to fuse hydrogen into helium, the density of the Sun’s core rises and, in turn, causes the Sun’s nuclear furnace to burn more efficiently. The Sun’s ongoing nuclear burning process means that the Sun’s luminosity (brightness) is considerably greater today—according to initial estimates, as much as 25 to 30 percent greater––than when life first appeared on Earth some 3.8 billion years ago.

At the time of life’s origin on Earth global mean temperatures—if atmospheric conditions were the same as today’s—would have been only 7° F or -14° C. Even 1.5 billion years later (or 2.3 billion years ago) the Sun’s radiance was so faint that global mean temperatures on Earth would have been below the freezing point of ocean water (<27° F or -3° C). The word “paradox” refers to the apparent contradiction between the seemingly freezing conditions on the early Earth and the obvious evidence for an abundance of life on Earth in that extremely inhospitable time.²

The standard naturalistic solution to the faint Sun paradox has been to propose that the early Earth had sufficient greenhouse (heat-trapping) gases in its atmosphere to retain sufficient solar heat to support life. Proponents of this solution have held that as the Sun grew older and brighter, these greenhouse gases dissipated at a rate that perfectly, or nearly perfectly, compensated for the change.

In the April 1 (2010) issue of Nature a team of four geologists published their challenge to this standard explanation of the paradox.³ They demonstrated that Earth’s early atmosphere would have lacked sufficient greenhouse gases to compensate for the faintness of the early Sun—if it were as much as 25 to 30 percent dimmer. Specifically, these geologists showed that the mineralogy of Archaean sediments, especially the ubiquitous presence of iron (II-III) oxides in banded iron formations, is inconsistent with high greenhouse gas concentrations in Earth’s early atmosphere. Furthermore, metabolic constraints on the methanogens (microorganisms that produce methane as a byproduct of their metabolic activity) of that early era would have rendered such creatures incapable of pumping out sufficient quantities of methane (a powerful greenhouse gas).

The four geologists advanced an alternative solution to the faint Sun paradox. They suggested that early Earth possessed a much lower “albedo,” or reflectivity, than it does today. With less continental land area on Earth’s surface at the time and less cloud cover (because clouds are, in large part, “seeded” by Earth’s abundant microorganisms), Earth––largely ocean-covered––would have reflected less light and absorbed more heat, presumably enough to compensate for dramatically lower solar luminosity.

I see a major problem with this attempted solution, and I anticipate it will soon be challenged by my fellow astronomers, if not by other geologists. While the team’s assertion about the early Earth’s exposed landmass appears consistent with the evidence (see Figure 1), their suggestion that the oceans would have absorbed more light (hence heat) than continents would have remained highly debatable. As anyone who has ever gone surfing or water skiing in daylight can attest, liquid water is a highly efficient reflector.

Over the past 4.57 billion years plate tectonic activity transformed Earth’s surface from a water world to its present 29 percent coverage by continents and islands.
Credit for background image: NASA

Today, Earth’s continents absorb significantly more solar radiation than do its oceans. The only way continents could serve as better reflectors than oceans is if they were covered with snow and/or ice or by highly reflective plant life.

The fundamental problem with the geologists’ proposed solution to the faint Sun paradox lies in its core assumption—a failed model of the Sun’s developmental history. The particular model they used overlooks the fact that the Sun initially lost mass (and thus brightness) when it first formed. From its first 1.0 to 1.5 billion years, the Sun dropped between 4 and 7 percent of its total mass.⁴

Physics explains why. As with all stars, the Sun’s luminosity is proportional to nearly the fourth power of its mass. In other words, even a small difference in mass makes a huge difference in luminosity. So, despite the advancing fusion of hydrogen into helium in the Sun’s nuclear furnace, which worked to increase the Sun’s luminosity, that advance couldn’t keep pace, initially, with the early Sun’s loss of mass. As a net effect of these two factors, the Sun’s brightness declined by roughly 15 percent during its first 1.0 to 1.5 billion years. Since then, ongoing mass loss in the Sun has declined to just 0.002 percent every billion years. Thus, over the past 3.0 to 3.5 billion years the Sun has regained its lost luminosity.

At its beginning the Sun initially brightens by accreting mass from its original molecular gas cloud. During its youth the Sun becomes dimmer as a result of mass loss. During its middle age it begins to brighten again as a result of the fusion of hydrogen into helium in its nuclear furnace. Eventually, the Sun’s increasing luminosity will make Earth uninhabitable. The dotted blue line indicates the present.

Taking into account a more accurate history of the changes in the Sun over its 4.57 billion-year history, we see that the Sun’s luminosity today exceeds that of the era when life first appeared (roughly 3.8 billion years ago) by about 15 percent, not 25 to 30 percent. Therefore, the resolution of the faint Sun paradox requires neither the high greenhouse gas quantities assumed (and disproven) nor the (implausibly) low reflectivity proposed. Instead, it demands something far more complex.

The resolution of the faint Sun paradox requires the exquisite fine-tuning and continuous adjustment of a long list of features not just at the beginning of life but also throughout Earth’s history. The characteristics identified thus far include:

1. plate tectonic activity level
2. continental landmass erosion rate
3. nutrient recycling rate and extent
4. volcanic outgassing level
5. microorganism kinds
6. microorganism quantity
7. microorganism diversity
8. plant and animal life kinds
9. plant and animal life quantity
10. plant and animal life diversity
11. mass extinction events’ number, extent, and timing
12. mass speciation events’ number, extent, and timing

If any one of these parameters were to deviate from the narrow requirements, Earth’s surface would either have suffered a runaway freeze-up or a runaway evaporation of all its water, resulting in permanent global sterilization, or, death to all life-forms. How reasonable is it to conclude, then, that the solution to the faint Sun paradox is some lucky mega-coincidence in which the different physical features of Earth and of Earth’s life line up perfectly and remain in appropriate ranges for a few billion years?

Given that these characteristics must perfectly align with all the others in changing values throughout the past 3.8 billion years, it seems more reasonable to suggest a purposeful Agent’s activity. Someone who knows and understands the physics of the Sun evidently intervened regularly to guarantee that all twelve, and perhaps many more, of such features manifested and retained the just-right values throughout life’s history on Earth. This maintenance would mean adding, subtracting, and replacing various life-forms to keep things in balance as the Sun’s luminosity changed.

A glimpse of this scenario may be seen through the window of Psalm 104:27–30:

“These [creatures] all look to you [LORD]
to give them their food at the proper time.
When you give it to them,
they gather it up;
when you open your hand,
they are satisfied with good things.
When you hide your face,
they are terrified;
when you take away their breath,
they die and return to the dust.
When you send your Spirit,
they are created,
and you renew the face of the earth.”