The Faint Young Sun Paradox: An Unsolved Mystery

The origin of life seems to require liquid water. Geological evidence indicates that our planet was warm enough for liquid water to have existed about 3.5 billion years ago, when the first earthly life appeared. But models of solar evolution lead scientists to conclude that the sun gave off much less heat billions of years ago. Judging only from the sun's evolution, our planet should have been completely frozen three billion years ago. This discrepancy is known as the faint young sun paradox.

Below is a diagram from a scientific paper by Shani and Shtanov discussing the faint young sun paradox. As you can see from the diagram, if we assume (for the sake of simplicity) that our planet has had its current atmosphere for the past 3 billion years, then our entire planet should have been frozen until about 1.7 billion years ago.

Scientists have advanced some explanations for the faint young sun paradox, but none of them is problem-free. One explanation is that billions of years ago our planet's atmosphere was vastly different. It could have been that billions of years ago our atmosphere had vastly more carbon dioxide and methane. That would have produced a greenhouse effect many times greater than the greenhouse effect currently produced by man-made pollutants.

But there are some problems with this theory. There would have needed to be almost ridiculous amounts of carbon dioxide and methane in the atmosphere for the greenhouse effect to have been sufficient to explain the paradox. For example, in this link there's a discussion of some scientists hinting that they have solved the faint young sun paradox through a greenhouse gas explanation. But they imagine carbon dioxide levels of 20,000 parts per million – fifty times greater than today's level. Why are such high levels of greenhouse gases implausible? A good explanation is given by this scientific paper by Shani and Shtanov:

While CO₂ [carbon dioxide] can be removed from the atmosphere by bacteria and plants during photosynthesis and also by the weathering of rock, the enormous concentrations referred to earlier seem difficult to account for. Indeed, large amounts of CO₂ in the early atmosphere would have led to the formation of the iron carbonate based compound siderite (FeCO₃). However, analysis of billion year old paleosols challenges this picture by finding no FeCO₃. Found instead are iron silicates which support a much more moderate presence of CO₂ in the early atmosphere. Moreover, it would be difficult to make CO₂ disappear almost entirely from the Earth’s atmosphere since much of it, after reacting with rocks and being used by shell-forming organisms to form CaCO₃ shells, gets deposited on the ocean floor and makes its way back into the atmosphere via volcanic activity and plate tectonics . The presence of other greenhouse gases is equally problematic.

The paper makes the point that if you had once had fantastically high levels of carbon dioxide (CO₂) in the atmosphere, and that had slowly disappeared, much of that carbon dioxide would have been stored in what are called carbon dioxide “sinks.” But over the eons the carbon dioxide in those sinks would have been slowly re-released by geological processes – which would have left us with more carbon dioxide in the atmosphere than we have today.

What solution do Shani and Shtanov suggest for the faint young sun paradox? They suggest a varying gravitational constant. The gravitational constant is the fundamental constant that indicates the strength of gravitation throughout the universe. It is one of several fundamental constants that are crucially important in determining the behavior of the sun. The authors suggest that if the gravitational constant had varied by 2 percent over the past few billion years, that could explain the faint young sun paradox.

This explanation can be called something of a “Hail Mary pass.” Scientists don't quite have a rule of “if all else fails, suggest the laws of nature have changed,” but assumptions about changing laws of nature or changing physical constants are generally regarded as a kind of last resort. This paper by Pitjeva and Pitjev claims to set a limit on changes in the gravitational constant. The paper (based on spacecraft observations) concludes that the gravitational constant changes by less than 1 part in 20,000,000,000,000 per year. This conclusion is hugely inconsistent with the hypothesis of Shani and Shtanov that the faint sun paradox can be explained by assuming a 2 percent variation of the gravitational constant. If Pitjeva and Pitjev are correct, the gravitational constant could not have varied by more than 1 part in 10000 during the past five billion years, a limit which rules out a 2 parts in 100 (2 percent) variation as imagined by Shani and Shtanov.

That apparently shoots down a varying gravitational constant as an explanation for the faint young sun paradox. But what about other fundamental physical constants that the sun's behavior depends on, such as the fine-structure constant? That's not an area scientists prefer to be fiddling with, for two reasons. First, it has already been concluded that the sun is extremely sensitive to changes in the fine-structure constant, and that if it were only the tiniest bit different, the sun would be some other type of star such as a red dwarf or a blue giant. For example, this paper by a physicist finds that changing the fine-structure constant by 2 parts in 1000 would cause the earth to freeze, and changing the fine-structure constant the other way by only a few percent would cause the earth's water to boil. Given this extreme fine-tuning of the fine-structure constant, do we really want to imagine some other fine-tuning, some just-right slow change in the fine structure constant to resolve the faint young sun paradox? Also, a study based on earthly isotopes indicates that the fine-structure constant is totally invariant over the eons, varying by less than 1 part in 10,000,000,000,000,000 per year.

So where does that leave us in regard to the faint young sun paradox? We are left with an unsolved mystery. Somehow our planet was warm billions of years ago, but we really don't know why it wasn't completely frozen. Considering the matter, we should be humbled, and remember how fragmentary and incomplete our knowledge of nature is.