Apr 26 2012

The Faint Young Sun Paradox

Earlier this week I wrote about paradoxes in science, about how they are good things pointing the way toward new research and possibilities. Science deniers, however, exploit them to cast doubt on established science, without creating a viable scientific theory of their own. I gave as an example the solar neutrino problem – the fact that in the 1980s and 1990s neutrino detectors were detecting 1/3 to 1/2 the solar neutrinos than the standard model of particle physics predicted. Creationists used this to argue that the entire nuclear fusion model of stars was wrong.  It wasn’t long, however, before the missing neutrinos were discovered and the paradox resolved.

Recently I was asked about another sun-based paradox that creationists use to argue for a young earth – the faint young sun paradox. This was first pointed out by Carl Sagan and George Mullen.  Our models of stellar evolution indicate that the sun has been getting steadily brighter and hotter over the last four billion years. As hydrogen is fused into helium and helium therefore builds up on the core of the sun, it has to burn a little hotter in order to maintain equilibrium. The sun is burning about 30% hotter today than it was four billion years ago.

The sun is the major source of heat for the earth’s surface, and therefore a colder sun in the past would mean that the earth was colder, by about 25 °C.  By this factor alone the earth should have been mostly a ball of ice and snow up until 1-2 billion years ago. However the geological evidence points strongly to there being liquid water on the earth even when it was young.

There is the paradox – stellar models tell us the early earth was too cold for liquid water, but geological evidence tells us there was liquid water on the early earth. Paradoxes point the way to new information that will both resolve the paradox and improve our understanding of the universe. When two pieces of scientific information conflict that standard resolution is that either one or both pieces of information are incorrect, or there is a missing piece of information that changes the picture.

So – one way to resolve this paradox is to argue that our models of stellar evolution are incorrect. Perhaps the early sun was not colder than the current one. These stellar models, however, are rooted in fairly solid physics and astronomical observation. It seems unlikely at this point that they are far off from reality. However there is a hypothesis that the early sun had more mass and that this compensated for the other factors, resulting in an early sun that was hot enough to account for a warm earth.   The second way is to question the geological evidence for a warm early earth. There are multiple independent lines of evidence for liquid water on the early earth, however, including sedimentary rocks, mud cracks and ripples, and algae.

The paradox, by the way, also holds true for Mars. Mars also would have been cooler due to a fainter young sun. There is also evidence for liquid water on the early Mars. Solutions to the paradox that affect the entire solar system would resolve it for both planets. If the solution is planet specific, however, then each planet would require its own solution.

If both our stellar models and the evidence for a warm early earth are fairly solid, then we must consider a third possibility – that there is some other factor at work that can resolve the apparent paradox. The standard hypothesis is that the atmosphere of the early earth was different than it is now in a manner that would increase the greenhouse effect and result in a warmer earth despite a cooler sun. This seems like an obvious enough solution. Carbon dioxide, ammonia, and methane are all powerful greenhouse gases and may have been present in larger amounts in the early atmosphere. Life did radically change the composition of the earth’s atmosphere, introducing oxygen, for example. The net effects, however, still need to be sorted out.

A related hypothesis not involving greenhouse gases is that the early earth had a lower albedo, meaning that it absorbed more energy from the sun. This could be due to a darker ocean and/or less cloud cover. (The cloud cover hypothesis has recently been refuted, however.) There are also other minority hypotheses, such as the cosmic ray hypothesis. Cosmic rays may have a cooling effect on the earth, and the early sun may have shielded the earth from more of these cooling cosmic rays.

A recent study of fossilized raindrop impressions in volcanic ash suggests that the atmosphere was not thicker billions of years ago. This means that a thicker total atmosphere is likely not the solution to the faint young sun paradox. This was not really a major hypothesis in any case, and I am also not sure how definitive this raindrop evidence is. It is just one more bit of data in the complex story of this paradox.

Young earth creationists, however, have been quick to pounce on this study to “paradox monger.” The Institute for Creation Research recently published an article called, “Ancient Raindrops Argue for Young Earth.” I’m not sure if the irony of this headline is lost on them or not, but I am pretty sure they don’t care. All they do care about is that there is a paradox, and they can exploit that to sow doubt about scientific findings they find inconvenient. They cannot help, however, to hopelessly contradict themselves.

The apparent paradox results from sciences that also point to an ancient earth, solar system, and universe. The same stellar models that create the paradox also point to a sun that is billions of years old. The same fossil evidence that points to a warm early earth also points to an ancient earth. Creationists, however, want the paradox without the underlying science, resulting in a self-contradicting mess of an argument. They are not interested, however, in creating a viable theory. They just want to poke apparent holes in established science.

I don’t know what the solution to the faint young sun paradox will turn out to be. There are many viable hypotheses, but we simply lack definitive information to know which one or ones are correct. That’s science. This controversy will rage for a while, but if history is any guide eventually we will sort out what the correct resolution is.

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