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Our future, our universe, and other weighty topics

Saturday, June 4, 2016

“Nuke Your Way to Life” Theory Isn't Convincing

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.

Some scientists have tried to solve this problem by suggesting that there were high levels of carbon dioxide in the early atmosphere, causing global warming through the greenhouse effect. But (as discussed in this paper) for such a theory to work, global carbon dioxide levels would have needed to be fifty times larger than today. Such high carbon dioxide levels would have left traces in the fossil record, traces that have not been discovered.

Recently in the news there was discussion of another theory to explain the paradox. The theory is that billions of years ago there were more solar flares: “frequent and powerful coronal mass ejection events from the young Sun—so-called superflares.” A coronal mass ejection is when the sun shoots out lot of particles. Nowadays really powerful coronal mass ejections occur only once every 30 years. But advocates of this solar flare theory claim that billions of year ago such really powerful coronal mass ejections may have occurred as often once a day. Such advocates suggest that if such flares had been enough to melt ice, they might have helped to make life possible.

Helpful solar flares? That doesn't sound very sensible when you read this description of a coronal mass ejection from another web site:

Coronal mass ejections (CMEs) are violent ejections of solar gas, plasma and electromagnetic radiation that can propel more than ten billion tons of solar matter outward from the sun’s atmosphere with the power of over a billion hydrogen bombs....They can extend billions of miles into space. Once jettisoned from the sun’s hold, they can accelerate to several million miles per hour and can reach Earth within one to three days.

A billion hydrogen bombs? So basically this attempt to resolve the early sun paradox by imagining very powerful solar flares is a kind of “nuke your way to life” theory.

There are several reasons why a straightforward version of such a theory seems to make no sense:
  1. We have no known cases of polar ice that was melted because of solar flares.
  2. If ice were to be melted by a short-lived solar flare zapping our planet, it would very quickly freeze again (presumably within hours), killing off any life that may have formed while liquid was available.
  3. The same level of solar flare intensity needed to melt ice would have lots of life-killing radiation intensity.
A more subtle version of the solar flare theory claims that solar flares may have caused nitrous oxide (N20) to have formed in the atmosphere. Nitrous oxide is a greenhouse gas 300 times more powerful than carbon dioxide. Could more nitrous oxide in the atmosphere have warmed up the planet? Probably not, because in section 5.4 of this paper giving a very thorough discussion of the faint young sun paradox issue, we read the following: “Warming by nitrous oxide (N2O) has been suggested [Buick, 2007], but N2O is rapidly photodissociated in the absence of atmospheric oxygen [Roberson et al., 2011], making it an unviable option for the Archean.” Referring to the Archean eon (the the period between 4 billion years ago and 2.5 billion years ago), this statement rebuts the idea that “helpful solar flares” may have caused a greenhouse effect through a production of nitrous oxide. The objection here is that if nitrous oxide had been produced in the early atmosphere, it would have been rapidly destroyed by sunlight.

The same objection is made by scientist James Kasting. A New Scientist article says this:

Kasting thinks that ultraviolet light from the sun might have destroyed nitrous oxide before it could mix into the atmosphere. “It would take a convoluted mechanism to produce that high up in the atmosphere and then get enough of it into the lower atmosphere to produce a good greenhouse effect,” he says.

It would seem, therefore, that the faint young sun paradox is still very much with us. At the end of this scientific paper, the author makes clear that scientists have been knocking their heads on the faint young sun paradox for 40 years, but the problem refuses to go away.