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.
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:
- We have no known cases of polar ice that was melted because of solar flares.
- 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.
- 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:
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.
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