Irradiating the Early Earth

It is about to be summer in the northern hemisphere, the time for barbecues and beaches and sunscreen. Sun burns are not any fun, and radiation is constantly being sent towards the Earth from the Sun.
Have you ever wondered what that radiation was like billions of years ago?
Probably not, but some scientists have. Today’s article is “Habitat of early life: Solar X-ray and UV radiation at Earth’s surface 4–3.5 billion years ago” by Ingrid Cnossen, Jorge Sanz-Forcada, Fabio Favata, Olivier Witasse, Tanja Zegers, and Neil F. Arnold. Published in the Journal of Geophysical Research in 2007, this article looks at the Sun’s radiation and its affects on early life.
We don’t know exactly when life showed up on Earth, but evidence preserved in the geological record put it somewhere at 3.8 to 2.7 billion years ago. Ultraviolet (UV) and x-ray radiation give energy to chemical reactions early life would have needed, but it can also be harmful, even fatal, to living beings. From looking at stars like our own sun, we know that younger stars give off more UV and x-ray radiation, but are also fainter. This faintness leads to the faint young sun paradox: there is evidence of water on early Earth, but the sun was 25 to 30 percent weaker, and should have left the Earth cold and icy. Although there are multiple solutions to this paradox, the most common one is that the composition of early Earth’s atmosphere was different than it is now, and this compensated for the faint young sun through the greenhouse effect. So just how much was early life subjected to radiation from this young sun?
The electromagnetic spectrum.
The electromagnetic spectrum. Note how little is actually visible to human eyes, and where UV and x-rays lie in the spectrum. Image taken from Principles of Physics, page 823.
How much radiation actually gets from the sun to Earth depends on different factors, one of which is the atmosphere and its composition. The atmosphere can absorb radiation, trapping it there and not letting it reach the surface. The atmosphere also causes Rayleigh scattering, in which light ‘bounces’ off molecules in the atmosphere, scattering it in all directions. This not only causes the sky to be blue, but also scatters some radiation away from the surface. How much absorption and scattering occurs depends on the type of light and the composition of the atmosphere. Some things in the atmosphere do much more scattering than absorbing, or vice versa.
For this paper, the authors focused on early Earth 4 to 3.5 billion years ago, which for the most part is in the Archean era, and near when the earliest signs of life appear. They modeled the young sun as it is expected to have behaved at about half a billion years old, as it would have been  in the Archean.
The geologic time scale. Taken from Portrait of a Planet, page 443.
The geologic time scale. Taken from Portrait of a Planet, page 443.
For their model, the authors assumed that half the light would move towards the surface of the Earth after Rayleigh scattering, while the rest was scattered away. Absorption was calculated, and cloudless skies were assumed (clouds affect the amount of radiation the surface of the Earth receives).
The models used different compositions for the atmosphere, giving levels for nitrogen, methane, ozone, water vapor and carbon dioxide based on what likely constituted the atmosphere during the Archean. The levels of carbon dioxide, methane, ozone, and water vapor were varied in the simulations. The authors found that methane and water vapor had only a slight effect of the amount of radiation that reached the surface. Ozone, which is known to absorb radiation, had only a minor effect as there was much less of it in the atmosphere of early Earth. However, the models showed that the amount of radiation that was transmitted through the atmosphere was highly effected by the amount of carbon dioxide. The more carbon dioxide, the less radiation got to the surface.
Even stars of the same type vary somewhat, and they could give five times more or five times less radiation than what was used for these models. This factor, however, and even a modeled solar flare, made little difference in the amount of radiation reaching the surface compared to the effect of the concentration of atmospheric carbon dioxide. The range of radiation levels that early Earth could have received still vary broadly, but it received a lot more than modern Earth. Most likely, early life would have found shelter from the harmful radiation, possibly in the oceans. The surface was probably not friendly to life until oxygen became a factor in the composition of the atmosphere, about 2.2 to 2 billion years ago. Like many things in life, change in the atmosphere is slow. Luckily for us, modern Earth is much kinder to its inhabitants, at least in terms of the levels of harmful radiation.
Though that does not mean you should skip the sunscreen.

 

Bibliography
Sources used to write this article.
Cnossen, Ingrid, et al. “Habitat of early life: Solar X‐ray and UV radiation at Earth’s surface 4–3.5 billion years ago.” Journal of Geophysical Research: Planets (1991–2012) 112.E2 (2007).
Today’s featured article.
Serway, Raymond A., and John W. Jewett. Principles of Physics: A Calculus-Based Text, Volume 1. 4 edition. Cengage Learning, 2005. Print.
My intro physics textbook, used for the image of the electromagnetic spectrum.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Company. Print.
My geology textbook, used for the diagram of the geologic time scale.
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