A Hazy Shade of Earth

Hello all! Today we are once again travelling back in time to the early Earth. The period known as the Archean spans 3.8 to 2.5 billion years ago. Geochemical evidence, more specifically data from sulfur isotopes in the rock record, suggest that the atmosphere of the Archean was at times hazy. This hydrocarbon haze would have interacted with UV and other forms of radiation to affect the planet’s climate. There were probably three to five intervals of haze, lasting less than a million years and a longer one between 3.2 and 2.7 billion years ago.
The haze’s effect on climate has been previously studied, but not looked at as a whole with other habitability conditions. That brings us to today’s article, “The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth by Giada Arney and eight others. Published in the journal Astrobiology in late 2016, this article takes the reader through a model of the early Earth atmosphere and its results.
Titan as seen by the Cassini spacecraft A moon of Saturn, Titan is known for its haze. Image Credit: NASA/JPL-Caltech/Space Science Institute.
Titan as seen by the Cassini spacecraft A moon of Saturn, Titan is known for its haze. Image Credit: NASA/JPL-Caltech/Space Science Institute.
Using constraints suggested by the geochemical evidence previously mentioned, a model of the Archean conditions is created. It is a variant of a model that has been used many times before, though improvements and adjustments were made. Unlike some previous haze models, this one took larger haze particles to be fractals. Fractals are structures in which each part repeats in similar or identical patterns at a smaller scale. Think of a snowflake branching, or a river, where the smaller streams and rivers mimic the pattern of the larger river and tributaries. Fractals are not uncommon in nature and used often in modelling. In this case, fractal haze particles interact differently with radiation than their spherical counterparts in such a way that they produce less cooling for the surface below.
Previous studies have indicated that surface temperatures of under 273 Kelvin (just below freezing) would result in a completely frozen, uninhabitable Earth. However, more recent studies have suggested that Earth could have gotten as cold as 248 K (-25 Celsius, or -13 Fahrenheit) would still have open ocean. Using this lower temperature, all modeled scenarios resulted in a hazy Archean Earth that was cold, but still habitable. Three out of four of the modeled versions of the atmosphere were above 273 Kelvin as well. They also found that there was a link to the size of the particles and the temperature of the atmosphere; a hotter atmosphere led to larger particles.
Fractal hazes are better at absorbing short wavelengths, which makes them a potential shield against deadly UV radiation. This kind of radiation at the time would have been potentially able to sterilize early Earth, but tests from the model indicate that with the protection of the haze, organisms naturally hardened against UV radiation would have still been able to survive at the surface. There is evidence that some organisms did live on the surface or near the surface at this time.
Diagrams explaining spectra. Taken from page 151 of The Solar System, by Bennett et al, 7th edition.
Diagrams explaining spectra. Taken from page 151 of The Solar System, by Bennett et al, 7th edition.
Another thing to consider is spectra. Spectra are from interaction of light and gas in the right conditions; it can leave either a series of dark lines in a rainbow or bright lines in something otherwise dark. These are unique to each element, so spectra can be used to find which elements and chemical compounds are in a gas. When looking at the spectra of exoplanets (planets outside our solar system), there should be telltale signatures in the spectra that would signal a haze. The James Webb Space System, scheduled to launch in 2018, should be able to detect these spectra. The haze would of course change the appearance of the planet, should it be imaged; while Earth has famously been called a pale blue dot, pale orange dots may also be habitable.
Despite all of this, however, there are still several questions remaining. Based on comparisons with other models, the results here underestimate the surface temperature of the Earth by three to five degrees Celsius (5.4 to 9 degrees Fahrenheit), likely because this model uses only one dimension. A 3-dimensional model could be of much use. Additionally, it is still unknown how exactly hazes form from a chemistry standpoint. While the haze of Saturn’s moon Titan is currently being studied, it is not likely the same as the haze around early Earth. There is still a lot of work to be done before our Archean Earth questions are answered.

 

 

Bibliography
Arney, Giada, et al. “The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth. Astrobiology. November 2016, 16(11): 873-899. doi:10.1089/ast.2015.1422.
Today’s main article.
Bennett, Jeffrey, Megan Donahue, Nicholas Schneider, and Mark Voit.  The Cosmic Perspective: the Solar System. 7th ed. Boston: Addison-Wesley, 2014. Print.
My astronomy textbook, which I used for reference on, and the image of, spectra.
Jet Propulsion Labratory and California Institute of Technology. “Titan’s Building Blocks Might Pre-date Saturn. JPL/CalTech, 23 June 2014. Web.
An article discussing Titan, from which I took the first image in this post.
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