The ancient Earth would have looked alien to us as it was engulfed in a thick orange haze. By studying Earth’s past, scientists are deciphering what a similarly hazy exoplanet would look like in future space-based telescopes.
Imagine if scientists in the future invent a time machine, and someone wishing to study the Earth in the distant past could simply step into the machine and then step out in another era. If they went past the time of the dinosaurs, past the time of strange monsters in our oceans, and back to a period of time where microbial life was just gaining a foothold, what would they find?
Such a scientist stepping out into the Archean Eon – between 3.8 and 2.5 billion years ago – would first of all need some sort of protective suit as the atmosphere was mostly devoid of oxygen. Not only that, but some of the earliest known bacteria were methanogens, meaning that they produced copious amounts of methane. If enough methane was released into the anoxic atmosphere, an organic haze could surround the planet.
The scientist would also be able to detect sulfur photochemistry in the atmosphere caused by UV light penetrating the atmosphere. This type of chemistry doesn’t occur in our atmosphere today, as the ozone layer stops the UV light required to drive it. But in the Archean, there was no oxygen and therefore no ozone layer to stop the sulfur photochemistry from occurring. A photochemical sulfur “signal” was deposited in the rocks and preserved in time for analysis by scientists who have to make do without a time machine. The sulfur signal is also the key to inferring the existence of the haze.
As methane production increased, the organic haze would get thicker and block out some of the UV light needed for the sulfur photochemistry. This change in the sulfur signal caused by attenuated UV light coupled with rock record evidence indicating contemporaneous enhanced biological methane-production throughout the Archean is the geological evidence for the haze, and the rock records show that there were between three and five intervals of such haze during the Archean. The hazes eventually stopped after the “great oxygenation event” some 2.5 billion years ago when oxygen built up in the atmosphere.
Would the time-travelling explorer also need Arctic gear to survive freezing temperatures? Previous studies have suggested that the thick haze would block out so much sunlight that surface temperatures would plummet, causing a major glaciation and creating a major obstacle for life. However, new research by non-time-travelling scientist Giada Arney from the University of Washington (now at NASA Goddard Space Flight Center), and her colleagues, has shown that the freezing climate may not have been extreme as previously thought.
They used computer models to simulate the Archean atmosphere and discovered that the haze thickness – and thus the planet’s temperature – would eventually stabilise, thereby preventing catastrophic cooling. Their results also showed that with the haze blocking enough harmful UV light in the atmosphere, conditions would have been more favourable for the organisms below.
They also used their models to determine what the spectra of exoplanets with similar hazes would look like – something that future space-based telescopes such as the James Webb Space Telescope might be able to detect. From their spectra, they were also able to tell what the colour of the sky and planet would be, and for a planet with a haze similar to the Archean Earth, it would be orange. This means that the early Earth would have looked more like Saturn’s moon Titan.
“Archean Earth may have occasionally had a Titan-like organic haze, but it still would have been a very different type of world,” explained Arney. “Archean Earth would have been MUCH warmer than Titan, and its atmosphere would have contained a lot more CO2, which fundamentally changes the type of atmospheric chemistry that’s possible. Still, the types of chemical reactions that lead to haze formation on early Earth may be similar to some of the types of reactions occurring in Titan’s atmosphere today. And, of course, both would appear orange!”
Carl Sagan famously referred to the Earth as the “pale blue dot”, but if we are to search for exoplanets similar to the early Earth, then we need to be looking for pale orange dots instead. “If we were able to observe a large statistical sample of exoplanets, we might be able to catch planets at different stages of evolution, and if hazy periods and oxygen build-up are common planetary phenomena, we might see examples of several atmospheric stages,” added Arney.
Such observations would mean that we wouldn’t need a time machine to study the early Earth, as we might gain a window to our past by studying other planets.
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