Original | Evan Gough
Translate | Linvo
The Earth has gone through a long and complex history since its formation about 4.5 billion years ago. Initially it was a molten ball, which later cooled and differentiated. Planetary differentiation refers to the process of separating the internal components based on the physical or chemical behavior of planetary bodies. The Moon was formed by a collision between the Earth and a protoplanet. Then the oceans formed, and at some point in time (about 4 billion years ago) simple life appeared.
These are rough outlines, and scientists have been working to refine a detailed timeline of Earth's history. However, on this timeline, there are many important and little-known periods. One of them is about ultraviolet radiation and its effects on early life.
A new study explores the effects of ultraviolet radiation on early life forms on Earth and how it shaped our world.
The sun bombards the earth with ultraviolet radiation, which is harmful to life. Ultraviolet radiation accounts for about 10% of the radiation from the sun. Much of this is not part of ionizing radiation, but it can still damage DNA, leading to sunburn and skin cancer.
Fortunately, Earth's ozone layer is a barrier against ultraviolet rays. The ozone layer provides varying degrees of ultraviolet protection on the Earth's surface, changing with the seasons and latitudes. Overall, it prevents 97 to 99 percent of medium-frequency ultraviolet light from reaching the Earth's surface. However, the effectiveness of the ozone layer depends on the amount of oxygen in the Earth's atmosphere, which fluctuates over time. The most significant change in oxygen levels in Earth's atmosphere occurred during the Great Oxidation Event (GOE), which is the focus of the new study.
The new study is titled "Revising Lower Estimates of Ozone Levels in earth's oxidation history." The study's lead author is Gregory Cook, a PhD researcher at the University of Leeds' School of Physics and Astronomy. The paper was published in the royal society open science journal.
GOE has raised the amount of oxygen in Earth's atmosphere from near zero to about 21 percent of what it is today, all of which occurred about 2 billion to 2.4 billion years ago. Scientists attribute this phenomenon to the emergence of cyanobacteria, known as blue-green algae. Cyanobacteria appeared 2.7 billion years ago and use photosynthesis to produce energy. A byproduct of photosynthesis is oxygen.
The Great Oxidation Event (GOE) began as early as 2.33 billion years ago, a period scientists believe marks the beginning of the permanent presence of oxygen in the atmosphere.
Earth's oxygen levels fluctuate up and down over time, but GOE is the most important event in Earth's oxygen levels in history. Another event, named "Neoproterozoic oxidation," may also have played an important role in boosting Earth's oxygen. However, it is not fully understood, or even unanimously recognized, as GOE. But in any case, as the oxygen content increases, so does ozone.
Previous studies have shown that ozone protects the planet from harmful radiation when oxygen levels are as low as 1 percent. But the new study draws a different conclusion. It shows that the atmosphere needs 5 to 10 percent of Earth's oxygen today to protect life from ultraviolet radiation.
What does this mean for early life on Earth?
"This may have had a fascinating impact on the evolution of life."
"We know that if life is exposed to too much ultraviolet radiation, it can have catastrophic effects," first author Cook said in a press release. "For example, it can cause skin cancer in humans. Some organisms have effective defense mechanisms, and many organisms can repair some of the damage caused by ultraviolet radiation. ”
"While increased UV radiation won't stop life from emerging or evolving, it can act as an alternative pressure that better motivates organisms exposed to more UV radiation to gain an advantage," Cook said. ”
A historical model of Earth's climate is the basis for this research. These models suggest that previous estimates of UV levels on the Earth's surface may have underestimated UV exposure. Instead, the Earth may be exposed to ten times more ultraviolet light than we think.
Over the past 2.4 billion years, the level of ultraviolet radiation reaching the Earth's surface may have changed.
The modeling in this study is different from earlier modeling. Previous studies relied on one-dimensional models of Earth's oxygen. But the study takes into account more complexity. The authors explain in their paper that the study used "... A complete atmospheric chemistry-climate model to model the change of three-dimensional ozone with oxygen concentration under Proterozoic and Proterozoic conditions applicable to the Earth". "We showed the three-dimensional effect of oxygen on the ozone layer (its size and spatial variation) and discussed how this affects the estimation of habitability (the ability of life to survive on the surface)."
The study used a "climate model of intact atmospheric communities (WACCM6)". WACCM6 combines submodels of atmosphere, land, ocean, land ice, and sea ice. The team conducted 12 different simulations in their study.
Schematic diagram of the WACCM6 Earth system model.
Scientists use "Dobson units" to measure atmospheric ozone levels. 1 Dobson unit is the number of ozone molecules required to generate a layer of pure ozone 0.01 mm thick at a temperature of 0 degrees Celsius and 1 atmosphere.
The figure shows the Dobson units of earth's ozone content density.
If life on Earth were exposed to more ultraviolet light than previously thought, it would become an integral part of natural selection. Creatures that adapt in a certain way (such as hiding underground, repairing damage, etc.) cannot defeat other mobs without these adaptations.
"If our model shows atmospheric scenarios in Earth's oxidation history, then for more than 1 billion years, the Earth may have been bathed in more intense ultraviolet radiation than previously thought," Cook said. ”
"This may have had a fascinating impact on the evolution of life." When animals appeared, and what kind of environment they encountered at sea or on land, we don't know. However, depending on the oxygen concentration, animals and plants may face harsher environments than they do today. We hope that the evolutionary implications of our findings will be fully explored in the future." ”
Everything about Earth is much more complicated than one or two simple factors. The strength of the ozone layer and its ability to protect life from ultraviolet radiation depends heavily on oxygen levels. But there are many other factors at play, such as atmospheric mixing and the intensity of the sun's output. Our industrial activities and other biological processes also affect the ozone layer. Interested readers can go to the original paper.
There are several interesting takeaways from this work: First, we wouldn't be here without enough UV protection; if early life hadn't been exposed to more UV light, we probably wouldn't have been here. So can we call it "the two sides of ultraviolet light"?
Another exciting discovery is about exoplanets and our growing interest in them. Thanks to the launch of the James Webb Space Telescope (JWST), scientists will be able to study the atmospheres of exoplanets in more detail. The presence and abundance of gases such as oxygen and ozone in these atmospheres will help us understand the potential habitability of exoplanets. The modeling in this study may be part of explaining the results of JWST and understanding the puzzle of exoplanet and the possibility of life.
original:
https://www.universetoday.com/153972/the-early-earth-was-really-horrible-for-life/