The sun is getting brighter. If you traveled back in time to the dawn of the solar system 4.5 billion years ago, you would find a star that was 30% dimmer than it is today. It is a function of the nuclear fusion process that takes place in the cores of stars like our own, and it will continue to do so until the end of its life.
The original sun should have caused disaster here on Earth. The planet would be too cold for liquid water to flow if it were placed under that sun.
And yet it did. We know that our planet had liquid water on its surface as early as 4 billion years ago, and maybe even earlier. Soon afterwards, single-celled life seems to have sprung up. Despite a few close calls, the planet's water and life have persisted.
How could the sun be so low?
Scientists have been vexed by the faint young sun paradox for decades. Recent work has led many researchers to believe that we have a solid hold on the problem. Old ideas have been refined, while new ideas have helped to fill the few remaining gaps in our understanding.
The faint young sun paradox has important implications for our understanding of how life on Earth might come to be. Did our world, even in its optimal location, produce life by only the slimmest of margins? Benjamin Charnay, a planetary scientist at the Paris Observatory, said that it was a fundamental question about the habitability of Earth over all its history.
The history of our world has been opened up by the exploration of the faint young sun problem. We're discovering that the reason for our existence may be revealed by what was once a paradoxes.
The mechanics that drive the evolution of stars like our sun were understood by scientists in the 20th century. In a star's core, hydrogen and helium combine to produce energy. The fusion rate is boosted by the amount of hydrogen decreasing. Over time, the star gets brighter.
The British astronomer Fred Hoyle and the German American astrophysicist Martin Schwarzschild used the same knowledge to arrive at the same conclusion. Evidence of water on Earth was found in the 1960s. Earth should not have been warm enough under the young sun to have liquid water. One paper in 1965, in an effort to solve the discrepancy, suggested that either the sun was older than we thought, or the model of our sun's evolution may need some modification to permit higher luminosities.
The first detailed analysis of the faint young sun problem was performed by Carl Sagan and George Mullen. They suggested that a thicker atmosphere might have kept the planet warm enough to support liquid water. They put ammonia in the greenhouse gas.
Ammonia is destroyed by solar ultraviolet radiation, according to a climate researcher at the Potsdam Institute for Climate Impact Research.
In 1981 carbon dioxide's possible effects were explored by Kasting and his colleagues. The large greenhouse effect should melt the ice cover in a geological way.
We can't just grab a sample of the atmosphere from 4 billion years ago. Other possibilities emerged in the absence of geological corroboration. The amount of reflective low clouds that would have bounced sunlight back into space might have been reduced by high levels of carbon dioxide. If you increase the mass of the sun by 5%, it would be just as bright as it is now, and we wouldn't. Most researchers are skeptical of this idea.
The Hadean eon, which lasted from 4.5 billion to 4 billion years ago, and the Archean eon, which ended 2.5 billion years ago, were the earliest periods of Earth. To find out when water and life first arose, scientists needed to get a handle on Earth's early atmosphere. Fortunately, answers were close.
An object the size of Mars barreled into the young Earth at the dawn of the Hadean. The moon was formed by the collision and our planet was reset to zero. It is likely that it sent temperatures on Earth soaring, with a vast ocean surrounding the planet.
A more recognizable planet could be formed by the cooling of the magma ocean. There is evidence from tiny crystals called zircons.
Zircons have ages that go back as far as 4.2 billion years.
Some of the first evidence of life appears just 300 million years later. In 2015, Elizabeth Bell and colleagues from the University of California, Los Angeles, found carbon from biological sources inside zircons.
We don't have reliable measurement of atmospheric carbon dioxide. We can see that there was a lot of it. Owen Lehmer, a planetary scientist at NASA's Ames Research Center in California, and his colleagues published work analyzing the composition of meteorites from 2.7 billion years ago. The meteorites preserved a record of the atmospheric composition. It may have been 70% carbon dioxide or more, compared to just 0.04% today, according to the researchers.
Adding a bunch of carbon dioxide is in line with keeping the young Earth warm and preventing a giant snowball.
It wasn't always so warm. Feulner said the thermostat failed spectacularly on several occasions. The glaciers pushed all the way to the equator around 2.4 billion years ago.
Our planet may have been able to reverse the process because of additional carbon dioxide from volcanoes. The sun became warm enough to support liquid water about 2 billion years ago. The snowball Earth events are thought to have happened 700 million and 635 million years ago. The beginning and end of each period may have been caused by volcanic eruptions.
Over the past decade, advanced modeling of our planet's carbon cycle has suggested that early in Earth's history, less carbon dioxide may have been needed. The production of methane by living things may have helped boost the greenhouse effect.
Ren and his colleagues from the Max Planck Institute for Solar System Research in Germany came up with another potential source of heat. The moon was 15 times closer to Earth than it is today. The moon's gravity would have had a huge impact, creating huge tidal waves that towered 2 kilometers above any magma or liquid-water oceans present. It would have pushed and pulled Earth's interior, generating extreme tidal heating that increased the planet's temperature. The moon could have given Earth a vital boost over the first 100 million to 300 million years of our planet's existence, increasing Earth's temperature by several degrees and helping to drive volcanic activity across the surface.
If we include tidal heating, it becomes less severe.
The faint young sun may have been a lifesaver. Earth might have become too hot if the sun had possessed less than 98% of its present luminosity 4.5 billion years ago. Our planet might have shared Venus' fate if the early sun hadn't been so faint.
Upcoming missions to Venus planned by NASA and the European Space Agency may be able to tell us if this idea is correct by looking for signs of ancient water. We should be grateful if they don't find any. The oceans would have been boiled away by a hot sun.
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