The sun is thought to have once been far fainter than it is today, which should have left Earth frozen as a global snowball. That it wasn鈥檛, a discrepancy known as the faint young sun paradox, has long plagued astronomers, but now we might have an answer: the moon helped keep Earth warm.
Models of the sun鈥檚 past suggest it was up to 70 per cent dimmer in its first 100 million years. 鈥淭he surface of the Earth should have been frozen for at least a billion or even 2 billion years,鈥� says Ren茅 Heller at the Max Planck Institute for Solar System Research in G枚ttingen, Germany.
Geological evidence, not to mention the evolution of life, shows this didn鈥檛 happen. We know Earth must have had water back then thanks to a mineral called zircon, some crystals of which have survived for 4.3 billion years and retain evidence of water at the time. Life may have even emerged around a similar time.
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But the early Earth did have an important difference to today. When both the moon and Earth formed about 4.4 billion years ago, our satellite began life as little as 20,000 kilometres away from our planet, compared with an average of 380,000 kilometres now. Earth was also rotating much faster back then, as quickly as once every 3 hours.
Heller and his colleagues have calculated that these two factors mean the gravitational interaction between the two bodies would have been much stronger 鈥� enough to produce tidal heating from the gravitational squeeze. This would have slightly warmed Earth and could have triggered the eruption of volcanoes, giving our planet a thicker atmosphere that could trap more heat.
鈥淭he classic example in our solar system is [Jupiter鈥檚 moon] Io, which is spectacularly volcanic because of the tidal heating from Jupiter,鈥� says Rory Barnes at the University of Washington. 鈥淭he moon could have turned early Earth into something like Io for tens of millions of years.鈥�
Finding out how Earth was able to hold liquid water back then could thus be crucial in our search for life on other worlds, says Ludmila Carone at the Max Planck Institute for Astronomy in Heidelberg, Germany. 鈥淲e are not entirely sure why Earth was habitable,鈥� she says. 鈥淲e have the possibility to go back in time and think about the early Earth as a kind of exoplanet.鈥�
Other solutions for the faint young sun paradox include Earth having a thicker carbon dioxide atmosphere at the time as a result of the planet being molten following the giant impact that formed the moon, trapping more heat. Another is that the planet鈥檚 orbit brought it closer to the sun at times, warming it up, or that the sun had more mass at the time and was brighter than we think.
All these ideas have numerous unknowns, says Barnes, but while the tidal heating hypothesis is a good fit, it isn鈥檛 perfect. The amount of energy produced directly by the moon鈥檚 gravity would have been small, requiring it to cause other processes like volcanic eruptions to also take place, which we don鈥檛 have any direct evidence for.
鈥淭he amount of tidal heating required to have a climatological effect is very great,鈥� says Kevin Zahnle at the NASA Ames Research Center in Mountain View, California. The moon moved away from Earth quickly, limiting the duration of the tidal heating to just 10 to 20 million years, he says 鈥� not enough to warm Earth sufficiently. Further modelling of the early Earth could help better understand the different factors at play, says Heller.
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