It is accepted that the ancient Sun was considerably cooler than our local star is today, so much so that Earth a few billion years ago should have been a lifeless frozen ball. But scientists have also shown that the planet was not frozen—shallow seas warmer than any modern ocean abounded with microbial life. A recent study, detailed in the journal Nature, is a good example of the sometimes convoluted, even improbable reasoning is used to get a handle on earthly climates during eons long vanished. Using the fossilized impact dimples from rain drops that fell 2.7 billion years ago, researchers have calculated new limits on the density of Earth's atmosphere. This, in turn, has implications on the development of the ancient atmosphere and what role greenhouse gases may have played in warming the young Earth.
The paradox of the Faint Young Sun has vexed scientists for decades, having first been raised by astronomers Carl Sagan and George Mullen in 1972. Applying the observation made by astrophysicists, that main sequence starts like the Sun grow steadily warmer as they age, and working back in time from current day conditions they posed an awkward question: why did the ancient Earth not freeze solid billions of years ago? In their new paper, “Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints,” the problem is framed this way by Sanjay M. Som et al.:
According to the ‘Faint Young Sun’ paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate1. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening or enhanced greenhouse gas concentrations. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen. But no field data constraining ground-level air density and barometric pressure have been reported, leaving the plausibility of these various hypotheses in doubt.
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