In the Archean Eon about 2.5 billion to 4 billion years ago, before the first advanced life appeared on the planet, the sun was only about 70 percent as bright as it is today. This means the amount of heat felt on Earth was much less, and Earth's surface should have been frozen.
However, ancient rocks at Isua near the southwest coast of Greenland indicate liquid water and even life was present on Earth about 3.8 billion years ago. "So Earth's climate had to be somewhere between the freezing point and boiling point of water, and probably pretty close to the temperature we have today, which sustains life," said researcher Emily Pope, an isotope geochemist at the Natural History Museum of Denmark in Copenhagen.
The contradiction between the cold Earth that apparently should have existed and the temperate Earth that apparently did exist is known as the "faint young sun paradox." Until now, the most popular explanation for this enigma was that there was a higher concentration of "greenhouse gases" such as carbon dioxide in the atmosphere than today. These gases absorb heat from the sun, helping warm the planet.
"Just like the average temperature of Earth is getting higher today because there are more greenhouse gases than there were before the Industrial Revolution, or even before the invention of agriculture, the presence of high concentrations of carbon dioxide and methane should have kept the early Earth warm," Pope said. [Early Earth Was Purple]
For greenhouse gases to explain the faint young sun paradox, their concentrations would need to have been extremely high, hundreds to thousands of times as much as today.
"If levels of carbon dioxide were that high, they would be recorded in ancient soils and sediments in the rock record," Pope said. "If levels of methane were that high, they would actually form a kind of organic haze in the atmosphere that blocks the sun's rays and would counteract its properties as a greenhouse gas."
Now scientists analyzing relatively pristine 3.8-billion-year-old rocks from Isua find no evidence that greenhouse gas levels were high enough to explain the faint young sun paradox, further deepening the mystery, Pope told LiveScience.
Specifically, researchers looked at serpentine mineral deposits, which form when ancient seawater interacts with deep ocean crust (the outer layer of Earth). These deposits record details of the water such as the hydrogen and oxygen isotope ratios found within, which rely in part on ocean size. Isotopes are atoms of the same element, like hydrogen, with differing numbers of neutrons. Light hydrogen isotopes are more likely to be found in the air and escape into space than heavier ones; the smaller the oceans, the more their waters will have slightly lower concentrations of light isotopes.
The rocks suggest that the oceans were up to 26 percent larger in the past. These shrunk over time to present-day volumes — seawater became trapped in newly formed continental rocks, and hydrogen that is one of the key ingredients of water instead escaped to outer space.
The rate of hydrogen loss to space is linked to atmospheric levels of methane and carbon dioxide; both these greenhouse gases can interact with hydrogen and other gases such as oxygen in complex ways. The hydrogen loss rate the researchers estimated based on these findings suggests that concentrations of these greenhouse gases were nowhere near high enough to reconcile the faint young sun paradox. [Stunning Images of the Sun]
"We have new concrete data that characterizes the early oceans," Pope said. "This will hugely help our ability to put realistic constraints on our models of how Earth's oceans and atmosphere first evolved."
An alternative explanation for the faint young sun paradox is that early in Earth history, there were fewer continents because a number had not formed yet; less land mass would have meant less cloud cover, because there weren't biologically generated particles such as pollen and spores that could behave as seeds around which the clouds could form.
"The result was that the planet, covered mostly by oceans, was darker, and like an asphalt road on a hot day, could absorb a lot more heat, enough to keep the Earth clement," Pope told LiveScience.
The scientists detailed their findings online March 5 in the journal Proceedings of the National Academy of Sciences.