Washington, Apr 7 (IANS): Four billion years ago, the sun radiated only 70 to 75 percent as much energy as it does today. Now, scientists are trying to explain what compensated for the reduced solar output and kept the earth's water liquid.

A popular theory holds there must have been higher concentrations of greenhouse gases in the air, most likely carbon dioxide, which would have helped retain a greater proportion of the solar energy that arrived.

But a team of earth scientists, including researchers from Stanford University in California, have analysed the mineral content of 3.8-billion-year-old marine rocks from Greenland and concluded otherwise.

"There is no geologic evidence in these rocks for really high concentrations of a greenhouse gas like carbon dioxide," said Dennis Bird, professor of geological and environmental sciences at Stanford.

The crux of the theory is that because oceans are darker than continents, particularly before plants and soils covered landmasses, seas absorb more sunlight.

"It's the same phenomenon you will experience if you drive to Wal-Mart on a hot day and step out of your car onto the asphalt," Bird said. "It's really hot walking across the blacktop until you get onto the white concrete sidewalk."

Instead, the team proposes that the vast global ocean of early earth absorbed a greater percentage of the incoming solar energy than today's oceans, enough to ward off a frozen planet.

Because the first landmasses that formed on earth were small - mere islands in the planetary sea - a far greater proportion of the surface was covered with water than today, said a Stanford release.

The study was published in the April issue of Nature.