"Soon after the planets were formed 4.5 billion years ago, Earth, Venus and Mars probably all had water," says Maura Rabbette of NASA's Ames Research Center. "How did Earth manage to hold onto all of its water, while Venus apparently lost all of its water?"
Rabbette and her colleagues think they have an answer, and it comes from studying an unusually warm pool of water in the Pacific Ocean northeast of Australia, as well as the atmosphere above it.
At sea surface temperatures above 80 Fahrenheit (27 C), evaporation loads the atmosphere with a critical amount of water vapor, the researchers say. Water vapor is one of the most efficient so-called greenhouse gases.
The vapor allows solar radiation from the sun to pass through, but it absorbs a large portion of the infrared radiation coming from the Earth.
If enough water vapor enters the troposphere, a layer of the atmosphere where lots of the planet's weather is generated and controlled, it will trap thermal energy coming from the Earth, increasing the sea surface temperature even further.
The effect should result in a chain-reaction loop where sea surface temperature increases, leading to increased atmospheric water vapor that leads to more trapped thermal energy, the researchers say, based on computer models created with the data they collected.
This would cause the temperature increase to 'run away,' causing more and more water loss through evaporation from the ocean. This runaway greenhouse, as it has been called, was long ago put forth as a possible explanation for Venus' current hot state, in which temperatures are higher than would be expected simply based on the planets proximity to the Sun.
Some have suggested that Venus is a model for what could happen on Earth if human-caused greenhouse gases are not controlled. For now, however, a controlling factor seems to keep the same thing from happening on Earth.
Here, sea surface temperatures never reach more than about 87 Fahrenheit (30.5 C), and so the runaway phenomenon does not occur.
But the scientists are not sure why it does not occur.
"What is limiting this effect over the warm pool of the Pacific?" asked Richard Young, another member of the research team.
Young said cloud cover may affect how much energy reaches or escapes Earth, or that the ocean and atmosphere may transport trapped energy away from the local hotspot. "If we can model the outgoing energy flux, then maybe we can begin to understand what limits sea surface temperature on Earth," he said.
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