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Earthshine Could Reveal Oceans On Other Planets

April 15, 2009

Earthshine ““ sunlight reflected from the Earth toward the Moon ““ may provide a way for researchers to seek out oceans on other planets, according to researchers.

Last week, researchers from the University of Melbourne and Princeton University reported finding that the difference in reflection of light from the Earth’s landmasses and oceans can be seen on the dark side of the moon.

Sally Langford, from the University of Melbourne’s School of Physics, said the brightness being reflected varied as the Earth rotated, which showed how the light being reflected is dimmer from land and much stronger when being cast from water.

“In the future, astronomers hope to find planets like the Earth around other stars. However these planets will be too small to allow an image to be made of their surface,” said Langford, whose study appears in the international journal Astrobiology.

“We can use earthshine, together with our knowledge of the Earth’s surface to help interpret the physical make up of new planets.”

Langford worked alongside Edwin Turner of Princeton University to measure the earthshine reflected back toward the Earth using a small telescope and camera on three days out of the month when the moon would crescent.

Turner and colleagues first devised the concept of using earthshine to confirm the existence of other Earth-like planets in 2001.

Langford noted that the amount of light reflected was reduced by as much as 23 percent during an hour of observation, which points to dimming of earthshine through transition.

What’s more, she told BBC News, the spectrum of light changed over time, causing the reflection of the ocean to become “reddened” as land and plant life began to absorb the light.

“When we observe earthshine from the Moon in the early evening we see the bright reflection from the Indian Ocean, then as the Earth rotates the continent of Africa blocks this reflection, and the Moon becomes darker,” Ms Langford said.

“If we find Earth sized planets and watch their brightness as they rotate, we will be able to assess properties like the existence of land and oceans.”

Langford and Turner believe that the use of future space-based telescope missions such as NASA’s Terrestrial Planet Finder (TPF) or the European Space Agency’s Darwin to show dimming and reddening would indicate the presence of a landmass.

“I call it looking for exobeaches, rather than exoplanets,” Professor Turner told BBC News.

“Whether we could see this or not depends partly on luck – if the exoplanets are relatively nearby and are bright, if they rotate slowly – and different geometries are more helpful than others. There are many ways the luck factor could help or hurt, but you have to hope you’re lucky.

“It’s important nevertheless to have information like this because it drives the design of experiments like [TPF and Darwin]. We’re just at the beginning of what we hope will be a major, long-term research program.”

Image Caption: Moon lit by Earthshine captured by the 1994 lunar prospecting Clementine spacecraft. Clementine’s camera reveals (from right to left) the Moon lit by Earthshine, the Sun’s glare rising over the Moon’s dark limb, and the planets Saturn, Mars and Mercury (the three dots at lower left).

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