How to spot oceans on alien Earths

Sunlight scattered from Earth and reflected off the side of the Moon facing away from the Sun is known as Earthshine. The phenomenon has garnered considerable attention in recent years because it can help determine the Earth’s energy budget: take the amount of energy that the Earth reflects away from the total hitting us and you end up with the amount the Earth must absorb. The way that figure is changing is an important factor in climate change.

But exoplanet hunters are interested in Earthshine for another reason–it is a measure of the Earth’s brightness and therefore analogous to observations of an exoplanet.

But Earthshine is not constant over a 24 hour period because specular reflections from oceans are far stronger than those from land. So as the Earth rotates, its Earthshine varies dependng on whether sea or land is facing the Moon. At least, that’s the theory.

Now Sally Langford at the University of Melbourne in Australia and a couple of cobbers have measured this effect for the first time. They say they’ve been able to detect a drop in Earthsine as well as a reddening of the spectrum, as the Indian Ocean swings out of view giving way to Africa’s east coast.

The drop in Earthshine occurs as the ocean gives way to land and the reddening is presumably the result of the well known “red edge” associated with photosynthesis that astronomers have already hypothesised could be used to spot vegetation on other planets.

Now it looks as if they can use variations in the brightness of Earth-like planets to spot oceans too.

Ref: arxiv.org/abs/0904.0845: Photometric Variability in Earthshine Observations