Twinkle, Twinkle, Alien Ocean

View of crescent Earth from space. A glint of sunlight (marked with yellow lines) appears just west of the Galapagos and South America. Photo

The phenomenon that causes a diamond to sparkle could be used to find large bodies of water on rocky, Earth-like planets, says Darren M. Williams, lead author of a paper in Icarus that describes the process.

The trick, he says, is to look for planets when they are in crescent phase, ideally in orbits that lie at an edge-on angle to Earth. In that position, the glare bouncing off the water would make the planet seem unusually bright.

"Crescent phase is where the starlight would be glancing off the edge of the planet toward our telescopes,” says Williams, an associate professor of physics and astronomy at Penn State Erie. “That would be when the light is coming at the surface at a very steep angle, and the specular reflection would be the strongest and most intense."

Williams ran simulations of idealized, cloud-free planets with three types of surfaces: unfrozen land, snow and ice, and water. His goal was to see to what extent the presence of water would contribute to the light coming from a planet in another solar system.

He found that for slightly tilted systems the reflection from water would be the most powerful signal, particularly if the planet were observed in crescent phase. When a star appears directly over a body of water, almost all the light is absorbed. But from a glancing angle, most of the light is reflected. The shape of the light curve would help distinguish Earth-like planets with water from those without.

This image of Earth was taken by Mars Global Surveyor, in orbit around Mars. Surface features and cloud cover can be distinguished over North America.
Credit: NASA/JPL/Malin Space Science Systems

More than meets the eye

Oceanic glint is shown as an unusually bright patch (yellow) on the dark ocean surface.
Credit: D.M. Williams, E. Gaidos.

At the University of Florida, Dr. Eric Ford has also been learning about how light can reveal the presence of ice, sand, vegetation and water. Asymmetry in a planet’s light curve over time could indicate major seasonal transformations or weather patterns caused by the ebb and flow of clouds.

Warmer, ice-free planets, for example, might experience a large blooming event of plants or oceanic algae, changing the way the planet reflected light. Scientists could detect those changes by looking for certain signatures in the planet’s colors and measuring their intensities. While fluctuations would signal some type of surface feature, scientists would need a lot more follow-up before they could say for sure what was causing it.

“It could be water and land, it could be some other liquids, particularly where it’s further out where you might have methane, like on Titan,” says Ford. But, he explains, scientists could rule out many alternatives using other methods.

“If we have an idea of how far away the planet is from the star, and how bright the star is, then maybe we could say methane would not be liquid at those temperatures,” for example. Scientists could narrow the possibilities by comparing their findings and might conclude the only logical explanation is the presence of water, “particularly if there was spectroscopic evidence of water vapor in the atmosphere,” says Ford.

Experiments from Interplanetary Space

The sparkle of sunlight may be described as “specular reflection,” a mirror-like reflection in which incoming light from a single direction is reflected in a single outgoing direction.
Photo Credit: Tammy Knight (2005)

Looking at Earth-sized planets beyond our own solar system will require advanced new telescopes using modern technology to its full potential. Two proposed efforts, NASA’s Terrestrial Planet Finder (TPF) and the European Space Agency’s (ESA’s) Darwin mission, will both have instruments capable of picking up the sparkle of an ocean.

The way those instruments are designed and built will be based, in part, on the results of what Williams sees of Earth from Mars and Venus.

Williams is using observing time on ESA’s Venus Express and Mars Express orbiters, to look at Earth from interplanetary space. From Mars, all of Earth’s phases can be seen, but from Venus, only Earth’s gibbous (more than half-full) and full phases are visible. Regardless, Williams can use the results to test his predictions.

"That’s the most exciting application from my standpoint,” he says. “The observation of extreme glint off the Earth’s oceans as seen from Mars Express."

Looking at terrestrial planets while they are in crescent phase, when only a small fraction of their disk is illuminated, will be extremely difficult, perhaps taking two or even three weeks of observing time. But, as Williams points out, of all the extremely difficult measurements astronomers plan to do, it might be one of the easiest.

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