What would Earth look like to alien astronomers? If they had access to telescopes far more powerful than our own, it might look a lot like what the Deep Impact spacecraft recently saw from its vantage point 50 million kilometres away.
Over the course of one Earth day in May, the spacecraft snapped images every 15 minutes to produce a movie of the Moon gliding in front of our home planet, whose swirling clouds and continents rotated in and out of view.
"Making a video of Earth from so far away helps the search for other life-bearing planets in the universe by giving insights into how a distant, Earth-like alien world would appear to us," says Michael A'Hearn of the University of Maryland, College Park.
Point of light
A'Hearn is the principal investigator for an extension of the Deep Impact mission, which sent an impactor into Comet Tempel 1 on 4 July 2005 and watched the debris fly. Now, in an extended mission called EPOXI, the spacecraft will search for extrasolar planets on its way to a flyby of a comet called Hartley 2.
The Moon has been seen 'transiting' the Earth previously by other spacecraft, but never in such good detail. "To image Earth in a similar fashion, an alien civilisation would need technology far beyond what Earthlings can even dream of building," says EPOXI team member Sara Seager of MIT.
Alien astronomers around the nearest stars, which lie a few light years away, would glimpse just a single point of light if they wielded telescopes like the ones terrestrial researchers are now planning.
But even that single point of light could yield valuable data. Previous research has shown that it would be possible to tell how fast the Earth is rotating, and make a rough map of its continents and oceans, by observing how that point of light changes over time.
"The video will help us connect a varying point of planetary light with underlying oceans, continents and clouds â?? and finding oceans on extrasolar planets means identifying potentially habitable worlds," Seager said in a statement.
Team member Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Maryland, agrees. "A 'Sun glint' can be seen in the movie, caused by light reflected from Earth's oceans, and similar glints to be observed from extrasolar planets could indicate alien oceans."
Also, the team used a near-infrared filter to make the video, which made the continents stand out more clearly from the bodies of water.
That's because plants and some microbes reflect near-infrared light â?? apparently because absorbing it would cause them to overheat during photosynthesis â?? and that causes land masses to appear bright at these wavelengths.