An extraterrestrial spacecraft lurking in a satellite's orbit near Earth would be able to see city lights and pollution in our atmosphere. But what if it searched for signs of life on Earth from afar?
This question has great pertinence to those searching for other Earths outside of our solar system. NASA's Kepler space telescope is among a fleet of telescopes and spacecraft searching for rocky planets similar to our own. Once the size and location of these worlds are plotted, the next step is examining the chemical composition of their atmospheres.
From afar, Earth-like worlds appear as tiny points of light, making it hard to imagine ever finding out much about them. The best we can do with telescope technology at the moment is to examine some atmospheric components of worlds that are larger than Jupiter. But that doesn't mean we should discount the possibility of ever finding a planet similar in size to our own, researchers say. Telescopes are only getting more powerful. [The Search for Another Earth (Video)]
"We’re trying to think about how to use observations of the Earth itself to understand the kinds of things we’ll be able to do in the future with possibly the next generation of telescopes," said Robert Fosbury, an emeritus astronomer with the European Southern Observatory (ESO) who participated in the research.
Fosbury and leading researcher Fei Yan, an astronomer with ESO and the University of Chinese Academy of Sciences, examined the shadow of the Earth during a lunar eclipse. While there is no facility at ESO that is dedicated to astrobiology, Fosbury said the researchers are thinking closely about the implications for life beyond Earth.
The paper, "High resolution transmission spectrum of the Earth’s atmosphere: Seeing Earth as an exoplanet using a lunar eclipse," is available on the pre-publishing site Arxiv and has been accepted in the International Journal of Astrobiology.
Shadow glance
Observations took place during a total lunar eclipse on Dec. 10, 2011. A lunar eclipse appears as the Earth moves between the moon and the sun, and is visible anywhere the sky is dark and clear with the moon above the horizon.
A lunar eclipse is easier to observe than a total solar eclipse, which appears when the moon passes between the Earth and the sun. During a solar eclipse, the moon's shadow is so small that it creates a brief few minutes of totality and a small "track" of shadow visible from the Earth's surface. [Solar Eclipses: An Observer's Guide (Infographic)]
In this study, the researchers made observations with the High Resolution Spectrograph mounted on a 2.16-meter telescope at Xinglong Station, China, and focused the telescope near the moon's Tycho Crater because that is where the moon has high reflectivity.
The researchers hoped to learn more about the Earth's spectrum, which is shown in the moon's reflection. A spectrum is the band of colors that makes up visible light, and is most readily recognized in a rainbow. Certain elements preferentially emit certain wavelengths of light, and absorb others. By using a spectrograph to examine another planet, for example, you can see what atoms or molecules are present in its atmosphere or surface.
Watching the Earth's light reflected by the moon is similar to watching an exoplanet transit across the face of its parent star, the astronomers said. In both cases, finding the absorbing molecules in the atmosphere is a process of subtraction. In the case of an exoplanet, astronomers compare the molecular absorptions in the starlight during and after the transit. In the case of the moon, astronomers compared the elements found in the Earth's shadow, and when the moon was clear of the shadow.
During the eclipse, the science team took spectra when the moon was in the shadow (umbra) of the Earth. The moon turns red during this time because most of the light you see is a refraction of sunlight through the Earth's atmosphere (it's all the sunsets and sunrises on the Earth seen at once). The scientists also compared the spectrum of the moon when it was completely out of the shadow.
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