October 14, 2008
Scientists using European Space Agency's (ESA) Venus Express are trying to observe whether Earth is habitable. Silly, you might think, when we know that Earth is richly stocked with life. In fact, far from being a pointless exercise, Venus Express is paving the way for an exciting new era in astronomy.
Venus Express took its first image of Earth with its Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) soon after its launch in November 2005. About a year after the spacecraft established itself in Venus's orbit, David Grinspoon, a Venus Express interdisciplinary scientist from the Denver Museum of Nature & Science, Colorado, suggested a program of sustained Earth observation.
"When Earth is in a good position, we observe it two or three times per month," says Giuseppe Piccioni, Venus Express VIRTIS co-principal investigator, at IASF-INAF, Rome. The instrument has now amassed approximately 40 images of Earth during the last 2 years.
The images of Earth cover both visible and near-infrared regions of the spectrum. Astronomers can split the images into spectra to search for the signature of molecules in Earth's atmosphere.
Earth spans less than a pixel in Venus Express cameras so it appears as a single dot with no visible surface details. This situation is something that astronomers expect to soon face in their quest for Earth-sized worlds around other stars.
"We want to know what can we discern about Earth's habitability based on such observations. Whatever we learn about Earth, we can then apply to the study of other worlds," Grinspoon said.
Since 1995, astronomers have discovered more than 300 of these extrasolar planets. As they've refined their observation techniques and taken data continuously, they've discovered smaller and smaller planets.
Now, with CNES-ESA's Convection Rotation and Planetary Transits (COROT) and NASA's Kepler missions, the prospect of discovering Earth-sized worlds in Earth-like orbits around other stars is better than ever. "We are now on the verge of finding Earth-like planets," Grinspoon said.
When astronomers find an earth-like planet, they apply innovative methods to separate the planet's feeble light from the star's overwhelming glare.
One thing has become obvious from the study of Earth using Venus Express: determining whether a planet is habitable is not going to be easy. "We see water and molecular oxygen in Earth's atmosphere, but Venus also shows these signatures. So looking at these molecules is not enough," Piccioni said.
Instead, astronomers are going to have to search for more subtle signals, perhaps the so-called red edge caused by photosynthetic life. "Green plants are bright in the near infrared," says Grinspoon. The analysis to see whether this red edge is visible is just beginning.
The team also will compare spectra of Earth's oceans with those taken when the continents face Venus Express. "We have initiated the first sustained program of Earth observation from a distant platform," Grinspoon said. Although the observations may not tell us anything new about Earth, they will allow us to unveil far-off worlds, making them seem more real than simply dots of light.