Paris, Oct 11 : In a surprising move, scientists are using ESA's (European Space Agency's) Venus Express to observe the Earth's habitability, which might be helpful to search for life on other planets.
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 and Science, Colorado, suggested a programme of sustained Earth observation.
"When the Earth is in a good position, we observe it two or three times per month," said Giuseppe Piccioni, Venus Express VIRTIS Co-Principal Investigator, at IASF-INAF, Rome, Italy.
The instrument has now amassed approximately 40 images of Earth over the last two years.
The images of Earth cover both visible and near-infrared regions of the spectrum and can be split into spectra, in order to search for the signature of molecules in the Earth's atmosphere.
The value of the images lies in the fact that Earth spans less than a pixel in Venus Express's cameras. In other words, 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 the Earth's habitability based on such observations. Whatever we learn about Earth, we can then apply to the study of other worlds," said Grinspoon.
With CNES-ESA's 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," said Grinspoon.
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," said Piccioni.
Instead, astronomers are going to have to search for more subtle signals, perhaps the so-called red edge caused by photosynthetic life.
The team will also compare spectra of the Earth's oceans with those taken when the continents are facing Venus Express.
"We have initiated the first sustained programme of Earth observation from a distant platform," said Grinspoon. "Although the observations may not tell us anything new about the Earth, they will allow us to unveil far-off worlds, making them seem more real than simply dots of light," he added.