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Earth light: Terrestrial vegetation detected in the spectrum of the earthshine

January 23, 2002

A team including Pierre Riaud and Jean Schneider of the Observatoire de Paris and Luc Arnold, Sophie Gillet and Olivier Lardie're of the Observatoire de Haute Provence detected for the first time the color characteristic of the terrestrial vegetation in the "Earthshine", i.e. the dark part of the Moon only hit by the Earth light. To observe the light of the Earth by reflexion on the Moon enables to characterize the aspect of our planet as seen by a remote observer. It prepares the detection of extra-solar planets similar to the Earth.

The observations were made from April 2001 (and some are still being done) at the 80 cm telescope of the OHP. At the time of the observations reported here (last December) in the morning the Moon sees Asia and in the evening it sees America (Northern and Southern). Similar observations were made at the Steward Observatory (Arizona) from June 2001.

Thanks to observations, a simulation of the image of the Earth seen by the Moon was obtained for the first time, in the evening of December 20, 2001. The color of the Earth by reflexion on the Moon is characterized by an abrupt increase in intensity in the spectrum of the earthshine, starting from 750 nm in wavelength. The increase longwards of 750 nm is characteristic of the average spectrum of the terrestrial vegetation and is interpreted as being a reflexion of vegetation color on the Moon. The long-term purpose of these observations is to test the detectability of the vegetation on the extra-solar planets by the space missions of the DARWIN type, modified for observations in visible light, in project for years 2015.

Why not be satisfied with the direct observation of the terrestrial vegetation seen from satellites of the SPOT type or others? Because those do not see the Earth from a remote enough point, thus do not have an instantaneous global vision of it. Moreover they have only a quasi vertical vision of the terrestrial ground and thus do not take account of all angles of sight, and of the effects of absorption due to the oblique crossing of the atmospheric layers. So the conditions of observation similar to that of a remote extra-solar planet are not met. On the contrary, the Moon being far enough from the Earth, it sees it overall; moreover the roughness of its surface makes that it reflects in all the directions the terrestrial light, thus mixing the luminous rays resulting from all the terrestrial areas, which makes it possible to have the average color of the Earth.

Moreover, the Earth is the only planet of the solar system to have the blue color characteristic of its atmospheric scattering; the atmosphere of Mars is too tenuous and that of Venus too opaque to have this blue color. This color will thus be a simple, but invaluable indicator for the studies of exobiology, of the state of the atmosphere of an extra-solar planet.

Observatoire de Paris and CNRS