ESA Science & Technology30-Jun-2005 10:56:13

Mars Express Analyses the Earth

SPICAM Detects Ozone

SPICAM/Mars Express detects strong quantities of ozone in the atmosphere of the Earth, allowing life on this planet.

On July, 3, 2003, a spectrum of Earth illuminated by solar light was recorded by the Ultra-Violet spectrometer SPICAM, on board Mars Express. Taken from a distance of 7 million km, the spectrum shows only some signal at 310 nanometers, and nothing at shorter wavelengths, though the instrument sees plenty of UV light from another star. This is what is expected if the atmosphere of the Earth contains ozone, a gaseous species made of 3 atoms of oxygen, known to provide a natural screen to harmful UV solar radiation.

"Together with the OMEGA infrared spectrometer detection of water vapour and oxygen, the detection of copious amounts of ozone in the atmosphere indicates that this planet that we call Earth could sustain life", says Jean-Loup Bertaux, a French CNRS scientist from Service d'AĆ©ronomie/IPSL, responsible for the SPICAM instrument.
The main scientific objective of SPICAM is to observe both ozone and water vapour in the atmosphere of Mars. But the quantity of ozone on Mars is about 100 times smaller than on Earth, and solar UV radiation reaches the ground of Mars with little attenuation only, making life difficult to persist at the surface of Mars under present conditions.

Blue line: The spectrum (intensity of light as a function of wavelength) of the Earth recorded by SPICAM in the Ultra-violet. This light comes from solar light scattered by the atmosphere of the Earth back to outer space. Only the peak just above 300 nm is significantly above noise.
In contrast, the spectrum of a star measured by SPICAM (red line), somewhat resembling the sun, shows an important signal above 200 nm. The absence of UV light in the Earth's spectrum is due to the presence of ozone in the atmosphere of the Earth, which absorbs UV photons below 300 nm.

The interpretation of SPICAM spectrum is completely supported by simulations of the effect of the atmosphere on the solar radiation which is scattered by the Earth (with MODTRAN software). Ozone absorbs all solar radiation between 200 and 300 nm, preventing harmful effects on the  DNA molecules of all living species exposed to the Sun.

Red line: Simulation of the solar light scattered by the atmosphere that would be seen by SPICAM if there were no atmospheric absorption. Blue line: simulation with a realistic Earth atmosphere containing ozone. The difference between the two spectra is due to the presence of the ozone layer in the upper atmosphere of the Earth.

This curve represents the attenuation factor (below unity) of the scattered solar radiation by ozone in the atmosphere of the Earth. It's obtained from the ratio of the two curves in the previous figure.

Credits: SPICAM is a dual UV-near IR spectrometer , designed and built by Service d'AĆ©ronomie (France, BIRA (Belgium), and IKI Space Research Institute (Russia), under the leadership of Jean-Loup Bertaux, Paul Simon and Oleg Korablev.

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