On the night of 3 July 2003, the Mars Express spacecraft was pointed backwards to obtain a view of the Earth-Moon system from a distance of 8 million kilometres while on its way to Mars. During a series of instrument tests, the OMEGA spectrometer on board Mars Express acquired 'spectra' of the Earth and the Moon, in visible and near-infrared light. This particular spectrum corresponds to the entire Earth's illuminated crescent, dominated by the Pacific Ocean, and indicates the molecular composition of the atmosphere, the ocean, and some continents. As the peaks in the image indicate, water (H2O) and carbon dioxide (CO2) dominate. Molecular oxygen (O2) is also identified, as well as ozone (O3), methane (CH4) and several other minor constituents. During the observations, the Earth rotated so as to offer a varying observed surface and atmospheric composition. These Earth observations by OMEGA have several unique features. In fact, OMEGA provided a global view of the Earth's disc from a high-phase angle, contrary to low-orbit observations by previous space missions. Such global disc spectra are useful not only for observations at Mars, but also to prepare future observations of Earth-like planets, such as for the Darwin mission.

Credits: ESA/Institut d'Astrophysique Spatiale (Orsay, France)