We continue our discussion of Mars Express, focusing now on the instrumentation and various discoveries that have been made.
OMEGA is a mineralogical mapping spectrometer that observes in both visible and infrared light. It gives spectra of the surface and the atmosphere between 0.35 and 5.2 microns (µm) with a ground resolution varying from 350 m to 10 km. Such observations allow mapping of the main minerals, which has allowed a map to be created of the surface composition in 100 meter squares. The spectral resolution (from 13 to 20 nm) is also sufficient to study atmospheric phenomena, such as the abundance and variability of minor components, aerosols, etc.
OMEGA was developed by the Institut d'Astrophysique Spatiale (IAS; Orsay, France) and the Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA) (affiliated with l'Observatoire de Paris), with the support of the Centre National d’Etudes Spatiales (CNES; the French space agency) and the participation of Istituto di Fisica dello Spazio Interplanetario (IFSI; Rome, Italy) and the Space Research Institute (IKI; Moscow, Russia). The instrument was originally developed for the Russian mission Mars-96, which was lost during the launch in November 1996.
The above image is a map of Mars' south pole, as derived from OMEGA infrared spectral images. Astronomers have known for years that Mars possessed polar ice caps, but early attempts at chemical analysis suggested that the northern cap was composed of water ice while the southern cap was composed of carbon dioxide ice ("dry ice"). Recent space missions suggested that the southern ice cap, which exists year round, could be a mixture of water and carbon dioxide. OMEGA has confirmed that this is indeed the case.
OMEGA measured the amounts of sunlight and heat reflected from the Martian polar region. The results showed that hundreds of square kilometres of "permafrost" surround the south pole. Permafrost is water ice, mixed into the soil of Mars, and frozen to the hardness of solid rock by the low Martian temperatures. This is the reason why water ice has been hidden from detection until now - because the soil with which it is mixed cannot reflect light easily and so it appears dark.
The south polar region of Mars can now be split into three separate parts. Part one is the bright polar cap itself, a mixture of 85% highly reflective carbon dioxide ice and 15% water ice. The second part comprises steep slopes known as "scarps," made almost entirely of water ice, that fall away from the polar cap to the surrounding plains. The third part was unexpected and encompasses the vast permafrost fields that stretch for tens of kilometres away from the scarps.
The OMEGA observations were made between 18 January and 11 February of 2004, when it was late summer for the Martian southern hemisphere and temperatures would be at their highest (approximately -130° Celsius).
In the above image, the polar cap, which is rich in carbon dioxide, is colored light pink. The water-rich ice, free of carbon dioxide, is colored green to blue.