The European Mars Express orbiter has been looking beneath the surface of Mars, at a particularly odd area known as the Medusae Fossae. The Medusae Fossae roughly forms the divide between lowland and highland regions along the Martian equator and has long intrigued geologists because the material it is composed of has been seen to absorb radar waves, leading to them been referred to as called "stealth" regions. Until now, no one has been sure how thick these deposits are, or what they might be composed of. The only reasonable certainty is that the material is relatively new, at least in a geologic sense, as there is little sign of disturbance by impact craters. Now the Mars Express Orbiter, which uses longer wavelengths than Earth-based radar experiments, has been able to make some intriguing observations.
The material it turns out is in places up to 2.5 kilometres (1.4 miles) thick in places, but there is still some uncertainty as to their composition. They could be volcanic ash deposits from now-buried vents or nearby volcanoes, or perhaps deposits of wind-blown materials eroded from Martian rocks. Most excitingly, they could be ice-rich deposits, somewhat similar to the layered ice deposits at the poles of the planet, but formed when the spin axis of Mars tilts over, making the equatorial region colder. Unfortunately, this later scenario seems the most unlikely, as the water vapour pressure on Mars is so low that any ice near the surface would quickly evaporate. The electrical properties of the layers suggest that they could be poorly packed, fluffy, dusty material, but this also has its detractors, since if it hard to understand how 2.5 kilometres of dust could retain such a lose composition. So the mystery of the Medusae Fossae endures, but we are step closer to understanding it.
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