Artwork depicting Mars during its Hesperian Period, roughly 3700-3000 million years ago. The period is characterised by extreme volcanism which led to catastrophic flooding. These latter helped carve the extensive outflow channels seen on Mars - wide, long surface features resembling huge rivers and deltas (top right). The canyon Valles Marineris (centre right) was formed during this period, as were the famous Tharsis volcanoes such as Olympus Mons (upper left in this image). This was also a transitional period for Mars, during which it lost much of its atmosphere and surface water, and became colder and dryer than during the previous Noachian Period.