As time went on, this belief dimmed. Nevertheless, until the '60's many astronomers believed that seasonal changes in the surface markings could be due to changing vegetation patterns. All of these beliefs changed dramatically with discoveries from the Mariner and Viking missions. The Viking missions provided the most comprehensive and detailed information we presently have about Mars. Mars has global asymmetry, with most of its younger surfaces on one hemisphere and most of the older surfaces on the other.
Volcanism appears to have occurred throughout the planets' history, possibly until the present.
The surface has also preserved a long record of impacts, despite having been extensively modified by wind, water, and ice. The results of these effects have been surprisingly different to the effects of them on Earth. Huge volcanoes have accumulated, extensive fault systems disrupt the surface, and vast canyons have formed. Mars differs greatly to Earth in that it is smaller in size, its lack of tectonism, and its climate. Mars has a radius of 3,393 km, compared with 6,378 km for Earth.