The study of climatology on Mars began with the Mariner 7 pictures, for some of them exhibited suggestions of clouds-a hint that was proved by later missions, which detected many clouds. To begin with, it had been fitted with a retro-rocket using hypergolic fuels to enable it to be placed in orbit round Mars, and two large propellant tanks had been added for the nitrogen tetroxide and monomethyl hydrazine. Geologists will be keen in the next century to get samples from these layers, for they will not only provide a record of the sequence of volcanic episodes on Mars, but they will also provide a means of dating these episodes by physical means, using ratios of radioactive isotopes. Several scientists have concluded that the channels have an extremely ancient provenance. None is younger than 100 million years old and the origin of many stretches back almost 4 billion years, close to the formation of Mars.