As recently as 50 years or so ago it was believed that the surface of Mars was very smooth, or at most a gently undulating landscape.
Very little could be inferred about the climate, and it was suspected that certain dark areas on the surface of the planet might be some form of organic matter, perhaps even vegetation.
Then early in 1965 the spacecraft Mariner 4 skimmed past Mars at a distance of 6,000 miles. It sent back images which revised our notions of that other world, but they provided only a fleeting glimpse. Mariner 4 was followed in 1969 by Mariners 6 and 7, both of which added to our still meagre knowledge of our nearest neighbour.
But the real Martian breakthrough began just 30 years ago, on November 14th, 1971.
On that day Mariner 9 entered into orbit round the red planet, becoming the first artificial satellite of another body in the solar system. It was to transform our notions about Mars.
The career of Mariner 9 began with a surprise. As the spacecraft approached its target a small swirl of what seemed like cloud over the planet's dusty surface grew and grew, until it became a giant dust storm.
It covered the entire planet, and hid the surface from the spacecraft's view for a full month. But when the dust had cleared, Mariner 9 proceeded to reveal a planet very different from the kind expected, a planet that boasted gigantic volcanoes, one of them three times higher than Mount Everest, and a grand canyon stretching for 3,000 miles across the Martian landscape.
More surprisingly, the images clearly showed what could well be giant river valleys, or canyons channelled out by floods.
But since the spacecraft also told us that the average temperature on Mars was something like minus 50 degrees Celsius, it was obvious that they were not of recent origin.
The images continued to arrive regularly for nearly a year, more than 7,000 of them altogether, covering the entire Martian surface, and providing the first full photographic atlas of another celestial body.
After nearly 700 orbits, two per day and approaching to within 900 miles of the Martian surface, the spacecraft was shut down in October 1972, by which time it had opened a new window for us on the solar system.
It was the 20th century equivalent of the moment immortalised by Keats:
"Then felt I like some watcher of the skies/When a new planet swims into his ken;/Or like stout Cortez, when with eagle eyes/He stared at the Pacific/Silent upon a peak in Darien."