The funny thing about the discovery of water on Mars is that back in the late 19th century the Italian astronomer Giovanni Schiaparelli announced that he had discovered canals on Mars - a network of straight lines in the equatorial regions of the planet. His discovery was confirmed by other astronomers around the world.
Schiaparelli never claimed that there was water in these canals. He used the Italian word canali, which means channels - not necessarily water channels - but the word was widely translated into English as "canals", which, to English-speakers, meant channels of water.
The discovery of the Martian canals was hailed as a huge and important discovery, and was the subject of intense debate among scientists for more than a decade.
Maps of the Martian surface were published, emphasizing the canals and giving them names from mythology. By 1900, the existence of the Martian canals had been confirmed through observation by leading scientists and astronomers all over the world. Their reality was the scientific consensus.
All kinds of explanations were proposed for the canals. The fact that they were straight lines seemed to indicate that they were made by intelligent beings. The renowned astronomer Percival Lowell interpreted the canals to mean that there was an advanced civilization living on Mars, which, on the basis of "logical deduction", he described in great detail. He envisioned the Martian landscape to be divided up into continents, with great cities, transport routes and so on. The British naturalist Alfred Russel Wallace was moved to write a book rejecting Lowell's claims. Among other things he pointed out - correctly - that the atmospheric pressure on Mars was too low for liquid water to exist on the planet's surface.
In the early 1900s, as telescopes improved, it gradually became apparent that Schiaparelli's "canals" were an optical illusion.
It turned out that if you looked at the surface of a planet through a poor quality telescope, the dots on the surface - craters, pock marks, mountain ridges and so on - gave the illusion of being in straight lines. The mind of the observer, in other words, joined up the dots.
The "Martian Canals", which had been discussed and argued about endlessly, and which had been the subject of numerous books and articles in scientific journals, didn't actually exist.
Thus, the Martian Canals became one of most embarrassing episodes in the history of science, and thereafter astronomers were careful not to even mention the possibility that there might be water, let alone water channels, on Mars. Which is kind of ironic. Because now we know that there are, even if they are not the ones observed by thousands of astronomers at the turn of the 20th century.
The Martian canals they saw didn't exist. But the Martian canals they didn't see, did. I'm sure Schiaparelli would be kicking himself if he were here today.