That planet has a considerable but moderate atmosphere. So that the inhabitants probably enjoy a situation in many respects similar to ours.

— William Herschel, The Gentlemans's Magazine and Historical Chronicle, 1784.

The present inhabitation of Mars be a race superior to ours is very probable.

— Camille Flammarion, French astronomer and founder of the French Astronomical Society, La plan — te Mars et ses conditions d'habitabilit — , 1892.

Sketches of Mars by Huygens, (L to R) 1659, 1672 & 1683.

It is well to fetter the wings of our fancy and restrain its flights. It is quite possible we may have formed entirely erroneous ideas of what we actually see. The greenish gray patches may not be seas at all, nor the ruddy continents, solid land. Neither may the obscuring patches be clouds of vapor. Man is too quick at forming conclusions. Let him but indistinctly see a thing, or even be undecided as to whether he does actually see it and he will then and there set himself to theorizing, and build immense castles of conjecture on a foundation, of whose existence he is by no means certain.

— Edward Emerson Barnard, Mars: His Moons and His Heavens, an unpublished manuscript in the Vanderbilt University Archives, 1880.

Speculation has been singularly fruitful as to what these markings on our next to nearest neighbor in space may mean. Each astronomer holds a different pet theory on the subject, and pooh-poohs those of all the others. Nevertheless, the most self-evident explanation from the markings themselves is probably the true one; namely, that in them we are looking upon the result of the work of some sort of intelligent beings… . The amazing blue network on Mars hints that one planet besides our own is actually inhabited now.

— Percival Lowell, address to the Boston Scientific Society, printed in the Boston Commonwealth. This was before he want to Flagstaff and viewed Schiaparelli's network of canali for himself. 22 May 1894.

Are physical forces alone at work there, or has evolution begotten something more complex, something not akin to what we know on Earth as life? It is in this that lies the peculiar interest of Mars.

— Percival Lowell, Mars, 1895.

Irrigation, unscientifically conducted, would not give us such truly wonderful mathematical fitness [as we observe in the Martian canals]… . A mind of no mean order would seem to have presided over the system we see — a mind certainly of considerably more comprehensiveness than that which presides over the various department of our own public works.

— Percival Lowell, c. 1908.

Mars, therefore, is not only uninhabited by intelligent beings such as Mr. Lowell postulates, but is absolutely uninhabitable.

— Alfred Russel Wallace, last sentence of his book Is Mars Habitable, 1907.

There are celestial sights more dazzling, spectacles that inspire more awe, but to the thoughtful observer who is privileged to see them well, there is nothing in the sky so profoundly impressive as the canals of Mars. Fine lines and little gossamer filaments only, cobwebbing the face of the Martian disk, but threads to draw one’s mind after them across the millions of miles of intervening void.

— Percival Lowell, Mars as the Abode of Life, 1908.

[scientists have] discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the center of the primary planet exactly three of its diameters, and the outermost five; the former revolves in the space of ten hours, and the latter in twenty one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the center of Mars; which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies.

— Jonathan Swift, Gulliver's Travels, the moons of Mars could not be observed at this time, 1726.

The logistic requirements for a large, elaborate mission to Mars are no greater that those for a minor military operation extending over a limited theatre of war.

— Wernher von Braun, The Mars Project, written in German in 1948, published as Das Marsprokekt in 1952, first English edition published in 1953.

In 1492 Columbus knew less about the far Atlantic than we do about the heavens, yet he chose not to sail with a flotilla of less than three ships… . So it is with interplanetary exploration: it must be done on the grand scale.

— Wernher von Braun, The Mars Project, written in German in 1948, published as Das Marsprokekt in 1952, first English edition published in 1953.

Today we haved touched Mars. There is life on Mars, and it us us — extensions of our eyes in all directions, extensions of our mind, extensions of our heart and soul have touched Mars today. That's the message to look for there: We are on Mars. We are the Martians!

— Ray Bradbury, science fiction author, speaking at The Search for Life in our Solar System, a symposium at the Jet Propulsion Laboratory, Pasadena, California, 8 October 1976.

We have your satellite if you want it back send 20 billion in Martian money. No funny business or you will never see it again.

— Reportedly seen on a wall in a hall at NASA's Jet Propulsion Laboratory, Pasadena, California, after losing contact with the Mars Polar Lander, 1999.

If a dog had shit on the ground one meter from a Viking lander, it would never have detected it.

— Fraser Fanale, regards the Viking 1 technology, NASA meeting on possible Martian biology, 1997.

We have concluded that the rocks here were once soaked in liquid water. It changed their texture, and it changed their chemistry. We've been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion.

— Steven Squyres, science team leader for the Mars Rover mission, news conference at NASA headquarters in Washington D.C., 2 March 2004.

We are all … children of this universe. Not just Earth, or Mars, or this system, but the whole grand fireworks. And if we are interested in Mars at all, it is only because we wonder over our past and worry terribly about our possible future.

— Ray Bradbury, Mars and the Mind of Man, 1973.

‎If it's a new planet, sign me up. I’m tired of driving around the block, boldly going where hundreds have gone before in orbit around earth — give me a place to go and I’ll go.

— Neil deGrasse Tyson, astrophysicist and director of the Hayden Planetarium, regards a manned mission to Mars. TV interview on The Daily Show With Jon Stewart, 18 January 2011

It was obvious to me that we could never colonize Mars without reusability, any more than America would have been colonized if they had to burn the ships after every trip.

— Elon Musk, SpaceX. Quoted in Fortune magazine, 9 December 2013.

It’s a fixer-upper of a planet but we could make it work.

— Elon Musk, SpaceX, regards Mars. Speaking at the D11 conference, 29 May 2013.

The first human beings to land on Mars should not come back to Earth. They should be the beginning of a build-up of a colony/settlement, I call it a 'permanence'.

— Buzz Aldrin, Reddit interview, 8 July 2014.

It might be helpful to realize, that very probably the parents of the first native born Martians are alive today.

— Harrison 'Jack' Schmitt, Apollo 17 moonwaker.