Astronomers Saw!
In the late 1800's astronomers were focusing their telescopes and attention to Mars. Astronomers such as:  Rev. Pietro Angelo Secchi, (1818-78);  Giovanni Virginio Schiaparelli, (1835-1910); Percival Lowell, (1855-1916); William Henry Pickering, (1858-1938); Andrew Ellicott Douglass, (1867-1962); Edward Emerson Barnard, (1857-1923); William Wallace Campbell, (1862-1938); William Huggins, (1824-1910); Camille Flammarion, (1842-1925); Eugène Michael Antoniadi, (1870-1944); Henri Perrotin, (1845-1904), Earl C. Slipher, (1883-1964); to list just a few. What the astronomers of the late 1800's were discovering was canals on Mars, first called canali by Rev. Secchi in 1858 and Giovanni Schiaparelli in 1877.  Many of these astronomers verified the canals and also noted seasonal changes on the surface that corresponded with the melting of the polar ice (Mars Orbiter Laser Altimeter (MOLA) Science Investigation 1, The Encyclopedia of Astrobiology, Astronomy, and Spaceflight 2 or Astronomy Picture of the Day Archive, NASA)3.  It seems at the end of 1800's early 1900's astronomers were recording canals, seasonal surface changes, theorized to be changes in possible plant growth and water dispersion.  "In 1888...Perrotin4 described having observed dramatic changes in a feature, named "Libya" by Schiaparelli, and assumed to be a continental land mass. "Clearly visible two years ago, it no longer exists today," and "By 1892, Perrotin5 had switched his attention to watching for "bright projections" on Mars, reporting three in the summer of that year". By 1909 no signs of the canals existed according to Antoniadi6.
In 1894 Percival Lowell 7 noted a tawny colored cloud of dust that extended over 300 miles and Percival Lowell from his research estimated the temperature on the surface of Mars to be an average of 48°F noted from the MOLA Science, NASA 8.  Percival Lowell's temperature was later verified by the Russian Mars 3 lander.  In 1971 the Mars 3 recorded temperature readings on the surface of Mars "the coldest point proved to be the north polar cap, where the temperature was -110°C (-166°F); elsewhere the values ranged from -93° to 13°C (-135°-55°F), depending on the latitude and time of day" (Sheeman, Chapter 12) 9.   According to a Mars data web page, "Recent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars' atmosphere now seems to be both colder and dryer than measured by the Viking landers", (SEDS, The Nine Planets, Mars) 10.  The Space Telescope Science Institute (STSci) report, MARS: A Cooler, Clearer World 11, states the fact several times that the Mars surface temperature has changed since Viking landing missions, constantly dropping in temperature.  Fact, the Viking 1 landed on Mars on July 20, 1976, continuous recording data until Nov. 11, 1982, over six years of recordings.
Mariner 9 12 waited until the cloud storm of 1971 had subsided somewhat to activate the camera. A similar cloud storm was observed in 1956. At that time Mariner 9 13 recorded the canyon Valles Marineris whereas the canyon had never been viewed as a canyon prior to Mariner 9. The canals had been viewed, surface changes had been viewed but never a massive canyon.
According to Planetscapes, VIKING ORBITER VIEWS OF MARS, M.H. Carr, et. al., NASA SP-441 14:
-The canyons do not form a well-integrated drainage system
-Indicate downward subsidence of canyon floors along faults
-Impact craters, which are so numerous on other Martian terrains, are scare within Valles Marineris
-No evidence of flow of water has been found within Valles Marineris, although some channels on the adjacent upland are abruptly truncated by steep canyon walls
-Because these canyons are poorly linked with one another, and their floors not a regularly graded slope, they could not have formed as water drainage features.
In 1954 National Geographic 15 sponsored a Mars observation project at Lamount-Hussey Obervator, Bloemfontein, South Africa on Naval Hill an altitude of 4888 feet. From this National Geographic sponsored expedition 20,000 pictures were taken of Mars. The pictures themselves stand as their own interpretation.  The National Geographic article 16 resulting from this project was "New Light on the Changing Face of Mars, A Huge Green Area Almost the Size of Texas Appears in Photographs Made by National Geographic Society-Lowell Observatory Mars Expedition to South Africa", September 1955, pages 427 to 436 written by Dr. E.C. Slipher. The picture in the National Geographic article reveals the areas mentioned as blue-green seasonal change. September 1954, two years before the 1956 planet wide dust storm.
Various observers over those earlier years with differences in equipment, varying observation periods but still these anomalies on Mars were verified by other researchers.
4 -5
9 -12-13
15-16 The National Geographic, The Complete National Geographic 112 Years, "New Light on the Changing Face of Mars, A Huge Green Area Almost the Size of Texas Appears in Photographs Made by National Geographic Society-Lowell Observatory Mars Expedition to South Africa",  Dr. E.C. Slipher, 1955, pg 427-436.