Engineers designed Mariner 9 to provide the most complete view of Mars ever obtained. Launched on May 30, 1971, it became the first spacecraft to orbit another planet.
Scientists designed Mariner 9 to return more data on atmospheric structure, composition, density, and pressure, but equally important, mission goals included mapping over 70% of the Martian surface. Using infrared radiometry, the spacecraft could also look for signs of volcanic activity as inferred by any heat anomalies found on the surface. Engineers also designed Mariner 9 to study Mars’ two moons, Phobos and Deimos. As an orbiter, Mariner 9 could refine the findings of previous missions and take away the veil of uncertainty which had so far concealed most of the Martian surface. Ironically, when Mariner 9 arrived, a global dust storm obscured the surface from view for about a month. With the dust clearing, Mariner 9 revealed a planet much more interesting than previously expected from the limited views provided by prior flyby missions....
Here is the striking image of Olympus Mons, a giant volcano 22 kilometers high and 600 km wide - about the size of the state of Arizona. One reason volcanos are important is that they can contribute to the development and evolution of a rocky planet’s atmosphere.
Ice & Clouds
Water on Mars exists as ice in the polar caps, as ice or possibly in liquid form beneath the surface, or as scant water vapor in the atmosphere. Here is an image of the north polar cap, which Mariner 9 scientists determined is comprised of water ice and frozen carbon dioxide thanks to spectroscopy. Gray swirls represent frost-free areas. A huge field of dark dunes surrounds the cap. Images like this one indicate a dynamic climate on Mars.
Moisture in the atmosphere condenses when it is lifted high up into the atmosphere. Here we can see clouds forming around volcanos in the Tharsis region. By analyzing the spectral lines, Mariner 9 scientists determined that the clouds are comprised of mostly water crystals.
Past evidence of running water
Mariner 9 gave us an entirely new perspective about the history of Mars. Its images showed ancient river beds and many other flow features indicating free-flowing water on its surface in the ancient past. Mariner 9 provided clear evidence of channels, which looked like they’d been cut by running water through similar processes as we see on Earth.
Valles Marineris is 4000 km (2500 mi) long and reaches depths of up to 7 km (4 mi)! For comparison, the Grand Canyon in Arizona is about 800 km (500 mi) long and 1.6 km (1 mi) deep. If on Earth, this canyon system would stretch from San Francisco to New York.
Below is Mariner 9's view of the "labyrinth" at the western end of Vallis Marineris. Grooves, and crater chains dominate this region, along with a number of flat-topped mesas. The image is roughly 400 km across.
Evidence provided by the channel-like features suggests Mars once had a much thicker atmosphere, warmer conditions, and a lot more water on the surface than we see today. Mariner 9 atmospheric experiments showed current atmospheric pressures ranging from 2.8 to 10.3 mbar (water would rapidly evaporate at this temperature). With evidence of flow features, and therefore the possibility of a time when water was in liquid form on the surface of Mars, scientific interest in the question of the existence of past or present life on Mars intensified.
This image shows upclose, visual evidence of a thin atmosphere. It was thought to be comprised mainly of carbon dioxide crystals:
Another exciting thing we discovered by being in a long-term orbit around the planet: things were changing before our eyes. We saw high winds (up to 200 mph), and seasonal changes due to weather that corrected the assumption of early observers that these variations could have been due to vegetation. The image below shows dust storms which can carry micron-size particles high into Mars' atmosphere. The low-density atmosphere requires high wind speeds for the movement of particles on Mars.
Mapping of Mars
After 349 days in orbit, Mariner 9 had transmitted 7,329 images, covering 100% of Mars' surface. These images were manually combined to form a full map:
The results of the Mariner 9 mission paved the way for the Viking program, a set of two orbiters and the first two landers. Mariner 9 confirmation of atmospheric pressure allowed engineers to design the Viking landers for a safe descent, and the fantastic images and data retrieved fueled the fire for further Mars exploration.
Analysis of findings
For more details about the Mariner 9 mission, we have a video below from NASA's archives which covers all of the findings mentioned above and more: