Mars 1971

Telescopic views of Mars (above) show the varying size of the south polar cap.

The Southern Hemisphere of Mars experiences late spring and summer when the planet is at (or near) perihelion. In other words, in the south, the warmth resulting from being tilted toward the sun is compounded by the planet's relative nearness to the sun.  Relatively warm conditions, and the midnight sun, causes the polar cap to shrink rapidly. The sublimation of frozen CO2 leads to high pressure in the south polar region. The gas shoots northward to fill the partial vacuum of the rest of the planet. High wind velocity, coupled with the low gravity of Mars, stirs up great quantities of dust, leading to the legendary dust storms.

In 1971, Mars was at perihelion on September 8. Not surprisingly, a dust storm broke out around then, and soon encompassed the entire planet. This was disadvantageous for the US Mariner 9 spacecraft. Launched in May 1971, it began orbiting Mars in November for the purpose of mapping that world. Initially, a dusty atmosphere obscured the Martian surface.

The dust gradually settled, revealing many interesting features on Mars, including its huge volcanoes.

First seen in 1971, Nix Olympica is the largest volcano in the solar system. Mars also has the Vallis Marineris, a big canyon complex named after the spacecraft which detected it.