A calendar of important Mars observing dates is also enclosed. The dark surface markings were once thought by some astronomers to be great lakes, oceans, or vegetation, but space probes in the 1970's revealed the markings to be vast expanses of rock and dust.
Windstorms sometimes move the dust, resulting in both seasonal and long-term changes.
Among the areas where yearly variations have been recorded are Trivium-Elysium, Solis Lacus, Syrtis Major, and Sabaeus Meridiani. The Syrtis Major is the planet's most prominent dark area. It was subsequently found on Viking Orbiter photographs taken in 1975, apparently undetected by Viking scientists. This is an example of the importance of ground-based observations of the Solar System. Another feature that is of great interest to professional Mars researchers is the Trivium Cerberus, on the southern rim of the Elysium shield.