Mars From Earth
Photo by Brian H. Day, Fremont Peak Observatory
Astronomers have long looked for geologic features on Mars, making drawings of light and dark areas, noting changes, and speculating on possible scenarios for what they believed they saw. During what appeared to be Martian dust storms in 1924 and 1956, astronomers noted a dark spot within the obscuring clouds, possibly a protruding tall object. When the dust abated, this feature appeared as a bright area along with three more spots over which an atmospheric brightening called the "W-cloud" often occurred. Visiting spacecraft, Mariners 4, 6, and 7, however, saw only a cratered surface much like that of the Moon.
Mariner 8 and 9 were sent to orbit and map the planet, one from polar orbit
the other equatorial, but Mariner 8 failed en route. Mariner 9's mission planners then changed its orbital inclination to cover as much of Mars' surface as possible. It achieved orbit Nov. 14, 1971 in the middle of another Martian dust storm and for the first few months saw only indistinct features which created dust eddies downwind of their obscured objects. Pictures of this atmospheric disturbance taken along the limb of the planet indicated that these clouds reached 70 km high.
As the storm abated, four dark spots in a T-shaped pattern appeared as if islands in the surrounding dust sea. These protrusions became enormous shield volcanoes rising from the Amazonis Planitia. Olympus Mons, the largest of the volcanoes, at 29 km high and 500 km across, is three times larger than Mauna Loa in Hawaii. The other three peaks lie on the 3 to 5 km high Tharsis Ridge. Ascraeus Mons, Pavonis Mons and Arsia Mons, the Tharsis Montes, are each 400 km wide and because of their location on the ridge. almost as high as Olympus Mons which rests on the ridge's western flank.
Central Caldera of Olympus Mons
Mariner 9 photo
Other volcanic features mapped by Mariner include many small, simple domes called tholi which dot tile vast plain called Amazonis Planitia. The Planitia appeared to be an extensive lava mare with flow patterns and scarps similar to Lunar mare features and Earth's basaltic flows. Some of these flows appear to have buried many craters at least partially, which would indicate the flows were relatively recent in Mars' geologic history.
Mariner 9 had accomplished a great deal when the spacecraft ran out of fuel and its mission came to an end Oct. 27, 1972. The mission raised more questions than it answered but the spacecraft had sent a great deal of information as well to aid in the planning of the Viking missions to follow (NASA, 1974).
Viking Orbiters 1 and 2 launched in 1975, provided the data for a mosaic map of most of Mars' surface. This data is the source of all planetary images used in this paper unless otherwise noted (TRW, 1991).
Content assembled by Brian H. Day, Margaret McCrary, and Bill Frye.
Web Development by Brian H. Day.
Responsible NASA Official: Donald James
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