Manned Mission to Mars. `Mars is . . . a world of wonders'-- Carl Sagan, astronomer

AS Cornell University astronomer Carl Sagan notes, ``Mars is . . . a world of wonders.'' Volcanic mountains soar two to three times higher than the highest structures on Earth. A great canyon 2,800 miles long and three times deeper than Arizona's Grand Canyon may have the Red Planet's geological history ``written'' on its walls.

Stream beds and river valleys, along which liquid water once ran, reflect a different climatic regime than today's glacial desert. Mars appears to still have abundant water hidden away beneath its surface or locked in the polar caps. Spread as an ocean over the planet, there might be enough water to form a layer 300 to 600 feet deep.

The Martian atmosphere is very thin. Surface pressure is only about a hundredth of sea level pressure on Earth. Yet Mars has weather. At times, raging winds raise huge dust storms. Regular seasonal changes bring expansion and shrinking of the polar caps.

The atmosphere is 95.3 percent carbon dioxide -- which can form ``dry ice'' snow -- and 2.7 percent nitrogen, with traces of other gases including a little water vapor.

The fourth planet, Mars orbits 1.5 times farther from the Sun, on average, than does Earth. With a diameter of 4,218 miles, compared to 7,926 miles for Earth, it has only a tenth the mass and 38 percent of the surface gravity of our planet. A day on Mars is only 2 percent longer than on Earth. But the Martin year is 1.881 Earth years.

Mars has two tiny irregularly shaped moons -- Phobos (fear) and Deimos (dread) -- respectively measuring 11.2 miles by 13.7 miles and 7.5 miles by 8.1 miles in their shortest and longest diameters. They may be captured carbonaceous chondritic (containing organic compounds) asteroids.

Overall, Mars seems to be a kind of half-and-half world. Parts appear to be geologically active. Other parts seems more like the barren cratered surfaces of Mercury or the Moon.

Whether there is life on the planet is an open question. But the Viking landers found no sign of it.