Still, of 29 previous missions launched to Mars over the last four decades by the United States, the former Soviet Union, Russia and Japan, fewer than a third can claim full success. But among the most recent was NASA's successful Mars Pathfinder mission. Layered accumulations of dust and ice in the barren, windswept north and south polar regions of Mars began to pique the attention of scientists more than 25 years ago when images were transmitted to Earth by the U. S. Mariner 9 orbiter. Among Mariner 9's revelations was a dust storm, reversing a prevailing notion that there was little dynamic about the Martian weather. Fueled by findings from bygone missions and those of NASA's camera-laden Mars Global Surveyor, which began orbiting the planet in 1997, scientists say the region may hold proof that the Martian poles function as geologic sponges, retaining frozen reservoirs of underground water.
Some experts say water once flowed and pooled on the now arid surface of Mars.
According to a competing theory, the water is released periodically in response to warming events linked to Mars' orbital track and subtle changes in the tilt of the planet's axis. Both factors could alter the amount of sunlight and heat reaching the polar caps. The first big question is whether there is water present. If so, that really strengthens the arguments that the water that was present early in Mars' history on the surface is still on the planet, it's just under the surface. The finding would also bolster the 1996 claims of a small team of NASA scientists that a meteorite of Martian origin harbors the fossilized remnants of microbial life.