The landing site, picked from a list of four potential locations, was chosen for its mysterious martian polar layered terrain which scientists think may hold secrets about the climate and surface evolution of the planet. It is very different from the ground that covers the planet's mid-latitudes.

Observed only at the poles, this rough undulating terrain is composed of layers of compressed dust, sand and water ice, said project scientist Richard Zurek, of NASA's Jet Propulsion Laboratory.

The built-up bands of surface material suggests that "there has been repeated, frequent climate change in the recent geologic history of Mars," Zurek said. He explained that the study of these layers could teach scientists about the history of martian climate much the same way tree rings reveal a great deal about climate and biology on Earth.

Scientists believe the surfaces of the martian north and south poles are relatively young. These areas appear to be free of craters that pock the rest of the planet's surface.

However, this does not mean they are smooth. The layered features seem to have been built up or eroded irregularly over hundreds of thousands, perhaps millions of years. The terrain varies dramatically throughout the polar regions. Some areas are rolling mounds of gently sloping hills, other regions are pocked with king-size potholes, while jagged hillocks and cliffs mark other places.

The landing site announced today was relatively flat for the region, with an overall slope no steeper than 10 degrees, Zurek said.

After landing, scientists hope to get busy digging through the surface -- provided the lander doesn't alight on permafrost.

The Mars Polar Lander has a robotic arm that can trench up to two feet below the surface, excavating the layers and analyzing the chemical composition of martian material as it digs. The end of the arm carries a microscopic camera that can zoom in on material as the shovel-like arm scratches through the ground. It can examine minute dirt or ice particles.

The main camera on the polar lander is a copy of the stereoscopic imager that the Mars Pathfinder carried to Mars in July 1997. The craft also has a descent imager that will take pictures of the ground as it falls to the ground.

Parachutes will slow the spacecraft's fall after it enters the martian atmosphere, then thrusters will bring it to a near standstill before it plops to the ground at about 5 mph.