Oct. 8, 2004 — The Mars rover Opportunity has found new evidence of past existence of water on the Red Planet, NASA said Thursday.

Opportunity keeps finding new clues of past water as it continues probing Mars, said mission scientists at the U.S. space agency's Jet Propulsion Laboratory in Pasadena, Calif.

The new discoveries increase the possibility that the area around the "Endurance" crater, which Opportunity explored, was once submerged before drying out.

Some rocks appear to have become wet a second time, possibly after a meteorite hit the planet, creating the crater, NASA said.

The flat rocks, dubbed "Escher," were found inside and outside Endurance, which is the size of a stadium. Networks of cracks crisscross the surface of these rocks, dividing them into patterns similar to cracked mud on Earth, NASA said.

Scientists cited possibilities for why the cracks formed. A meteor's crater could have fractured the rocks. Or, the original wet environment that formed the rocks dried up and left behind the polygonal cracks.

"When we saw these polygonal crack patterns, right away we thought of a secondary water event significantly later than the episode that created the rocks," said John Grotzinger, a rover-team geologist from the Massachusetts Institute of Technology in Cambridge, Mass.

"Did these cracks form after the crater was created? We don't really know yet," Grotzinger said.

But where did the water come from? Melting frost during climate changes, for one, as Mars wobbled on its axis over tens of thousands of years. Or, Grotzinger said, underground ice melted, or enough underground water was released to create a small lake within the crater.

Meanwhile, halfway around the globe, Spirit is climbing higher into the Columbia Hills, where it has found plenty of signs of rocks altered by water.

"We haven't seen a single unaltered volcanic rock since we crossed the boundary from the plains into the hills, and I'm beginning to suspect we never will," said Steve Squyres of Cornell University in Ithaca, N.Y., principal investigator for the science payload on both rovers.

"All the rocks in the hills have been altered significantly by water. We're having a wonderful time trying to work out exactly what happened here."

Layered rock outcrops above Spirit could hold more clues to what happened at Columbia Hills, Squyres said.

"Just as we worked our way deeper into the Endurance crater with Opportunity, we'll work our way higher and higher into the hills with Spirit, looking at layered rocks and constructing a plausible geologic history," Squyres said.

Opportunity and its twin rover Spirit landed on Mars early this year on a mission to search for signs of past life. Already well past their original 90-day missions, the rovers continue to do sound science through the Martian winter.

Jim Erickson, rover project manager at JPL, said in the press release: "Both Spirit and Opportunity have only minor problems, and there is really no way of knowing how much longer they will keep operating.

"However, we are optimistic about their conditions, and we have just been given a new lease on life for them, a six-month extended mission that began Oct. 1. The solar power situation is better than expected, but these machines are already well past their design life. While they're healthy, we'll keep them working as hard as possible."