The two worlds, created at about the same time in the history of the solar system, and similar in size and composition, both started along the path to a livable environment. The findings by NASA's twin Mars exploration rovers this year have made that assumption clear.

Spirit and Opportunity have been exploring the surface of the red planet now about three times longer than their designers expected, and the more they look, the more evidence of past liquid water they discover.

New findings, for example, suggest the area Opportunity has been roving since late January, once contained quite a bit more than a smidgen of water. Long ago, before it dried out and softened into a wide, flat expanse, Meridiani Planum was soaking wet - which means temperatures on Mars, in some form, must have exceeded 32 degrees Fahrenheit (0 degrees Celsius) - the freezing point of water.

It could be that liquid water existed under a heavy layer of ice, something that happens now at Earth's south pole, where Vostok, a huge freshwater lake, lies hundreds of feet under a massive Antarctic glacier.

Another possibility is the Martian atmosphere once might have been much thicker than it is now, possibly containing enough carbon dioxide to sustain a greenhouse effect and keep in enough heat to melt the ice.

Because Mars orbits the sun nearly 50 million miles, on average, farther out than Earth, its atmosphere also would have to be considerably thicker than Earth's to trap enough heat to melt ice. Even Earth, without its atmosphere, would have an average surface temperature of about 0 degrees F (minus 18 degrees C).

There are two contemporary precedents for the heavy Martian atmosphere scenario: The planet Venus and Saturn's moon, Titan, have atmospheres that are more dense than Earth's, even though both bodies are less massive.

Whatever the situation, streams, rivers, lakes and, possibly, oceans were temporary phenomena on Mars. Over time, vast quantities of water left the surface for somewhere, whether underground or into space.

The planet is too close to the sun for water ice to cover its non-polar surface permanently, and if there once was an atmosphere heavily laden with CO2, it is long gone.

As Earth eventually teemed with living, thriving organisms - so much so that repeated, devastating impacts could not extinguish all of them - Mars withered away to barrenness. What is left is only the tantalizing evidence of a once-wet planet.

For example, Opportunity recently found a flat rock - called Escher by mission scientists - near the bottom of the football-stadium-size Endurance crater.

The rock's surface is fractured, and the scientists suspect those fractures, which look very much like a dried-mud river bottom, could have been caused when water left over from the rock's formation dried up.

Another intriguing possibility is that, much later, after the rock was formed - and after the impact crater was created - the rock became wet once again, then dried up and developed the 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.

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

If they did, one possible source of moisture could be the accumulation of frost, which melted periodically during climate changes, when Mars wobbled on its axis. Grotzinger said another possibility could be underground ice melting or the release of underground water in large enough quantities to pool into a lake within the crater.

A second curious rock, called Wopmay, also found within Endurance, further strengthens the case for a wet Martian surface. Opportunity's instruments show the rock contains a crust of water-soluble minerals.

Meanwhile, halfway around Mars, along the equator, Spirit is beginning to uncover water evidence as well, as it explores the periphery of Gusev crater. Spirit is climbing higher into the Columbia Hills, which ring the crater's flat, wide bottom.

Spirit has driven about 2 miles (3 kilometers) from its landing site to reach exposed bedrock along the cliffside that looks very much like it has been altered by water.

NASA scientists had ordered the robotic rover to look for relatively unchanged rock to compare with the altered rock, but even the freshest-looking rocks show signs of pervasive water interaction.

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, the 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.

Squyres thinks Spirit might have to climb further up the hills before it reaches unaltered rocks for a baseline chemistry comparison.

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.

The only possible hitch is the aging of the robots.

Both Spirit and Opportunity have only minor problems, and there is really no way of knowing how much longer they will keep operating, said Jim Erickson, rover project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. 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.

Erickson said the condition of both rovers' solar panels 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.

Perhaps, as the rovers drive on, the tragedy of Martian water - and its lost prospects for becoming a true cousin of Earth - will yet be revealed. Scientists will learn why Mars is not a far, far better place than it has become.

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