Opportunity, which has been exploring an area near the Martian equator called Meridiani Planum since late January, first discovered rocks that formed as a result of the chemical actions of water. Then, NASA scientists announced on Tuesday the robotic craft had located rocks most likely shaped by a body of gently flowing seawater.

"We think Opportunity is parked on what was once the shoreline of a salty sea on Mars," said Steve Squyres of Cornell University, Ithaca, N.Y., the principal investigator for the instruments aboard both Opportunity and its twin Mars Exploration Rover, Spirit.

Those revelations, which today are taken for granted, also were monumental in their day. Prior to their disclosure, people confidently -- even fervently -- believed Earth was the immovable center of the universe, surrounded by all the heavenly bodies, each of which was a perfect, featureless sphere. Galileo's announcement was considered so shocking at the time he was charged with heresy by the Roman Catholic Church.

Opportunity's findings have been treated more matter of factly, with NASA officials holding a news conference and bubbling over with enthusiasm at the images Opportunity has transmitted, and members of the media duly reporting the information and displaying the rover's images.

Yet the importance of this finding cannot be overstated.

Until now, we have known for sure of only one planet on which liquid water has flowed -- and water is absolutely essential for supporting life as we know it. There are no chemical processes that will permit the formation of the long, complex organic molecules composing living organisms other than in the presence of water.

It is an extremely simple rule: No water, no life. As long as Earth was the only planetary body containing liquid water -- and, more particularly, seawater -- then it was the only place in the universe where life was possible.

Now, suddenly, there are two.

"This dramatic confirmation of standing water in Mars' history builds on a progression of discoveries about that most Earthlike of alien planets," said Ed Weiler, NASA's associate administrator for space sciences. "This result gives us impetus to expand our ambitious program of exploring Mars to learn whether microbes have ever lived there and, ultimately, whether we can."

NASA's Administrator Sean O'Keefe put it more succinctly.

"When you dare to ask profound questions about the universe in which we live, you may very well receive some rather profound answers," O'Keefe said at the briefing on Tuesday. His statement might be tremendously prophetic.

Two months and 10 days ago, President George W. Bush announced a new policy for the U.S. space program, involving a renewed effort to return humans to the moon, stay there, and push outward into the solar system.

The discovery by Opportunity provides a bigger impetus for that effort, as does the recent finding, by the European Space Agency's Mars Express orbiter, that Mars retains water still -- in the form of ice at its south pole.

O'Keefe said Opportunity's discovery coincides with the administration's strategy of altering decisions about its new space policy to accommodate the findings of existing missions.

In the case of Opportunity's discovery at Meridiani Planum, suddenly that site has become the prime target for NASA's next Mars rover mission, the Mars Science Laboratory. About the size of a VW Bug, the nuclear-powered craft is currently planned for launch in 2009.

There may be other, major discoveries awaiting the rovers or their successors, such as finding fossils of ancient Martian organisms or, even more dramatic, finding living creatures.

Still, the seawater-sculpted shapes on the rocks photographed by Opportunity promise to furnish the material for a new chapter in Burke's classic book, "The Day the Universe Changed." Because now that it has been shown there are two planets where water once flowed, there no longer is a reason to doubt hundreds -- or even thousands -- more might exist right within our own Milky Way galaxy.

Farther out in the universe -- which, thanks to images from the Hubble Space Telescope, contains perhaps several hundred billion galaxies -- there could be a huge number of planets holding water even now. It is intriguing to imagine water lapping against unimaginably distant shorelines upon which water-based, DNA-structured beings build their vacation homes and dock their sailboats.

Once any condition can be shown not to be unique, it can be commonplace.

That is a profound realization by anybody's standard.

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