Mars Critics Say Billions Are Ill-Spent

Mission accomplished, said James Garvin, NASA's lead scientist for Mars and lunar exploration.

In a finely layered rock, Opportunity detected concentrations of jarosite, an iron sulfate mineral that forms with water, as well as layers of salts that match evaporation sequences found on Earth when briny water pools dry up.

Visual examination also showed several features of rocks formed in watery environments, including signs of dissolved salt crystals, BB-sized spheres of minerals and crossbed patterns of rock layers.

Spirit found evidence that waterborne minerals were deposited inside cracks in a volcanic rock dubbed "Humphrey." However, the amount of water suggested by the data is far less than what Opportunity found.

"This really is the smoking gun of a watery past for Mars," said David Grinspoon, principal scientist at the Southwest Research Institute in Boulder, Colo., and author of "Lonely Planets." "We're not just chasing ghosts."

Not that future exploration necessarily will be accomplished by humans. Some NASA critics, like Colorado astrophysicist Robert Zubrin, strenuously argue that humans should go to Mars soon - traveling light and making their fuel from rocks on the Martian surface for the trip home.

But the current thinking is that robots and computers can do a cheaper, safer job in a hostile environment. The very thin Martian atmosphere contains almost no oxygen and it exerts only a trace of the pressure that helps make Earth habitable.

"We're in a new phase and one we had better get used to, Krauss said. "The more adventurous we get, the more we have to count on robots."

The value of robotic exploration is one area in which Mars supporters and critics like Etzioni and Earle can agree - up to a point.

After all, submersibles have been trolling the oceans for decades. Earle argues that remote marine studies have found that life - our lives, really - is not guaranteed as the oceans decline.

Most of the seas' big fish - tuna, sharks and swordfish - have been depleted. Half of the coral reefs are dead or dying. Around the world, runoff pollution has created more than 50 "dead zones" in coastal waters.

Sea levels are rising, and the oceans' role in the planet's changing climate is poorly known.

Real oceans need scientific attention more than the dried-up remnants on Mars, Earle contends.

"Every time I jump into the ocean I see things I've never seen before," she said. "We have better maps of Mars than our own ocean floor. That's just not right."