After more than a century of wild speculation, decades of serious searching, and years of collecting increasingly compelling evidence, there is suddenly a scientific and media buzz over whether Mars is a planet sculpted by water.
Thing is, that question is already answered in the minds of most Mars experts.
For the first billion years or two, Mars was almost surely warmer and wetter, most scientists agree. What lingers is a big, multipart mystery of what happened to all the water, how long ago it disappeared, and whether it was around long enough — and under the right conditions — to have possibly incubated life.
The latest news — a previously unheralded mineral called hematite has been detected by NASA's Opportunity rover — dribbled out late last week and over the weekend. It might turn out to be a key moment in Mars exploration history, but some scientists think it is more likely to represent just another piece in a huge puzzle of a planet that could remain largely enigmatic for years to come.
Hematite is particularly tricky.
"It's not going to tell us anything definitive," Craddock told Space.com, "unless we get really lucky and we're able to put the hematite into some sort of mineralogical context."
Hematite is common on Earth. It is sometimes used to make jewelry. It's essentially rust, the stuff that forms when iron is exposed to water and oxygen. The name comes from the Greek word for blood, and in its powdered form, hematite is indeed reddish. It gives Mars its overall ruddy color. (Most of Earth's iron sank to the core when the planet was young. Being less massive, more of Mars' iron ended up at the surface.)
"Red and gray iron oxides on Mars are really just different forms of the same mineral," explains Arizona State University geologist Victoria Hamilton. "If you ground up the gray hematite into a fine powder it would turn red because the smaller grains scatter red light."
Opportunity's landing site was chosen for an apparent abundance of gray hematite. An initial examination by the rover's spectrometer, which splits invisible infrared light into its various colors and then analyzes chemical signatures, is encouraging.
"I think the preliminary evidence is consistent with hematite forming at low temperatures in chemical reactions with water," Philip Christensen, an Arizona State University geologist who leads the hematite investigation, said Saturday. That means it might not be the volcanic variety.
Further investigation is planned and may be under way as you read this.
If Opportunity determines the hematite is filling cracks in rocks, then geologists might assume it was carried along by groundwater -- a significant scientific finding, said Craddock, who is a geologist by training and is not involved in the rover missions. If, however, the hematite is embedded in layers of ash, then scientists would know it was generated in a volcanic process; water hunters would be disappointed.
Further, Opportunity's landing site was hand-picked because Christensen and his colleagues had strong evidence, from orbiting spacecraft, that hematite was there. Craddock said any discoveries at Meridiani Planum, the equatorial location where the rover is working, will be akin to a geologist picking up the strangest rock in a field and trying to draw broad conclusions from that one rock.
"We're going to find out what this anomalously weird area of Mars is like, but we're going to have a hard time putting it in the context of 99 percent of the planet."
Opportunity's landing site might be unusual just as Yellowstone National Park is on Earth. Perhaps the now-barren landscape was once was peppered with hot springs, scientists speculate. And these warm watery areas might have served as an incubator for life.
"We're very interested to know if this region could have been like Yellowstone, with hot springs, so we'll be looking to see if there are other minerals in the area such as those at Yellowstone," said Joy Crisp, project scientist for the Mars Exploration Rover Project at NASA's Jet Propulsion Laboratory.
Water does not mean life, scientists point out, but it is the key ingredient, and so NASA's mantra for Mars exploration has long been "follow the water."
NASA's Viking missions in the 1970s were designed to find water and life, if any existed. The Viking probes of water ice in the northern polar cap as well as trace amounts of water vapor in the Martian atmosphere.
Scientists have since concluded that the northern cap is loaded with water ice. More recently, water ice has been found at the south pole. And NASA's Odyssey orbiter discovered over the past couple of years just under the surface away from the permanently frozen caps.
But organisms as we know them cannot live by ice alone. They need liquid water, at least now and then, so they can emerge from dormancy to repair their cells and procreate. Some bacteria on Earth are known to stay dormant for up to 40,000 years. Other creatures thrive in sub-ice havens where liquid water exists in pockets, sometimes only briefly.
Perhaps more intriguing than evidence for frozen water, then, are the countless photographs taken during the past three decades showing evidence of past rivers, lake and oceans. Collectively the body of photographs is not entirely conclusive, but few researchers argue that water long ago scoured the surface of the now-dusty world.
Some scientists cautiously note that the channels and canyons of Mars might have been formed by lava or flowing carbon dioxide, perhaps even gaseous carbon dioxide mixed with dust, and not necessarily by water. Others, particularly those making the investigations, frequently say that only water could sculpt the features they see.
Craddock, who is dubious of the most recent evidence suggesting water has carved modest features in recent geologic history -- meaning it might still flow there -- has little doubt about ancient Mars.
"It's almost undeniable" that there was water in the early history of the planet, Craddock said.
In June of 2000, researchers at Malin Space Science Systems released pictures from NASA's Mars Global Surveyor showing that appeared to be created by recently flowing water in several dozen locations. But the Malin scientists admitted their conclusion presented a perplexing problem: Logic dictated the freezing temperatures at Mars should have prevented what they saw from happening.
In December of 2000, images from Mars Global Surveyor showed that suggested ancient lakes. The layering had been seen by the Mariner missions of the 1970s, but never in such detail. Scientists said the discovery would be fascinating if it held up, but they admitted it would be heavily debated.
In 2001, an extensive analysis of Mars data led a team of geologists to conclude that a of water in one part of Mars had sculpted huge gorges and left water trapped in numerous subsurface reservoirs. That adds to the case for ancient water and helps explain where some of it might have gone.
And fresh analysis in late 2002 of dark streaks spotted , first noted in the Viking era, suggested to other researchers that water is still running on Mars, at least in brief spurts.
Then earlier this month, European Space Agency scientists put out one of the first images from their new Mars Express orbiter. It provided a high-resolution they think was carved by a river long ago. At the bottom of the channel is a dark region, not a shadow but some sort of deposit that's been laid down.
"There is no other phenomena that we know of that would produce these effects," said Gerhard Neukum of Germany's Free University of Berlin. "It is fair to say we are sure this is evidence of once-flowing water on the surface of Mars."
Other studies, however, have held that if water sculpted Mars, it probably did so in -- not the sort of conditions prone to fostering the development and sustenance of life. Supporting this less glamorous scenario, research last August found a relative called carbonates that should have been left behind if there were ever serious lakes or oceans.
Craddock said the lack of carbonates might be easily explained. They're sometimes hard to detect on Earth, he said, so scientists might simply not be seeing them on Mars when looking down from orbiting spacecraft. Also, Mars' thin atmosphere lets in more ultraviolet radiation, which is known to destroy carbonates.
(Water would also be useful to future manned missions, both for drinking and to generate hydrogen, which could be used for fuel on a return flight.)
With all the excitement of late, and some media hype, it might seem almost inevitable that Mars was once a life-bearing planet. Most mainstream scientists are very cautious on this point, however. Some are very doubtful.
"I think it's very unlikely that Mars ever had life," Craddock said. "I would argue that the evidence for Bigfoot or UFOs is better than for life on Mars."
He nonetheless supports planned missions that would scour the Red Planet for signs of biology, in part because even a failed search, via a bit of twisted logic, could along the way answer the huge question of how life began on Earth — another gaping hole in scientific knowledge.
"If we don't find life, we might find the prebiotic chemistry that went into life on Earth," Craddock explained. "That's more important, because we