Mars was wetter earlier, drier later

Among the reports at last week's Lunar and Planetary Sciences Conference, dozens dealt with what we've been learning from this most intensive period ever of Mars exploration, with two active rovers and three orbiters hard at work.
Here is a summary of what the latest information seems to indicate about the planet's climatic history, based on interesting talks by Tim Parket and Matt Golombek of NASA-JPL.
The flood of images and data from the Mars rovers over the past 14 months suggests that Mars may have been even wetter in its infancy than most scientists had believed, but may have dried out faster and earlier than had been thought.
Prior to the landing of the twin NASA rovers in January, 2004, the prevailing view was that Mars had a wet phase early on, but that this may have been relatively brief, episodic, and that the largest bodies of water that ever accumulated might have been lakes that froze over quickly. And while everyone agreed that the planet had dried up early on, there were some signs of episodic changes in climate that might have returned more moisture to the planet at some brief periods.
But close-up examination of the soil and rocks suggests otherwise, indicating that Mars may have had very little moisture, or much activity at all, for most of its existence. But at the same time, the case has strengthened for the controversial view that in its early years Mars may have had a vast ocean covering its northern plains. The signs of an even wetter early wet period, followed by an even longer and drier dry period, were presented last week by some members of the rover science team in a series of reports at the annual Lunar and Planetary Sciences meeting in Houston.
In its early years, the chemical, mineralogical and imaging data strongly suggest "either playas [shallow seas] or a deep ocean," said rover scientist Matt Golombek of NASA's Jet Propulsion Laboratory. "In either case, it was wet. There was a period of rapid erosion that produced layered sedimentary rocks."
But since then, there is little indication of any large-scale water action on the surface, and strong indications that the main thing that has been happening is just a very slow compacting of the desert into a hardpan surface, through the very slow removal of finer particles by the scouring action of the wind. "It's possible that nothing has happened in 3 billion years, except 12 centimeters of deflation," Golombek said.
That conclusion is based on analysis of the properties of the soil at both the Gusev Crater and Meridiani Planum sites, where the soils seem to be very tightly compacted, with only a very shallow layer of loose material on top -- and at Meridiani hardly any. Any active process should have produced more physical and chemical disturbances of that top layer than can be seen, he said.
Meanwhile, although it remains controversial, the idea that the incredibly flat northern plains may have been the floor of an ancient ocean has gained stength as the rover Opportunity continues its long drive across Meridiani.
Tim Parker, another rover scientist at JPL, says the surprisingly flat and hard surface of that plain, adorned only with long, shallow parallel ripples, is hard to account for with a shallow intermittent sea, but is consistent with currents in a long-lasting ocean.
Parker said the ripples across the plains "suggest sustained currents and directions" over a long period. And, he said, the hematite "blueberries" form a uniform coating, a hard pavement across the surface so the wind no longer can pick up any fine material to alter the profiles, he said. Yet orbital images show that coating extends over a large area, even across wide variations in topography, and that topography is hard to explain through wind erosion and must mean the current profile is very close to the original topography.
But that relief does not include any boundary to the north to contain an ancient sea, but dips down into the basin that may have been an ocean. He suggests that the plateau of Meridiani could have been a kind of platform built up within the ocean, similar to the way carbonate platforms form in Earth's oceans. But the chemistry of such platforms on Mars would have to be quite different and remains unexplained, he said.