Muddying the Water? Orbiter drains confidence from fluid story of Mars

Evidence for liquid water on some parts of Mars—now or in the past—looks leakier than researchers had supposed, according to an analysis of the sharpest images ever taken of the Red Planet from orbit. But in other places, the new images bolster the case that water once flowed.

High-resolution pictures and infrared spectra recorded by NASA’s Mars Reconnaissance Orbiter (MRO) indicate that fresh, bright streaks on two steep gullies don’t signify a recent flow of water, as scientists suggested just a year ago (SN: 12/23/06, p. 416). MRO’s instruments neither detected minerals that might have been left behind as salty groundwater evaporated from those regions nor found changes in the shapes of the deposits since the gullies were last imaged 15 months ago. Such changes could have occurred if the bright deposits were frost created by an underground supply of water rushing to the frigid surface in recent years or months.

Instead, the deposits on the steep gullies could just as easily have been formed by landslides of dry, sandy material, assert Alfred McEwen of the University of Arizona in Tucson and his colleagues in one of five reports of MRO findings in the Sept. 21 Science. Other gullies, however, whose slopes are too shallow to permit landslides, do offer clear evidence of watery flows. These flows might have occurred several million years ago, when the Martian climate was warmer.

McEwen, Windy Jaeger of the U.S. Geological Survey in Flagstaff, Ariz., and their collaborators reexamined a 300-kilometer-long system of channels called Athabasca Valles, which in lower-resolution images taken by previous spacecraft showed some resemblance to a frozen sea. Although not ruling out water might have carved the system, the new images suggest that the structure is more likely a remnant of a “once-swollen river of lava,” the researchers say in another of the five reports.

Theorists have invoked the presence of liquid water for “nearly every imaginable time and place on Mars,” notes McEwen. But the interpretation of planetary features “can completely change with better observations,” he adds.

Although the MRO images cover only 0.2 percent of the Martian surface, they reveal features as small as 50 centimeters across, about 10 times the resolution of previous satellite images. MRO reached Mars last year and is now recording a terabyte of data a week—more than many missions do during their entire lives.

Elsewhere on the Red Planet, McEwen notes, water might have played a more important role than had been indicated. The MRO camera reveals branched channels and fanlike deposits adjacent to several large craters. The channels and deposits suggest the flow of water, perhaps when meteoroids slammed into an ice-rich crust, McEwen suggests.

As well as excavating craters, the impacts would have created transient, wet microenvironments in which liquid water sculpted the channels and left behind the deposits, he says. Images of the features support the hypothesis that ancient Mars had sporadic episodes of warm, wet weather at specific sites but not a warm, planetwide climate for long periods.

Some scientists in the past may have been too quick to jump on the bandwagon for a wet Mars, comments Philip Christensen of Arizona State University in Tempe. The new findings add to the evidence that the planet had liquid water “only for brief periods” during an otherwise cold and dry history, he says.