Now that they know the composition of tiny spherical objects that litter Meridiani Planum, landing site of the NASA rover Opportunity, Mars scientists are eager to figure out how deep the water ran.
And so far they don't agree.
The BB-sized spherules, or "blueberries," are packed with hematite, a mineral that supports the case for water in Mars' past. But their composition still does not answer the question of whether the Martian water once existed as a lake or was solely locked up in the form of groundwater.
"I think, for now, the surface water story and the groundwater story are two different ones which may have a common happy ending," Andrew Knoll, a science team member with the Mars Exploration Rover mission, said during a press conference Thursday. But the hematite in the Martian blueberries, he added, "really speaks to groundwater very specifically."
The sedimentary rock containing the hematite-bearing spherules also contains other minerals that not only had to be placed by water, but most likely by groundwater, he added.
But wait ...
Other mission scientists, however, are not so quick to look past the idea that standing water, such as a shallow lake, sat atop Meridiani Planum during the formation of the blueberries —which are, for the record, gray in color.
"I'm becoming more convinced, from the evidence, that there's been a standing body of water on Mars," said Philip Christensen, principal investigator for the miniature thermal emission spectrometer aboard Opportunity. "It probably extends the lifetime of water at this site."
Christensen told Space.com that the formation of hematite concretions isn't a geological process that happens rapidly, such as crystals that can grow in weeks or months.
"It's a process that measured in years and decades, not weeks or weekends," he said, adding that on Earth such objects have taken thousands of years to form. "I don't know if it's that long on Mars or not," he cautioned.
Christensen spoke via telephone Thursday from the Lunar and Planetary Science Conference in Houston.
Knoll, however, said that standing water was not a prerequisite for the hematite finding, citing that similar processes have taken place here on Earth, such as in Utah, where sandstone outcrops were gradually saturated by groundwater as the water table rose over time.
The composition of the blueberries was a mystery when they were first detected because the spherules were too small — just a tenth of an inch (3 millimeters) in diameter — to target with the Opportunity's instruments.
Earlier this month, scientists surmised they were concretions of material dissolved in the water that soaked through the rocks of an outcrop near the rover. Over time that material built up into the round, gray objects dubbed blueberries, some of which remained embedded in the rock while others weathered out and onto the Martian floor.
Opportunity ultimately examined a group of blueberries that had weathered out of its sedimentary prison and collected in an area called the "Berry Bowl." The grouping allowed the rover a large enough sample area to use its iron-sniffing Mössbauer spectrometer, as well as its miniature thermal emission and alpha particle X-ray spectrometers.
By studying a berry-rich area in the "Berry Bowl" as a whole, then a berry-free patch nearby, scientists were able to filter out sedimentary material until they were left with a strong hematite reading from the "blueberries."
More to learn
Hematite is a mineral that typically, though not always, forms in water. Orbital studies of the Meridiani Planum site indicated large tracts of the mineral across the region, which led mission planners to select the area for Opportunity's visit. The region itself is about the size of Oklahoma, and mission scientists are eager to see if "blueberries" there suggest the same kind of weathering on the plains as seen at Opportunity's outcrop, which sits inside a small crater.
"If you look at this outcrop, it's about 30 centimeters [12 inches] of sediment that contains maybe 2 to 3 percent of hematite," Knoll said. "Perhaps the whole floor of Meridiani Planum is covered with them, and if that's true, then a much larger volume of rock was stripped away by erosion through time."
Knoll, Christensen and other mission scientists are anxious to unleash Opportunity onto the Meridiani plains to search for more spherules, hematite and evidence of past water on Mars.
"I think there's a decent chance that we'll find more spherules outside the crater," Cornell University's Steven Squyres, principal investigator from the rover missions, said in an e-mail interview. "We clearly see spherules in the soil of the crater wall above the outcrop, and unless they can be blown uphill, that means they rolled down from above."