Now that NASA's Opportunity rover has established a locale on Mars where rocks were once drenched in water, it is in a race against time and battery life to learn how widespread and deep the water was.
Was there a giant sea, or did less conspicuous groundwater percolate locally through the subsurface of Meridiani Planum?
Opportunity has yet to creep out of the shallow depression in which it fortuitously landed. Additional observations, potentially crucial, will be made in the next week or so at home base. And then the robot will hit the road, with plans for two lengthy journeys in search of additional rock outcroppings.
If successful, the road trips could add more spellbinding early pages to a book scientists are writing about the history of water on Mars.
Big mysteries remain
The rest of Opportunity's mission will be fashioned to build on what it has learned.
Here's what's known: A curb-height shelf of bedrock at the landing site has been chemically altered in the presence of soaking amounts of water, NASA scientists said Tuesday. The rock is infused with "enormous amounts" of salt, which on Earth occurs when salts precipitate out of water over time, and especially when the water evaporates.
There are two scenarios that could explain the finding, said Stephen Squyres, Mars Exploration Rover principal investigator from Cornell University:
- Volcanic eruptions generated layers of ash with lots of pore space. Subsurface water then percolated through the pores.
- Or there was a salty sea of water with currents and waves. As the water evaporated, salt settled out along with other sediments and built the layers.
Either scenario would have provided an environment suitable to microbial life. But while some scientists are leaning toward the sea idea, Squyres isn't ready to commit. Nor would he speculate on when the water existed or how long it lasted.
Bruce Jakosky, a geologist at the University of Colorado at Boulder, helped pick the rover landing sites but has not been directly involved in the science explorations. He offered some hints of the time frame for the presumably ancient water.
"Probably not the day before yesterday," Jakosky told Space.com, stating the obvious. He said the water was probably present during the "early to middle Martian history, but there's a lot of wiggle room there, a lot of uncertainty." Mars, like Earth, is about 4.5 billion years old.
Opportunity must now do what it was built for: rove beyond its landing site to seek a deeper understanding.
Baby steps
Before the robot wheels up onto Meridiani Planum, a windswept plain the size of Oklahoma, it will examine two more interesting spots at the landing site over the next week or so, including a part of a rock face called "Big Bend."
The rover will then examine what Squyres called the "blueberry bowl," a small spot where several of the ubiquitous spheres are collected.
The unexplained BB-sized spheres, referred to as blueberries, are also embedded in the rocks, and their shape and relation to surrounding rock layers provided one important line of evidence convincing scientists of the past presence of water. But the composition of the spheres is not yet known.
Once out of the landing area depression, likely within two weeks, Opportunity will investigate grains in the soil made from a mineral called hematite. The mineral usually, but not always, forms in the presence of water. The landing site was chosen in part because a lot of hematite had been spotted by an orbiting spacecraft.
More hematite exists up on the plain than in the depression, and Squyres' team hopes to figure out which flavor of the mineral they are dealing with.
Squyres hinted that the next two weeks could generate crucial information showing whether the rock layers were laid down in water — implying a lake or sea — or whether the water bubbled through the rocks later. However, he admitted, "We may never know."
Rock on
Then Opportunity will head eastward to a crater named Endurance, carved long ago by an asteroid impact. It's a trek of nearly a half-mile (740 meters) that will take several weeks. Squyres said Opportunity might eventually outdistance its twin, Spirit, which has made several long jaunts on the other side of the planet.
Endurance is about 525 feet (160 meters) wide. Opportunity has already photographed its rim, seen as a bright embankment on the horizon. The rover should be able to peek down into the 100-foot-deep (30-meter) crater.
"That is going to be one heck of a view," Squyres said.
More important is whether the rim is a more extensive and telling version of the outcrop just studied.
"We're interested in finding out whether that bright rim material is the same as the outcrop rock or something different," said Joy Crisp, rover project scientist at NASA's Jet Propulsion Laboratory. "It may have a different water story." In addition, she said, rocks down inside the crater could be older those already examined by Opportunity.
Then Opportunity will try to travel about 1.2 miles (2 kilometers) south of Endurance, to a broad area of dark-and-light mottled terrain. Again, geologists want to know if the light areas are rocks that have a similar composition and tell a similar water story to what they've already seen.
Time is limited
But with winter approaching, the days are getting shorter and less sunlight reaches the rover's solar panels. Dust collects on the panels, too, limiting the amount of electricity the craft can generate. It was designed to operate into April, but Squyres has said it could survive several weeks longer. Eventually its batteries will die.
"Our rover might not last that long," Crisp said of the trip south.
Opportunity could get some investigative assistance from Spirit, operating inside the huge Gusev Crater. Spirit is currently heading toward Bonneville, a crater within the crater. If it were to detect similar clues to a watery past in Bonneville, then a more global picture of Mars' water history might emerge.
Squyres told Space.com that Spirit's mission probably would not change based on the things Opportunity has found, however.
"Each rover is already being used just about as effectively as it can be to explore its particular landing site," he said.
One chapter of many
Meanwhile, it's important to remember that this week's landmark announcement was not the first solid evidence for ancient water on Mars, said Jakosky, the University of Colorado geologist.
Previous photographic evidence has provided a virtually airtight case for past episodes of running water and strong signs of lakes or oceans. And laboratory studies of a Mars rock found on Earth — the infamous Allan Hills meteorite — have shown geochemical evidence for water. (The rock, also called ALH 84001, is most known for its as-yet-unproven signs of Martian life.)
"We know an awful lot about the history of water on Mars," Jakosky said. "There was liquid water on Mars. It was present in the crust at least in some places at some times. There's reason to think it was widespread. We know substantial quantities of water have been lost to space" as the planet's climate changed, and owing to its gravity, which is 38 percent that of Earth.
Today there remains some water vapor in the Marian atmosphere, as well as significant amounts of water ice at the poles and underground at high latitudes. But scientists don't know if that water ever shows up nowadays in liquid form, or how it cycles through the atmosphere, Jakosky said. "We're getting clues, but we really don't understand the basics yet."
Scientists have spoken several times recently about writing a book on the water history of Mars. How is that project going?
"We're writing Chapter One," Jakosky said.