On January 24, at about 9:05 p.m. Pacific Standard Time, NASA's second rover is scheduled to arrive on Mars. Opportunity will land near the equator, on a plain known as Meridiani Planum. It'll be halfway around the planet from Gusev Crater, where its twin, Spirit, is already feeding eager scientists as much data as it can transmit.
Meridiani Planum interests scientists because it contains an ancient layer of hematite, an iron oxide that, on Earth, almost always forms in an environment containing liquid water. The site appears dry now. So how did the hematite get there? Was there once water in the area? If so, where did it go?
"There are five or six hypotheses to explain the hematite on Mars, but none of them are a slam-dunk," says NASA's Mars Landing Site Science Coordinator John Grant. "We have to go there to find out which is correct."
Right: The distribution of hematite in Sinus Meridiani, where Meridiani Planum is located. Reprinted from "Global mapping of Martian hematite mineral deposits: Remnants of water-driven processes on early Mars," by P.R. Christensen et al., J. Geophys. Res., 106, 23873-23885. [More]
It's possible, for example, that the hematite was produced directly from iron-rich lavas, a process that would not require liquid water. But if water was involved--and that's considered most probable--then, most likely, the hematite either formed from the iron-rich waters of an ancient lake, or it formed when Martian groundwater percolated through layers of volcanic ash.
Opportunity's suite of spectrometers, cameras, microscopes, and sampling tools should allow scientists to figure out where the hematite came from. For instance, if a mineral called goethite is found among the hematite, that would mean that the hematite formed in watery conditions. On the other hand, if magnetite is found and goethite is not, a watery past is unlikely.
Just being able to look at the way the hematite is distributed will provide some answers. If the hematite occurs as a thin layer within a pile of layers, then it's likely to have formed in a long-ago lake, says Grant. If, on the other hand, it occurs in more discrete veins, deposited between cracks in rocks, "then it's much more likely to have been associated with groundwater."
Above: An artist's concept of Opportunity approaching an interesting rock. [More]
If you look in the Earth, he says, in places where the groundwater percolates through the subsurface, "you see evidence for life all over the place." This mission, Grant emphasizes, is not seeking evidence of Martian life. It's looking for environments that were favorable for life, and in which evidence of life may have been preserved.
Knowing how the hematite formed will help determine if Meridiani Planum is that kind of environment.
Meridiani Planum is unique on Mars because there's so much exposed hematite there, according to data gathered by NASA's Mars Global Surveyor spacecraft. "Localized deposits also exist in two other sites: the deep canyon Valles Marinaris and a place called called Aram Chaos," notes Grant, "but neither are accessible based on the current landing system." Meridiani Planum has more hematite and it's a safer place to land.
Right: An artist's concept of ancient hot springs on Mars where hematite might have collected. [More]
Meridiani Planum is also attractive because the site appears to be eroding, with once-buried craters that are now half-revealed. Opportunity might be able to inspect layers of ground that would otherwise be hidden, affording a glimpse into the area's distant past.
"There's so much we don't know about Mars," says Grant. "But I really think we're going to come out of this mission with a better understanding of what Mars has been like over time, and where we might go for our next step."
Mars Exploration Rovers -- NASA's home page for the exploration of the red planet by the twin rovers
The Lure of Hematite -- (Science@NASA) On rusty-red Mars, a curious deposit of gray-colored hematite (a mineral cousin of common household rust) could hold the key to the mystery of elusive Martian water.
The Case of the Missing Mars Water -- (Science@NASA) Plenty of clues suggest that liquid water once flowed on Mars --raising hopes that life could have arisen there-- but the evidence remains inconclusive and sometimes contradictory
Making a Splash on Mars -- (Science@NASA) On a planet that's colder than Antarctica and where water boils at ten degrees above freezing, how could liquid water ever exist? Scientists say a dash of salt might help.
Sedimentary Mars -- (Science@NASA) Mars Global Surveyor images reveal sedimentary rock layers on the Red Planet that may have formed
Layers of Mars -- (Science@NASA) Mars Global Surveyor has spotted terrains on Mars that look like sedimentary rock deposits. If the mysterious layers formed underwater, as some scientists suspect, they may be a good place to hunt for Martian fossils.