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“Follow the water” is the mantra for NASA’s Mars exploration program. But present-day Mars is so cold, and its atmosphere so thin, that liquid water cannot exist on the planet’s surface.

What NASA can look for, though, is evidence that water was present and active on Mars in the distant past. There are strong indications, in images taken by cameras aboard orbiting spacecraft, that features of the Martian landscape have been carved by water. But some scientists argue that these features could have been caused by short-lived torrents of water – flash floods.

More interesting, because the ultimate goal of Mars exploration is to look for signs of life, would be to find areas where water flowed over long periods of time, either continuously or episodicaly; or where water ponded for long periods of time, such as in a lake.

Convincing evidence for long-term water acitivity is difficult to nail down with photographs taken from orbit, however. That’s the job of landers and rovers, like the rovers NASA plans to send to Mars later this year.

The upcoming missions are referred to as MER-A and MER B – MER stands for Mars Exploration Rover – and, yes, it’s missions, plural. MER A will launch in late May or June, MER B in June or July. Both rovers will touch down on Mars in January 2004.

The MERs are equipped with a host of instruments designed to investigate the mineral compositions of rocks nearby the landing sites. By studying mineral compositions, scientists will be able to learn a great deal about how the rocks were formed, particularly what role water played in their formation.

Only once before has NASA sent a spacecraft to Mars capable of roaming about on the surface. Pathfinder, which arrived on July 4, 1997, carried within it a small rover, Sojourner. For all the excitement Sojourner generated, it traveled a grand total of about 100 meters (about 330 feet) before its mission was terminated.

The MERs are larger than Sojourner, contain more advanced scientific instruments and can travel farther – up to half a kilometer (just under one-third of a mile).

But just where on Mars is the best place to ‘follow the water’? Although Mars’s diameter is only about half that of our home planet, it has as much surface land mass as Earth – because it’s all land mass. That’s a lot of real estate to choose from.

Two landing sites, very different from each other, are the leading candidates. One, in Meridiani Planum (formerly referred to as Terra Meridiani), is notable because it contains a large deposit of gray hematite, an iron-rich mineral that on Earth is usually formed by the long-term action of water. The other, Gusev Crater, is intriguing because it appears to be the site of an ancient lakebed, where layers of sediment were deposited by a long-term flow of water into the basin formed by the crater.

This article will focus on the Meridiani Planum site; a subsequent article will discuss Gusev Crater.

On contemporary maps of Mars, Meridiani Planum is in dead center. What makes it an exciting spot is that it is the site of a vast deposit of gray hematite.

Hematite is iron oxide. It comes in two forms: red and gray. The red kind you’re probably familiar with. It’s rust, and it’s everywhere on Mars. It forms readily whenever iron is exposed to air. It gives the planet its red color.

Gray hematite has a dark gray metallic luster. You’ve probably seen it in curio shops and jewelry stores. Unlike red hematite, gray hematite usually forms over long periods of time, in the presence of liquid water, often standing bodies of liquid water.

Gray hematite has been found on Mars in only three places, Meridiani Planum being one of them. The other two locations, Aram Chaos and Valles Marineris, although even more interesting than Meridiani Planum in some respects, are too hazardous for a MER spacecraft to land in.

The gray hematite was identified by TES, the Thermal Emission Spectrometer, aboard the Mars Global Surveyor (MGS) spacecraft that has been orbiting Mars for nearly four years. TES is an infrared spectrometer, an instrument that measures infrared light reflected by the Martian surface. The data it gathers enable scientists to draw maps of the mineral composition of that surface. But the maps are rather crude. That’s why researchers are anxious to send instruments to the surface, so that they can study Martian minerology in more detail.

Phil Christensen, of Arizona State University, is the principal investigator for TES. He has come up with five possible scenarios for how the hematite in Meridiani Planum formed. Four of these involve liquid water; the fifth is a volcanic process that Christensen considers unlikely. Although each of these scenarios could account for the hematite seen from orbit by TES, each would appear very differently to the instruments onboard MER.

Christensen explains the various possibilities this way.

“If you get there and you picked up a rock and it had some banding layers of hematite in it, that’s a smoking gun for a lake deposit.

“Another possibility is that, say, I have a sediment formed in a lake – or however it formed – and hot water flowed through that. Then I would think you would see that as almost like a hematite cement, filling in the pores of this pre-existing rock.

“If it’s a volcanic ash that got oxidized without water, then the hematite should be really fine-grained, dispersed through the whole rock, and it looks like an ash that oxidized to hematite.

“Another possibility is leaching. On the Earth, massive amounts of rainfall basically dissolve away most things and leave behind the hematite. And what happens there is the hematite, the iron will dissolve out of the near surface stuff, leach down, and re-precipitate. So you sort of get this glob coating of hematite coating everything else.”

And what is Christensen’s favorite scenario?

“I have come to think that it’s probably a hydrothermal process” that formed the hematite. He imagines a frozen lake covering layers of iron-bearing sediment. Some underground heat source melts the ice, causing water to flow through the sediments. As it flows through, it dissolves iron from the sediments. Downstream it re-precipitates the iron as hematite.

It is not the hematite, however, but the water in the hematite-formation process that has Christensen intrigued.

“The hematite itself is not particularly interesting. We know it’s there; we’ve mapped it. So what? I argue that it’s a beacon that says, water was here, okay? And so now if you’re looking for the most interesting places to go land, there’s a beacon that says, hey, there was mineral evidence of water here, go there. And you look in detail and see what else is there.”

Recently, Wendy Calvin, of the University of Nevada, Reno, uncovered evidence for a second aqueous mineral, in both Meridiani Planum and Aram Chaos. Ironically, it comes not from NASA’s most recent missions to Mars, but from two of the oldest: Mariner 6 and 7. These two spacecraft, which flew past Mars in the summer of 1969, also contained infrared spectrometers and, by co-incidence, recorded data for Meridiani Planum and Aram Chaos.

Wendy Calvin, of the University of Nevada, Reno, recently re-examined the Mariner data, which contained information about a different portion of the infrared spectrum than that collected by TES. She discovered the signature of a second aqueous mineral, the location of which correlates closely with that of the hematite found by TES. This finding adds significant weight to the argument that the hematite was, indeed, formed by water.

Just what that second mineral is, though, Calvin can’t say.

The data collected by the Mariner spacecraft contain enough information to determine three important facts about the mystery mineral:

o First, it contains a lot of water. The Mariner data show a very strong water signal in the locations where the unidentified mineral appears.

o Second, it is something other than hematite. Hematite doesn’t have a detectable signature in the part of the spectrum where this second mineral is detected.

o Third, it isn’t a typical silicate or a carbonate. Typical silicates – quartz, for example – are among the most common rocks on Earth. Carbonates, such as limestone, are also very common on Earth, especially where minerals have precipitated out of water. If the unknown mineral were a typical silicate or a carbonate, TES would have been able to detect it.

That doesn’t leave many options. The mineral that Calvin believes best fits these constraints is a type of clay, black or dark green in color, known as a ferrous silicate.

“They’re clays that were formed on Earth before there was a lot of oxygen in the atmosphere,” says Calvin. “They’ve got lots and lots of iron in them.”

This is consistent with what is known about Mars’s atmosphere. “Mars doesn’t have a lot of oxygen in the atmosphere – never did. Early earth didn’t,” either, says Calvin.

According to Christensen, the instruments aboard the MER will be able not only to identify Calvin’s mystery mineral, but also to reveal the process that formed the hematite detected by TES.

“If you learn fifteen minerals,” he says, “you can know 99 percent of what forms in rock. But which four or five happen to be in that rock tell me the temperature, the pressure, the pH, the oxygen. So these five will form under these conditions, and these five will form under those conditions.

“I want to get on the ground and see, what are the minor minerals that we can’t detect in orbit that are there, that are associated with hematite?” Christensen says. “And then we’ll be able to say, ah-hah, look, these little trace minerals only form under these conditions. Or these trace minerals form under a different set of conditions. That’s what I’m hoping.”

Will Meridiani Planum turn out to be a good place to look for life on Mars? Perhaps not, says Christensen.

“It may turn out that [the hematite] formed in a hydrothermal system that had a pH of 1 [highly acidic], that was the most inhospitable place you can imagine, okay? And all it had going for it was water. And the biologists, if we land there and look, they may say, Geez, you know, life couldn’t have possibly started there. We don’t want to go there [to look for further evidence of life].”

But, he argues – and his arguments appear to have convinced the scientific community that studies Mars – “You look at Mars. And there’s so few places where someone can stand up and say, Hey, here’s mineral evidence for water.”

What’s Next

Although Meridiani Planum and Gusev Crater are the favorite sites among scientists, rigorous analysis of whether they are safe enough to land in remains to be completed. Once the safety analysis of these two sites, along with two alternates, is complete, it will be up to NASA Headquarters to make a final decision. That decision is expected in April.