Its surface mineral distribution seems very bland and uniform -- in part because of the wind-blown soil mixed evenly over the planet by its dust storms. Moreover, the minerals its operators had most looked forward to detecting -- carbonates -- have so far failed to turn up at all.

This absence is one of the major mysteries of Mars, since according to the favored theory Mars lost its early dense CO2 atmosphere because it dissolved in the planet's liquid surface water and then reacted with its surface silicate rocks to form permanent deposits of solid carbonates - whereas on Earth the carbonates are eventually dragged deep underground by our planet's crustal tectonics and heated to re-release their CO2.

Perhaps some atmospheric process destroys carbonates on Mars' upper surface -- leaving big deposits of them a short distance underground -- or perhaps we're seriously misinterpreting the planet's early history.

But there was one glaring exception to Mars' mineralogical blandness: a huge area in the Sinus Meridiani region, 350 by 750 km wide, which shows clear spectral signs of a coarse-grained variety of the iron oxide mineral hematite. Since then, MGS has found several much smaller patches of coarse hematite, all in areas likely to have been exposed to water during Mars' early days.

The Sinus Meridiani hematite patch is in a smooth plains area, which seems to consist of several different layers of differing rock deposited at different times, with the hematite just being one of those layers.

At first it looks undistinguished in orbital photos, but William K. Hartmann has noted something odd about the craters on its surface: they fall into two radically different types.

It is more densely covered than almost any other part of Mars with extremely eroded, ancient craters obviously left from the Solar System's earliest days when the planets were still being heavily bombarded with asteroids -- most such craters have been completely erased on most other parts of Mars by billions of years of wind erosion, particle deposition and the constant erosive rain of very small meteors.

But the hematite region has few craters any newer than that -- in fact, just a sparse sprinkling of small, very sharp and uneroded craters apparently laid down on its surface just within the past 10 million years.

The conclusion seems to be that, after Mars' earliest days -- the "Noachian Period", which ended roughly 3.9 billion years ago, in which Mars underwent its early heavy asteroid bombardment and also had its apparent dense atmosphere and possible friendly surface temperatures -- the hematite-rich rock layer was covered over by other material (windblown or deposited in other ways), and only re-exposed by a change in wind erosion patterns about 10 million years ago.

It is therefore the only really ancient surface left over from Mars' earliest days which has been both preserved from destruction and only recently re-exposed, thus enabling landers to examine its geology.

And if -- as seems very likely -- this rock layer does indeed contain a lot of coarse-grained hematite, that's very interesting in itself.

There are several ways this mineral can be produced, but most of them involve prolonged exposure to liquid water in some form.

Perhaps the most likely source is iron dissolved in a large body of water on Mars' surface, in which the dissolved iron is gradually oxidized into insoluble hematite before precipitating out to mix with the lake bottom sediment, which then later hardened into rock.

Alternatively, it may have been laid down by a large field of ancient volcanic hot springs, or even by extensive percolation of liquid groundwater through the local underground rock.

There may have been many deposits of similar ancient water-deposited hematite all over the planet, but most of these have either long since been eroded away to scattered dust or were covered over and are still buried. In Sinus Meridiani, we may have a unique "snapshot" of what the watery regions of ancient Mars were like.

And if the hematite was laid down by liquid water -- of any form -- that water could also have served as a promising dwelling place for ancient Martian microbes.

Hematite deposits on Earth are very good at preserving microscopic fossils of the microbes that were trapped in the sediment before it hardened into rock -- there's hardly any material that is better.

For these reasons, a subcommittee of the NASA Astrobiology Institute has picked the hematite region -- or, as it's starting to be called, the "Meridiani Formation" -- as being, on balance, the most promising of all the proposed 2003 landing sites as an area to look for fossil biological evidence.

Click For Part Four

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