On the floor of Gusev Crater, Spirit is only a little more than 300 meters from the western spur of the "Columbia Hills", which once seemed so distant on the horizon of its original landing point 2.6 km away.

As the months passed, and the rovers continued to perform well, the operating team has learned how to make better time, driving the vehicles.

This includes revising their "Auto-Nav" software so that they can use their "Hazcam" hazard-detection cameras to identify genuinely dangerous rocks and slopes without shying excessively from small and harmless objects.

As a result, Spirit now regularly drives 120 meters a day, whereas at the time of landing, 100 meters a day was considered a wildly optimistic goal for it. Its arrival at that western spur- referred to as "West Spur"- is now imminent.

It's also scientifically fortunate. Gusev was selected as a landing site because it seems to have been the drainage point for Ma'adim Valley, which seems to have been one of ancient Mars' biggest long-lived watercourses.

During Mars' middle or "Hesperian" Age, when it was in the process of losing almost all its atmosphere and thus freezing over from loss of its original dense air's greenhouse effect), it was speckled by large number of "catastrophic outflows" - places where the pressure of the advancing sheet of underground permafrost seems to have exerted great pressure on the planet's remaining deposits of underground liquid water, untimely forcing them violently to the surface in floods dwarfing any ever seen on Earth. But these were very short-lived, lasting at most a few months apiece before choking off freezing solid on the surface and later vaporizing into the planet's new ghostly-thin air.

By contrast, Ma'adim -like many of Mars' much smaller winding "valley networks" from its oldest Noachian Era - seems to have had water flowing through it, at least intermittently, for very long periods, and so it's entirely possible that Gusev served as a long-lived water-filled (and perhaps ice-covered) lake that could have made it a good abode for ancient microbial life.

Indeed, it's possible that Ma'adim had water flowing periodically through it into Gusev through most of the Hesperian Era, perhaps up to only two billion years or so ago. During that period, both Ma'adim and Gusev may have been ice-covered.

But Gusev has been a very frustrating object of study for Spirit. Its possible days as a lakebed - or at least as a plain periodically drenched by repeated water flows from Ma'adim - ended two billion years or more ago, and the rover has discovered that since then any sediment deposits that were laid down on most of its floor and hardened into layers of sedimentary rock have long since been buried by more conventional Martian surface material.

Not only is Gusev's main plain covered by windblown dry basalt-lava sand - whose high content of the mineral olivine proves that it can never have been exposed to liquid water for other than extremely brief periods - but all the rocks Spirit has analyzed seem to be made of the same never-wet basalt.

Some of these basalt rocks have a thin water-weathered crust, suggesting that they have been repeatedly exposed to thin films of liquid water over the eons during Mars' endless hundred-thousand-year "obliquity cycles" of increasing and decreasing axial tilt, which have dramatic effects on the planet's seasonal weather even today.

But we are still talking about only very small amounts of liquid water, not the kind of extensive surface or near-surface liquid water which scientists hope to find evidence that Mars had during its earliest days, and which the two rovers were in fact designed to look for (and which Spirit's twin "Opportunity" has in face found on the opposite side of the planet).

Gusev's floor- since at least the Hesperian Era- has not been exposed to large amounts of liquid water from Ma'adim. During the succeeding two to three billion years, any sedimentary rocks it acquired have not only been covered over by wind-blown dry basalt sand - and possibly by repeated eruptions of volcanic ash from the big Apollinaris volcano 250 km to Gusev's north during the first half of Mars' history - but, it now appears, may also have been covered by actual basalt lava flows from some volcanic vents either on the floor of Gusev or near the crater's edge during the same period.

Alternatively, The floor of Gusev's crater may have been largely covered by basalt rocks, thrown into the crater as ejecta, from several other large impact craters, formed nearby during this period.

So, after Spirit had thus discovered that Gusev's main floor is distressingly typical of most of Mars' present-day surface, its operators decided to waste little time continuing to explore the rocks and soil on the Gusev plain in detail, and instead to have the rover hightail it for the Columbia Hills - for that clump of little hills, no more than 90 meters tall, seeming from orbital pictures to be an entirely different kind of surface: a surviving remnant of the surface laid down during Gusev's early days.

This "Etched" surface type -as it's been named on the basis of orbital photos of Gusev - can be found in little hilly clumps sticking up above Gusev's flat present-day surface, mostly in the crater's southeast parts. But the denser accumulation of small meteor craters on it indicates that it is actually a considerably older surface than the flat "Plain" material.

Apparently the Etched hills represent the remains of original much taller hills on Gusev's original surface - or else patches of relatively hard material in that original surface, which stubbornly resisted being eroded away as much as the surrounding original material - and thus they remain defiantly sticking up today above the later lava flows and windblown sand that later flowed over and deeply covered the other deeply eroded remains of Gusev's earliest surface.

And that surface, as we've seen, may well be composed not of boring basalt lava but of ancient water-deposited sedimentary sandstone or shale, which would be a very promising possible target for future landers to look for microbial fossils.

(Just proving conclusively that Mars has substantial layers of ancient sedimentary rock laid down by water rather than being hardened out of windblown sediment would be enormously important for that search, even if the particular patches of it that the near-future landers explore for fossils are nowhere near Gusev.)

Indeed, the orbital photos of the Columbia Hills - like those of many other Etched hills and small crater walls inside Gusev - show clear evidence of layering. There are at least four clearly visible separate strip-shaped flat terraces of rock exposed on the Hills' southern slope - a clear sign that the Hills are made of separate layers of rock with somewhat different compositions and hardness's, laid down at separate times, which have gradually eroded at somewhat different rates since then.

But it still leaves open the possibility that the Hills are not composed of layered sedimentary rock of varying composition and hardness, and made out of layers of sediment deposited on Gusev's floor by Ma'adim during its apparently repeated periods of water flow.

They may, instead, be a surviving remnant of still earlier repeated and stacked flows of lava across Gusev's early floor, or even layers of hardened volcanic ash from early nearby volcanic eruptions.

Some geologists - such as MER science team members James Rice and Hap McSween - think that Ma'adim's ancient water outflows never actually filled Gusev, but only flowed shallowly northward across its western part, producing a long strip of deposited aqueous sediment and a bunch of small temporary lakes and ponds in that region while leaving the rest of Gusev, including Spirit's landing area composed of dry volcanic rock.

As you can imagine, however, Spirit's scientists are eager to find out. Thus their long but increasingly fast trek southeast across the plain toward the Hills since the end of March - after spending the first three months on what turned out to be a wild-goose chase 350 meters northeast to the edge of 200-meter-wide "Bonneville" crater- turned out to be much too shallow to have dug up any possible sedimentary rocks from the older floor material, buried dozens of meters deep below the present surface of Gusev plain.

At this point - while Spirit's own pictures of this side of the Hills still fails to show any visible sign of layering - it can see a very dense scattering of loose boulders on the hill slopes, many of which have rolled down onto the nearby plain to form what geologists call "talus". So - instead of the rover having to make an impossible climb up the hill slopes to sample their possible different layers of rock - the mountain has come down to the rover.

By comparing infrared Mini-TES spectra and 13-color near-IR Pancam photos of the hills at different heights with similar spectra of the detached boulders at the hills' feet, Spirit can identify boulders that originally came from the different layers, and approach them directly to analyze them with the other compositional instruments on its robot arm.

(The latest photos also show one especially steep slope near the foot of "Husband Hill" - the highest of the Hills, and the one which the rover is approaching - where loose debris and soil seem to have completely slid off the slope face to expose bare outcrops of the underlying bedrock of the hills. These are inaccessible to the rover except for its remote sensing cameras and Mini-TES spectrometer, but will be carefully examined by those.)

The rovers were originally designed to operate for a minimum of only three months, but Spirit is still going strong after five - working virtually perfectly except for occasional delays due to correctable software errors, such as the one that paralyzed it for two straight weeks in January and two more that held it up for a total of five days over the past few weeks.

Both rovers have now officially been assigned "extended missions" that call for them to work for at least eight months, and may very well work somewhat longer than that. At the rate Spirit is going, this will give it time to do a considerable amount of poking around the flanks of the Columbia Hills.

Eventually, though, it will suffer a fatal power loss due to the combination of the slow accumulation of windblown dust on its solar panels and the fact that Gusev is now entering Martian winter with its shorter days and colder, power-consuming nights.

The tentative plan at this point is for Spirit - after one brief final stop to check out the rocks at the rim of a small crater called Tecopa - to reach the edge of the hills' West Spur next week. It will then make its way about half a kilometer further southeast along the foot of the hills - carefully inspecting them and analyzing the detached rocks at their feet - until, about two weeks after reaching West Spur, it climbs a particularly gentle slope to a place called "Lookout Point" on this side of the hills about 15 meters above the plain.

This will give it an excellent view further east into a large "basin" at which the lower plain bites into the west side of the hills. From this point, it can get a good view of the four terraced layers of rock that have been spotted from orbit, on the hillside that makes up the north side of this basin.

It cannot reach them directly; but after completing this relatively high-altitude map of the surrounding terrain, it will proceed eastward into the basin and skirt the hillside at its outer rim, curving east, then south, then west again and out of the basin - and then as far down the edge of the hills as it can manage.

Further details of this exploration plan - and, for that matter, the current details - will of course depend on what interesting things it discovers at the hills. If scientists discover that even the Etched hills of Gusev consist of just older layers of volcanic rock and the rover never finds the hoped-for signs of sedimentary rock layers in this part of Gusev, they will of course be disappointed - but this was always considered a risk when this crater was picked as a landing site.

And even in that case, Spirit's studies will have provided invaluable "ground-truth" data to allow a more accurate interpretation of the tantalizingly ambiguous orbital photos and spectra obtained by the American and European Mars orbiters currently mapping the planet - and, particularly, the far more detailed maps that America's "Mars Reconnaissance Orbiter" is scheduled to make after it reaches the planet in 2006.

The second part of this series will look at the action about to take place at Endurance Crater. Due to a mix up in article processing, this article was mistakenly entitled as an Endurance Crater report. Sorry for the confusion.

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