One crater that has long been considered very promising is Gusev, 160 km wide, which has a great and apparently long-lived outflow valley running into it that may even have left a delta of sediment at the outflow's mouth on the crater floor. But while this remains a top-priority landing spot for MER-A, recent MGS photos suggest that it may not be as good as once thought.

The crater's floor seems to be largely covered with windblown deposits that may have covered up its original lakebed sediments; and -- because the original MER-A landing-error ellipse is located in the middle of the crater, in order to make sure that the rover will land within the crater walls even if it touches down at either end of the long ellipse -- the ellipse is too far north for the rover to reach that intriguing "delta" on the crater floor's south side.

So, while that landing ellipse still remains one of MER-A's second-priority sites, and the team will also soon try to pick a better-situated landing ellipse within Gusev that will be one of the rover's seven top-priority sites -- more attention is being focused on another crater: Gale.

Gale -- 170 km wide -- is the site of one of the most spectacular displays of that multiple sedimentary-rock layering, found here and there on Mars, which Malin and Edgett described in their famous paper in "Science" last year.

Fully 11 major rock layers, subdivided into hundreds of thinner ones -- totaling 2.3 km thick, twice as thick as the Paleozoic layers of the Grand Canyon -- are visible sloping shallowly along the inner sides of its crater walls.

Malin and Edgett think it likely that these layers were laid down by alternate episodes of flooding and drying-out of the crater during hundreds of millions of years of early Martian history.

But it's possible that they are instead made of layers of volcanic ash thrown out by occasional giant Martian eruptions, or layers of wind-deposited dust laid down during repeated periods when Mars' changing climate has caused its atmosphere to thicken somewhat and traces of water to move from its polar caps into the atmosphere and then refreeze in the equatorial regions. In either case, the dust or ash could have been gradually cemented into rock by relatively small traces of soil water.

Nonetheless, Gale is the best display of Malin-Edgett layering accessible to either of the 2003 rovers. Again, the MER-A landing ellipse must be planted in the middle of the crater so that the rover can be sure of landing within the crater even if it touches down at either end of the ellipse -- but while this makes it hard for the rover to have any chance of reaching the main displays of rock layers along the crater walls, there are signs that "this layered material may extend into the mid-regions of the crater."

And if MER-A is able to confirm that the layering was laid down by a long-lived lake in the crater during Mars' earliest days, those layers of sedimentary shale would, once again, be a splendid place for later sample-return missions to collect rock samples for examination by Earth scientists for possible microfossils.

For this reason, Gale crater is probably the second most favored landing site for one of the 2003 rovers. The NASA Astrobiology Institute group regarded it as a close second to the Meridiani hematite plain, and it's another of MER-A's seven top-priority sites.

However -- besides Gale and Gusev -- four sites in three other possible crater lakes are also in the Lander's second-priority list.

Moreover, it should be remembered that if these really are crater lakes, the accessible sediment on their floors apparently doesn't date from Mars' early, and apparently more friendly, Noachian Period, but from the later Hesperian Period.

During this later period there may have been repeated short episodes of dense atmosphere and surface liquid water, but Mars on the whole was already a more hostile place with a far colder and thinner atmosphere.

As such, the Sinus Meridiani Hematite Plain and the runoff areas around the Isidis Rim are the only rover landing sites where we can be confident of finding ancient Noachian rocks.

Click For Part Six

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