Their color "Microscopic Imager" cameras, for example, only have a resolution of only about 1/3 of a millimeter, which is about as good as a hand-held lens, and as such only good for studying rock crystals and sediment grains, but essentially useless for detecting something as tiny as bacterial fossils.

But -- by both visual analysis and three different kinds of chemical analysis -- the 2003 Rovers can confirm whether or not an area really is the sediment bed of an ancient lake, and whether it's a good hunting ground for later, more sophisticated, landers which could return samples of it to Earth for infinitely more sophisticated studies that could firmly establish whether or not it contains any evidence of ancient life.

In particular, the rovers' "Mossbauer Spectrometers", which can analyze iron minerals in detail, can tell whether the commonest mineral in this area really is hematite, and how it was laid down physically -- and perhaps even whether (as an outside chance) some of the hematite was directly produced by ancient Martian bacteria.

And on top of all that, this area is a very smooth plain, safe for landing a spacecraft. So it's hardly surprising that it did end up as the definite favorite of the assembled scientists as a landing area for one of the rovers.

In the end, they decided on a list of 12 top-priority landing site candidates -- which will be the subject of further high-resolution imaging by Mars Global Surveyor to confirm their landing safety, even if MGS has to be commanded to tilt itself in order to point its bottom-mounted camera at them -- and 20 second-priority sites, which MGS should photograph but only when it's flying over them anyway.

The other seven possible landing ellipses were finally rejected on the grounds of excessive landing risk or inadequate scientific interest.

Of the finalists, landing ellipses in the Meridiani Formation ended up as five of the 12 top-priority choices and four of the 20 second-priority ones.

The general feeling now is that one 2003 rover should be sent to the Meridiani Formation because it's the safest and flattest of the candidate landing regions, while leaving open the the other Rover will be sent to a somewhat more rugged place on Mars.

But which of the two rovers should go to the hematite plain? Well, MER-B -- which is scheduled to land 35 Earth days after its predecessor -- suffers from a whole set of disadvantages. It can't land farther than 10 degrees south of the equator, which means that a great variety of interesting landing sites in the 10-15 degree S. zone -- including most of the possible crater lakes and Marineris Valley sites -- can be reached by Rover A but not Rover B.

Because of its later arrival, it's likely to have a somewhat shorter lifespan and thus travel less distance on the Martian surface than Rover A, because fall will be coming on in the south at about the time the rovers land, lowering the temperatures which the rovers must survive and steadily shortening the daylight periods during which they can receive solar power.

And Earth will be farther from Mars by the time Rover B lands, decreasing its landing accuracy -- so that its landing ellipses will be 10-15 km wider than Rover A, which in turn means that it must be aimed at bigger smooth areas on Mars to be sure of a safe landing -- and also somewhat reducing the rate at which it can telemeter science data back to Earth during the last stage of its mission.

For all these reasons, it seems very likely that MER-B will end up being sent to the hematite plain. Rover B's big landing ellipses fit better within it; since the area is especially smooth, it's likely that Rover B can travel a longer distance across it during its limited lifetime; and given the Meridiani Formation's uniform geological nature over long distances, it's more likely that the rover can make all its desired scientific observations there during its shorter lifetime.

Indeed, three of MER-B's five top-priority landing ellipses and two of its four second-priority ones are located in various parts of that large smooth plain.

The other two top-priority candidate sites are located near the south edges of the Elysium and Isidis Plains -- where ancient runoff valley networks from the bordering mountainous highlands have spilled onto the flat lower bottoms of those plains.

One of the remaining second-priority sites for rover B is also located on the Isidis rim -- with the other in a part of Mars' ancient southern highlands -- but the Isidis landing sites are less likely in any case because the European Space Agency's little Beagle 2 Mars lander is now officially scheduled to land in the Isidis Plain, about five degrees north of the Mars 2003 Isidis candidate sites.

Rover A, on the other hand, is both capable of surviving longer and capable of landing in geological areas which vary more from place to place, maximizing the scientific usefulness of a longer traverse.

Only two of MER-A's seven top-priority landing ellipses and two of its 16 lower-priority ones are located in the Meridiani Formation, with all the rest being in those other interesting areas. But in which of them should it land?

The Workshop group quickly homed in on two alternative types of landing sites: the floors of possible impact crater lakes, and sites on the floors of the canyons of the Marineris Valley -- with the craters coming first.

Click For Part Five

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