This would allow us, for the first time, to determine how accurate our current techniques are for estimating the age of different places on Mars by counting the number of meteor craters accumulated on their surfaces.

Up to now, all our estimates of the age of the surface of Mars in different places - that is, the time since it was it was originally laid down either as lava flows or as sediment deposits (or since it was exposed by a period of erosion in Mars' past) - have been based on such counts of accumulated craters both large and small, with the cratering rate of the Moon's history as a guide.

This is crucial if we are to understand both the history of Mars' internal geological activity over time, and the history of its climate changes - including the real time in its early history when it apparently lost its initial thick CO2 atmosphere, which may have made it warm enough for liquid water and thus life to exist on its surface. We can't judge how accurate our theories of Martian history may be until we have such reasonably good dating of its surface features.

Mars' geologic history has been divided into three main "ages". The oldest, the "Noachian", is the early period during which Mars was still undergoing very heavy bombardment by big objects - and during which (perhaps not coincidentally), it seems to have had a much denser atmosphere than today, perhaps even one capable of warming its surface temperature to the point that liquid water and life could exist there.

Judging from the counts of big craters, only a relatively small part of Mars' current surface still dates from that time; but that part contains most of Mars' "valley networks" which may be the beds of ancient rivers.

Most of Mars' southern highlands have few very large craters, but still have a large number of smaller ones - indicating that they date from the "Hesperian" age, during which the rate of bombardment of the planets slacked off dramatically, and during which most of the planet's atmosphere also disappeared.

But this was also an era during which the number of huge, short-lived "catastrophic outflow" floods from underground increased - and during which most of the great "Tharsis bulge" and the associated Marineris Valley formed on one side of Mars.

And virtually all of Mars' northern lowlands contain very low crater populations, and are thus said to present surfaces from the most recent, "Amazonian" era - during which the cratering rate dropped still lower, after the north had been resurfaced by great lava flows and/or by massive deposits of water- or gas-borne sediment.

The four great shield volcanoes on the summit of the Tharsis bulge also have flanks covered with lava flows that are uncratered, and thus must have occurred during this period.

But there are tremendous uncertainties in our current estimates of the periods these ages actually cover, because we are similarly uncertain of the rate at which cratering bombardment did slack off on Mars.

We have excellent knowledge of the ages of different terrain on the Moon, because we have returned samples that can be radioactively dated for comparison with the number of craters on their origin areas. But since Mars is so much closer to the Asteroid Belt, we are very uncertain how fast cratering slacked off for it.

Estimates of the end of the Noachian Age, vary from 4.3 to 3.85 billion years ago - and since cratering slacked off very dramatically for the Moon about 3.8 billion years ago but we don't know whether this also happened on Mars, estimates of the end of the Hesperian and the start of the Amazonian vary wildly from 3.8 billion years ago to a mere 1.3 billion years ago! And our estimates of the ages of individual areas within those terrains are of course comparably murky.

Dating the ages of rocks involves very complex analyses of elements existing in only traces within them, and it's usually been regarded as requiring the return of Mars samples to Earth labs.

But Plescia and his co-experimenters are convinced that even getting the ages of Martian surface areas down to an uncertainty of 15 percent would be a huge increase in our knowledge of the planet, and that we now have miniature instruments which could be carried on Mars landers to do just that.

Thus Urey, which would soft-land and age-date the rocks in a single place on Mars - so that, by comparing that age with the local crater count, we could finally get a reliable estimate of how the overall cratering rate on Mars has compared with that of the Moon, providing a "benchmark" from which we could estimate the ages of Mars' other surface formations with far greater confidence.

To do this, Urey will try to land in an area where virtually any rock it analyzes is likely to be the same age - but most of Mars' surface is covered with rocks and soil that have been carried there from other places by wind, water, or the ejecta from distant giant craters, making any estimate of the region's age from just a few analyzed samples far less reliable.

So it will aim for the Cerberus Highlands, a region covered with some of the most recent lava flows on Mars - which judging from their sparse cratering, are not only recent but all about the same age.

The lava flows there may be very young indeed; William H. Hartmann estimates from his crater count that they may be as little as 10 million years old, but Plescia thinks they're more likely about 200 million years old, and that some of the youngest rocks found among the Mars meteorites may come from here.

This also means that dating the rocks here could only give us a measurement of Mars' cratering rate in recent times, and there would be uncertainties in how it applied to earlier ages - but Plescia still thinks that, given the uniformity of age in any rock samples analyzed here, this is the best spot on Mars for the purpose, especially given the fact that they seem to have very little wind-blown dust covering them.

Urey's rover would use a drill originally developed for the Mars sample return mission to collect rock cores about two centimeters long, and then date them with two different techniques for comparison. In both, the rock would be struck with a small but very high-powered laser, hot enough to boil some of the more volatile elements in them into gas whose isotopes would be analyzed by mass spectrometers.

In one, the laser system would be used to measure the radioactive potassium-40 in the rock - after which the rock core would be ground into powder and roasted in a tiny oven to release argon-40 produced by the potassium's decay and then trapped inside the rock.

Since any argon produced by this process before the rock solidified out of lava would have escaped, by measuring the ratio of the two elements, we could estimate how long the rock has been in a solid form from which the argon couldn't escape. The tiny British Beagle 2 lander will try to use a cruder version of this technique when it lands on a sediment-covered plain in 2003.

In the other technique, the very narrow laser beam would be fired at several different spots on the rock core, thus analyzing grains containing different types of minerals with different amounts of the element rubidum.

Rubidium-87 is mildly radioactive and slowly decays into strontium-87 - so, by measuring the amounts of rubidium in the various grains and also measuring the differing ratios of strontium-87 to strontium-86 in the same grains, we can judge how long the grains have been solid and thus trapped different amounts of strontium-87 within themselves.

The original plan was for Urey to be a copy of the two 180-kg Athena rovers that NASA plans to land on Mars in 2004 using the Pathfinder airbag system, so that it could then crawl as much as a kilometer across the landscape to collect its samples.

But Plescia now has some doubts that this rover design could carry those larger age-dating instruments without major modifications - and since all the rocks in Cerberus are likely to be the same age, he is considering a cheaper alternative design which (like CryoScout) would use the unflown Mars Surveyor 2001 lander, and collect all its rock samples from just a few dozen meters away using a much smaller rover that would collect the samples with its core drill and then return them to the dating instruments on the main lander. The rover might even be tethered to the lander by a signal cable, allowing its control computer to stay on the main lander.

Urey would also carry other instruments. Depending on the design, either the big Athena rover or the stationary Surveyor lander would carry a stereo camera and a mineral-mapping near-IR spectrometer on a mast - and either the big rover or the small one would carry an arm with two other instruments to analyze rocks on the spot.

One would combine a magnifying camera, an X-ray spectrometer to measure rock elements, and an X-ray diffractometer to analyze specific minerals and which, unlike most diffractometers, wouldn't need to grind up the rock first.

The other would use UV fluorescence to look for tiny traces of organic compounds in the rocks - and while it couldn't tell whether such organics came from ancient microbial fossils or from nonliving sources, just knowing whether some traces of organic compounds can survive on Mars' surface despite the destructive chemical processes discovered by the Vikings would be important.