for age dating purposes.

from: http://ida.wr.usgs.gov/html/m23012/m2301255.html

This is a Northern Lowlands segment at 33.7/181.67 that appears to be part of a flooded plain on which a number of craters and circular formations have formed.

In the upper part of the view is an obvious fresh impact crater, with an astonishing fan of ejecta sprayed around it. This ejecta is remarkably dark, and it does not appear to be composed of material excavated from the crater. Given the discovery of pitch black asteroids and comets (viz: Mathilde and Halley, et al) it could be that the dark material is the shattered remains of the impacting body, a bolide which may have been nearly pure carbon. A rough ring around the crater, about a crater diameter wide, seems to be free of impact debris. This ring may actually be covered with material excavated from the crater.

Just below and to the left of the crater is another formation that looks almost identical in size and shape to the upper crater -- but without the fan of debris. Indeed, there is no debris of any kind visible around this crater. The rim of this crater is not circular, but is more scalloped. Despite the superficial resemblance between these craters, they may be the result of different origins, one certainly the result of impact, the other possibly the result of internal forces.

The surface here appears to be relatively freshly deposited volcanic material. We can see plenty of flow lines and ridges.

Below the craters previously described, there are two more craters, one of which appears to be a classic bowl-shaped impact crater (though lacking visible ejecta) and the other, nearly identical in size, appears to be a ring formations, like thousands of others on the Martian surface, with no ejecta visible, and an apparently previously fluid interior. Again, we may be seeing two similar features caused by different forces -- one internal the other external.

Next we see something remarkable: a figure 8 shaped pit, with what looks to have been a once fluid interior. There is no sign of impact origin here. But nearby is a soft-looking circular pit with a ragged rim. It's origin is a mystery.

Further down the scene we find a relatively large ring formation with a fan of debris inside the rampart. Exactly what this debris is is a mystery. Its fan-shape is mirrored in thousands of other craters, many of which look as if they might have filled from below. There is some ejecta associated with this formation, mostly visible to the right.

To the right and below this ring formation are a number of small circular formations, one of which looks like a bowl shaped crater, the others are barely visible. One appears to be a mesa!

Signs of flow here are strong.

Further on we see ropy flow margins and more craters, each of which seems to be somewhat unique, none of which show definite signs of impact origin. All seem to have a very smooth interior.

In this segment of the Northern Lowlands, only one of the 40 or so circular formations visible can definitely be ascribed an impact origin. As the surface material here appears to have once been fluid and to have flowed, covering a former surface, it's reasonable to assume all the circular formations postdate the flow.

Some of these craters appear to have formed in the surface when it was soft and partially fluid, but how they formed cannot be discerned from the image here.

All but the single impact crater may be pits formed by internal forces.

This is not necessarily a representative area, and of course the features here are quite small. But if only one in 40 circular formations here are actually impact craters, and if a roughly similar ratio (even if it's only 1:4) of impact to other origins is found elsewhere on Mars, then ages derived from crater counts may be well off the mark.

And what we might find then is that surface activity on Mars, including the flows that formed the surface here and in many other places, some of the channeling and rifting, and a good deal of the volcanic activity is orders of magnitude more recent than widely assumed by reference to traditional age-dating by crater counting.

And if it is more recent, then potentials for biology and subsurface reservoirs of liquid water (among other things) may be a good deal higher than is generally believed.

When all (or almost all) circular formations on the Martian surface are identified as impact craters by default, or are simply assumed to be impact craters without further bother, age-dating by crater counting can be inappropriately and wildly skewed toward "ancient," when in fact surfaces may be much younger.

But I could be wrong! ;-)

Cheers, BFelix