Bob Clark wrote:
Then the reason why so many found Mars meteorites are young is that they stem from equatorial regions on Mars. It is known that the equatorial Tharsis volcanoes have experienced geologically recent eruptions. Perhaps most of the Mars meteorites stem from these lava flows.

Alex writes:
That's a good working assumption, but without absolute dates (as opposed to "relative" dating from stratigraphy and superposition) for Martian surface features we cannot be sure. The "geologically recent eruptions" discerned in Tharsis are based on cratering statistics, which I have confidence in, but which are nevertheless based on isochrons that have been calibrated and fit to the lunar production function. Clearly, what is needed are absolute dates, either by in situ measurement (e.g., Plescia's Urey mission concept for 2007 Mars Scouts) or by sample return. Interestingly, I recently had a tete-a-tete with Bill Hartmann regarding the issue of dateable samples, as opposed to in situ dating. Regardless of which approach is adopted, we both agreed that the best case to calibrate the entire cratering chronology sequence would probably be a broad, homogeneous area large enough to give good crater statistics, and not too modified; maybe middle Amazonian. These specs sound like a broad lava flow areas. Yet that might be the dullest, from the point of view of looking for signs of Noachian life. So we have a conundrum: go for the fossils or go for the age calibration? In my mind, one of the greatest scientific legacies of the Apollo lunar expeditions was the enabling of absolute dating for the lunar stratigraphic column.

As for Tharsis (e.g., Arsia Mons), although Hartmann et al. have formal solutions that give < 10 Myr dates for certain surfaces on Mars, they are more comfortable citing their result as saying that they have identified Mars lava flows less than 100 Myr. This is because the formal solutions depend on extrapolating the crater size distribution down into the diameter range < 200m, and on Gerhard Neukum's conclusion that he can actually see the "production function" of craters at those sizes, for example on asteroids.

At any rate, Hartmann and Neukum have a seminal paper in the works on this subject, which will appear within a few months in a special edition of Space Science Reviews. This issue will consist of papers on "Chronology and Evolution of Mars" from a series of workshops at the International Space Science Institute (ISSI) in Bern, Switzerland. The Kluwer publishers will then put a hard cover on that and it'll be sold as a book.

Reference:

Hartmann, W.K. and G. Neukum, Cratering chronology and the of Mars, Space Sci Rev, (in press).

Finally, I'll close with a quote from Bill Hartmann (from a recent conversation) that I believe distills the entire issue:

"...the point I try to get across to audiences is that it really doesn't matter if the answer is 10 My or 100 My in terms of impact on Mars science. The real issue is: Does Mars have geologically young volcanism? Because this truly affects our geophysical/geological understanding of the planet. A 100 My eruption happened within the last 2.5% of geologic time, and it is unlikely volcanism lasted 97.5% of Mars history and then shut off just before we got there. So either way, we have strong evidence (from crater counts AND dates) that thermal activity is continuing. (People have trouble getting this point because they focus just on the numerical result, and say 'Wow, a factor 10 error!')"

Alex R. Blackwell
University of Hawaii