The New Scientist is reporting that a method used for dating ages for the surface of Mars is seriously flawed. Nadine Barlow, the person who has catalogued 42,000 craters on the Martian surface, says that recent images done by the Mars Odyessey project suggest that crater counts overestimate the number of impacts on Mars. This in turn would mean that most martian surfaces are somewhat younger-- and Mars has therefore been somewhat more active-- than has been calculated in the past.
The age of the surface of Mars is calculated by counting craters . Heavily cratered regions have been geologically still for a longer period, whereas lightly cratered regions have somehow "erased" their allotment. To assign a date to these counts, the crater numbers have been compared to regions on the moon , which in turn have been precisely dated by lunar rock samples.
Barlow's comment is that cratering occurs differently on Mars than on the Moon. Even smallish Martian craters have extensive rays of ejected material, suggesting ejection of much more material than calculated, and the creation of millions of small secondary craters as the "splash" from the first impact falls back down. Thus regions might acquire cratering faster than previously recognized, and a given region should be assigned a younger age than a strict Moon dating scheme would give.
I'm a little confused by this news, and I have not been able to pick it up anywhere except New Scientist. In fact, Dr. Barlow was apparently saying just the opposite in 2003: that Mars makes fewer secondary craters than the moon, arguing, at the time, because of subsurface ice. What is abundantly clear from Google is that she's the expert, so it may just be the reports which are confusing me.
In the meantime, check out the unfortunately named PIGWAD interactive map of Mars. I launched the Mars Craters survey and could zoom from the whole globe down to features of 5 km. Now if I could only use this to find my glasses....got 'em!