A week or so ago, I prognosticated that some craters on Satturn's moons looked like they were from gentler impacts. Lo, for a Nature article promptly arrives saying the same thing, as does a Mars Global Surveyor shot of secondary craters in western Arabia Terra.
The Nature authors (Edward Bierhaus, Clark Chapman and William Merline of Lockheed Martin’s Space Exploration Systems [link features a cool GoogleMaps-like satellite imagery navigator] division and the Southwest Research Institute) are not shy about the implications:
We now return to questions of crater distributions in the Solar System in general, and the implications for impactor size distributions and age dating. Laboratory-scale experiments demonstrate that impacts into ice and rock targets yield ejecta fragments with steep SFDs [size–frequency distributions], with ice-impacts generating ejecta more efficiently. [...] The Moon and Mars both possess a steep branch in their crater SFD at sizes below a few kilometres. We integrated our derived secondary cratering efficiency over the visible large (10 km < D < 64 km), post-mare, primary impact craters on the Moon. We calculate that the steep branch of the lunar crater SFD at small diameters could be fully accounted for by secondary cratering, even if the Moon’s secondary cratering efficiency is less than Europa’s — that is, even if rock targets generate ejecta less efficiently than ice targets.
Our work raises doubts regarding methods that use the lunar small-crater distribution to calibrate other inner Solar System surface ages (for example, Mars). If, as on Europa, lunar and martian secondaries are 95% of the small crater (less than a few kilometres) population, the error bars (and thus derived surface ages) on any residual primary crater population become large (uncertainties are 20 times the measured density value). This uncertainty applies to both the measured population on a martian surface unit and the lunar SFD that supposedly represents absolute age. [emphasis mine]
It always causes scientists angst when a tool that they “know” is accurate turns out to be nothing of the sort, but the resulting demise of complacency generally results in a sustained effort to find a replacement which actually works, and they’re more inclined to actually test the new theory — give it a bit of a rattle to see if anything shakes loose — before recklessly building other hypotheses upon it.
There is generally also a scramble to defend now-baseless pet theories as well, of course, to make them sit comfortably with the replacement tool even if they obviously don’t and can’t. This is a bit sad to see but also kind of inevitable given human nature, and of course sooner or later truth will break out despite the best efforts at painting over the cracks.
One phrase in the above made me pause for thought. “Small” craters are regarded as being “less than a few kilometres” across. It’s a bit awe-inspiring to imagine an event which the mere shrapnel from could obliterate everything between West Perth and the Causeway, inclusive.