Hello Nick -

Always interesting to muse about, but the detailed work for this is either the job of others or their avocation. My own contributions to the field are limited to retrieving some of the records of historical impacts, something which I have the skills and ability to do, along with some reporting, serving as the eyes and ears of others.

With that warning, I don't know if any rule of thumb holds up for Mars, given the clathrates. Given the energy release set off by impacting those clathrates, even converting crater size over to energy is tough, and then you get into impactor composition and relative velocities.

Given my pretty complete lack of skills here, I'm afraid I can't even guess what caused that couple percent which is observed. So I'll just wish everyone involved good luck with it, and wait for their results.

best wishes
EP