Prof. Za’arshi (zi of the explanation of the Blackbird Incident) is amused and alarmed that we have discovered the remnant radioactivity on Mars. Fortunately for zi’s peace of mind Mars was never anything but a remote base with a few installations concentrated in a few places, so we’re unlikely to find much even if we investigate the craters. Mind you, if anybody was outside the base when it was plastered with nukes, there are probably a goodly number of (ahem!) interesting artifacts scattered over the Martian surface, but we’re unlikely to find them with battery-powered, remote-controlled rovers.
Briefly: Sixty-five million years ago there was an interstellar war. Just who the other participants were isn’t clear, but the intelligent race of the Professor’s planet was one of the belligerents — and was utterly wiped out, to the point of extinction of practically all land-based life, which is why zi and his fellow sophonts look like octopi. They evolved from ocean-dwelling molluscoids who were among the few survivors of the war higher than an amoeba, and have as little in common with their predecessors as we do with the dinosaurs.
Earth was involved as collateral damage. The war involved both nuclear weapons and asteroid bombardment, but the weapon of choice is The Bomb because it can be delivered quickly by ship, as the Enemy outpost on Mars discovered. Hitting things with rocks is a fine way to destroy them, but it’s a slow process leaving plenty of time for anti-asteroid measures, including diversion. If the Chicxulub object had been allowed to hit Mars it would look very different today. Nobody ever intended that it hit the Yucatan, but when you’re in a hurry and trying to defend yourself otherwise-undesirable things become likely. Think of it as a stray round in a firefight.
Current scholarship suggests that Earth-human life evolved from the genetic material of the Enemy, none of whom actually survived but whose blown-up remnants drifted to the surface and began the long, slow process of rising from the muck. Arguments from genetics are of no account. All of us in this volume of space have the same biochemistry, and (within broad limits) the same genetic base. Why that should be is still a mystery, but it’s at least suggestive that the Professor’s home planet had a Cambrian Explosion that exactly coincides with ours, plus or minus a million years or so.
The Professor’s race doesn’t consider humans the Enemy, but having observed the devastation of land-dwelling life on their own planet they’re inclined to be cautious. “Warlike” and “violent” don’t come into it — if anything, their history is nastier than ours in that respect. (One of the reasons they began exploiting the land and building spaceships is that nuclear explosions in the ocean leave unpleasant effects that tend to linger.) However, if humans are descended (however distantly) from the Enemy, it’s best to take precautions, so they’re careful to avoid discovery and/or contact as they study us. All that business with Cthulu was a matter of renegades evading the regulations, long since settled by executing or jailing the perps. We have nothing to fear…