I’m more interested in apparent magnitude. If we had a patch of the Sun one micron wide on Mars, it wouldn’t be visible from Earth.
You also have to take into account the fact that most of the radiation from a nuclear weapon is not in the visible range, but detonating it above the surface would cause local heating and re-radiation, so hopefully we could see that.
Another factor is the diffraction limit associated with the human eye - if a flash is too small, the human eye would spread the flash out into a diffuse blob, making it difficult or impossible to see. I expect modern electronic telescopes would be able to get round that limitation somewhat, but there would still be a limit on how small a bright explosion could be and still be detectable.
Another problem is that Mars always has its daylight side pointing towards Earth. The brightness of the flash has to be brighter than the reflected sunlight in the region that the flash occurs in by a significant margin, once you allow for diffraction effects.
Of COURSE you can see the result. Look at this clear documentary example of what an atomic detonation on Mars looks like. Why, the mushroom cloud is almost as high as the diameter of Mars!
https://www.amazon.com/Topps-Bubbles-Attacks-Earth-Bombs/dp/B016CHVYNQ
I expect that by the time we are ready to attack Mars in the Bubblegum era, we will have bigger nukes.
Got a small strike two days ago. (“Small” being a relative term.)
I’d heard about two of those, but I’m not surprised there’s been more. Looks like they’ve been keeping a sharp lookout for them (video recorders help a whole lot). “They” being amateur astronomers here.
Shouldn’t we all be?