I assumed that paste-up explosion representeed the crash-landing of the spaceship.
Bradbury was all over the place in his drafts, in terms of how the aliens were represented. He's not consistent from draft to draft; he alludes obliquely to spiders and lizards, gossamer hair, glitter, smoke, etc. He evidently didn't want to be put in the position of actually flat-out describing the aliens, which, of course, presented problems for the poor guys who actually had to make them. Yes, they had a solid alien--the frowning chicken eye with the big dome--but what >else< do we see of it? Bob Burns and Bob Skotak are both convinced that the aliens had long, spindly arms with spidery hands; I never saw that myself, but I do see what they're referring to. There was a lot of smoke involved, plus "angels' hair"--the spun glass once frequently used for holiday decorations. But there also seems to be a jelly-like mass sometimes, or something that looks like gelatin with craters in it (this is the stuff reaching around the miners). When I was a kid, I was determined to draw a cartoon-like picture of the aliens; I concluded they had a hard, shell-like body part, with jelly-ish stuff below that, something like a tall snail with one eye. I'm fairly well convinced that the idea was to present an alien that couldn't really be described--which returns us to what Bradbury was doing.
The studio actually had another, quite diferent, Xenomorph made--it was included in the Collier's article on Universal-International monsters, turns up in the last Hitchcock scene in "The Sign of Satan" (Alfred Hitchcock Presents), and finally ended up in the collection of William Malone. He gave me a photo of it to include in the new edition of KWTS. And the design of >that< one was reshaped into the head of the Mutant in THIS ISLAND EARTH. It has two eyes that look rather like they could extend on stalks; the eyeballs don't look like earthly eyeballs at all. There is no mouth, just a textured area where the mouth would go. It's a solid piece, with the top part made to look as though it's semi-transparent.