Oneiric travel to Mars in rocket ship made from scrap

Question

This was possibly published in Omni, or some other magazine, about 1979.

A group of boys build a rocket ship from scrap in their backyard. They enter into it for playing, and when they get out they are in a very John Carter-ish version of Mars. They even think they see a green, four-armed giant warrior.

They come back to the rocket and return to Earth. One of them realizes he lost his penknife during the tour. They never comment on their experience, thinking it was just a dream.

Decades later, the boy who lost his penknife becomes the first astronaut to walk on the airless and dry real Mars. Inspecting the rocks, one brilliant object draws his attention. He retrieves it and realizes that it is the penknife that he lost when he was a child.

Answer 1

I believe this may be "The First Mars Mission" by Robert F. Young, which first appeared in the May 1979 issue of "The Magazine of Fantasy and Science Fiction."

Your memory of "valles marineris" appearing verbatim lead me to a fragment of a story in Google Books that did not particularly look like it matched (and searches steadfastly refused to both name the exact issue or the author or any details more than about a paragraph of text), but since it did look like it was in a magazine from 1979, I did some digging and found the title, and searching on that, I found this site which describes the story:

Between the Hard SF and Planetary Romance stories of Mars falls this one interesting tale that serves as a sort of bridge between the two: "The First Mars Mission" by Robert F. Young (1979). "The First Mars Mission" tells of a group of young boys who build a space ship in their backyard and take a fanciful trip of the imagination to Mars. But when one of the young boys grows up to be an astronaut and travels to Mars for real, he discovers that his boyhood fantasy journey may not have been so fantastic after all. This story—half fantasy, half science fiction—first appeared, appropriately enough, in The Magazine of Fantasy & Science Fiction.

Which looks like it matches pretty closely to your own description.