Simply Technical
That is, Robinson's writing style is both simple and technical. Yes, that is an oxymoron, but no less accurate for being so.
Just consider this small example:
All these adapted abilities are coded and passed along in genes. If the genes mutate, the organisms change. If the genes are altered, the organisms change. Bioengineers use both these forms of change, not only recombinant gene splicing, but also the much older art of selective breeding. (4.1.2)
Robinson is discussing some pretty heavy science concepts here—you could fill biology books with the genetic concepts and theories he's riffing on. In fact, they have.
But Robinson keeps the sentences straightforward and the words simple (minus the necessarily long and, let's say, science-y words). Although the writing style requires technical details, the style manages to maintain readability and artistic quality that'd you'd expect from a literary novel. It's like if a science textbook and a novel had a baby. That baby would be Red Mars.