Readers always seem to prefer the short answer, but invariably they only trust the longer one.
So, to answer the question in the title of this essay, I will first try to provide that simple answer everyone cherishes. Then, I will work through a longer answer, attempting to avoid excessive appeals to your emotions. To put it concisely, rhetoric of science is a metaphor. And, like all metaphors it invites us to view and conceptualize science from a particular motivating perspective. So, words like invention, style, ethos, topoi, narrative, and genre that are mainstays in any good-faith discussion of rhetoric can now be used to explain scientific discourse.