The phrase the literate is what is known as a fused Modifier-head noun phrase (fused Modifier-head NP, for short).
This noun phrase has an elipted noun which, although missing, is understandable from the context. The head word in this noun phrase is the word literate. Althought is the Head of the noun phrase, it is also understood as a Modifier of the missing word.
If we wanted to we could model the noun phrase like this:
Here it is easy to see that the word literate is an adjective even though it is Head of the noun phrase. If we want to modify this adjective—in other words, we want to say that the people are literate in a scientific way, that they are literate about science—then we need an adverb, not another adjective:
- the scientifically literate [
- *the scientific literate [
people] (ungrammatical with this meaning)
- the emotionally literate [
- the emotion literate [
The Original Poster asks if we can modify nouns with adverbs. The answer to this question was previously thought to be "no". However, in the past decade or so research by John Payne, on occasion in collaboration with Huddleston and Pullum, has shown that adverbs can, in fact modify nouns. However, they can only post-modify them (they must appear after the noun they are modifying). There are other types of restriction too. Here's an example with the noun people and the adverb globally:
- People globally are becoming concerned with what is happening to their local environment.
Here is a link to an important paper by Payne, Huddleston and Pullum where they show that adverbs do in fact sometimes post-modify nouns: