impenetrableness vs impenetrable

impenetrableness is a noun but impenetrable is not a noun.

Word	Noun	Adjective	Verb	Adverb
impenetrableness	Yes	No	No	No
impenetrable	No	Yes	No	No

impenetrableness (noun)	impenetrable (noun)
incomprehensibility by virtue of being too dense to understand

impenetrableness (adjective)	impenetrable (adjective)
	not admitting of penetration or passage into or through
	impossible to understand
	permitting little if any light to pass through because of denseness of matter