Is it possible to impeach a Pope?
God will impeach him.
Is it possible to impeach a Pope?
How about a recall.
No, but condoning heresy causes him to lose the office automatically, after which the Cardinals can elect a new one.
It's never actually happened before is the problem...so though it seems to be a generally accepted theological principle that the Pope can lose his office for heresy, there's no "case law" on how the process unfolds.
Not coincidentally, next month there just so happens to be a conference in Paris on the Deposition of a Pope.