I am looking for a paperback novel I read in the mid 1980s. The cover had astronauts looking at a frozen monolith on Pluto.
The story takes place in the future where mankind has learned to live for centuries at a time, but can only remember the last few years.
There is an expedition to Pluto, and everybody thinks it is the first time humanity has been out that far. When they get to Pluto they find evidence that humans had already been there many years before. The main plot of the book revolves around this.
There is a possibility that I mis-remembered and instead of Pluto it is a moon of Jupiter or something similar.