what is the type of atmosphere is created in part 1?

Although Marlow is an experienced sea-man, the African Congo is a very alien world to him. We see the people and landscape as if we were there. Conrad describes the scenes rather than tells as if he is entering some strange planet,

{"Black shapes" crouch on the ground, and "creatures" walk on all fours to get a drink from the river. They are called shadows: reflections of humans, not substantial enough to be real. Marlow observes the piece of white string on a young man, and he is taken aback by how much the whiteness stands out against the darkness, thinking about the string's probable European origin. He cannot seem to conceive of mixing black and white.}

Conrad paints a picture with a definite sense of "otherness" which is maintained through much of the novella. This is, of course, quite disturbing to a reader. Human rights were non-existent and the suffering and indignities the natives felt is overwhelming.

what is the pupose which conrad to achieve?