Matter-of-Fact
Part of the distance of tone is achieved by the matter-of-fact prose. Lem describes the most impossible, miraculous, or terrifying situations in the same flat voice. Check it out:
But the spasms resumed, and again I had to hold her down. Now and then she swallowed drily, and her ribs heaved. Then the eyelids half closed over the unseeing eyes, and she stiffened. This must be the end. I did not even try to wipe the foam from her mouth. A distant ringing throbbed in my head. (9.118)
That's Kelvin describing the gruesome death by liquid oxygen poisoning of the love of his life, using language appropriate to describing a rather uninteresting experiment. "But the spasms resumed." "This must be the end." That is one low-key horrific death scene.
The matter-of-fact dryness becomes almost like a parody of itself in the passages where Kelvin goes on at length about Solaris history and Solarist theories. But it can also attain a kind of blank poetry, as in the final paragraph, when Kelvin's longing and confusion becomes more poignant because of the understatement:
I hoped for nothing. And yet I lived in expectation. Since she had gone, that was all that remained. (14.69)
You feel more is expressed here because Kelvin, as always, expresses so little in his clipped, simple sentences. Unmodified by descriptive language, the chasm between his hope and expectation looms in its fullest possible size.