On Solaris, there is an ocean which is a monstrous, biological fluid with amazing capabilities. It exerts a gravitational pull, correcting the planets misguided orbital path around the two suns. But the most fascinating aspect of the ocean is that it is able to produce life, gleaning knowledge of the scientist's deepest hidden thoughts and recreating living, breathing people which mimic or stand in for lost relations to Kelvin, Snow, and Sartorius. This red ocean is able to produce a "ceaseless mutation of forms", a byproduct of the scientist's minds who inhabit the station on Solaris. The created likenesses are memories from the deepest recesses of people's minds taken from dreams. Then when Kelvin and the others awake, the person, flesh and blood, is there next to them in a form that is immortal and ageless-a god. These creations become their masters, their gods, whom they must placate at any cost.
Rheya, the alien reproduction of Kelvin's deceased ex-wife, does not understand why she cannot stop following him, even to the point of her tearing the door off it's hinges to be near (this is what the picture illustrates, her confusion at acting the way she does; in fact, she doesn't even know why she's bleeding). Lem's entire novel reflects the ideas of Luctretius: religion as a byproduct of the human mind and nature as the ultimate creator.
Whilst human kind
Throughout the lands lay miserably crushed
Before all eyes beneath Religion—who
Would show her head along the region skies,
Glowering on mortals with her hideous face—
A Greek it was who first opposing dared
Raise mortal eyes that terror to withstand,
Whom nor the fame of Gods nor lightning's stroke
Nor threatening thunder of the ominous sky
Abashed; but rather chafed to angry zest
His dauntless heart to be the first to rend
The crossbars at the gates of Nature old.
And thus his will and hardy wisdom won;
And forward thus he fared afar, beyond
The flaming ramparts of the world, until
He wandered the unmeasurable All.
Whence he to us, a conqueror, reports
What things can rise to being, what cannot,
And by what law to each its scope prescribed,
Its boundary stone that clings so deep in Time.
Wherefore Religion now is under foot,
And us his victory now exalts to heaven.
-Lucretius
Rheya etymologically comes from the goddess named Rheia, the titan mother to all Olympus gods. While the ocean produces the substance that makes Rheya "real", it is Kelvin's mind that produces the her characteristics which form of a familiar, comforting deity. When it is time for her to leave (try not to spoil it too badly), it is nearly impossible for Kelvin to let go. The destruction of the gods mirrors Lucretius' denunciation of the pantheon. Rheya is the embodiment of what Lucretius is questioning in De Rerum Natura.
The ocean echoes Lucretius as it presents a disruption to the entirety of human experience and knowledge as it creates life at will. This is in complete absence of a god or creator in the most common sense, instead posing a natural force as the ultimate source of life.
Whence Nature all creates, and multiplies
And fosters all, and whither she resolves
Each
in the end when each is overthrown.
-Lucretius
Just as the red fern, like the red ocean, conjures up the proposition that the world and all of creation has not changed but rather the way in which we are seeing it has evolved. The fern was an object or symbol within my mind but had not been fully illuminated until experienced fully. Likewise, the ocean on Solaris is a symbol for the expansion of human knowledge which is never completely realized until Kris Kelvin arrives there and truly understands the pure natural power of the ocean. The ocean lives, creates, moves, a godlike organism which tests the reality of Kelvin's world and the realm of possibility.