Uos is a small icy moon orbiting the gas giant Pentas, its surface is covered by a layer of black ice very rarely punctuated by small rock formations. Its surface easily reaches -270°C and it has no atmosphere, none can survive there save Sailor Uos herself.

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The liquid of the vast ocean covering the entire moon is mixed with black earthen minerals which give it its black appearance, even when not frozen. The ice itself is about 5.5 kilometres deep before reaching a layer of liquid (an almost water without oxygen mixed with soil and oil) and then about 3 kilometres deeper before rock, and lastly in the centre is a rather cool magna core. Eruptions are extremely rare on the moon.