I grew up in love with astronomy, and the Solar System, to the point that I memorised the order of all the (then nine) planets by distance from the Sun, and could repeat this order, even without the mnemonic my mother taught me. As a budding lover of sci-fi and fantasy, I loved the idea that ancient civilisations existed on each of these planets, and that their citizens had rulers with special powers.
Of course, I also knew that the planet Mercury, as the closest planet to the Sun (so close it takes a mere eighty-eight Earth days to revolve around the sun!) was “completely dry” and rocky. So, it made no sense that Sailor Mercury’s element is water.
I puzzled about it for a long time. The Sailor Moon franchise offers zero explanation for why this is so, as far as I can tell, but it’s so crucial, that the colour (which I remember thinking of as brown as a child, for some reason, but is apparently grey) of the planet Mercury is rendered as blue in her transformation sequence in the newest anime adaptation Pretty Guardian Sailor Moon Crystal:
In Japan, though, her element is actually not a mystery. Sailor Mercury, like most of her teammates, gets her element from the Japanese name of her planet. In Japanese, the planet Mercury is called 水星 (すいせい、suisei), which literally means “water star”. Not only does she get her element from this, but also her surname: 水野 (みずの、Mizuno). Though mizu and sui are pronounced differently, they’re actually written with the same kanji (Chinese character): 水.
Mystery solved, huh?
With the mystical elements of Sailor Moon, it’s not all that much of a stretch to believe that maybe Mercury was indeed a watery planet in the time of the Silver Millennium, and that, when this ancient kingdom fell, the planet became the dry wasteland it is today. Or maybe, in the Sailor Moon universe, the planet is still a watery world when Ami is reborn on Earth…
Perhaps we’ll never know…