All of them are. 'Mizuno' - water, for instance. 'Aino''s an easy one.
Oh it gets better. All last names are based on the Japanese names for each celestial body.
月 Tsuki/Moon -> 月野 Tsukino (Moon field, but "no" can mean "of" in Japanese as well.)
地球 Chikyuu/Earth -> 地場 Chiba
水星 Suisei/Mercury (water) -> 水野 Mizuno
火星 Kasei/Mars (fire) -> 火野 Hino
木星 Mokusei/Jupiter (wood) -> 木野 Kino
土星 Dosei/Saturn (soil) -> 土萠 Tomoe
天王星 Ten'ousei/Uranus (sky lord) -> 天王 Ten'ou
海王星 Kaiousei/Neptune (sea lord) -> 海王 Kaiou
冥王星 Meiousei/Pluto (underworld lord) -> 冥王 Meiou
Sailor Venus is the odd one out since her last name is from 愛 Ai/love not 金星 Kinsei/Venus (gold/metal). The elements involved in each name also carry over to the powers of each character. This is why Jupiter has plant references in her attacks, as her element is supposed to be wood.
Also of note, the Japanese week follows the Latin week: Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn. Our week is a nordic adaptation of this, but we got lazy with "Saturday".