Urania is the muse of astronomy and astronomical writings, usually depicted with a globe and a rod or a caliper, sometimes a compass. As the muses in this deck are all depicted with a globe, the artist placed it in her left hand where she traditionally held it.
Urania was sometimes told to be clairvoyant, most likely due to the inextricable connection between astronomy and astrology in those days. However, it's a pretty far stretch to assume this might have any occult meaning for the Mantegna deck (although, in our times, I wouldn't be the least bit surprised if someone came up with it...;). Alas, the good lady didn't find her way into traditional Tarot decks.