Mars is a classical planet, traditional ruler of Aries and Scorpio, its red colour long linking it to a god of war. The familiar circle-with-arrow glyph read today as a shield and spear is attested in that basic form from at least the eleventh century, an arrow drawn across or through a circle, but it is not the planet's oldest recorded sign.
The Greek Oxyrhynchus papyri, among the earliest surviving sources for the classical planetary glyphs, show Mars marked with a different symbol altogether, evidence that more than one graphic convention for the planet circulated in antiquity before the shield-and-spear form eventually won out and stabilised through the Byzantine and medieval periods. Its warlike associations made the reading of a shield and projecting spear an easy, memorable gloss once the form had settled, whether or not that was the design's original intent, and the same association of Mars with iron and weaponry later carried the sign into unrelated technical vocabularies that had nothing to do with astrology. Mars's glyph is unusual among the classical seven for having a single dominant reading, shield and spear, agreed on across most modern popular accounts, where several of its companions, Jupiter's and Saturn's especially, are more honestly described as abbreviations whose pictorial gloss was added after the fact.