Mars
Because of the reddish hue, which the Romans felt like the blood, the nearest planet to Earth has been named in honor of their god of war, a symbol of the planet are a shield and spear of Mars. As this symbol of Tuesday (Tuesday word came from the Teutonic name of the god Tiu, which is identified with Mars: the latinskop this day was called dies Martis, "Mars Day".
The sign is also consistent with the iron, metal, most associated with this god.
Since the mid 18 century, this symbol to denote male biology.