Mars and Tiw share a day. They might also share a symbol: ♂︎ ᛏ. The earliest symbols for Mars the planet were of a pointed spear, or a spear with a shield drawn as a circle with a line running through it. We still use a shield and spear to mean Mars. He is a warrior like Tiw, both recognizable by their weapons, forever on a journey over the obscurity of night.
In Old English yr means only the name for this rune. A bow is a guess, a bow made out of yew. In Old Norse yr means the yew tree. The Icelandic Rune Poem says yr is “bent bow and brittle iron, and Farbauti (a giant) of the arrow.” Yew Bows from Britain were prized all over Europe, so I think a yew bow as an answer to this riddle makes the most sense from the minimal context we have. But in Old English yew is spelled eoh, so this rune could be describing something else. Some think this stanza describes a horn or a saddle bow or buckle. Whatever yr was, it was something to see. This is what this stanza is all about. Being something to see. Looking good on a horse. That’s sexy. Nobody wants to look bad on a horse.