The front axle doesn't have any boots -- it's all internal -- that is the steering cylinder and tie rod/joint you see in the background with a boot.
Did you check to see if the prop shaft from transmission to front axle is missing any covers? They usually have a protective cover over the joint(s), though it normally doesn't look like the boot in your hand in the picture.
Is it possible the mower, or some other implement, lost that boot?