Those white rings on Iowa floor mats are dried salt crystals left behind after brine and slush evaporate. They keep reappearing because the salt is still in the carpet fibers and backing — every time the carpet gets damp, it wicks back to the surface.
The only permanent fix is to dissolve the salt and extract the water, not just wipe it off.
Wiping or vacuuming removes crystals on the surface but leaves dissolved salt in the padding. Once the carpet dries again, the salt recrystallizes and the ring returns — usually in the exact same shape.
The professional sequence looks like this:
Rubber all-weather mats are the single best investment — they contain slush instead of letting it soak in. Knock snow off boots before getting in, and get a mid-winter Maintenance Detail ($140) so salt never has months to build up.
A diluted vinegar solution can dissolve light salt residue, but without extraction the salt stays in the padding and the stain returns once the carpet dries.
Repeated salt cycles stiffen and discolor fibers over time and accelerate corrosion under the carpet, so removing it each winter matters beyond appearance.
Once mid-winter and again in early spring. That cadence keeps salt from ever setting into the backing.