Here's another theory: Carbon dioxide freezes at -110 degrees F (-79 degrees C) to become dry ice.

What would happen if not only the water vapor in the jet exhaust crystallized, but the carbon dioxide crystallized, too? How long would the chem, er, contrails persist them?

Regardless, given the right conditions (relatively humid air) and cold temps, contrails can persist for many hours, and can actually "seed" additional condensation from humidity in the air, so they can actually grow into large contrails, particularly if the air temp is dropping (such as when you have high cirrus forming which may block some solar radiation).