It's not just the foam lining that absorbs impact. The shattering of the hard outer shell also is a form of impact absorbtion/energy conversion. So if the outer shell is cracked -- be that from an impact or from the desert sun -- the helmet is compromised. Definitely get a new one. A new helmet costs, what? As little as $30 for a perfectly good one. A new head can't be had for any amount of money (says she who'd have been a widow were it not for dh wearing a helmet when that car pulled a left right in front of him).