In Halo 3 what does each skull do?

[mitch_p_hoffman]

I've got all the skulls but i want to know what each one does both the gold and silver ones.

 

[legodps]

cowbell: the ark, silver skull, makes bigger explosions for grenades

 

[Mike]

they just do diffrent things i think for silver gives you achivments and if you press Y when at campigan it should be some cool stuff add me if yu want my gamertag is Xxgh0stlegendxX