What recent compositional finding forces a revision of the simple 'dirty snowball' model for some comets?

Answer

The amount of dust detected may be substantially greater than the amount of ice present

Observations from missions like Rosetta have indicated that in some comets, the bulk mass might be dominated by refractory, rocky, or dusty grains rather than being balanced evenly with ice.

What recent compositional finding forces a revision of the simple 'dirty snowball' model for some comets?
Spaceastronomycometdusticespace object