In planetary science, volatiles are the group of chemical elements and chemical compounds with low boiling points that are associated with a planets or a moons crust or atmosphere.
Very low boiling temperature examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, methane and sulfur dioxide.
In contrast with volatiles, elements and compounds with high boiling points are known as refractory substances.
On the basis of available data, which is sparse for Mars and very uncertain for Venus, the three inner planets then become progressively more depleted in K passing from Mars to Earth to Venus.