Volatile materials in protoplanetary disks are best described as which?

• A. Gaseous components that vaporize easily
• B. Solid metals with high melting points
• C. Ice crystals only
• D. Dark matter

Answer: (A)

Volatile materials are defined by how readily they move into the gas phase under the conditions found in a protoplanetary disk. They have low condensation or vaporization temperatures, so they exist as gas in the warmer parts of the disk and can condense to ices in the colder outer regions. This is why the best description is gaseous components that vaporize easily. Materials with high melting points, like metals and silicates, stay solid in much of the disk and are considered refractories rather than volatiles. Ice crystals are a form volatiles can take in the outer disk, but volatility refers to their tendency to vaporize rather than to be strictly icy. Dark matter is not part of the disk’s chemical composition.