Let’s Explore the Phases of Matter: Sublimation and Deposition

Image Credit: European Space Agency/David Hardy

The Basic Phases
Science recognizes four states of matter that we can find in every day life. These include solid, liquid, gas and plasma. Other states (I’ve written about a few of them here) such as Bose-Einstein, supercritical fluid, and degenerate gas occur in extreme conditions, but I’m focusing primarily on phases today. It is important to remember that matter remains the same substance regardless of which state it is in. For example, water is still water, regardless of whether it is ice or in a cloud. What makes the difference is the amount of energy in the matter. The more energy the atoms in the matter have, the further apart the atoms become. Thus, solids are denser than liquids, liquids are denser than gases and so on. The energy that excites atoms typically is heat. The phase change puzzle doesn’t include only energy, however. It also includes pressure. The higher the pressure, the harder it is for matter to expand in response to the extra energy present, so phase changes become more difficult as pressure increases.

Although matter usually goes through phase changes gradually as heat energy is added or taken away, in some situations, there is enough energy present that matter can go directly from the solid state to the gaseous state. This is called sublimation. Sublimation occurs most easily when pressure is low because the atoms have less resistance when trying to expand.

Deposition is perhaps the lesser-known cousin of sublimation. It occurs when matter skips from the gaseous state directly to the solid state. This requires energy to be lost quickly. A good example of this is frost. Deposition occurs most easily when pressure is high because the pressure makes it easier for atoms to come closer together to form a solid.

What Does This Have to Do with Astronomy?

Glad you asked!

Comets like the comet Hyakutake are an excellent example of sublimation at work. Comets consist mostly of what I think of as dirty ice and dust. If you live in an area where it snows, you’ve likely seen how snow and ice on roads turns dark as it’s mixed with dirt. Hence, dirty ice. Anyhow, as comets approach the Sun, the radiated heat warms and sublimates the large mass of ice found in the comet. As a result, gas is released in the form of a temporary atmosphere or cloud around the comet known as a coma. Because comets have virtually no gravity, this temporary atmosphere is unsustained by the comet. As a result, the coma is flung away from the comet and results in the streaming tail that you see behind comets as they soar through space. So this is a great example of sublimation occurring in our universe.

Examples of sublimation can also be found on many planets and moons in our solar system as well. Many of these masses have little or extremely low pressure atmospheres resulting in ice buildup on their surfaces. When the ice is heated, sublimation occurs if the pressure is low enough. We see this occurring frequently on Mars during the Martian summer season. The planet has polar ice caps that sublimate into the atmosphere during the Martian summer season at its’ poles.


Heiselberg, H. (2000). Phases of dense matter in neutron stars Physics Reports, 328 (5-6), 237-327 DOI: 10.1016/S0370-1573(99)00110-6

Image Credit: European Space Agency/David Hardy


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