Dry ice is the common term for carbon dioxide (CO2) in its solid form. It is a manufactured product widely used in commerce for temporary refrigeration in the shipment of temperature-sensitive goods. Because solid CO2 is significantly colder than regular water ice, it is also highly valued for creating dramatic, low-lying fog effects in the entertainment industry. Its widespread availability often prompts the question of whether it occurs naturally.
Understanding Solid Carbon Dioxide
The physical properties of solid carbon dioxide explain why its natural formation is so challenging on Earth. Unlike water ice, which melts into a liquid when heated, dry ice transitions directly from a solid state to a gaseous state, a process called sublimation. This unique behavior occurs because the compound’s triple point—the specific pressure and temperature where solid, liquid, and gas phases coexist—is above standard atmospheric pressure.
At Earth’s typical atmospheric pressure, solid CO2 cannot exist as a liquid. Instead, it maintains a temperature of -78.5°C (-109.3°F) as it rapidly absorbs heat from its surroundings and converts into an invisible gas. This characteristic makes dry ice an effective coolant that leaves no liquid residue behind. The requirement for such a low temperature and high pressure relative to CO2’s partial pressure is the scientific barrier to its natural occurrence.
Natural Occurrence on Earth
Dry ice almost never occurs naturally on Earth. While the required temperature of -78.5°C is sometimes exceeded in the coldest regions of Antarctica, the necessary atmospheric pressure conditions are not met. The air contains only a trace amount of carbon dioxide, resulting in a very low partial pressure for the gas.
To form solid CO2 directly from the atmosphere at the low partial pressure found on Earth, the temperature would need to be far colder than anything ever recorded, approximately -140°C. The coldest temperature ever measured on the surface of the planet, -89.2°C in Antarctica, is not cold enough under the existing atmospheric conditions. Any naturally occurring instances would be extremely rare and transient, perhaps in specialized, high-pressure geological pockets, but these are not sustained, large-scale deposits.
Abundance in the Solar System
In sharp contrast to its scarcity on Earth, carbon dioxide ice is a common and plentiful substance throughout the outer reaches of the Solar System. The conditions of space—low pressure and extremely low temperatures—are suited for its large-scale formation. This makes dry ice a significant component of many extraterrestrial bodies.
Mars is the most notable example, where carbon dioxide is the main constituent of the atmosphere and freezes out seasonally at the poles. The Martian polar caps consist of permanent water ice and a seasonal veneer of dry ice that expands and contracts with the planet’s orbit. During the winter months, up to a third of the atmospheric CO2 can precipitate onto the surface as dry ice snow and frost.
Dry ice is also a component of comets, which are often described as “dirty snowballs” composed of frozen water, carbon dioxide, and other volatile ices. When a comet approaches the Sun, the frozen CO2 sublimates, releasing gas that forms the comet’s visible coma and tail. The surfaces of many icy moons and distant dwarf planets also feature carbon dioxide ice, confirming its status as a common natural volatile in the solar neighborhood.
Industrial Production and Capture
Commercial dry ice is produced using industrial processes that capture and manipulate carbon dioxide gas. The raw material, gaseous CO2, is often sourced as a byproduct from large-scale industrial activities, such as the production of ammonia or fermentation processes in breweries. This captured gas is purified and then subjected to high pressure and cooling until it liquefies.
The liquid CO2 is then rapidly expanded by reducing the pressure, causing a sudden drop in temperature. This rapid cooling transforms a portion of the liquid into a finely powdered solid known as “carbon dioxide snow.” This snow is collected and compressed by hydraulic presses into the dense blocks, pellets, or slices that are recognized as commercial dry ice. This controlled, multi-step process ensures a consistent and reliable supply.