Precipitation occurs on Mars, but the nature of this snowfall is unlike anything commonly experienced on Earth. Martian snow involves exotic materials and atmospheric physics driven by extremely low pressure and frigid temperatures. This unique environment results in weather phenomena fundamentally different from terrestrial snowstorms, offering a glimpse into the Red Planet’s complex atmospheric cycle. The existence of this precipitation highlights how volatile compounds like water and carbon dioxide behave under non-Earth conditions.
The Two Types of Martian Snow
Martian snow comes in two distinct compositions: water ice (\(\text{H}_2\text{O}\)) and carbon dioxide ice (\(\text{CO}_2\)), commonly known as dry ice on Earth. The presence of \(\text{CO}_2\) snow is a direct consequence of Mars’ thin atmosphere, which is composed of over 95% carbon dioxide. Extremely cold temperatures over the Martian poles during winter allow this abundant atmospheric gas to freeze directly into a solid.
Water ice requires temperatures below \(0^\circ\text{C}\) to freeze, while carbon dioxide ice forms at temperatures plummeting to approximately \(-125^\circ\text{C}\) (\(-193^\circ\text{F}\)). This extreme temperature difference separates the two types of precipitation. The \(\text{CO}_2\) snow falls primarily in the polar regions, where seasonal temperature drops are sufficient to freeze the bulk of the atmosphere.
How Carbon Dioxide Snow Forms and Falls
The formation of carbon dioxide snow involves deposition, where gaseous \(\text{CO}_2\) transitions directly into a solid without passing through a liquid state. This occurs high in the atmosphere, particularly within thick clouds over the south polar regions during winter. The Mars Reconnaissance Orbiter (MRO), using its Mars Climate Sounder instrument, was able to peer through these clouds and detect the \(\text{CO}_2\) snow falling toward the surface.
The structure of these snowflakes is theorized to be fundamentally different from Earth’s six-sided water crystals. Because carbon dioxide molecules bond with a symmetry of four, scientists predict the resulting dry ice crystals will be cubic or cuboctahedral in shape. These particles are incredibly small, estimated to be smaller than the width of a human hair or comparable to the size of a red blood cell. If a person were standing on the surface, this precipitation would likely appear more like a dense, settling fog than the large, intricate flakes familiar on Earth.
The accumulation of \(\text{CO}_2\) snow contributes significantly to the seasonal polar ice caps, coating the surface with frozen gas. Unlike water ice snow, the dry ice snow is confirmed to accumulate on the surface. This process demonstrates an annual exchange where a substantial portion of the Martian atmosphere freezes and then sublimates again with the change of seasons. This \(\text{CO}_2\) cycle is a dominant factor in regulating the planet’s atmospheric pressure and climate dynamics.
The Unique Nature of Water Ice Snowfall
Water ice snow also falls on Mars, but its behavior is influenced by the planet’s extremely low atmospheric pressure. This pressure causes the frozen water crystals to bypass the liquid phase and transition directly from a solid to a gas, a process known as sublimation. Consequently, water snow often evaporates high in the atmosphere before reaching the ground, a phenomenon similar to terrestrial virga.
The Phoenix lander, situated near the northern polar region, provided direct evidence of this water precipitation in 2008. Its laser instrument detected water-ice snow falling from clouds approximately four kilometers above its landing site. However, the data showed that these ice crystals were sublimating about halfway to the surface, confirming the virga effect. This sublimation is why water snow accumulation on the ground is extremely rare and often limited to frost instead.
Recent observations of water frost on the towering Tharsis volcanoes near the Martian equator show that water ice forms in a thin layer atop the peaks due to unique microclimates. This suggests that while precipitation is challenging in the thin air, water still actively moves and condenses in localized atmospheric cycles. The water snow that does occur is often composed of tiny, microscopic particles, reflecting the struggle of ice crystals to survive the journey through the thin, dry atmosphere.