The common perception of Mars is that of a desolate, rusty-red world with a thin, cold atmosphere. While the Martian atmosphere is tenuous—less than one percent the density of Earth’s—and composed primarily of carbon dioxide, it is far from static. This thin envelope of gas hosts a dynamic system of weather that includes distinct clouds, demonstrating that the Red Planet’s sky is not simply a dusty void. These clouds are integral to the Martian climate and water cycle.
What Martian Clouds Are Made Of and How They Form
Martian clouds are fundamentally different from those on Earth because of the planet’s extreme atmospheric conditions, yet they form through the same basic process of condensation. The two primary compositions are water ice and carbon dioxide ($\text{CO}_2$) ice. Water ice clouds, analogous to terrestrial cirrus clouds, are the most common and form when the small amount of water vapor in the atmosphere freezes into tiny crystals.
The formation of these ice crystals requires extremely cold temperatures and low atmospheric pressure, but also demands a high level of supersaturation. Laboratory experiments recreating Martian conditions show that water vapor requires a relative humidity of up to 190% before it will condense into cloud particles. This suggests that cloud formation is more difficult to initiate on Mars than on Earth. For condensation to occur, water molecules need a nucleus to cluster around, often filled by mineral dust particles lofted from the surface. High-altitude clouds also use “meteoric smoke,” the microscopic dust left by space debris burning up in the atmosphere, as a seed for their formation.
The second type of cloud is composed of solid carbon dioxide ice, or dry ice, which is possible because $\text{CO}_2$ makes up about 95% of the atmosphere. These clouds form in the coldest regions of the planet, particularly in the polar night or at very high altitudes where temperatures can plummet below -130 degrees Celsius. These $\text{CO}_2$ ice clouds are a direct manifestation of the planet’s atmospheric mass cycling, as the main atmospheric gas freezes and thaws seasonally.
Different Types of Martian Clouds
Clouds on Mars are categorized by their altitude, composition, and the atmospheric mechanism that triggers their formation. The most visually striking are orographic clouds, which form when air currents are forced upward over large surface features like the massive Tharsis volcanoes. As the air rises over the slopes of mountains such as Arsia Mons, it cools rapidly, causing the water vapor to condense into distinctive, lens-shaped clouds. These stationary clouds, similar to lenticular clouds on Earth, are often found on the leeward side of the mountains.
At high latitudes, especially during winter, a broad, seasonal haze known as a polar hood forms over the receding polar ice caps. These extensive cloud systems are composed of fine water ice particles and can cover areas of over 1,000 kilometers.
High-Altitude Clouds
In the middle atmosphere, high-altitude clouds form a distinct equatorial cloud belt, particularly during the Martian aphelion season when the planet is furthest from the sun and temperatures are slightly cooler. These cirrus-like clouds can extend up to 80 kilometers in altitude. They are often composed of $\text{CO}_2$ ice crystals and are typically thin and wispy.
How Clouds Influence Martian Weather and Climate
The presence of clouds is a significant factor in regulating the Martian climate by interacting with solar and thermal radiation. Clouds have a dual effect on the planet’s temperature through their radiative properties. They reflect incoming sunlight back into space, which has a cooling effect, but they also absorb outgoing thermal radiation from the surface, which works to warm the atmosphere. This balance depends on the cloud’s altitude and composition.
High-altitude water ice clouds can contribute to a warming effect, similar to an atmospheric greenhouse, because they efficiently trap heat in the upper atmosphere. The formation and movement of clouds are also linked to the three major Martian cycles: water, dust, and carbon dioxide. High-altitude clouds transport water vapor across the planet, moving it from the polar regions to the equator.
The $\text{CO}_2$ ice clouds are directly involved in the planet’s seasonal pressure cycle. During the winter, the condensation of $\text{CO}_2$ into ice clouds and frost can remove up to a third of the atmosphere’s mass, leading to significant seasonal variations in surface air pressure. Dust particles, which act as cloud seeds, absorb solar radiation and heat the atmosphere, strengthening atmospheric circulation and influencing the formation and intensity of dust storms.
Studying Clouds from Orbiters and Rovers
Scientists use a variety of instruments on both orbital spacecraft and surface rovers to analyze the structure and composition of Martian clouds. Orbiters like the Mars Reconnaissance Orbiter (MRO) carry instruments that remotely sense the atmosphere. The Mars Climate Sounder (MCS) on MRO looks at the planet’s limb to measure vertical profiles of temperature, dust, and water ice content.
The European Space Agency’s Mars Express orbiter uses instruments such as the OMEGA spectrometer and SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) to identify the chemical signatures of cloud particles. Spectroscopy allows researchers to determine the composition of the clouds, confirming the presence of both water ice and $\text{CO}_2$ ice crystals. Rovers like Curiosity and Opportunity have captured images of clouds passing overhead using their Mastcam and Pancam imagers.
The Phoenix lander, positioned in the northern polar region, used a Lidar (Light Detection and Ranging) instrument to send laser pulses into the atmosphere. By measuring the light reflected back, Lidar provided precise data on the altitude and density of near-surface water ice clouds and detected snowfall. These combined observations from above and below the clouds provide comprehensive, three-dimensional data that improves climate models of the Red Planet.