Mars, often portrayed as a barren world, hosts clouds in its thin atmosphere. These clouds differ considerably from Earth’s in composition and appearance, offering insights into the planet’s atmospheric processes and past climate.
The Makeup of Martian Clouds
Martian clouds are primarily composed of two types of ice crystals: water ice and carbon dioxide (CO2) ice. Unlike Earth’s clouds, which are almost exclusively water, Mars’ atmospheric ice formations are unique. CO2 ice clouds form directly due to Mars’ atmosphere being over 95% carbon dioxide and its extremely cold temperatures.
Water ice clouds on Mars are similar to Earth’s cirrus clouds, often appearing thin and wispy. They form at lower altitudes where water vapor condenses into ice crystals. In contrast, CO2 ice clouds form at much higher altitudes, often above 37 miles (60 kilometers), where carbon dioxide freezes directly from gas into solid. The Phoenix lander detected CO2 snow falling from these clouds, though it sublimated before reaching the ground.
How and Where Clouds Appear on Mars
Martian clouds appear in various forms and locations, influenced by the planet’s atmospheric dynamics and topography. One prominent feature is the “aphelion cloud belt,” a band of water ice clouds that forms near the Martian equator when the planet is farthest from the Sun. This belt shows increased cloud formation and daily variability during Mars’ northern summer and early fall. The Curiosity rover has observed morning clouds that may represent an early stage of this cloud belt.
Orographic clouds form as moist air is forced upwards by elevated terrains like the Tharsis Montes volcanoes. These clouds, such as the elongated cloud near Arsia Mons, can be extensive, stretching over 1,100 miles (1,800 kilometers) and displaying dynamic daily patterns. Seasonal hazes, like “polar hood” clouds, appear over Mars’ polar regions during winter. These dense polar hoods are primarily water ice in the southern hemisphere and a mix of water and CO2 ice in the north, influencing the planet’s water cycle.
Detecting Clouds from Earth and Space
Martian clouds have been observed using various methods, from early Earth-based telescopes to advanced spacecraft. Earth-based telescopes provided the first glimpses, with historical observations of polar hoods dating back centuries.
Orbital spacecraft, such as NASA’s Mars Reconnaissance Orbiter (MRO) and ESA’s Mars Express, have advanced the study of Martian clouds. Instruments on these orbiters, including cameras and spectrometers like the Mars Climate Sounder, capture images and analyze cloud composition and vertical structure. The MRO, launched in 2005, provides data on atmospheric conditions and water and dust distribution.
Mars rovers like Curiosity and Perseverance provide ground-level perspectives, offering direct observations of clouds from the Martian surface. Curiosity has captured images of water ice and CO2 ice clouds, including iridescent “twilight” clouds illuminated by the setting sun. These ground-based images, combined with orbital data, help determine cloud altitudes, movement, and formation conditions, enhancing understanding of Mars’ atmospheric processes.