New Observation of Colourful Twilight Clouds on Mars  

Curiosity rover has captured new images of colourful twilight clouds in the atmosphere of Mars. Called iridescence, this phenomenon is caused due to scattering of light from the setting Sun by the dry ice aerosols present in the Martian atmosphere. This was fourth season of Curiosity’s twilight cloud observations which is location specific and is seen periodically. Observation of iridescence in twilight clouds provide an opportunity to study size and growth rate of particles in the Martian atmosphere. 

On 17 January 2025 (the 4426^th Martian day, or sol, of Curiosity’s mission), the Curiosity rover captured new images of noctilucent (night shining) or twilight clouds in the Martian sky that were tinged with red-and-green colours. Called iridescence, this phenomenon is caused due to scattering of light from the setting Sun by the ice aerosols present in the Martian atmosphere. Occasionally, the twilight clouds create a rainbow of colours, producing iridescence, or “mother-of-pearl” appearance.   

This was the fourth season of Curiosity’s twilight cloud observations. The phenomenon occurs periodically during early fall in the southern hemisphere. The Curiosity rover first captured images of iridescence (bright colour formations that change with movement) in the Martian clouds in 2019. Twilight clouds were first observed on Mars in 1997 by Pathfinder mission.  

Atmosphere of Mars is composed of 95% CO₂, 3% N₂, 1.6% Ar and traces of O₂, CO, H₂O, CH₄, and lot of dust. At higher altitudes and lower temperatures, the Martian clouds are made of frozen CO₂ or dry ice. These clouds produce iridescence. Observation of iridescence in twilight clouds provide an opportunity to study size and growth rate of particles in the Martian atmosphere.  

Of all rovers that successfully landed on Mars, only Curiosity (which is located south of the Martian equator) has spotted twilight clouds made of dry ice. Perhaps, this is location specific phenomenon.  

It is thought that atmospheric gravity waves on Mars are responsible for  cooling the atmosphere sufficiently to allow CO₂ to condense into dry ice which scatter sunlight giving rise to iridescence in the twilight clouds.  

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References:  

1. NASA’s Curiosity Rover Captures Colourful Clouds Drifting Over Mars. Posted on 11 February 2025. Available at https://www.nasa.gov/missions/mars-science-laboratory/nasas-curiosity-rover-captures-colorful-clouds-drifting-over-mars/  

2. NASA. Sols 4450-4451: Making the Most of a Monday. Posted on 11 February 2025. Available at https://science.nasa.gov/blog/sols-4450-4451-making-the-most-of-a-monday/  

3. Lemmon M., et al 2024. Iridescence Reveals the Formation and Growth of Ice Aerosols in Martian Noctilucent Clouds. GRL. Published: 29 November 2024. DOI:  https://doi.org/10.1029/2024GL111183  

4. Mars Science Laboratory: Curiosity Rover. Available at https://science.nasa.gov/mission/msl-curiosity/  

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• Perseverance: What is so Special About the Rover of NASA’s Mission Mars 2020 (2 August 2020)  

• PHILIP: Laser-Powered Rover to Explore Super-Cold Lunar Craters for Water (18 May 2020).  

• Artemis Moon Mission: Towards Deep Space Human Habitation (11 August 2022)  

• Mass Extinctions in the history of Life: Significance of NASA’s Artemis Moon and Planetary Defence DART Missions (23 August 2022).   

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