Halos on Mars

Last updated on September 16, 2023

Halos on Mars: A Spectacular Phenomenon in the Martian Sky

When we think of atmospheric optics, our minds often turn to the beautiful halos that grace the skies of Earth. But did you know that Mars, our neighboring planet, can also exhibit its own stunning halos? Halos formed by clouds of frozen carbon dioxide (CO2) create a mesmerizing display in the dusty Martian sky. In this article, we will explore the intriguing world of halos on Mars, shedding light on their formation, characteristics, and the exciting possibilities they hold for our understanding of Martian clouds and climate.

The Martian Atmosphere and Winter Polar Conditions

The thin atmosphere enveloping Mars is predominantly composed of carbon dioxide, with only minuscule traces of water vapor. During each Martian year at the winter pole, temperatures plummet to a bone-chilling minus 123°C (-189°F), causing a portion of the atmosphere to condense into towering clouds of CO2 crystals, often referred to as "dry ice." As winter progresses, CO2 snow descends upon the polar surface, forming a thick layer that can reach up to 2 meters in depth. These fascinating waxing and waning cycles of the polar ice caps can be observed from Earth using even a small telescope.

Could Mars Exhibit CO2 Crystal Halos?

Given the prevalence of CO2 crystals in the Martian atmosphere, one might wonder if these crystals could give rise to halos similar to those seen on Earth. While several conditions would need to align for such halos to form, it is indeed possible that Mars occasionally showcases exotic and colorful halos unlike any seen on our home planet. CO2 crystals possess the necessary transparency and optical quality to generate halos, but their size and distribution must also meet specific criteria.

The Potential Beauty of Martian Halos

In regions other than the winter pole, where darkness shrouds the majority of the clouds, sufficiently cold temperatures in the upper atmosphere could support the formation of CO2 clouds reminiscent of Earth's cirrus clouds. If these CO2 clouds possess the right characteristics, they could produce halos that are visually distinct from those found on our planet. Imagine the spectacle of vibrant and otherworldly halos adorning the Martian sky, adding an ethereal touch to the already captivating landscape.

The Quest for Martian Halos: The Mars Polar Lander Mission

The possibility of Martian halos captured the attention of researchers, leading to the inclusion of a search for these atmospheric phenomena as part of the Mars Polar Lander mission launched in January '99. Equipped with multi-spectral sky imaging capabilities, the lander aimed to observe and document any halos encountered during its mission. The detection of even fragments of halos on Mars would provide valuable insights into the nature of Martian clouds and its climate.

The Elusive Nature of Martian Halos

Unfortunately, our quest to witness halos in the skies of another world was met with disappointment. After an arduous 11-month journey, contact with the Mars Polar Lander was lost upon its entry into the Martian atmosphere. As a result, we were left waiting for further opportunities to observe and study halos on Mars. Despite this setback, scientists remain determined to uncover the mysteries surrounding Martian halos and continue their pursuit of knowledge about the Red Planet's atmospheric phenomena.

Conclusion

The existence of halos on Mars opens up a realm of possibilities for understanding the atmospheric dynamics and climate of our neighboring planet. While we have yet to witness these captivating halos firsthand, the potential for their presence within the Martian sky is an exciting prospect. As technology advances and future missions explore the Red Planet, we can eagerly anticipate the day when we gaze upon the breathtaking beauty of halos adorning the Martian atmosphere. Until then, we will continue to marvel at the wonders of atmospheric optics both on Earth and beyond.

Halos formed by clouds of frozen carbon dioxide shine in a dusty Martian sky. Familiar water-ice crystals form the inner 22� halo. CO2 cuboctahedral crystals produce the next of 26� radius and the outer 39� halo is from octahedra and cuboctahedra. Cuboctahedral platelike crystals generate multiple parhelia and the bright uppermost arc. Painting & HaloSim3 simulation by Les Cowley

The thin Martian atmosphere is mostly carbon dioxide with only minute traces of water vapour. Each Martian year at the winter pole it gets cold enough (minus 123�C) that some of the atmosphere condenses into tall clouds of CO2, 'dry ice' crystals. As winter deepens, CO2 snow falls and covers the polar surface until springtime with a dense 2m thick layer. The wax and wane of the 'ice' caps can be watched from Earth with a small telescope.

Could Mars have CO2 crystal halos? CO2 crystals are sufficiently transparent. They would also have to be reasonably large enough and optically good. Clouds over the winter pole are mostly in darkness but elsewhere, high in the atmosphere, it is sufficiently cold for cirrus like CO2 clouds. Many conditions need be met but Mars might sometimes have exotic and colourful halos quite unlike those of Earth.

Michael Schroeder and myself used an early version of the HaloSim software to predict them and the Mars Polar Lander launched in January '99 was scheduled to search for halos as part of its multi-spectral sky imaging program. Observation of even fragments of halos would tell us more about Martian clouds and climate. However, after an 11 month voyage, contact with the Lander was lost after it entered the Martian atmosphere. We shall have to wait longer to see halos in the skies of another world ...

More details in:

Forecasting Martian Halos by Les Cowley & Michael Schroeder,

Sky & Telescope, Dec '99. pp 60-4.

'Vorhersage von Mars-Halos', Meteoros, Jahrgang 3 Nr.6/2000

pp 103-8.

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