Several things may have contributed to this strange "cloud formation". One possibility is a meteoric impact event, where dust was spewed up into the atmosphere. Another could be a major dust storm, which are typical on Mars. While the other possibility is the more mundane, that these observations were caused by a mere optical illusion via a type of glint that occurred due to having just the right combination of lighting and atmospheric conditions. Some suggest volcanic activity, though this is unlikely given it has been 20 to 200 million years since lava has flowed on Mars."
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