NASA's Viking Orbiter 2 first recorded an unusually long cloud on Mars in 1977. The feature, known as the Arsia Mons Elongated Cloud (AMEC), has been imaged in greater detail by ESA's Mars Express spacecraft. The featured image comes from 2018.
The cloud forms only during Martian spring and summer, and only in morning hours. It consists of water ice. The cloud appears near the volcano Arsia Mons, then winds blow it downstream, creating a tail that extends over 1500 kilometers.
Scientists have puzzled over the cloud's formation mechanism. The standard process for cloud crystal formation around dust particles would fail due to the thinness of the Martian atmosphere and the scarcity of dust at high altitudes. Leading speculation holds that dust plays no role in AMEC formation.
Continued study of these clouds may provide insights into cloud formation on Earth.