The cloud, known as the Arsia Mons Elongated Cloud (AMEC), stretches for more than a thousand kilometres each morning before dissipating within hours. Traditional atmospheric models could not reproduce its thin, bright shape, typically producing either scattered haze or no tail at all.
Researchers led by Dr. Jorge Hernández-Bernal of the Sorbonne used the Mars Planetary Climate Model to simulate conditions around Arsia Mons in summer. They found that strong westward winds hitting the 20-kilometre-high volcano create gravity waves, standing ripples in the atmosphere, that cause the air to cool by about 30°C in less than fifteen minutes, dropping below -143°C.
At that point, water vapour molecules spontaneously freeze into hundreds of ice crystals per cubic centimetre. This homogeneous nucleation requires extremely high humidity and very few dust particles. In summer the Martian atmosphere near the volcano contains only about 0.1 dust grains per cubic centimetre, compared with millions on Earth, allowing relative humidity to soar to around ten million percent.
The resulting ice particles are then carried by winds into a long, narrow tail that remains stable for hours before dispersing.
The finding opens the possibility that homogeneous nucleation may occur elsewhere in the solar system, such as in the icy plumes of Saturn’s moon Enceladus or in Earth’s polar mesosphere. For now, AMEC is the only known natural example of cloud formation without condensation nuclei.