Scientists using Mars Express imagery and a high-resolution atmospheric model have explained how the Arsia Mons Elongated Cloud (AMEC) forms and behaves. The AMEC is a daily, spring-summer phenomenon downwind of the 20-km Arsia Mons volcano that stretches up to 1,800 km and appears as a long water-ice wisp each morning during Mars’s dusty season. Simulations required inclusion of homogeneous nucleation - water vapour freezing directly into ice without dust or other particles - to reproduce the cloud. Strong lee waves generated as winds flow over Arsia Mons lift moist air several kilometres in minutes, cooling parcels by roughly 30°C in about 10 minutes and producing extreme relative humidities that trigger spontaneous freezing and rapid cloud growth and decay.
Reproducing the AMEC represents a notable success for Martian meteorology because homogeneous nucleation has not been observed before in a planetary atmosphere and demands exceptional conditions. Mars Express’s Visual Monitoring Camera, High Resolution Stereo Camera and OMEGA provided the multi-timescale, high-resolution morning observations that anchored the model. While some modeled details still differ from images, the close match validates the mechanism and highlights that unusual microphysical processes can operate on Mars, with implications for understanding atmospheres elsewhere and for planning future observations.
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