Mars Express data point to dust-free ice formation in a 1,800-kilometre cloud
A study in Nature Geoscience suggests the cloud that forms each morning downwind of Mars’s Arsia Mons may grow through homogeneous nucleation: water vapour freezing directly into ice without dust. Models matched Mars Express observations more closely when this process was included, after winds lifted moist air and temperatures fell by about 30°C in 10 minutes. The mechanism is inferred from modelling, not directly observed, and some details of the simulated cloud do not match observations.
Bottom line — Further Mars Express observations will test whether homogeneous nucleation explains the cloud’s daily cycle.
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The cloud can stretch up to 1,800 kilometres during spring and summer in Mars’s southern hemisphere, then evaporates rapidly.
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Researchers say relative humidity in the model exceeds levels experienced at Earth’s surface by more than 100,000 times.
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Mars Express has monitored the cloud for years and can track changes over several hours, ESA project scientist Colin Wilson said.
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The study used observations from three Mars Express instruments; the authors say limited data on Mars’s atmosphere leaves the explanation uncertain.