ESA · Space

Mars's Arsia Mons cloud formation may involve rare physics

Scientists studying Mars's Arsia Mons Elongated Cloud (AMEC) have found that its formation might involve a rare process called homogeneous nucleation, where water vapor freezes directly into ice particles without needing atmospheric dust.

The AMEC is a striking, daily-forming cloud near the Arsia Mons volcano in Mars's southern hemisphere, stretching up to 1800 km. While previously identified as an orographic cloud, simulations struggled to replicate its formation until now.

Researchers found that including theoretical "exotic physics" in their models was necessary to reproduce the AMEC. This physics, known as homogeneous nucleation, has not been observed in a planetary atmosphere before.

Typically, clouds form via heterogeneous nucleation, requiring particles like dust for water vapor to condense onto. Homogeneous nucleation bypasses this, allowing water vapor to freeze directly into ice, a process previously only theorized for planetary atmospheres.

This rare process requires extremely high relative humidity levels, over 100,000 times those common on Earth. The study suggests Mars's atmosphere can indeed reach these conditions.

The unique topography of Arsia Mons creates powerful atmospheric waves that rapidly lift moist air, causing rapid cooling and a spike in humidity, enabling homogeneous nucleation and AMEC formation.

While the model isn't a perfect match for all observations, reproducing the AMEC to this degree is considered a significant success for the Mars Express mission and its data. The findings highlight the potential for unusual atmospheric processes elsewhere in the universe.

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