Jorge Hernández-Bernal's Model Reveals Hidden Mars Cloud Physics

A new computer model has provided the first successful explanation for a massive cloud that forms daily over Mars. The simulation, led by Sorbonne Université researcher Jorge Hernández-Bernal, shows that the cloud depends on a physical process not previously observed in any planetary atmosphere.

The Arsia Mons Elongated Cloud, or AMEC, appears during Martian spring and summer. It stretches more than 1,000 miles, a distance comparable to the straight-line span between New York and Kansas City. The cloud grows throughout the morning and then dissipates, a cycle that repeats over several months. A probe first detected the feature in 2018, but scientists had struggled to explain its formation until now.

Hernández-Bernal and his team published their findings in Nature Geoscience. The simulations only produced the cloud when the researchers included what they termed exotic physics. This process, known as homogeneous nucleation, involves water vapor turning directly into ice crystals without the aid of dust or salt particles. On Earth, clouds typically form when vapor condenses around existing debris. Hernández-Bernal described the Martian process as wholly unexpected, likening it to droplets forming in the middle of a room rather than on a window.

The formation of the cloud requires extreme atmospheric conditions. The model shows that temperatures must drop by 30 degrees Celsius within 10 minutes. Additionally, the relative humidity must reach levels more than 100,000 times greater than any value measured on Earth. The 65,000-foot Arsia Mons volcano plays a central role in creating these conditions. When wind strikes the peak, it generates a powerful wave that lifts moist air high into the atmosphere within minutes.

Scientists had long theorized that homogeneous nucleation might occur in the upper atmospheres of Venus and Earth, but they had never observed it. The extreme humidity required for the process likely explains its absence in previous observations. Hernández-Bernal noted that researchers did not previously know such high humidity existed on Mars. He emphasized that far less is known about the Martian atmosphere than Earth's, making the accurate reproduction of the cloud a notable achievement in planetary science.

The study confirms that the daily cloud is a result of this rare ice physics rather than standard condensation. The model aligns with the visual data recorded by the 2018 probe. The findings offer a new baseline for understanding atmospheric dynamics on Mars, where conditions can differ vastly from those on Earth.