Rare physics demystifies Martian cloud mystery

Just before dawn near the Martian equator, a strange cloud starts to form above a shield volcano. Over the next few hours, the cloud grows massively longer until it starts to drift west, slowly evaporates, and then disappears before the afternoon arrives.

The Arsia Mons Elongated Cloud, as it’s known, appears every morning for a few months during the dusty season. It can grow as long as 1800 km—long enough to be clearly seen from orbiting spacecraft, as shown in the figure.

Until recently, models of the Martian atmosphere couldn’t explain how the cloud forms. But now, by adding an uncommon physical process to their simulations, Jorge Hernández-Bernal, a postdoc at Sorbonne University in Paris, and colleagues have shown how the cloud likely develops. ¹

Typically, when clouds form below the triple point of water, at pressures too low for liquid to exist, water vapor turns directly into ice by gathering on small particles of dust that act as seeds. Researchers thought that the process, known as heterogeneous nucleation, was how all clouds above Mars formed. But they couldn’t reproduce the Martian cloud using standard atmospheric models.

Theoretically, ice can also form spontaneously in supersaturated air without a particulate host. That process, called homogeneous nucleation, was previously considered implausible in the Martian atmosphere. But once Hernández-Bernal and colleagues included the process in their model, everything lined up, and they were able to show how the cloud likely forms through the rare phenomenon.

“It’s a very curious case,” says Alexander Medvedev of the Max Planck Institute for Solar System Research in Göttingen, Germany. “It’s quite interesting in the sense that an improbable mechanism can be probable.”

Because Mars has a much more elliptical orbit than Earth, seasonal changes related to its distance from the Sun have a greater effect on the Red Planet’s climate. Mars’s dusty season occurs when the planet is closest to the Sun. A seasonal increase in solar radiation heats the surface. That temperature change kicks up dust storms and warms the atmosphere, enabling it to hold more water vapor. Those atmospheric conditions interact with the topography of the Arsia Mons volcano.

Winds passing over the 20-km-tall extinct volcano generate waves high in the atmosphere, forcing moist air upward where it cools from decompression. Just west of the volcano, about 45 km above the surface, a particularly cold pocket of air develops. At that altitude, the dust density is insufficient to seed the cloud, but the air is supersaturated with water vapor—enough to support homogeneous ice nucleation and lead to the creation of the unique elongated cloud.

The simulations also reproduce some amount of cloud formation through heterogeneous nucleation, which can produce a haze like the ones sometimes observed around the cloud. Similarly shaped clouds have been spotted at other volcanoes on Mars, but none as large or recurrent as the Arsia Mons Elongated Cloud.

Hernández-Bernal hopes that when future missions are sent to the planet, government agencies and private companies will be mindful of how their activity might alter the atmospheric dynamics. “If we made some changes in the climate of Mars in the future, probably this cloud will be easily lost because it’s probably very sensitive to changes in the climate,” he says. “Of course, on Earth we should be doing the same.”