Thousand-Mile-Long Martian Cloud Obeys Physics Never Seen on Earth

A cloud stretching over 1,000 miles (1,600 kilometers) above a 12-mile-tall (20-kilometer) extinct volcano on Mars differs greatly from any other clouds on Earth. The cloud emerges and then quickly disappears from the Martian sky every day, much like Australia’s rolling Morning Glory clouds—except the clouds on the Red Planet obey “exotic physics” that often appear in theory but rarely in nature.

A Cloud Like No Other

In 2018, the European Space Agency’s Mars Express orbiter spotted the mysterious cloud, dubbed the Arsia Mons Elongated Cloud, or AMEC, just south of Mars’ equator. Launched in 2003, the orbiter is still going strong two decades later and has long made tracking and monitoring the cloud part of its routine, among other activities like mapping the planet. The orbiter also serves as a backup data relay for surface missions, such as NASA’s Perseverance.

The cloud has appeared nearly every morning throughout Mars’ spring and summer, when solar radiation triggers strong surface winds, before fully evaporating within a few hours. AMEC can stretch up to 1,100 miles (1,800 km) long —about a twelfth of the planet’s circumference, or roughly the distance between New York City and Miami.

The Martian atmosphere is no stranger to giant dust clouds and “dry ice” clouds—frozen carbon dioxide tinged with vibrant red and green streaks from scattered sunset light. This is possible because the Martian atmosphere is 95% carbon dioxide. However, Mars rarely has puffy, Earth-like clouds because the atmosphere contains only about 0.02-0.03% water vapor. As a result, scientists struggled to explain how the cloud could be so large. When usual physics failed to answer the question, they looked beyond it.

Out-of-the-Box Physics Explains AMEC

In a study published Wednesday (October 7) in the journal Nature Geoscience, researchers used data from Mars Express to run new simulations that explain AMEC. The team, led by the study’s first author, Jorge Hernández-Bernal, a planetary scientist at the University of the Basque Country in Spain who was affiliated with Sorbonne University in Paris at the time, struggled to explain the phenomenon using standard meteorological principles.

Only when the team took an odd but out-of-the-box route did they succeed. To create the AMEC in their model, they included physics generally limited to textbooks and rarely seen in nature— also known as exotic physics. The puzzling phenomenon is governed by physics that defies everyday rules, not just on Earth, but across the entire solar system.

The AMEC is orographic—a cloud that forms when strong winds push water vapor up and over a raised structure like a mountain. The “Levanter” cloud, which sometimes forms above the Rock of Gibraltar in southern Europe, is an example of an orographic cloud on Earth. But scientists hit a wall when they tried to explain the Martian cloud with the same process. When they went past, a meteorological discovery stood on the other side.

Water Vapor Converts Directly into Icy Cloud Particles

On Earth, clouds form when moist air cools and condenses into ice crystals that form around tiny atmospheric particles like dust, pollen, soot, etc. These tiny particles are also called cloud condensation nuclei (CCN), and the process is termed heterogeneous nucleation. Although Mars has plenty of atmospheric dust, it doesn’t drive AMEC formation.

Instead, AMEC forms without any of the tiny atmospheric particles, or nuclei. Water vapor turns directly into ice, forming clouds without any intermediate step. It can be pictured as condensation droplets appearing in the middle of a room instead of on a window. Scientists call this process homogeneous nucleation. So far, AMEC is the first time scientists have recorded the phenomenon in a planetary atmosphere.

Scientists have predicted that homogeneous nucleation might occur in Earth’s upper atmosphere or in the skies above Venus, but the chances are very low. Even more surprising, Mars, a planet almost completely devoid of water, supports this process, which requires roughly 100,000 times the humidity found on Earth’s surface.

Mars Express Data Tells a Different Story

Arsia Mons is more than twice as high as Mount Everest, and winds rising along its slopes create a powerful “lee” wave that shoots air into the upper atmosphere at very high speeds. Researchers calculated that this air can cool by 54 degrees Fahrenheit (30 degrees Celsius) in just 10 minutes. The simulations show that this not only eases freezing but also amplifies the humidity within the wave, creating the perfect setup for homogeneous nucleation.

However, where does all the water come from on the already parched Red Planet? The researchers have a theory: some of it is already in the atmosphere, but some likely originates at the summit of the Arsia Mons, which is covered with a thick layer of frost. Interestingly, these clouds form on the mountain’s “leeward” side, which doesn’t face the wind.

The AMEC’s Impact on Studying Martian Weather

“We don’t have nearly as much information about Mars’ atmosphere as we do about Earth’s, so reproducing the AMEC to this degree is a big success for the model,” Hernández-Bernal said. Indeed, the simulations are a landmark achievement in our understanding of Martian meteorology. However, the scientists could not have achieved these findings without the Mars Express orbiter, one of the few Mars spacecraft to have seen Arsia Mons in daylight.

Three instruments on the spacecraft—the Visual Monitoring Camera, the High Resolution Stereo Camera, and the OMEGA instrument —mapped how the AMEC takes shape and evolves each day. Despite sharing similar basic principles, Earth and other planets have very different weather systems that require exotic physics. The discovery could significantly impact space exploration, our search for distant exoplanets, and possibly even the search for extraterrestrial life.