The Arsia Mons Elongated Cloud as seen by Mars Express. Credit: ESA

Every morning, for several months of the Martian year, something extraordinary happens near one of the tallest volcanoes in the solar system.

A white cloud appears above its slopes, unfurls across the landscape and stretches for up to 1,800 kilometers (1,100 miles), roughly the distance from New York City to Miami. Then, within hours, it vanishes.

Scientists have watched this peculiar daily performance for years. But they couldn’t explain how such an enormous cloud could form in the thin, dry atmosphere of Mars.

Now, a team of researchers believes it has found a crucial missing piece.

According to a new study, the cloud may form through an unusual process in which water vapor transforms directly into ice crystals without needing dust particles to get things started.

The mechanism, known as homogeneous nucleation, has long been understood in theory. But scientists have never confirmed that it actually occurs in a planetary atmosphere.

A Volcano With Its Own Weather3D rendering of the cloud’s positioning on the planet. Credit: ESA

The cloud has a name: the Arsia Mons Elongated Cloud, or AMEC.

Its birthplace is Arsia Mons, an extinct volcano rising roughly 20 kilometers (12 miles) above the Martian landscape. That’s more than twice Mount Everest’s height above sea level.

During spring and summer in Mars’s southern hemisphere, winds sweep across the volcano, forcing air upward. The mountain acts like an enormous obstacle in the atmosphere, generating powerful waves that send pockets of moist air soaring several kilometers in just minutes.

As the air rises, it expands and cools dramatically. The researchers calculated that temperatures can plunge by 30° Celsius in just 10 minutes, reaching below minus 143° Celsius.

Levanter cloud over the Rock of Gibraltar. Credit: Wikimedia Commons

The cloud forms about 45 kilometers above the planet’s surface. Winds then carry its icy particles westward, creating its astonishing tail.

Similar mountain-generated clouds occur on Earth, including the famous Levanter cloud over Gibraltar. But conventional weather models couldn’t reproduce the Martian cloud’s extraordinary length.

Earlier simulations published in 2022 successfully captured the powerful upward-moving air and rapid cooling. Yet the enormous white tail remained elusive.

Something was missing.

Ice Crystals Without Dust

Ordinarily, clouds need something to grow on.

On Earth, water vapor condenses or freezes around microscopic particles of dust, sea salt, soot or other material suspended in the atmosphere. Scientists call this heterogeneous nucleation.

Mars has plenty of dust, but the researchers found that dust alone couldn’t explain AMEC. So they tried something unusual.

“To create the AMEC in our modelling, we found that we needed to include some exotic physics… physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature,” said lead author Jorge Hernández-Bernal, a planetary scientist, in a statement.

Instead of relying on dust grains, the researchers allowed ice crystals to form spontaneously from water vapor itself. This is homogeneous nucleation, and it requires extreme conditions.

Normally, there simply aren’t enough water molecules in the air for them to cluster together and freeze without some help. But above Arsia Mons, the temperature drops so rapidly that the air becomes extremely supersaturated, even though Mars contains relatively little water.

Supersaturation means the air contains more water vapor than it could normally hold at that temperature. The colder the air gets, the less vapor it can support. Above the volcano, cooling happens so quickly that water vapor can temporarily build up to extraordinary levels relative to saturation.

Those conditions may allow ice crystals to form without the usual microscopic seeds.

When the researchers added this process to their simulations, the cloud finally appeared, complete with its characteristic elongated tail.

Observation vs. simulation. Credit: ESA

The model also reproduced several smaller features, including the cloud narrowing and eventually detaching from the volcano.

However, it wasn’t a perfect match. The simulated cloud was 30 to 50% narrower than observed, formed about 90 minutes too late and reached only about a quarter of the observed maximum length before detaching.

Nor have scientists directly witnessed individual ice particles forming through the proposed mechanism. The evidence comes from how well the simulations reproduce the observations.

Beyond MarsOverhead view of ARISE from Mars Express. Credit: ESA

The Mars Express spacecraft made this detective work possible. Its instruments have provided detailed observations of the cloud since 2018, although earlier spacecraft captured glimpses of the phenomenon decades ago.

The discovery also builds on earlier research into Mars’s surprisingly unusual water cycle. In 2011, scientists discovered that water vapor could persist in the Martian atmosphere at concentrations far above what its temperature should normally allow. A 2020 study subsequently found widespread water vapor supersaturation during the planet’s dusty season, helping explain how moisture can reach higher into the atmosphere than previously expected.

Scientists have proposed similar spontaneous ice formation in the upper atmospheres of Earth and Venus, but nobody has confirmed it there.

If the interpretation is correct, Mars may provide the first compelling real-world evidence that water-ice clouds can form directly from vapor without the help of dust or other particles.

The next question is whether Arsia Mons is unique or whether the same process helps form other Martian clouds, potentially influencing how water moves around the planet.

It could even prompt scientists to revisit how certain clouds form on Earth, Venus and other worlds.

For years, this enormous white plume seemed like an oddity of Martian weather. Now, it may be telling scientists something fundamental about how clouds form throughout the Solar System.

The study was published in the journal Nature Geoscience.