The Mars elongated cloud that forms without dust

Each morning during the southern spring and summer on Mars, a thin white streak called the Mars elongated cloud grows beside the volcano Arsia Mons, stretches about 1,800 kilometers westward, and then fades before noon. A study published on October 7, 2026, in Nature Geoscience finally reproduces that daily performance in a computer model, and the recipe calls for ice forming straight from water vapor, with no speck of dust for it to cling to. According to the research team, led by Jorge Hernandez-Bernal of Sorbonne Universite in Paris, the simulated cloud only appeared once the model allowed vapor to freeze on its own.

The Mars elongated cloud has been watched since the European Space Agency orbiter Mars Express first recorded it in 2018. It starts on the western slope of Arsia Mons at dawn, at an altitude near 45 kilometers, and expands westward at more than 600 kilometers per hour for about two and a half hours. Then it detaches from the volcano and evaporates as the morning warms. Few other spacecraft can catch it, because most orbiters pass over the region in the afternoon, long after the cloud has gone.

Why the old models kept failing

On Earth, clouds need a seed. Water vapor condenses or freezes around dust, salt, pollen, or soot, and those particles give ice crystals a place to start. Mars has plenty of dust, so modelers long assumed the same rule would hold there. Yet every simulation that allowed ice to grow only on dust grains produced a short, stubby patch near the volcano, nothing like the Mars elongated cloud in the photographs. The new work ran a regional weather model at a resolution near 10 kilometers, fine enough to resolve the wave that wind makes as it crosses the volcano, and that wave turned out to be the missing piece.

The extreme pocket above the volcano

Arsia Mons rises about 20 kilometers above the surrounding plains, and wind striking it sets off a powerful atmospheric wave. That wave lifts moist air several kilometers upward within minutes. In the model, the rising air cools by 30 degrees Celsius inside 10 minutes, and the relative humidity climbs to more than 100,000 times the values typically measured on Earth. Under those conditions, vapor freezes directly into ice crystals, hundreds of them in each cubic centimeter of air. Dust is scarce in that pocket, roughly one grain in every two teaspoons of air, so ice growing on dust cannot use up the excess vapor. Hernandez-Bernal described the result in a statement as condensation appearing in the middle of a room instead of on a window.

The match is close but not exact. The simulated Mars elongated cloud shows brightness and crystal sizes consistent with the spacecraft data, and it narrows and detaches from the volcano the way the real cloud does. It comes out noticeably narrower than the observed streak, forms later in the morning, and reaches only about a quarter of the observed length before breaking away. The authors are clear about the limit of the result: the finding comes from modeling, not from a direct measurement of ice forming inside the cloud, and some observed details still resist explanation.

Why one cloud matters beyond Mars

The stakes reach past a single Martian volcano. If vapor can freeze without a seed in the Martian atmosphere, the same shortcut may operate in the upper atmospheres of Venus and Earth, and in the skies of distant exoplanets, where cloud physics shapes what telescopes can see. The Mars elongated cloud offers a repeatable test case, since it returns each morning for months at a time. Confirming the idea will take targeted morning measurements, the kind that are hard to schedule because so few instruments look at Mars at dawn. The team drew on two decades of Mars Express observations from its Visual Monitoring Camera and other instruments, and the authors say fresh morning data would settle whether the extreme humidity their model requires is real.

For now, the Mars elongated cloud keeps its schedule. Each southern spring and summer morning it grows, stretches, detaches, and disappears, giving researchers another chance to watch physics that textbooks describe and planetary atmospheres had not previously displayed.

Sources on the Mars elongated cloud: Sky & Telescope reports on the Nature Geoscience study and the European Space Agency on the Mars Express cloud observations.