The more observations scientists collect, the better they can understand how Mars behaves today — and how its atmosphere may have changed over billions of years.
Why the Clouds Appear Around Sunset
Timing is critical.
These clouds are difficult to see during the middle of the Martian day because they are extremely faint.
They become much easier to detect when the Sun is low.
At twilight, sunlight can continue illuminating clouds high above the surface even after the landscape below has become relatively dark.
That creates the unusual effect of clouds apparently glowing against the darker Martian sky.
NASA describes these as “twilight” or “noctilucent” clouds because they can remain illuminated after sunset from the perspective of an observer on the surface.
The phenomenon is somewhat familiar to people on Earth.
But on Mars, the chemistry and atmospheric conditions make the spectacle distinctly different.
Instead of simply seeing familiar water-ice clouds illuminated by sunlight, Curiosity can sometimes observe clouds composed of carbon dioxide ice at extraordinary altitudes.
A Seasonal Phenomenon
The clouds also appear to follow a seasonal pattern.
NASA scientists have discovered that Curiosity's observations of these twilight clouds can be planned around a particular period of the Martian year.
Mark Lemmon explained to NASA that after several observing seasons, the phenomenon had become predictable enough for scientists to plan their imaging campaigns in advance.
According to NASA, the phenomenon occurs during early fall in Mars' southern hemisphere.
That predictability is scientifically useful.
Rather than simply waiting for an unexpected cloud to appear, researchers can prepare the rover's cameras to observe the atmosphere during the appropriate season.
This allows them to gather much more systematic data.
A Feather-Shaped Rainbow in the Martian Sky
Curiosity has also captured another extraordinary example.
In January 2023, the rover photographed a feather-shaped iridescent cloud shortly after sunset.
NASA described the feature as one of the colorful clouds seen during a follow-up campaign designed to study Mars' noctilucent clouds.
The image was constructed from 28 photographs captured by Curiosity's Mastcam.
NASA processed the images to emphasize the highlights and reveal the subtle colors.
The result looked remarkably delicate.
Against the muted Martian sky, the cloud resembled a small pastel-colored feather floating high above the planet.
It was another reminder that Mars is not simply a world of rocks and dust.
Its atmosphere, although thin, can still produce surprisingly complex phenomena.
The Mystery Scientists Still Want to Solve
The repeated observations have answered some questions, but they have also created new ones.
One of the biggest mysteries is why these high-altitude carbon-dioxide-ice twilight clouds have not been observed in the same way at other locations on Mars.
Curiosity is exploring Gale Crater near the Martian equator.
NASA says the lack of similar observations elsewhere on the planet is an ongoing scientific question.
That raises an intriguing possibility.
Perhaps local atmospheric conditions around Gale Crater play a role.
Perhaps geography influences the formation of the clouds.
Or perhaps similar clouds occur elsewhere but have simply been difficult to observe.
More observations could help answer that question.
Mars Is More Dynamic Than It Looks
Mars is often portrayed as a frozen desert.
That description is not entirely wrong.
The surface is dry and cold, the atmosphere is extremely thin, and liquid water is not stable on the surface under today's conditions.
But the planet is not completely static.