NASA describes them as soft puffs of ice crystals. After years of chasing these clouds, the Curiosity rover has finally managed to capture their formation in the thin, delicate Martian atmosphere. A grayish, iridescent clouds that if one does not know the context one might well think that it is a photograph of Earth.
Curiosity has been on Mars since 2012, although it has photographed clouds there in the past (it has photographed everything), it has had to wait these last two years for the right cloud-forming conditions. Clouds on Mars are not common, although they usually form at the equator during the cool season of the Martian year. Last Martian year (two years ago here on Earth) NASA noticed that clouds formed earlier than expected, so this time they were ready and prepared to capture them.
Usually clouds on Mars form at altitudes of about 60 kilometers from the surface. However, these ones captured by Curiosity recently are higher. This is mainly because they have formed earlier than expected, but it also has other consequences. For example, they are believed to be clouds formed by frozen carbon dioxide rather than water ice.
Different characteristics of the clouds allow researchers to establish the position and composition of the clouds. For example, clouds glow because light passes through the frozen crystals in them. This brightness, depending on when it occurs and where the Sun is in the Martian sky at the time, allows the altitude of the clouds to be determined.
On the other hand, researchers have also been able to see iridescent clouds of various pastel colors on Mars. Iridescent clouds form when the crystals in the cloud are all the same size because they have formed at the same time, i.e., at the beginning of the cloud. The result, in the case of Mars, is clouds with red, green, bluish and purple colors.
One of the main tasks of the Curiosity rover on Mars right now is to study the (most curious) Martian atmosphere. After completing its main mission years ago and far exceeding its expected lifetime, it is now being used for other NASA research. It is one of the most profitable rovers ever sent out there.