Carbon dioxide falls as snow on Mars, according to new data revealed by a Nasa spaceship orbiting the planet.
The Mars Reconnaissance Oribiter craft showed that frozen 'dry ice', which requires temperatures of about minus-193 degrees Celcius to form, can form and fall to the surface in the cold regions of the Red Planet.
If true, it would be the only place that carbon-dioxide falls as snow in the solar system.
The Mars Climate Sounder, one of six instruments on the Mars Reconnaissance Orbiter, was used to analyse clouds from above and to the side.
The data told scientists about the composition and temperature of particles in the clouds, and revealed the clouds are large enough to fall to the ground.
The snowfalls occurred on the planet's south pole in winter, Nasa said.
Its existence has been theorised for decades, and the Phoenix lander observed falling water-ice in 2008, but the new data provides new insights into its nature.
The research, published in the Journal of Geophysical Research, is a stunning new insight into the varied climate of Earth's closest neighbour.
"These are the first definitive detections of carbon-dioxide snow clouds," said the lead author, Paul Hayne of Nasa's Jet Propulsion Laboratory.
"We firmly establish the clouds are composed of carbon dioxide - flakes of Martian air - and they are thick enough to result in snowfall accumulation at the surface."