Mars is a very strange planet. The surface of the planet is inhospitable to human life, yet scientists have discovered evidence that in the past Mars could have been habitable. Evidence of liquid water flowing on the surface of the planet in the past is one of the main reasons Mars is such a focus for scientific discovery.
While we spent most of our time talking about the Curiosity Rover that landed on the surface of the planet not too long ago, another NASA scientific mission has been on station for much longer with the Mars Reconnaissance Orbiter zipping around the planet gathering data as it goes. Recently, the orbiter captured data that has provided scientists with the clearest evidence yet that carbon dioxide snow is falling on the surface of Mars.
The evidence collected by the orbiter hints at the only known example of carbon dioxide snow falling anywhere in our solar system. You may remember from science class that frozen carbon dioxide is dry ice and requires temperatures of about -193°F to form. Evidence of dry ice snow was discovered in clouds around the Martian south pole, and snowfalls occurred during the Martian winter.
Falling water ice snow was also discovered in northern Mars by the Phoenix Lander mission in 2008. The evidence of dry ice snow was discovered by looking at clouds over the southern pole from directly overhead and sideways use using the Mars Climate Sounder, which is one of the six instruments aboard the Mars Reconnaissance Orbiter. This particular instrument records brightness in nine different wave bands of visible and infrared light.
The data gathered provided scientists with information on temperature, particle size, and particle concentration. Scientists say that the south polar residual ice cap is the only place on Mars where frozen carbon dioxide persists on the surface year-round. Scientists have been unable to determine exactly how the frozen carbon dioxide from Mars' atmosphere is deposited on the ground. The question the team of scientists has been trying to answer is whether the frozen carbon dioxide is deposited as snow or by freezing out at ground level as frost. The new data indicates that snowfall is especially vigorous on top of the residual cap.