(Sen) - Carbon dioxide snowfall has been detected on Mars by NASA’s Mars Reconnaissance Orbiter, making the red planet the only place in the Solar System where such snowfall has been spotted.
Clouds blanketing the Martian south pole during winter are responsible for the carbon dioxide snowfall. Water-ice snow has previously been detected on Mars in the northern hemisphere by NASA's Phoenix Lander, however much lower temperatures are required for snow to form from carbon dioxide. Carbon dioxide freezes below minus 125 degrees Celsius, becoming what is known as “dry ice.”
It has been known for some time that carbon dioxide on Mars exhibits seasonal effects, as the amount of carbon dioxide ice at the poles changes according to the temperature. The ice cap at the south pole is the only place on Mars where frozen carbon dioxide remains on the surface permanently, but it was unknown if this ice cap existed due to freezing or snowfall. "The finding of snowfall could mean that the type of deposition - snow or frost - is somehow linked to the year-to-year preservation of the residual cap," said Paul Hayne of NASA's Jet Propulsion Laboratory (JPL) and lead author of the paper published in the Journal of Geophysical Research. Both Martian poles see carbon dioxide snowfall, but there is 50 per cent more of a build up at the south pole.
"These are the first definitive detections of carbon-dioxide snow clouds," said Hayne. "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."
The Mars Climate Sounder aboard the Mars Reconnaissance Orbiter was used to observe the clouds, and discern the types of particles and their sizes, as well as the temperatures in the atmosphere. The results showed a pillar of carbon dioxide cloud that stretched for 500 kilometres in diameter over the pole. In addition, smaller carbon dioxide clouds were also spotted lurking further south.
Clouds were also found by researchers at the Massachusetts Institute of Technology (MIT) by sifting through old data from the laser altimeter aboard the Mars Global Surveyor. Reflected laser beams sometimes returned to the spacecraft in an unusual time, suggesting they had encountered clouds along the way.
The snowflakes of carbon dioxide are much smaller than those made of water ice on Earth. The particle sizes are only the size of a red blood cell, meaning that snowfall on Mars would probably look like fog. The snow particles sizes also differ in the two hemispheres, with the size of the northern snow particles being larger than in the south.
"One line of evidence for snow is that the carbon-dioxide ice particles in the clouds are large enough to fall to the ground during the lifespan of the clouds," said co-author David Kass of JPL. "Another comes from observations when the instrument is pointed toward the horizon, instead of down at the surface. The infrared spectra signature of the clouds viewed from this angle is clearly carbon-dioxide ice particles and they extend to the surface. By observing this way, the Mars Climate Sounder is able to distinguish the particles in the atmosphere from the dry ice on the surface."