Snowstorms and Climate Change . . . On Mars

By Michael D. Lemonick

Scientists have known since the mid-1600s that the Red Planet isn’t entirely red. That’s when Giovanni Cassini described two whitish patches, one at each of Mars’ poles; a little more than a century later, William Herschel noted that they grew and shrank with the seasons. They must, Herschel said, be ice caps, just like the ones at Earth’s North and South Pole.

He was right — and now researchers have used NASA’s Mars Reconnaissance Orbiter satellite to make two key discoveries about how those ice caps grow and shrink.

An orbital view of the north polar region of Mars.
Credit: NASA

In the first, reported in the Journal of Geophysical Research, scientists at the Jet Propulsion Laboratory have detected the first evidence of the dry-ice snowstorms long believed to be crucial to the growth of the polar caps during the Martian winter. 

In the second, published in Icarus, an international team of scientists has showed that layers in the Martian polar ice caps can be used to understand Mars’ climate history, just as the ice on Greenland and Antarctica have helped to reveal Earth’s.

The discovery of snow came from an instrument on the orbiter known as the Mars Climate Sounder, which constantly looks toward the Martian horizon so it can take edge-on measurements of the Martian atmosphere from top to bottom, registering the presence of temperature, humidity and solid particles of dust and ice.

Layers covered with dust and frost captured by the Mars Reconnaissance orbiter.
Credit: NASA/JPL/UA

In this case, the ice is frozen carbon dioxide (CO2), which can only exist at temperatures below −109.3 °F. Scientists have long known that CO2 is a major component of the thin Martian atmosphere and of the ice caps. They’ve also known that dry ice, building in winter and evaporating in summer, is why the Martian polar caps expand and contract over Mars’ 687-day year. Until now, however nobody was sure it settled as frost, or fell as snow.

These new observations, however, clearly indicate dry ice clouds, along with particles streaming all the way to the surface. The particles are  “flakes of Martian air,” lead author Paul Hayne said in a press release, “and they are thick enough to result in snowfall accumulation at the surface.” (Mars’ atmosphere has water vapor as well, but very little: in 2008, NASA’s Phoenix lander detected ordinary snow, which evaporated before it reached the surface).

The discovery about Mars’ climate history, meanwhile, came mostly from the orbiter’s high-resolution cameras, which show distinct layers in the permanent ice caps after the seasonal snows have evaporated.