Much of the discussion surrounding science on Mars typically focuses on the search for water. Like the Earth, Mars has polar ice caps, made mostly of water ice. However, in the frigid wintertime and total darkness at Mars’s poles, it is so cold that Mars’s primary atmospheric component, carbon dioxide (CO2), freezes solid on the surface.
Now a new study by Piqueux et al. suggests that solid CO2 is more common on the rest of the Martian surface than previously thought and takes the form of CO2 frost. Its cyclical formation and sublimation could keep the planet’s soil dusty.
Searching for CO2 ice on Mars’s surface is a somewhat tricky task. Most of the orbiting instruments make their measurements on the basis of how sunlight reflects off the planet, but the solid carbon dioxide occurs only when temperatures plummet at night, when there is no sunlight. But the Mars Climate Sounder (MCS) on board the Mars Reconnaissance Orbiter (MRO) can measure the surface temperature from orbit by detecting infrared radiation. Depending on atmospheric pressure (and thus elevation), the freezing point of carbon dioxide ranges from about 130 K to more than 153 K on the Martian surface. Using temperature data from the MCS combined with elevation profiles, the researchers were able to map where the planet’s surface gets cold enough for CO2 ice to form.
The team found that as expected, most of the solid carbon dioxide formed closer to the poles, but surprisingly, there was evidence for CO2 frost in middle to low latitudes as well, especially at high elevations. These frost layers can reach thicknesses of up to several hundred micrometers. Also, because the deposits were so meager and CO2 is so plentiful in the Martian atmosphere, the scientists say that the frost’s formation is not contributing in any significant way to the global atmospheric pressure cycle. In all cases, CO2 frost was short-lived, sublimating within an hour after sunrise.
The researchers suggest that this regular freeze-sublimation cycling could act to stir the planet’s soil, making it more porous and fluffy, thus explaining why some regions of the planet stay dusty while others have a more tightly compacted or cemented surface. (Journal of Geophysical Research: Planets, doi:10.1002/2016JE005034, 2016)
—David Shultz, Freelance Writer