South polar CO2 ice could double Mars’ atmosphere

Results from the Shallow Radar (SHARAD) instrument on NASA’s Mars Reconnaissance Orbiter show that the south polar layered deposits on Mars hold about 30 times as much carbon dioxide ice as previously known. If this CO2 were released into the atmosphere, the researchers say, it could nearly double the quantity of the gas in the air and similarly increase the surface pressure.

SLICE OF ICE: SHARAD draws a profile across the southern polar layered deposits, revealing layers largely free of dust and other materials that cause radar reflections. These clear lenses and volumes are likely to be CO2 ice. (NASA/JPL image)

A team of scientists led by Roger Phillips (Southwest Research Insitutute, Boulder) reports in Science that ground-penetrating radar profiles of the south polar layered deposits using SHARAD have revealed thick deposits of CO2 ice in the form of buried lenses and pockets. These are several hundred meters (roughly 1,000 feet) thick. Moreover, the team says, the ice occurs within a stratigraphic layer that shows collapse features perhaps caused by CO2 escaping from beneath the surface in the past.

In the current climatic cycle for Mars, where its axis tilts at 25° to the planet’s orbit around the Sun, the polar regions are cold. However, because it lacks Earth’s large stabilizing Moon, Mars can change its axial tilt (obliquity), reaching angles of 35° and more. During such high-obliquity periods — for example, one which occured about 600,000 years ago — the poles become much warmer than they are today, while equatorial regions become the coldest places on the planet. The polar warmth causes volatile materials, such as water and especially CO2 ice, to come out of the ground and go into the atmosphere.

Once in the atmosphere, the volatiles migrate to the colder parts of Mars, condense out of the air, and become trapped at the surface, thus depositing ice near the equator. Then, when the obliquity swings back and the climate pendulum follows in step, the volatiles reverse the migration, leaving the equator and returning to the polar regions by way of the atmosphere.

The team says, “If released into the atmosphere at times of high obliquity, the CO2 reservoir would increase the atmospheric mass by up to 80 percent.” They add that this would lead to “more frequent and intense dust storms and to more regions where liquid water could persist at the surface without boiling.”

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