Planetary
scientists have discovered new details to explain a mysterious seasonal
asymmetry of a Martian polar ice cap.
Like Earth,
Mars has frozen polar caps, but unlike Earth, these caps are made of carbon
dioxide ice as well as water ice. During the southern hemisphere's summer at
Mars, much of the ice cap
sublimates, a process in which the water ice turns straight back into gas,
leaving behind what is known as the residual polar cap of mostly carbon dioxide
ice (what we know as "dry ice").
While the
winter cap is symmetrical about the red planet's south pole, the residual cap
is offset by some three to four degrees. Scientists had proposed one
explanation for this puzzling misplacement in 2005, suggesting that more snow
fell in the polar region's western hemisphere during the winter. In that way,
during the summer and spring seasons, some of the surface snow would remain on
one side of the pole.
But now new
details about the Martian weather suggest a slightly different mechanism for
the wonky ice cap. The complex answer involves an odd interaction between wind,
snow and the sun.
Snow and
frost
The new
study relied on information gleaned from the Planetary Fourier Spectrometer
onboard the European
Space Agency's Mars Express spacecraft.
Marco
Giuranna of the Istituto di Fisica dello Spazio Interplanetario CNR in Rome, Italy, and colleagues measured temperature and other conditions of the Martian
atmosphere above the south polar region, along with carbon dioxide accumulation
in the southern ice cap.
They found
the offsetting process starts with strong eastward winds at Mars'
mid-latitudes that blow straight into the Hellas
Basin, the largest impact structure on Mars with a diameter of 1,429 miles
(2,300 km) and a depth of about 4 miles (7 km). The crater's steep walls
deflect the winds and create giant waves in the atmosphere, which on Earth are
referred to as Rossby waves. These waves reroute the high-altitude winds on
Mars and force weather systems toward the south pole.
In the
western hemisphere of Mars, the result is a strong low-pressure system near the
south pole, and a high-pressure system in the eastern hemisphere, again near
the south
pole.
The low-pressure system in the western hemisphere makes for cooler air temperatures. And these
temperatures are just right for carbon dioxide to condense into snow. That
means in the western hemisphere of the south pole there is a buildup of both
snow and frost.
In the
high-pressure system in the eastern hemisphere, temperatures are always too
warm for snow to fall, so only frost coats the ground there, Giuranna said.
Frost is
found across the south polar region on Mars, regardless of hemisphere. "During
the polar night [on Mars] you always have condensation of CO2 on the surface. I
call it frost," Giuranna told SPACE.com. "When CO2 gas touches
the surface it freezes instantaneously."
Sunlight's
role
The story
is not just one of more ice cover (from snow and frost), however.
The areas
that have extensive snow cover reflect more sunlight back into space than does
the surface frost. Grains of frost tend to be larger than snow grains and have
rougher surfaces. With such rugged texture, the frost traps more sunlight,
driving the sublimation.
So the
western area of the southern polar cap, built of snow and frost, not only has a
larger amount of carbon dioxide ice deposited but also sublimates more slowly
during the summer. The western area built of frost disappears completely.
This explains
why the residual cap is not symmetrically placed around the south pole, the
researchers said.
The
research is set to be published in a forthcoming edition of the journal Icarus.
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