With its
thin atmosphere and scant moisture, Mars is often largely cloud-free. But new
observations reveal clouds of dry ice thick enough to cast significant shadows
on the red planet.
Dust storms
are known to shroud vast swaths of Mars. Clouds have been photographed
from the ground before, too.
The new
research finds that carbon dioxide, the main component of martian air, freezes
into clouds so dense they dim the sun by about 40 percent. Frozen carbon
dioxide is commonly called dry ice here on Earth.
"This
is the first time that carbon dioxide ice clouds on Mars have been imaged and
identified from above," said Franck Montmessin of the Service d'Aeronomie,
University of Versailles and lead author of a paper in the Journal of Geophysical
Research. "This is important because the images tell us not only about
their shape, but also their size and density."
Until now,
only indirect
information suggested what these clouds were made of. The new observations
were made by the European Space Agency's orbiting Mars Express.
The
carbon-dioxide clouds are surprisingly high, some 50 miles (80 kilometers) and
are several hundred miles wide. They're thicker than expected, according to Montmessin's
team.
Another
surprise: the clouds are made of particles that are larger than expected. The
particles are more than a micron wide (one-thousandth of a millimeter).
Normally, particles of this size would not be expected to form in the upper
atmosphere or to stay aloft for very long before falling back toward the
surface.
The clouds
"cast quite a dense shadow and this has a noticeable effect on the local
ground temperature," Montmessin said. "Temperatures in the shadow can
be up to 10 degrees Celsius [50 degrees Fahrenheit] cooler than their
surroundings, and this in turn modifies the local weather, particularly the
winds."
These
clouds were found mostly near the equator. The researchers figure they're the
result of extreme variations in daily temperature that occur near the equator.
"The
cold temperatures at night and relatively high day-time temperatures cause
large diurnal waves in the atmosphere," Montmessin explained. "This
means there is a potential for large-scale convection, particularly as the
morning Sun warms the ground."
Convection
— warm air rising — is at the root of
Earth's weather, too.
On mars,
bubbles of warm gas rise. At altitude, the carbon dioxide condenses, the
researchers explained. This releases heat, causing the gas and ice particles to
rise higher.
On Earth,
water vapor condenses around tiny particles, often dust or salt, to form
clouds. It's not yet clear what the martian moisture is condensing around. The
researcher said it could be dust, micrometeorites or tiny crystals of water
ice.
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