Nighttime
clouds detected for the first time on Mars help to keep the planet's surface
warm after sunset when temperatures drop, a new study suggests.
The
nocturnal clouds are five times thicker than their daytime counterparts and hover
close to the ground, almost like a fog.
The study,
conducted by researchers at NASA and the National Oceanic and Atmospheric
Administration (NOAA), is detailed in the Feb. 1 issue of Geophysical
Research Letters.
Clandestine
clouds
Martian
clouds-both the day and night variety--resemble the wispy and high-flying cirrus clouds
on Earth, except they are thicker and
found at more variable heights. Previous studies have detected daytime clouds
on Mars as high as 62 miles (100 km)
above the surface, making them the highest
flying clouds ever detected on any planet.
Nighttime
clouds are harder to spot. During the day, Martian clouds appear brighter than
the planet's surface because they reflect more sunlight. Daytime clouds [image]
also stand out in thermal imaging because they are much cooler than the Martian
surface.
At night,
these differences disappear. There is no longer any sunlight to reflect, and
surface temperatures drop until there is hardly any temperature contrast left
between Martian cloud and surface.
A
temperature anomaly
Nighttime
clouds [image]
on Mars are predicted from computer models, but none had been directly observed
until now. The researchers discovered the stealthy clouds after investigating a
temperature anomaly that occurs on the Martian surface at night picked up by
the now-lost
Mars Global Surveyor
(MGS).
"We found
that in certain regions the temperature didn't drop as much as we would've
expected it to drop," said study leader John Wilson of NOAA.
Using the MGS's
laser altimeter, the researchers created the first map of Martian clouds at
night. An altimeter works by bouncing a pulse of light off the planet's
surface. Depending on how the pulse behaves--whether it is absorbed, reflected
or scattered--scientists can distinguish between clouds
floating in the atmosphere and rocks
on the planet surface.
The measurements
showed that the parts of the planet's surface blanketed by clouds at night were
warmer by about 35 degrees Fahrenheit than those that were exposed and
unprotected by clouds.
This same
effect occurs on Earth. "A cloudy night won't be as cold as a clear night," Wilson told SPACE.com. "If there
are no clouds, the radiation from the surface just goes straight off into space
rather than being bounced back."
In Mars'
distant past, when the planet was much warmer and wetter than it is now, clouds
would have been more abundant and contributed to its greenhouse effect, the
researchers speculate.
"The water
ice clouds would be very prominent," Wilson said, "and may have had a very important role in affecting Mars'
climate, maybe helping to make the planet warmer."
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