Night Clouds Warm Red Planet
NASA's robotic Opportunity rover snapped a picture of a vast, sandy plain and high-flying clouds while exploring Mars' Victoria Crater last October.
Credit: Mars Exploration Rover Mission, Cornell, JPL, NASA,

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 "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."