Mars: A World Without Clouds
In fact, it might be much like Mars, says JPL planetary scientist David Kass. The Red Planet today has relatively few clouds compared to Earth. That's because the Martian atmosphere contains less than a tenth of a percent of the amount of water vapor found in Earth's atmosphere. Without much water vapor, and with temperatures averaging 80 degrees Celsius (176 degrees Fahrenheit) colder than on Earth, only thin ice clouds form. They tend to look like a thinner version of Earth's wispy cirrus clouds. "We don't think that clouds on Mars get to the point where you couldn't see the sun through them, but they might get thick enough that you could look at the sun through them without hurting your eyes," sats Kass.

Mars also has thicker clouds made of frozen carbon dioxide commonly called dry ice that form both high in the atmosphere and at the poles during winter, where the sun never rises for half the Mars year. These clouds are dense enough to dim the sun's light by about 40 percent, but because they are found only in limited regions near the planet's poles and equator, they are unlikely to affect the Martian climate as a whole. Scientists theorize that the relatively sparse clouds on Mars allow temperatures to rise and fall dramatically. Without the cooling effect of significant cloud shade or the insulating effect of thick cloud blankets, the surface of Mars heats drastically during the day reaching temperatures around 18 degrees Celsius at the equator before the temperature plummets at night to equatorial surface temperatures as cold as 130 degrees Celsius below freezing.

But researchers don't yet know for certain how exactly Martian clouds affect the planet's climate. "It's not clear yet how big a role clouds play in Mars' climate," says Kass. "This is really on the cutting edge right now." As planetary climate models become more sophisticated, they will include the radiative effects of the clouds seen in data from the Mars Climate Sounder on NASA's Mars Reconnaissance Orbiter. Kass says the modelers will be able to incorporate that data and examine cases with and without clouds to see their impacts. "We hope to know more soon," Kass adds.

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