Clouds on Mars?!

How can clouds exist on a planet that has a thin atmosphere and little moisture? Isn't Mars just a desert planet?

Most of the time, Mars is cloudless, but temperatures often plummet below the freezing point of carbon dioxide. Carbon dioxide, which is the most abundant gas in the Martian atmosphere, freezes into thick clouds due to the low temperature. The frozen carbon dioxide, called dry ice on Earth, can block the sun by 40 percent. The direct observations conducted by the European Space Agency's Mars Express have confirmed the clouds, whose existence has been previously suggested.

The clouds' density and makeup is very surprising. They are made up of CO₂ particles larger than a micron(a micron is one thousandth of a millimeter) across.The particles normally would not be expected to form in the upper atmosphere, or stay in the air before returning to the surface. The clouds are also very high, reaching heights of more than 80 km (or about 50 miles) above the surface of Mars. The Martian clouds can also span hundreds of kilometers.

The French scientists who made the discovery know that the carbon dioxide clouds are most common near the equator of Mars, most probably because of the temperature variations there. Small amounts of warm gas rise above the planet, cooling off and condensing as they reach higher altitudes. When they release latent heat (the energy absorbed or released during a change of state), they rise even further. What these scientists haven't confirmed yet is what they condense around. Cloud droplets condense around sand or dust on our planet, but on Mars, there are many different possibilities. Martian dust kicked up by winds, or micrometeorites entering the atmosphere, or maybe even tiny crystals of water ice.

The importance of the discovery is that scientists can now learn more about the past climate of Mars. The planet is theorized to have been warmer billions of years ago, with CO₂ clouds more covering up the atmosphere. The clouds could have played a big role in the global warming of Mars.

For more information, read the article on the ESA Mars Express website detailing the discovery. You can also read about Mars Express and the Climate of Mars on Wikipedia.