Atmosphere

The Martian atmosphere is less than one one-hundredth that of earth's at sea level and is primarily composed of carbon dioxide. It is persistently hazy due to suspended dust and particles that make the sky took gray-yellow.

The atmosphere contains little water vapor because water condenses at low temperatures. Most of the water supply is held in the polar ice caps or near the equator in soil grains on the surface. Carbon dioxide is the chief oxygen-bearing gas on Mars, though some trace amounts of atomic oxygen and ozone appear in the atmosphere as a result of chemical reactions with ultraviolet light. No sulfur-containing gases have been detected in the atmosphere but they may have been released by volcanoes in the past and became particles that eventually formed sulfate minerals. When water is released into the soil oxygen is released, showing us that the soil contains an oxidant, probably a peroxide. Because the atmosphere of Mars has much less ozone than that of Earth, it lacks the equivalent of a stratosphere.

The low temperatures of Mars is a result of the large amount of carbon dioxide, which is able to radiate away the energy that reaches the planet from the sun. The thin atmospheres does little to help with the transport of heat on the planet, which is why carbon dioxide condenses on the surface of the poles as dry ice during the winter. This decreases the pressure at the poles during the winter; the pressure may drop about 25% from fall to winter. The entire surface is very responsive to direct sunlight, so the temperature may drop tens of degrees after the Sun sets. When winds on the surface of the planet reach 50 to 100m per second sand grains about 100 microns across start a skipping motion called saltation. They strike other smaller dust grains that may remain suspended in the atmosphere for months before coming down again. Winds of some 150 kph are estimated to be the minimum for the initiation of particle movement. Because of these high speeds particles have a greater erosion capability than they would have here on Earth. A local dust storm can grow to global proportions in a matter of weeks, enveloping the entire planet in a dust shroud. After a while there is too much dust in the storm and the winds and storm begin to decay. They develop during the spring and early summer in the southern hemisphere, when the planet is closest to the sun and the winds are stronger.