VI.

The Atmosphere of Mars

A.

Composition

The atmosphere of Mars is 95 percent carbon dioxide (CO₂), nearly 3 percent nitrogen, and nearly 2 percent argon with tiny amounts of oxygen, carbon monoxide, water vapor, ozone, and other trace gases. Earth’s atmosphere is 78 percent nitrogen and 21 percent oxygen, with about 1 percent argon and only 0.04 percent carbon dioxide. The larger relative proportion of argon in the air on Mars may indicate that its atmosphere was much thicker in the past. Compared to other gases, argon is a relatively heavy gas that is not as easily lost into space over time. As a noble gas, argon does not combine chemically with other substances in the atmosphere or on the surface.

In 2004 scientists reported detecting small amounts of methane (CH₄) in the Martian atmosphere, at about 10 parts per billion. The observations are controversial, however, because such small levels of abundance are involved. If the methane abundance is real, then some recent or ongoing process on Mars may be releasing fresh methane, because otherwise the gas would quickly break down from the ultraviolet radiation from the Sun. Possible sources for the methane on Mars could include volcanic or hydrothermal activity, chemical reactions between water and minerals in the crust, ancient deposits of methane ice, or even biological activity. However, there is currently no strong evidence for active volcanic or hydrothermal processes on Mars, which should also release sulfur dioxide (SO₂), a gas not yet detected there. The apparent concentration of methane in places where subsurface water ice is thought to be present might argue more for a water-rock chemical reaction origin. At present there is no real consensus on the presence or origin of methane on Mars.

The pressure of the Martian atmosphere varies with the seasons, ranging from 6 to 10 millibars, or about 1 percent of the air pressure at Earth’s surface. The variation in pressure occurs because in the fall and winter the temperature gets so low at the poles of Mars that carbon dioxide snows out of the atmosphere and forms meters-thick deposits of dry ice on the surface. In the springtime as the surface warms up, the dry ice evaporates back into the atmosphere. The pressure also varies with altitude just as it does on Earth and is about ten times lower on the top of Olympus Mons than on the floor of Hellas Planitia.

B.

Clouds

Even though the Martian atmosphere contains less than 1 percent as much water vapor as Earth’s atmosphere, clouds and frosts form on Mars and have been studied in detail by telescopes and spacecraft. Wave clouds, spiral clouds, clouds formed near topographic obstacles such as volcanoes, wispy cirrus-like clouds, and a wide variety of hazes and fogs have all been observed. Along with the dust storms and related clouds, these features all reveal the Martian atmosphere to be quite dynamic.

Data collected by the Mars Global Surveyor spacecraft indicate that much denser water ice clouds can form on the night side of Mars not visible from Earth. These clouds are about five times as thick as the water ice clouds sometimes seen on the day side and form lower in the atmosphere, creating a foglike layer above parts of the surface. A clue to the presence of such clouds comes from the ground temperature on the night side—during the Martian summer some areas near the equator in the northern hemisphere can be up to 20°C (68°F) warmer than predicted. The clouds overhead prevent some of the heat built up during the day from radiating away into space at night. Most of these water ice clouds quickly disappear after dawn.

Clouds consisting of carbon dioxide ice crystals also form in the Martian atmosphere, mainly over the polar regions in winter when the temperature is lowest. The CO₂ ice crystals strongly scatter thermal radiation, reducing the loss of heat into space over the poles.

C.

Sky Colors

Rovers and landers on the surface of Mars have photographed the planet’s striking sky colors, including sunrises and sunsets. On Earth, the mid-day sky is blue because of the way air molecules efficiently scatter blue light in our dense, relatively clear atmosphere. When the Sun is low in Earth’s sky, sunlight travels through much more of the air and so scattering of red light by dust and haze particles becomes more important than scattering by air molecules and our skies turn reddish and pink. On Mars, the sky color is almost totally determined by dust (only a feeble amount of molecular scattering is possible in the thin Martian atmosphere). Based on their sizes and shapes, dust particles scatter different colors of light more efficiently in certain directions, however. The mid-day Mars sky is reddish to pink because scattering of red light by fine dust particles is most efficient in that geometry. When the Sun is low, scattering of blue light is more efficient. Thus, the pinkish mid-day skies give way to bluish sunsets. Martian sky colors are to some extent opposite to those of Earth.

D.

Winds and Dust Storms

During most of the year wind speeds are fairly low—about 7 km/h (about 4 mph)—but during dust storms they can exceed 70 to 80 km/h (40 to 50 mph). These winds often originate in large basins in the southern hemisphere and carry great volumes of dust from the basins to other regions. The dust is not sandy, as in a sandstorm on Earth, but has the consistency of flour. The largest of these storms can cover the entire planet and last for months. An unusually large dust storm covered the planet in 2001 and was the largest storm seen since 1971. A similar large dust storm occurred in 2007. Smaller local or regional dust storms can occur any time during the Martian year. Some scientists think dust storms may generate a strong static electric charge that affects the chemistry of the soil, creating hydrogen peroxide that would break down organic substances on the surface or methane in the air.

Dust devils are also an important feature of Martian weather. These swirling columns of dust and sand occur during the Martian summer when surface temperatures can warm to 20°C (68°F). Starting as heated air at ground level, Martian dust devils can reach heights of 10 km (6 mi) and rotate at 30 meters per second (70 mph). Although the force of such winds is weak because of the planet’s thin air, dust devils on Mars can grow vastly larger than dust devils on Earth—ten times as large as Earth tornados. Because the rotating winds transport dust into the atmosphere, dust devils may have a major effect on the planet’s overall weather and climate. Dust devils also may carry a strong static electric charge. Such electrified dust devils could pose a hazard to future robotic or human explorers, coating equipment with dust and sand, interfering with radio communications, and even discharging lightning.