Atmospheric Circulation

The similarity of Marsís rotation period, axial tilt, and atmospheric transparency to Earthís suggests a valuable parallel to our experience with terrestrial meteorology, but certain features unique to Mars lead to novel departures from familiar behavior. First, the main atmospheric gas on Mars condenses on the surface, leading to large variations in surface pressure. Second, the enormous thermal inertia of Earthís ocean and the massive transport of latent heat by both oceanic circulation and evaporation of ocean water are not present on Mars to moderate day-night temperature variations and latitude-dependent temperature gradients. Also, the eccentricity of Marsí orbit causes "seasonal" temperature variations that heat and cool the entire planet in concert. 

Temperature mapping reveal the driving force behind planetary-scale winds, also shows that the surface has an extremely small thermal inertia, requiring the presence of a fine-grained, poorly conducting dust with a very low gas pressure. The surface temperature therefore responds quickly to changes in insolation (irradiation by the Sun).

Circumstances are different when the atmospheric dust content is high. Dust is always a factor on Mars; even in the absence of dust storms. Absorption and scattering of sunlight is never truly negligible and is sometimes a dominant effect. Absorption of sunlight by dust can lead to formation of a strong inversion layer (a warm layer well above the ground that inhibits convection from below), which not only affects the daytime vertical motions and limits the daytime solar heating rate, but also radiates heat to the ground at night, further diminishing the day-night temperature range. Dust storms also dramatically warm the poles by the same mechanism.

The general (planetary-scale) circulation of the atmosphere is characterized by the average zonal (east-west) wind speeds, the average meridional (north-south) motions, near-stationary wave structures, and seasonally varying features due to traveling waves. Small-scale processes are also important, as is the rapid inflation and deflation of the atmosphere during the diurnal cycle. This phenomenon of atmospheric tides is exacerbated by the rapid radiative response of the atmosphere.

The Martian atmosphere exhibits a wide range of dynamical behavior that depends on the dust content of the atmosphere. The dynamics of the rise, maintenance, and decay of giant dust storms are poorly understood, although much interesting work is in progress. One intriguing factor is an apparently chaotic factor governing the onset of severe dust storms: very similar seasonal and meteorological conditions will generate severe dust storms in some years, transient storms in others, and no storms at all in others. It is likely that the process of storm growth is highly nonlinear and sensitive to rapid divergence in response to extremely small differences in initial conditions. This behavior may be deeply chaotic in the mathematical sense, subject only to statistical description over time scales longer than a few days.



The four hemispheric views shown above have been combined into a

full-color global map (called a Mollweide projection), showing the

regions of Mars imaged by the Hubble telescope during the planet's

closest approach to Earth. Latitudes below about 60 degrees south were

not viewed by the telescope because the planet's north pole was tilted

towards Earth during this time. This image is a composite of pictures

taken with three filters: blues (410 nanometers), green (502

nanometers), and red (673 nanometers). The Hubble telescope's resolution

is 12 miles per pixel (20 kilometers per pixel) near the Martian





Atmospheric Composition