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Planets with tenuous atmospheres, like Mars, display much larger fractional temperature changes than planets with thick atmospheres. For Mars AT/T is approximately 20%, compared to ~0.4% for Venus.

Condensation flow winds result from the condensation of CO2 over the fall/ winter pole and its sublimation over the spring/summer pole. Atmospheric pressure

Atmospheric conditions and evolution 900 t--

500-1-1-1-10° 45° 90° 135° 180° 225° 270° 315° 360° Ls

Figure 6.10 Atmospheric pressure varies on an annual cycle because of the condensation flow between the polar caps. This figure shows the pressure variation averaged over three years at the Viking 1 landing site near latitude 22.5°N. Highest atmospheric pressures occurred near northern winter (Ls=270°) with lowest pressures occurring near the end of northern summer.

increases as the CO2 moves into the fall/winter hemisphere and decreases as summer approaches and the CO2 migrates to the opposite hemisphere. The combination of these polar sinks/sources of CO2 and Mars' high orbital eccentricity produce a 20% variation in surface pressure throughout the year (Figure 6.10).

The condensation/sublimation of CO2 is a major driver of Mars' atmospheric circulation and is called the CO2 seasonal cycle (James et al., 1992). The CO2 cycle is weakly linked with the dust cycle since the upwardly directed wind associated with sublimation will transfer dust from the ice cap to the atmosphere while the downward wind at the condensing polar cap will deposit dust (Kahn et al., 1992; James et al., 2005).

The amount of solar insolation decreases during the fall and reduces to zero at the poles during the winter. The amount of energy lost at the top of the winter atmosphere due to thermal radiation is not balanced by the energy produced by atmospheric convection. The temperature can therefore drop to the condensation temperature of CO2 (148K), causing atmospheric CO2 to condense and form the seasonal polar cap. Latent heat from the condensation of CO2 is the major atmospheric energy source during polar night.

The CO2 seasonal cycle involves up to 30% of the martian atmospheric mass. Atmospheric circulation is strongly affected as this mass sublimes from the spring/ summer pole and is transferred to the fall/winter pole for condensation. This condensation flow adds ~0.5 m s-1 to the planet's Hadley-circulation-produced meridional winds (Read and Lewis, 2004).

Another contributor to the condensation flow winds is the H2O seasonal cycle (Jakosky and Haberle, 1992; Houben et al., 1997; Richardson and Wilson, 2002).

This cycle involves the exchange of H2O between atmospheric and non-atmospheric reservoirs. The non-atmospheric reservoirs include the seasonal and permanent polar caps, adsorbed water in the regolith, and surface or near-surface ice. Most of the atmospheric transport involves H2O vapor, although the white condensate clouds also contribute. As with CO2, the abundance of atmospheric H2O vapor varies with latitude and season, ranging over a factor of two.

The dominant processes which affect the abundance of H2O vapor in the atmosphere are sublimation from the seasonal and permanent polar caps, condensation on the fall/winter polar cap, desorption of H2O from regolith grains due to seasonal temperature changes, and diffusion of H2O vapor from the regolith into the atmosphere. The permanent caps are the dominant contributor to atmospheric H2O by pumping H2O into the atmosphere in the spring/summer and removing it in the fall/winter. This creates an imbalance in the amount of H2O between the hemispheres, which can be counteracted by the regolith absorbing or releasing H2O. Equilibrium between the atmosphere and regolith is achieved within a few years.

The general circulation of the martian atmosphere is driven by zonal and meridional winds, the Coriolis force, planetary waves, and seasonal condensation flow. Zonal winds and meridional circulation are produced by solar insolation through Hadley circulation, although the seasonal condensation flow also contributes to the meridional component.

Winds follow a curved path from high to low pressure areas because of the planet's rotation. For prograde rotating bodies like Mars, winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This deflection of wind patterns because of the planet's rotation is the Coriolis effect and the fictitious force causing the wind to curve is the Coriolis force. The Coriolis force (FC) is an expression of the conservation of angular momentum:

where 0 is the angular velocity of the planet and v is the velocity of the atmosphere. The magnitude of FC depends on the latitude f, being greatest near the poles and negligible near the equator:

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