How are the atmospheric dust variations and dust storms taken into account ?

The major factor which governs the variability in the Martian atmosphere is the amount and distribution of suspended dust. Some years may have low or moderate dust loading throughout the year, while others may have regional or global dust storms which engulf the planet.

Because of this variability, and since even for a given year the details of the dust distribution and optical properties can be uncertain, multi-annual model integrations were carried out for the database assuming various ``dust scenarios'', i.e. prescribing various amount of airborne dust in the simulated atmosphere.

Five dust scenarios have been used :

• A first scenario, named Mars Global Surveyor Dust Scenario based on recent spacecraft observations, which should be used for most aplications. It is our current ``best guess'' thought to represent the moderately dusty planet Mars as observed by Mars Global Surveyor.

• Two additional annual scenarios which are provided to bracket the possible global conditions on Mars (Figure 1) outside global dust storms.
  • The ``Viking Lander'' Dust Scenario : a relatively dusty year made by generalizing the Viking Lander dust opacity observations to the entire planet, but with the large dust storms removed.
  • The Low Dust Scenario : a very clear year (visible optical depth $\tau= 0.1$).

• Two additional dust storm scenarios which are provided only during the period during which such global events are known to occur (southern spring and summer) :
  • Dust Storm Scenario ($\tau=2$) : a moderate global dust storm
  • Dust Storm Scenario ($\tau=5$) : A severe global dust storm

Further details on the dust scenario can be found in Lewis et al. (2001b) .

Figure 1: A typical temperature profile observed by radio-occultation (thick solid line, february 1998, $L_s=271^o$, $11.7^o$S-$155^ o$E, local time : 4:30) compared to temperature profiles predicted by the database at the same time and location. Profiles from the ``MGS'' scenario are usually very close to the MGS observations, whereas the ``Viking'' and ``low dust'' scenarios yield warmer and colder temperatures profiles in the lower atmosphere, respectively. MGS data courtesy of D. Hinson, Stanford University.