Modeling of Martian atmospheric phenomena and atmosphere ...

Figure 1: A MLAM forecast of ground temperature (greyscale; in K) and 1.5 m level winds(arrows; in m/s; the arrow below the image denotes 20 m/s) over topography (contours; in mfrom Mars’ lowest elevation) in the VL1 and MPF landing region (the landing sites are markedwith “VL1” and “MPF”). Season is summer solstice (L s = 90 ◦ ) and local time is 1100 at 0 ◦ W(0900 at 30 ◦ W — the MPF longitude).5. AEROSOL PROCESSESBesides dust, also ice crystals are frequently observed in the form of clouds in the Martian atmosphere.We have studied their formation processes in Martian conditions, and compared withobservations and previous modelling results. The situation in the Martian atmosphere differsfrom the terrestrial conditions, since the atmosphere is nearly pure CO 2 vapour, which takespart in the processes of particle formation (nucleation) and subsequent growth. In a near-puresituation nucleation becomes non-isothermal and strong coupling of fluxes arises in multicomponentcondensation. These effects need to be taken into account in the theories used.We studied homogeneous and heterogeneous nucleation of CO 2 based on the work by Wood(1999), and H 2 O nucleation following the same schemes. We have compared the results to CO 2laboratory measurements (Glandorf et al., 2002). The H 2 O results were compared to those ofInada (2002). A more detailed description can be found in Määttänen et al. (2005). On Mars,homogeneous nucleation would require very high saturation ratios (10 8 ). Anyhow, the onsetsaturation ratio (the saturation ratio for nucleation rate of 1 s −1 ) of heterogeneous nucleation forCO 2 on dust particles of 1.0 µm radius is 1.32. This is in fair agreement with measured values(Glandorf et al., 2002). For constant concentration of CO 2 it gives a threshold temperature of145 K. For water with average concentration of 300 ppm the onset saturation ratio would be1.18 and the corresponding threshold temperature 200 K (Figure 2).We have modelled nucleation as a function of height at different locations, e.g., the MPF landing226