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Mars Mars is named after the Roman god of war. Its moons Phobos (fear) and Deimos (panic) are named after the sons of the Greek war god, Ares who is the Greek counterpart to Mars. Both moons are thought to be asteroids captured by Mars gravitational field. Today Mars is a dry planet, completely covered by dust, except for mixtures of dry-ice, water-ice and dust in the polar region.
Figure 1. Image of the Mars surface and atmosphere limb taken by one of the Viking Orbiters (Courtesy NASA/JPL). The diameter of Mars is about 6,800 km, its rotation period is 24.6 hours, its year is 687 days long, its aphelion is 205.5 million km, and its perihelion is 248.5 million km. Mars gravity acceleration is 3.7 m/s2, its surface temperature ranges from about 140 K to 300 K, and its surface pressure at the lowlands ranges between 7 and 10 hPa, less than 1% of the mean sea level pressure on Earth. It is interesting that about 30% of the Martian atmosphere freezes each winter into its polar caps. The freezing creates low pressure zones around the polar cap during the winter, while their sublimation in the spring creates high pressure zones. Because of the low heat capacity of the dusty Martian surface and atmosphere, large temperature variations are ubiquitous. The Meteorology instruments on the Mars Pathfinder (MPF) measured diurnal temperature variations of about 70 K, and air temperature increases rate of about 20 K per minute, in the morning. There is a large asymmetry between the Martian southern and the northern hemispheres. While the southern hemisphere is higher and littered with craters of various sizes, the northern hemisphere is lower by about 4 km and made up smooth, younger and less cratered plains. A topographical map of Mars is displayed in Figure 2.
Figure 2. Topography of Mars from the Mars Orbiter Laser Altimeter, MOLA (Courtesy of NASA/GSFC). Weather Systems Like on Earth, weather systems on Mars range from small scale phenomena such as dust devils to global circulations such as the Hadley cell. However, on Mars dust devils are much larger than on Earth and dust storms can be global in extent. Here, we briefly review Martian weather systems with focus on a non-mathematical description of the basic physics of dust devils and dust storms. A detailed mathematical description of them can be found on the various PowerPoint presentations linked to this site and the references given on them. Dust devils In contrast to terrestrial dust devils that have typical diameters of less than10 m and are seldom higher than a few 100 m (Sinclair, 1973), Martian dust devils have diameters between 100 m and 1 km, and are up to 10 km tall (Thomas and Gierasch, 1986; Malin et al., 1999, Ferri et al., 2003). Theory, modeling and observations show that strong dust devils frequently form in crater walls illuminated by the sun and move downwind as illustrated in Figure 3 (see Renno et al., 2004). The amount of dust pumped into the atmosphere by these vortices is tremendous. A single dust devil about 100 m in diameter and a few 1,000 m tall contains ~5,000 kg of dust, and pumps ~50 kg/s of dust into the atmosphere (Renno et al., 2004). Thus, large amounts of dust from crater walls and gullies are lofted by dust devils and deposited downwind of their sources.
Figure 3. Dust devil and dust devil tracks on crater walls (Courtesy NASA/JPL/MSSS). Link to movie of the formation of a dust devil pair on a crater wall. Charged dust particles produce electrical fields in excesses of 10 kV/m in terrestrial dust devils (Farrell et al., 2002, 2003; Krauss et al., 2002). Since Martian dust devils are larger and stronger than their terrestrial analogues (Thomas and Gierasch, 1985; Renno et al., 2000; Cantor et al., 2002), it is likely that they produce stronger electrical fields and, perhaps even glow discharges. Thus, electrically charged Martian dust devils and dust storms are potential hazards to Landers and could be dangerous to future astronauts exploring its surface. Indeed, the design of adequate mechanical and electrical systems for these Landers cannot progress effectively without a better understanding of Martian dust devils and dust storms. Moreover, ancillary phenomena associated with electrically charged vortices can ionize atmospheric gases and might have important implications to atmosphere chemistry. Measurements in dust devils and dust storms show negative charges aloft, which is consistent with the idea that negative charges are transferred to the smaller dust particles during collisions (Ette, 1971; Melnik and Parrot, 1998). Assuming that the larger particles stay in the saltation layer, while the smaller particles are lifted by the dust devil updrafts, we can estimate the electric field generated by them. The maximum charge of airborne dust particles can be calculated by assuming that, after energetic collision between dust and sand particles during saltation, the particles’ charging is limited by field emission (Bernhard et al., 1992). Then, a microdischarge (spark) occurs while the particles break away from each other and they are left with a residual charge of the order of that necessary to produce electric discharges (Renno et al., 2003). The negatively charged dust particles of a few µm in diameter then rise with the updraft producing the bulk electric fields observed in terrestrial dust devils, while the larger positively charged sand particles stay in the saltation layer. Then, knowing the dust particle concentration and the dust devil size, we can calculate the maximum electric field generated by them. In addition, we can calculate the atmospheric charging rate (current per unit area) produced by them by knowing the dust flux. It follows from the calculations of Renno et al. (2003, 2004) that the residual charge in terrestrial dust particles of ~10 µm of radius is qres ~ 3 x 10-15°C. This value is consistent with the results of laboratory experiments reported by Bernhard et al. (1992) and the observation of dust particles with charges of up to 10-12°C in terrestrial dust devils (Farrell et al., 2004). Terrestrial dust devils have dust concentrations np ~107 particles/m3 and dust fluxes F ~ 108 particles/m2s (Renno et al., 2003). Thus, they have maximum charge densities of ~10-8 C/m3 and can produce vertical currents I ~ 10-7 A/m2. Renno et al. (2004) approximated a dust devil by a cylinder of radius R and height H, and found that it can produce a near surface electric field gradients given by Er ~ (k π R2 np qres H) / [r (r2 + H2) 1/2], (1) where k = 9 x 109 N m2/C2 is Coulomb’s constant, and r (> R) is the distance from the center of the dust devil. It follows from equation (1) that the electric field near the boundary (at r ~ R) of a typical dust devil of H ~ 100 m is E ~ 0.1 x R2 kV/m, where R is in meters. Thus, a strong dust devil of radius R ~ 10 m produces a maximum near surface electric field of about 10 kV/m. This electric field is of the order of the ones observed in terrestrial dust devil of similar size (Link to PowerPoint presentation on electrical activities in Martian dust devils and dust storms). The Viking Gas Chromatograph Mass Spectrometer (GCMS) found no signs of organics at the surface samples of Mars, while the Gas Exchange Experiment (GEX) showed a rapid release of O2 when nutrients and water were added to the soil (Atreya et al., 2005). It was suggested that these seemingly contradictory results could be reconciled if the surface contained an oxidant such as hydrogen peroxide (H2O2). Hydrogen peroxide has been detected in the Martian atmosphere (Encrenaz et al., 2004; Clancy et al., 2004). Atryea et al. (2005) argues that the observed atmospheric mixing ratio of 20-40 ppb is too low to account for the Viking results, even if a large fraction of hydrogen peroxide could diffuse from the atmosphere to the surface. Atreya et al. (2005) and Delory et al. (2005) propose a new mechanism that can produce a substantially greater abundance of H2O2 in the Martian atmosphere, and even lead to the settling of “hydrogen peroxide dust" on to the surface. The mechanism proposed by Atreya et al. (2005) and Delory et al. (2005) is driven by triboelectricity generated by the collision of sand and dust particles during Martian dust devils and dust storms. Electric field gradients of up to about 25 kV/m have been estimated in Martian dust storms (Melnik and Parrot, 1998). Such fields result in a large production of CO/O- and OH/H- in the Martian CO2-H2O atmosphere. Using an electro-photochemical model, Delory et al. (2005) calculate that the abundance of H2O2 in the Martian atmosphere resulting from triboelectric fields in dust devils and storms can greatly exceed that produced by the solar UV-driven photochemistry. Even more importantly, a significant amount of H2O2 is expected to freeze on dust particles and precipitate to the surface as hydrogen peroxide dust. Unlike the gas phase, the residence time of H2O2 ice bound to dust particles could be very long. Atreya et al. (2005) and Delory et al. (2005) hypothesize that the lack of organics on the Martian surface is most likely due to the presence of this powerful oxidant, hydrogen peroxide or one its subproducts. Indeed these oxidants probably make the Martian surface inhospitable to life (Link to AGU posters of Atreya et al. (2004) and Delory et al. (2004)). Dust Storms Regional dust storms occur rather frequently on Mars. They are highly convective and there are suggestions that they are similar to terrestrial hurricanes or polar lows (Gierasch and Goody, 1973). A pair of these storms is illustrated in Figure 4. Sometimes these regional dust storms grow and became global in extent as illustrated in Figures 5 and 6. Gierasch and Goody (1973) proposes that this growth is due to a positive feedback between solar absorption and dust in the storm core. The fact that regional and global dust storms frequently form near the edge of the south Martian polar cap during spring is consistent with the hypotheses that they are convective heat engines similar to dust devils, waterspouts, hurricanes, and polar lows. The Mars Observer Camera (MOC) Wide-Angle Camera (WAC) monitored a regional dust storm on orbits 50-54 (27 November to 2 December 1997). This was the southern spring which is the period of the on-set of maximum dust storm activity (Greeley at al., 1992). This active period includes the time of Mars perihelion passage and extends past the solstice into the southern summer. Malin et al. (1999) discovered various small dust storms forming along the margin of the retreating south polar cap in association with this regional dust storm (about 2,500 km wide) while it was developing. Moreover, the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) detected a region of enhanced dust abundance in the Hellas Basin that persisted for more than a week, before the largest dust storm detected by the MGS mission started at that region on June 26, 2001 (Figure 5). These observations are consistent with the theory for convective vortices proposed by Renno et al. (1998, 2000). This theory suggests that dust devils and dust storms have a higher probability of occurrence and are potentially more intense in regions of large temperature gradients and/or sloping terrain. In fact, it predicts that the most intense convective vortices forming near the edge of the Martian polar cap have maximum windspeed of about100 m/s. These winds are strong enough to produce energetic saltation and therefore intense dust storms.
Figure 4. A pair of baroclinic dust storms near the edge of the polar cap (Courtesy NASA/JPL/MSSS).
Figure 5. Martian dust storm activity detected by the TES instrument. Note that a region of enhanced dust abundance occurs at the Hellas Basin and persists for 10 days before the dust storm starts (Courtesy NASA/JPL/ASU).
Figure 6. Images of Mars during a clear day and during a global dust storm |
