Local Dust Storms Research Articles

The Martian Dust Storm of Sol 1742

After nearly five Earth years on Mars, the Mutch Memorial Station (Viking Lander 1) finally witnessed a local dust storm that eroded trenches, conical piles, and other disturbed surfaces in the sample field and near the lander. The event, called the Dust Storm of Sol 1742, occurred late in the third winter of lander observations between Sols 1728 and 1757. Analyses of tiny new wind tails and movement of materials indicate that the eroding winds were variable but more northeasterly than those that had previously shaped the surface. Pebbly residues and movement of 4–5 mm clods suggest drag velocities or friction speeds of the winds were about 2.2–4.0 m/s. Wind speeds at the height of the meteorology boom (1.6 m) were probably about 40–50 m/s. Much of the observed erosion could have occurred in a few to several tens of seconds, but somewhat longer times are suggested by analogy with the erosion of terrestrial soils. Most of the erosion occurred where preexisting equilibrium conditions of surface configurations and surface material properties had been altered by the lander during landing and during surface‐sampler activities, but thin layers of bright fine‐grained dust were also removed and redistributed. Surfaces where preexisting equilibrium conditions were unaltered appeared to be uneroded.

The clouds of mars as seen by viking

UV and IR observations of Martian cloud activity by Viking orbiters are reviewed. Localized spiral storms were detected at high northern latitudes during the summer, and both dust and water ice have been associated with the occurrences. Local dust storms formed in spring and summer in the southern hemisphere, and storms along the polar cap edge moved away from the ice covered area on thermally driven downslope winds originating from local variations in thermal inertia. Global dust storms happen nominally during the perihelion of the Martian orbit, although deviations from the pattern have been observed. Condensate clouds were most noticeable around the north polar hood and the Tharsis-Valles Marineris region, with most activity in the spring and summer. Wind speeds were 40 and 85 m/sec in the lower and upper layers of the atmosphere, respectively, with waves having wavelengths from 2-60 km and extending over hundreds of km.

Local Mars dust storm generation mechanism

On Ls = 340°, first Mars year of Viking on the surface, a local dust storm was observed at the Viking Lander #1 site by Viking Orbiter A. The storm lasted less than one martian day (sol) with the dust raised affecting the site for about three sols. It is concluded that this storm was caused by baroclinic waves and that the threshold wind speed for saltation was 25‐30 m sec−1.

A local dust storm in the Chryse Region of Mars: Viking Orbiter observations

A local dust storm was observed near the Viking Lander 1 site by Viking orbiter l in September, 1977, when the areocentric longitude of the sun, Ls, was 340° (shortly before vernal equinox). The orbiter observations, which consisted of a time sequence of pictures, show that the storm moved at about 50 m/sec to the ENE from the Lunae‐Planum region into the Chryse basin. Both baroclinic waves and topography may have been associated with the generation of the storm.

The opacity of some local Martian dust storms observed by the Viking IRTM

In this paper we analyze some Viking infrared thermal mapping (IRTM) measurements of local Martian dust storms observed in the southern tropical region of the planet between L s=225 and 262°. The derived opacities of these storms show that in the most opaque regions of the cloud, the optical thickness may be ≈6. Away from the individual clouds, the opacity is ≈2, which is still about four times the background level of dustiness in the Martian atmosphere. We find considerable structure in the derived opacity which will create corresponding variations in the atmospheric heating, which in turn may have an important feedback upon the local winds.

A numerical model of the Martian polar cap winds

An investigation of the Martian polar cap winds and their response to a variety of factors is carried out by a series of numerical experiments based on a zonally symmetric primitive equation model. These factors are the seasonal thermal forcing, mass exchange between polar caps and atmosphere, large-scale topography, and polar cap size. The thermal forcing sets up a circulation whose surface winds adjust to achieve angular momentum balance, with low-latitude easterlies and high-latitude westerlies. The maximum westerlies occur roughly where the horizontal temperature gradients are largest. This pattern changes when cap and atmosphere exchange mass. Corriolis forces acting on the net outflow or inflow produce easterlies at the surface during spring (outflow) and westerlies during winter (inflow). Topography appears to have a small effect, but cap size does play a role, the circulation intensity increasing with cap size. Peak surface winds occur when outflow or inflow is a maximum and are 20 m sec −1 during spring and 30 m sec −1 during winter for the northern hemisphere. The model results show that surface winds near the edge of a retreating polar cap are substantially enhanced, a result which is consistent with the Viking observations of local dust storm activity near the edge of the south polar cap during spring. The results also indicate that the surficial wind indicators near the south pole are formed during spring and those near the north pole during winter. The implication is that the high-latitude dune fields in the northern hemisphere are formed at a time when the terrain is being covered with frost. It is therefore suggested that the saltating particles are “snowflakes” which have formed by the mechanism proposed by Pollack et al. The model results for the winter simulation, which have formed by the mechanism transport by large-scale eddies, compare favorably with general circulation model (GCM) calculations. This suggests that the eddy transports may be less important than those associated with the net mass flow, and that 2-D climate modeling may be more succesful for Mars than Earth.

The influence of the Martian polar caps on the diurnal tide

The large horizontal heating gradients that exist near the edge of the Martian polar caps during spring are shown to be capable of exciting large oscillations in the diurnal tide. To a lesser extent, the daily mass cycling between cap and atmosphere can also contribute. The calculations which demonstrate this are based on classical tidal theory as applied to the cylindrical coordinate system. This is done to facilitate the representation of the heating function. Results are presented for the horizontal surface winds only. They indicate a circulation at the cap edge somewhat analogous to the smaller scale terrestrial sea breeze. The amplitude of the zonal component is largest and is increased from 1 to 10 m sec −1 by the modeled influence of the polar cap. When coupled with the basic flow these cap-edge tides can produce strong surface winds during spring. Such a mechanism may contribute to the ability of the south polar cap winds to generate the local dust storms observed near the cap edge at this season.

Properties and effects of dust particles suspended in the Martian atmosphere

Direct measurements of the optical depth above the two Viking landers are reported for a period covering the summer, fall, and winter seasons in the northern hemisphere, a time period during which two global dust storms occurred. The optical depth had a value of about 1 just before the onset of each storm; it increased very rapidly, on a time scale of a few days, to peak values of about 3 and 6 with the arrival of the first and second storms, respectively; and it steadily decreased shortly thereafter (≳ few days to few weeks) for both storms, with the decay occurring more rapidly during the initial period of decay. We have also carried out further analyses of observations of the sky brightness made with the lander cameras during the summer season to obtain improved estimates of other dust particle parameters, including the cross section weighted mean particle radius, several shape factors, and the imaginary indices of refraction. These results have been used to define the radiative properties of the suspended dust particles at solar wavelengths. Particle properties inferred by Toon et al. (1977) from their study of the Mariner 9 Iris data were employed to define the radiative properties of the dust at thermal wavelengths. In an effort to understand the effect of dust content on atmospheric temperatures and winds, we have incorporated the derived radiative properties of the dust into a 1 D radiative convective model. When only radiation and thermal convection are taken into account, these calculations fail to reproduce the temperature structure determined during the descent of the landers to the surface. However, satisfactory agreement is achieved when allowance is made for the effects of vertical motions induced by large scale atmospheric dynamics. The sign and magnitude of the required vertical velocities are crudely consistent with those found by Pollack et al. (1976a) in their general circulation calculations. The diurnal temperature variations predicted by the 1 D calculations for the observed optical depths are also in crude agreement with values inferred from orbiter and lander measurements. The 1 D model predicts that the diurnal temperature change and daily mean temperature, averaged over the entire atmospheric vertical column, steadily increase as the optical depth of the dust increases to a value of several, and then subsequently change little. Thus, for small and moderate values of optical depth, there is a positive feedback between atmospheric dust content and both tidal and seasonal winds, but little feedback occurs for very large values of optical depth. A negative feedback exists between dust content and thermally driven topographic winds. The occurrence of these feedback effects are supported by several Viking observations. It is suggested that they play a role in the generation of certain types of local dust storms, in the development of local dust storms into global ones, and in the decay of global storms. At present, dust is being preferentially deposited in the north polar region due to scavanging of dust by CO2 ice precipitation. Using the results of this paper and those of Pollack et al. (1977), we obtain an estimate of the sedimentation rate of dust and water ice in the polar regions. These results imply a time scale of about 105 years to form individual laminae in the polar laminated terrain, a result consistent with astronomical theories of their origin. Analogous calculations imply that equatorial and mid‐latitude regions are being eroded at a rate of about 7 m per million years.

A new look to the Martian atmosphere

Although the Martian atmosphere is at present only about 1% as dense as the Earth’s atmosphere it has been revealed as a dramatically active environment by the observations made during 1971-72 by theMariner9 and SovietMars2 and 3 spacecrafts which arrived at the planet during a major global dust storm. Local dust storms were seen to change in intensity on a daily basis and other evidence for winds were seen in cloud patterns and in visible streaks on the planet’s surface. Cloud layers composed of both CO2and water ice have been observed. The variations in the albedos of surface markings are probably caused by the wind blow dust. At the present time the lower atmosphere is found to consist mainly of CO2with traces of CO, O3, O2and H2O. Geological evidence of channels and gullies suggests erosion by water at some stage of the planet’s development although it is also possible that wind erosion has played a role in sculpturing these features. Periodic variations in the Martian climate may be created through variations in the planets obliquity and orbital eccentricity over time scales of 25000 years. Mars today, then, may be in a temporary ice age.

Mechanisms for Mars Dust Storms

Characteristics of the Mars global dust storm are reviewed. At the Mariner 9 encounter, the dust consisted of highly absorbing particles distributed rather uniformly up to great height (about 50 km). These observations together with temperature distributions inferred from the Mariner 9 IRIS by Hanel et al. (1972) are used to estimate global wind systems during the dust storm. The global distribution and direction of light surface streaks indicate that the axisymmetric circulation was a dominant part of flow during the dust storm. The axisymmetric winds may become strong enough to raise dust over wide areas of Mars' tropics under unusual conditions: the incoming solar radiation must be near its seasonal maximum, the static stability must be low, and the atmosphere must be able to absorb and re-emit a sizeable fraction of the incoming radiation. Strong winds around the periphery of the retreating south polar cap would be driven by the temperature gradient at the cap edge and by the mass outflow due to subliming CO2. These polar winds could generate local dust storms, raising the general level of dustiness, and providing the conditions necessary for onset of a global dust storm.

Weather patterns in Southern West Pakistan

Southern West Pakistan is an area of transition between the Indian summer monsoon system to the east and the winter cyclonic system of southwest Asia to the west. As a transition area, it receives scanty, unreliable rainfall averaging less than 10 in. (254 mm) per year from several storm types. Six main weather patterns cross the region: the large subtropical anticylonic high pressure cell which predominates most of the year; western depressions originating over the Mediterranean Sea; Arabian Sea cyclones; local thunderstorms and dust storms; a modified monsoon pattern; and eastern depressions originating over the Bay of Bengal or central India. A discussion of the physical and synoptic characteristics for each weather pattern and storm type is presented and summary charts of the weather patterns are included.