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 scavan...

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Data obtained by the Pioneer 10 vector helium magnetometer are presented along with models of the intrinsic magnetic field of JupiteL and its magnetosphere. Data acquired between 2.84 and 6.0 Rj, where the intensity of the planetary field ranged between 1900 and 18,400 'y, were used to develop a six-parameter eccentric dipole model of the field. The dipole so derived has a moment of 4.0 G Rfi and a tilt angle With respect to Jupiter's rotation axis of 11 ø. The system I!I (epOch 1957) longitude of the magnetic pole in the northern hemisPhere,•which is a north-seeking pole, is 222 ø. The dipole is displaced from the center of Jupiter by 0.11 Ra in the direction of iatitude 16 ø and system ili longitude 176 ø. The dipole tilt and the longitude of the pole ar e in goød agreement with values inferred from radio astronomy measurements. The magnetic moment and the offset derived from the Pioneer measurements represent a significant improvement in our knowledge of the planetary field. A model of the Jovian magnetosphere is presented in which the essential feature is an eastward current sheet that forms an annulus with Jupiter at the center. At Iarge distances from the planet the current sheet is nearly parallel to Jupiter's equator but, in general, does not lie in it. The Current sheet is warped, so that it is above the equator on one side and below it on the other. The current Sheet rotates with the planet, more or less lik e a rigid body; this behavior causes an apparent up and down motion and periodic crossings of the current sheet by Pioneer. The origin of the current sheet appears to be the very large centrifugal force, associated with Jupiter's great size and rapid rotation, acting on trapped low-energy magnetospheric plasma. The density of this plasma is estimated to be approximately 1 particle cm -3. A retrograde spiraling of field lines out of meridian planes is also observed, presumably as a result of azimuthal drag forces exerted on the outer magnetosphere. INSTRUMENT DESCRIPTION The magnetometer on board Pioneer !0 is an advanced version of the vector helium magnetometer previously used on the Mai'iner 4 and 5 missions to Mars and Venus [Slocum and Reilly, 1963; Connor, 1968]. The essential elements of the magnetometer include a helium lamp, a circular polarizer, a helium absorption cell, ß Helmholtz coils, a lens, and an !R detector. The basic operation of the instrument depends upon the effect of the magnetic field direction on the efficiency with which metastable helium can be optically pumped. The presence of an ambient magnetic field causes a sine wave modulation of the IR radiation passing through the gas cell at the fundamental frequency of the applied circular sweep field. The sensor output consists of the nulling currents that are applied in order to cancel this field 'signal.'The magnetometer measures the three field components over a frequency range from 0 to 10 Hz. At encounter the data rate was 1024 bits/s and the magnetometer sampling rate, corresponding to one vector measurement every 3/16 s, ...

Observations of the Martian sky, Phobos, and the sun were taken with the Viking lander imaging cameras to obtain information on the properties of the atmospheric aerosols. Atmospheric optical depths were derived from the observations of the brightness of the celestial objects. Information on the absorption coefficient, mean size, and shape of the aerosols was derived from studies of the sky brightness. For this purpose we used a multiple‐scattering computer code that employed a recently developed technique for treating scattering by nonspherical particles. By monitoring the brightness of the twilight sky we obtained information on the vertical distribution of the particles. Three types of aerosols are inferred to have been present over the landers during the summer and fall season in their hemisphere. A ground fog made of water ice particles was present throughout this period. It formed late at night during the summer season and dissipated during the morning. We infer that during the summer the frost point temperature was 195°K and the water vapor volume mixing ratio equaled about 1× 0−4 near the ground at VL‐2. Assuming that condensation occurs only on suspended soil particles, we estimate that the average particle radius of the fog was about 2 μm and that the fog's depth equaled approximately 0.4 km. A higher‐level ice cloud was prominent only during the fall season, when it was a sporadic source of atmospheric opacity at VL‐2. The formation of upper level water ice clouds during the summer may have been inhibited by dust heating of the atmosphere. Suspended soil particles were present throughout the period of observation. During the summer they constituted the only major source of opacity in the afternoon and most of the night. The cross‐section weighted mean radius of these aerosols is about 0.4 μm. They have a nonspherical but equidimensional shape and rough surfaces. These soil particles have a scale height of about 10 km, which is comparable to the gas scale height, and they extend to an altitude of at least 30 km. The principal opaque mineral in these particles is magnetite, which constitutes 10%±5% by volume of this material. We propose that soil particles, as well as any associated water ice, are eliminated from the atmosphere, in part, by their acting as condensation sites for the growth of CO2 ice particles in the winter polar regions. The resultant CO2‐H2O‐dust particle is much larger and therefore has a much higher fallout velocity than an uncoated dust or water ice particle.

Preliminary Ames-magnetometer data from Explorer 35, the lunar orbiter, show no evidence of a lunar bow shock. However, an increase of the magnetic field by about 1.5 gamma (over the interplanetary value) is evident on Moon's dark side, as well as dips in field strength at the limbs. Interpretation of these spatial variations in the field as deriving from plasma diamagnetism is consistent with a plasma void on the dark side, and steady-state (B = 0) magnetic transparency of Moon.