A regression curve for the north polar cap of Mars is extracted from the photographs obtained at Kwasan Observatory and at Hida Observatory during the 1979–1980 apparition. The north polar cap was first observed at Ls = 26°, and the latitude of the edge of the cap was about 66°N. The retreat of the cap stopped until Mars reached Ls = 57° when the cap started to shrink. The edge reached a latitude around 80°N at Ls = 85° and then became almost stationary. The roughly constant dimension of the north polar cap before mid‐spring appears to be a regular phenomenon. But the onset time when the cap starts to shrink after the temporary stop appears to be different from year to year. The measurements of the edge of the permanent cap showed an asymmetric shape in relation to the pole. The cap extends farther to the south for longitudes near 0° than for those near 180°. The shape of the permanent cap for Ls = 84°–101° in 1980 agrees very well with that for Ls = 97° in 1972 obtained by Mariner 9.

In the photographic observations of Mars at the Kwasan Observatory and at the Hida Observatory during the 1977–1978 apparition, the north polar cap was first observed at Ls = 14°, and the latitude of the edge of the cap was about 65°N. This edge of the cap changed little until Mars reached Ls = 50°, and then the cap started to shrink. The edge was observed near 74°N at Ls = 70°. The relatively constant dimension of the north polar cap observed before Ls = 50° appears to be a regular phenomenon in the behavior of the north polar cap. The photographs obtained at the Hida Observatory during the 1975–1976 apparition were examined to study the nature of the north polar cap near Ls = 0°. Red filter photographs taken in the period from Ls = 340°–0° revealed dark surface features of the northern high latitudes, when blue filter photographs show the extensive polar brightening (i.e., the polar hood). The latitude of the northernmost dark feature observed was about 65°N. A wide spread north polar cap, which is predicted by most of the existing models, was not observed in our observations in 1975–1976.

Abstract. We investigated diurnal variations of water-ice clouds in the Tharsis region of Mars in the three apparitions from 1995 to 1999. The Tharsis morning cloud, centered near a point (120• W, 10• N) between Olympus Mons and Tharsis Montes, was the brightest among the morning clouds in Tharsis. Its optical thickness was about 0.6 near 9.5 h Martian local time and reduced to a minimum of about 0.2 around local noon in 1995. The minimum optical thickness was 0.4 in 1997, which also appeared around local noon. The optical thickness of the Tharsis morning cloud increased again in the early or mid afternoon in 1995 and 1997. In the late morning when the brightness of the morning cloud declined, bright afternoon clouds appeared over the major volcanoes in Tharsis. The optical thickness of the afternoon cloud over Olympus Mons was about 0.7 in the early afternoon in early summer, 1997. The Olympus afternoon cloud seems to be more active in early summer than in the first half of late spring. Those morning and afternoon clouds repeat the diurnal variation from late spring to at least early summer every Martian year.

A set of wavelength-dependent scattering phase functions for Jupiter's clouds has been obtained; the Mie theory was employed to simulate well-defined scattering phase functions deduced from an analysis of the Pioneer 10 photometry data (Tomasko et al. 1978, AAA 021.099.011). A reasonable fit to the cloud-phase functions for the bright zone (STrZ) was obtained with a particle effective radius, reff, of 0.96 μ m and the real part of the refractive index, nr, being 1.591 for the blue and 1.535 for the red, slightly higher than the nominal values for ammonia ice. The derived Mie phase functions were tested against several STrZ data. The appropriateness of these phase functions has been demonstrated through the fact that they reproduce not only the observed limb-to-limb intensity profiles, but also the correct wavelength dependence of the haze optical thickness for a wide wavelength range from visible to near-infrared.

No abstract