Monitoring Mars with the Hubble Space Telescope: 1990-1991 Observations

Images of Mars obtained using the Planetary Camera on the Hubble Space Telescope during 1990 and 1991 are described and analyzed. Multispectral images of Mars record the martian season between Ls = 348° and 60° corresponding to late winter and spring in the northern hemisphere. The wavelengths of these observations varied between 230 nm in the ultraviolet and 890 nm in the near infrared. We use these images to discuss atmospheric and polar phenomena, to constrain the aerosols and ozone in the martian atmosphere, and to compare surface albedo features within this data set as well as with previous observations. Two methods are used to constrain the opacity of the martian atmosphere: comparisons of ultraviolet images with atmospheric scattering models and examination of contrast changes of surface features observed at visible wavelengths. Our observations are consistent with a Mars which was quite different from the planet when it was viewed by Viking; we estimate an upper limit of 0.1 for equatorial dust opacity near vernal equinox, compared to values of 0.4 to 0.5 measured in Chryse by Viking Lander 1. Later, at Ls = 60°, an upper limit of 0.2 was found for dust in the southern hemisphere. Except for suggestions of local dust activity near the north polar cap in late spring, there was no visual evidence of dust storms on the planet. The Hartley band of ozone is within the F230W filter bandpass, and the 230W/336W ratio is therefore sensitive to ozone. High latitude ozone is observed at a concentration similar to that observed by Mariner 9; a seasonal dependence in ozone concentration was observed with concentration decreasing at northern arctic latitudes and increasing at southern midlatitudes as Ls increases from 350° to 60°. Despite the difference in dust loading, the behavior of condensate clouds and the recession of the north polar cap are consistent with those observed during Viking years. Late winter images of the north polar region reveal an active north polar hood with a maximum (UV) opacity of about 0.25 near 50° latitude; the hood seems to have an annular form, with the central, polar regions having smaller opacity. Clouds observed in the northern hemisphere to the south of the hood boundary are suggestive of fronts similar to those observed during the Mariner 9 mission. A diurnal variation in the hood clouds which is (anti) correlated with the amount of water vapor in the atmosphere is noted. Spring images show significant clouds in the Tharsis, Elysium, and Arabia regions; the most optically thick components of these clouds are those near Elysium, Alba Patera, and Tithonius in the afternoon, although these regions also are bright on the morning limb. The cloud associated with Elysium had the largest optical depth for a cloud in the UV, 0.4. Most of the HST monitoring sequences were targeted on the Syrtis Major region, an area which was intensively studied during the Viking Mission. During Viking there were major changes in the region which were attributed to the two major 1977 global dust storms on Mars. Our series of images was projected onto map grids and photometrically corrected in order to study changes in the Syrtis albedo features. To first order, Syrtis does not change over the time period of these observations, as would have been expected if they followed major global dust storms during the preceding season. Images were deconvolved using the Richardson-Lucy iterative algorithm to remove the defocussing effects of the spherical aberration of the HST primary. Even when Mars subtended less than 5 arcsec, in May 1991, the images were scientifically useful and comparable to photographic images obtained at opposition. Although the data are subject to the solar pointing constraint for HST, the relatively slow movement of the allowed observation window, which is more than one half martian year, with respect to the martian seasons will permit us to address the question of major variations in the martian atmosphere.