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. Multispect ral images of Mars record the martian season between L(S) = 348-degree s and 60-degrees corresponding to late winter and spring in the northe rn hemisphere. The wavelengths of these observations varied between 23 0 nm in the ultraviolet and 890 nm in the near infrared. We use these images to discuss atmospheric and polar phenomena, to constrain the ae rosols and ozone in the martian atmosphere, and to compare surface alb edo features within this data set as well as with previous observation s. Two methods are used to constrain the opacity of the martian atmosp here: comparisons of ultraviolet images with atmospheric scattering mo dels and examination of contrast changes of surface features observed at visible wavelengths. Our observations are consistent with a Mars wh ich 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 ve rnal equinox, compared to values of 0.4 to 0.5 measured in Chryse by V iking Lander 1. Later, at L(S) = 60-degrees, 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 con centration similar to that observed by Mariner 9; a seasonal dependenc e in ozone concentration was observed with concentration decreasing at northern arctic latitudes and increasing at southern mid-latitudes as L(S) increases from 350-degrees to 60-degrees. 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 Vikin g 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-de grees latitude; the hood seems to have an annular form, with the centr al, polar regions having smaller opacity. Clouds observed in the north ern hemisphere to the south of the hood boundary are suggestive of fro nts similar to those observed during the Mariner 9 mission. A diurnal variation in the hood clouds which is (anti) correlated with the amoun t of water vapor in the atmosphere is noted. Spring images show signif icant clouds in the Tharsis, Elysium, and Arabia regions; the most opt ically thick components of these clouds are those near Elysium, Alba P atera, 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 mon itoring sequences were targeted on the Syrtis Major region, an area wh ich was intensively studied during the Viking Mission. During Viking t here were major changes in the region which were attributed to the two major 1977 global dust storms on Mars. Our series of images was proje cted onto map grids and photometrically corrected in order to study ch anges in the Syrtis albedo features. To first order, Syrtis does not c hange over the time period of these observations, as would have been e xpected if they followed major global dust storms during the preceding season. Images were deconvolved using the Richardson-Lucy iterative a lgorithm to remove the defocussing effects of the spherical aberration of the HST primary. Even when Mars subtended less than 5 arcsec, in M ay 1991, the images were scientifically useful and comparable to photo graphic images obtained at opposition. Although the data are subject t o 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 t he question of major variations in the martian atmosphere. (C) 1994 Ac ademic Press, Inc.