Hubble Space Telescope observations of the Martian aphelion cloud belt prior to the Pathfinder mission: Seasonal and interannual variations (original) (raw)
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New insights into Martian dust distribution and water-ice cloud microphysics
Journal of Geophysical Research, 2002
1] In this paper we use a direct method which combines microphysics and photometric simulations of the Martian atmosphere. This approach allows us to reproduce vertical profiles of the light scattered by the haze at the limb of Mars. Subsequent results are compared to a multiple-color image provided by the Viking Orbiter camera. The ''free parameters'' of our study are related to vertical transport and dust size distribution. A sensitivity study has been conducted, encompassing a relevant range of parameters, in order to obtain the most satisfying multispectral modeled profile with respect to the one inferred from data. Except for an unlikely distribution of submicron particles (r eff $ 0.2 mm), no consistent fit can be obtained. This implies that size distributions of dust suggested by previous studies are not in agreement with the present analysis. This apparent discrepancy can be resolved by adding a separate peak of submicron particles to the size distribution previously extracted from Viking Lander images. Only in that case can spectral and vertical structures of haze be successfully matched. While a bimodal distribution would change the current picture of Martian dust, such distribution is a common representation of soil-derived aerosol size function in Earth deserts. We also attempted to derive information from the cloud shown in the Viking limb image. Our estimates suggest that cloud particle effective radius is around 1.2-1.8 mm, while the cloud visible opacity is 0.02.
Icarus, 2003
We report high-spectral-resolution (/␦ ϭ 800 -2300) near-infrared mapping observations of Mars at L s ϭ 130°(April 1999), which were obtained by drift-scanning the cryogenic long-slit spectrometer at the KPNO 2.2-m telescope across the disk. Data were reformatted into calibrated spectral image cubes (x,y,) spanning 2.19 to 4.12 m, which distinguish atmospheric CO 2 features, solar lines, and surface and aerosol features. Maps of relative band depth between 3.0 and 3.5 m trace water ice clouds and show the diurnal evolution of features in the persistent northern summer aphelion cloud belt, which was mapped contemporaneously but at fixed local time by the Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES). Cloud optical depth, particle sizes, and ice aerosol content were estimated using a two-stream, single-layer scattering model, with Mie coefficients derived from recently published ice optical constants, followed by a linear spectral deconvolution process. A comparison of data and model spectra shows evaporating nighttime clouds in the morning followed by afternoon growth of a prominent orographic cloud feature on the west flank of Elysium Mons. Cloud optical depth at 3.2 m evolved to 0.28 Ϯ 0.13 and ice aerosol column abundance to 0.9 Ϯ 0.3 pr m in the afternoon. Column abundances as large as 0.17 pr m were retrieved in nonorographic clouds within the aphelion cloud band around midday. These clouds exhibit a modest decline in optical depth during the afternoon. Results show that ice particle radii from Ͻ2 m to Ͼ4 m exist in both cloud types. However, large particles dominate the spectra, consistent with recent MGS/TES emission phase function measurements of aphelion cloud aerosol properties.
Viking Orbiter imaging observations of dust in the Martian atmosphere
Journal of Geophysical Research, 1979
More than 20 local Martian dust clouds and two global dust storms were observed with the Viking orbiter camera. Sixteen of the local clouds were imaged in two colors or were observed with other instruments confirming their identification as dust clouds. These Viking results are compared with earthbased observations of Martian dust storms and with Mariner 9 data. Most of the dust activity seen by Viking occurred during southern hemisphere spring and early summer, when Mars was near perihelion and insolation was near maximum. About half the local clouds occurred near the edge of the southern polar cap, where winds are presumably enhanced by a strong regional temperature gradient. The other half occurred mainly in the southern hemisphere near regions where circulation models incorporating topography predict positive vertical velocities. Although dust clouds observed from earth show a similar partial correlation with models, some ambiguity exists concerning interpretation of regions near Hellespontus that have spawned the most spectacular Martian dust storms on record.
Water vapor saturation at low altitudes around Mars aphelion: A key to Mars climate
Icarus, 1996
The combined analysis of microwave temperature and water profiling of the Mars atmosphere indicates that low- to mid-latitude water vapor saturation typically occurs at much lower altitudes (below 10 km) during northern spring/summer than observed during this Mars aphelion season in the dusty, warm period of Viking observations (above 25 km). Temperatures profiles of the 0-60 km global Mars atmosphere are retrieved from microwave CO spectra around Mars aphelions in 1980, 1982, 1989, 1991, 1993, and 1995. These microwave temperature retrievals are 15-20 K colder than the Viking temperature measurements at the same season in 1976 and 1978, implying dust-free, radiative-convective conditions for the global Mars atmosphere at the aphelions of the microwave measurements. Mars water profiling from very large array water and Kitt Peak water isotope spectra were obtained in the 1993 and 1995 Mars aphelion periods. Their analysis indicates that Mars water vapor at low to mid latitudes was confined to altitudes below 10 km during these aphelion periods, in agreement with the low altitude of water vapor saturation predicted by the cold microwave temperature profiles. The existence of such low-altitude water vapor saturation for the aphelion Mars atmosphere is corroborated by HST ultraviolet and violet cloud imaging of the Mars atmosphere in 1991, 1993, and 1995. These images display a previously unidentified, global belt of moderate opacity (τ ˜ 0.2-0.6) clouds covering the ˜10°S-30°N latitude region around Mars aphelion (solar longitude, Ls˜ 60°-100°) for three consecutive Mars years. The center of this low-latitude cloud belt corresponds to the region of upward advection within the summer solstice Hadley circulation. These cold atmospheric temperatures, low altitudes of water vapor saturation, and low-latitude cloud belts are observed only around Mars aphelion, which presently occurs during northern late spring/early summer (L s= 71°). This behavior reflects the highly elliptical Mars orbit in which global surface and atmospheric temperatures vary by 20 K with orbital distance from the sun. The perihelion of Mars (southern late spring/early summer, Ls= 251°) is recognized as the season of global dust storms, which result from the higher solar flux incident at perihelion (e.g., Zurek and Martin 1993). We argue that the aphelion period exhibits a similarly distinct climate (cloudy and cold), which was not as apparent during the unusually dusty Mars years of the Viking observations. We further argue that this aphelion climate may be the key to understanding the large north-south hemispheric asymmetries of Mars water vapor and the residual polar ice caps. The orbital dependence of the altitude of water vapor saturation can couple with the solstice Hadley circulations of the Mars atmosphere to create a non-linear atmospheric water pump toward the aphelion summer hemisphere. It is even possible that this process accounts for the origin of the polar layered deposits, as the hemispheric direction of this water pump alternates every ˜25,000 years due to the orbital progression of the season of Mars perihelion. We also point out that an increased importance for global cloud formation in the Mars atmosphere suggests important non-linear relationships between atmospheric water and dust in the current Mars climate, which may contribute to the extreme interannual variations of Mars dust storm behavior and the current albedo and compositional differences of the north and south polar ice deposits.
Martian ice cloud distribution obtained from SPICAM nadir UV measurements
Journal of Geophysical Research, 2007
1] The Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument on board Mars Express has successfully performed one Martian year of measurements. Nadir UV (200-310 nm) measurements allowed it to build maps of ice cloud optical depth distribution for all seasons. The development and decay of the aphelion cloud belt (ACB) and polar hoods were observed. The characteristic values of the cloud optical thickness were 0.1-0.3 at the early stage of the ACB formation in the solar longitude range L s = 20-60°. After L s = 93°, the well-developed ACB showed cloud optical thicknesses varying between 0.3 and 0.8. The ACB quickly decayed after L s = 140°. Both polar hoods were observed during their development and decay stages, showing cloud optical thicknesses of about 0.35. The north polar hood started to develop at L s = 160°and the south one at L s = 330°. Estimates of water content in the ice clouds gave values of 0.35-1.8 gm À2 for ACB and 0.4 gm À2 for the polar hoods. A comparison with water vapor abundance showed that only a small fraction (10-20% for ACB and 30% for the polar hoods) of total water content in the atmosphere was accumulated in clouds. The Martian surface albedo at the wavelength 300 nm appeared very low (0.004-0.018) and exhibited anticorrelation with the visual albedo consistent with optical properties of iron oxides abundant in Martian soils. The investigation of a regional dust storm allowed the estimation of dust optical parameters at the wavelength 300 nm (asymmetry factor g d = 0.8 and single scattering albedo s d = 0.6).
Radiative effects of water ice clouds on the Martian seasonal water cycle
Water ice clouds form and dissipate in the Martian atmosphere and though their abundance is small compared to the bulk atmosphere, they play a significant role in the regulation of the net annual meridional transport of water (Clancy et al., 1996; Montemssin et al., 2004;. Here we show that the radiative effects of water ice clouds play an important role in the determining the thermal structure of the atmosphere, and the wetness of the seasonal global water cycle.
Icarus, 2019
Using spectral images recorded by the OMEGA instrument on Mars Express (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité), we are able derive physical properties of aerosols in water-ice clouds on Mars for a distribution of pixels over an observed cloud formation. These properties, mean effective radius, eff , and optical depth (at 0.67 m), i , were used to estimate the water ice-column (WIC), and we found an empirical relationship between the WIC and an ice cloud index (ICI). The overall mean of retrieved eff is ∼ 2.2 m, with a standard deviation of 0.8 m, and cloud formations with eff between 4.4 and 5.4 m are observed. The optical depth varies between 0.2 and 2.0. The OMEGA spectra are primarily sensitive to water ice mass due to absorption, and we find that the ICI, very easy to compute, is a good proxy for the mass of the water-ice column (WIC) along the optical line of sight. Our retrieval of physical properties is limited in time (to before 2010) by the exhaustion of coolant for one of the OMEGA channels, and in space (to equatorial observations between 140 ∘ − −90 ∘ E) by the availability of surface albedo measurements. However, we used the ICI to compute WIC values for the entire OMEGA data set, which has near-global coverage for Mars years 26-32, and we present a climatology of the WIC derived from the OMEGA data, which features enhancements on the order of 1.2-1.6 pr. m over the aphelion cloud belt,
Eight-year climatology of dust optical depth on Mars
Icarus, 2015
We have produced a multiannual climatology of airborne dust from Martian year 24 to 31 using multiple datasets of retrieved or estimated column optical depths. The datasets are based on observations of the Martian atmosphere from April 1999 to July 2013 made by different orbiting instruments: the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor, the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey, and the Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO). The procedure we have adopted consists of gridding the available retrievals of column dust optical depth (CDOD) from TES and THEMIS nadir observations, as well as the estimates of this quantity from MCS limb observations. Our gridding method calculates averages on a regularly spaced, but possibly incomplete, spatio-temporal grid, using an iterative procedure weighted in space, time, and retrieval uncertainty. In order to evaluate strengths and weaknesses of the resulting gridded maps, we associate values of weighted
Recent Ice Ages on Mars: The role of radiatively active clouds and cloud microphysics
Geophysical Research Letters, 2014
Key Points: • Simulations of recent ice ages are performed using an improved climate model • Cloud radiative effect and coupling to the dust cycle control snow deposition • The location of predicted ice deposits is consistent with geologic evidence Correspondence to: (2014), Recent ice ages on Mars: The role of radiatively active clouds and cloud microphysics, Geophys.
Infrared Spectral Imaging of Martian Clouds and Ices
Icarus, 1999
Multispectral images of Mars, taken at the NASA Infrared Telescope Facility (IRTF) near and at the 1995 opposition, are used to identify and track its atmospheric clouds and ground ices. Band depth mapping is used to help distinguish between the composition of volatiles and provide a check for the techniques of principal components analysis (PCA) and linear mixture modeling (LMM). PCA/LMM are used to create maps that track clouds and volatiles, a technique that requires no a priori spectral information in order to create these maps. Band depth maps at 3.33 µm, which have been shown to trace CO 2 frosts, show some transient features which could indicate polar CO 2 clouds at the time of these observations. We show that band depth maps at 2.25 µm are good tracers of H 2 O frosts and that band depth maps at 3.69 µm can distinguish between coarseand fine-grained water frosts. These maps have allowed the detection of fine-grained water frosts in the north polar region and along the morning and evening limb regions. From the PCA technique we find that just two principal components can account for over 99% of the data variance. The first of these is an infrared albedo unit and the second is an ice/thermal unit. Plotting the spectral data cubes in this new vector space, we find that most of the martian disk can be modeled by spectrally mixing three endmember spectra having extreme values of these principal components. The morning and evening regions of Mars are composed of 40-60% of the north polar ice/thermal component endmember, indicating a frost component there consistent with the band depth mapping results. With a combination of these techniques it is possible to not only identify the extensive martian clouds, but to also determine composition. These new results are particularly relevant in light of recent Mars Pathfinder descent temperature profile data that indicated upper atmosphere temperatures below the CO 2 frost condensation point, implying that CO 2 ice clouds may be an important radiative component of the current martian climate.