Polar Hood Research Articles

Relationship between frontal dust storms and transient eddy activity in the northern hemisphere of Mars as observed by Mars Global Surveyor

We have compiled a catalog of frontal dust storms in the northern hemisphere using Mars Orbiter Camera daily global maps spanning ∼2.3 Martian years of Mars Global Surveyor (MGS) observations (from 1999 to 2003). The most vigorous frontal storms that flush dust to the low latitudes occur in early‐mid fall and mid‐late winter, away from the northern winter solstice. While many streaks are observed in the polar hood during the winter solstice period, no frontal dust storms are observed in the vicinity of the north polar region. We have also analyzed simultaneous MGS Thermal Emission Spectrometer (TES) temperature data and found statistically significant negative temperature anomalies associated with frontal storms. In the lowest scale height of the atmosphere, the geographical and seasonal distributions of temperature standard deviations associated with transient variations agree well with the distributions of frontal storms. The correlation deteriorates with increasing altitude, suggesting that lower‐level temperature waves are associated with the frontal dust storms. Specifically, eastward traveling m = 3 waves with periods of 2–3 sols appear to be closely related to the development of flushing frontal storms.

2003 Mars Report from Cooperative Observation Networks. II. After the Opposition

This is the second report on Mars in 2003 by two networks for cooperative Mars observations in Japan: Nishi-Harima Astronomical Observatory Mars Cooperative Observation and Association of Lunar and Planetary Observers in Japan. This paper is a summary of 4145 data for 226 nights collected by the two networks during the second half of the last apparition from 2003 September 1 through 2004 June 22, corresponding to a period of $L_{\mathrm{s}} \sim 252 {{}^{\mathrm {\circ }}} \hbox{--} 51 {{}^{\mathrm {\circ }}}$ ($L_{\mathrm{s}}$ is the areocentric longitude of the Sun). During this period, many interesting phenomena were observed: (1) Albedo features did not show a large difference from the first half of the period. (2) Blue clearings due to the opposition effect and due to the cloud effect were, respectively, observed. (3) South polar cap (SPC) recession was usual. The recession rate in $\sim 210 {{}^{\mathrm {\circ }}} \hbox{--} 270 {{}^{\mathrm {\circ }}}\mathrm{W}$ increased and that in $\sim 30 {{}^{\mathrm {\circ }}} \hbox{--} 60 {{}^{\mathrm {\circ }}}\mathrm{W}$ decreased after $L_\mathrm{s} \sim 230 {{}^{\mathrm {\circ }}}$. (4) North polar hood (NPH) had a topography-synchronous wave structure with $\mbox{a wave number} = 1$ a wave number$= 1$. (5) Many water ice clouds (WICs) were observed. Some particular regions frequently had prominent WICs. Some of them were related to topography. (6) Dust storm “2003c” was detected on December 9 ($L_\mathrm{s} = 314 {{}^{\mathrm {\circ }}}$) in Chryse. The storm grew rapidly to a very large dimension in the southern mid-latitudes, encircling over half of Mars. Another dust core occurred on December 23 around Aurorae Sinus. (7) Another dust storm is suspected in $L_\mathrm{s} = 20 {{}^{\mathrm {\circ }}}$ over the south polar region.

Water clouds and dust aerosols observations with PFS MEX at Mars

Observations of water ice clouds and dust are among the main scientific goals of the Planetary Fourier Spectrometer (PFS), a payload instrument of the European Mars Express mission. We report some results, obtained in three orbits: 37, 41 and 68. The temperature profile, and dust and water ice cloud opacities are retrieved from the thermal infrared (long-wavelength channel of PFS) in a self-consistent way using the same spectrum. Orographic ice clouds are identified above Olympus (orbit 37) and Ascraeus Mons (orbit 68). Both volcanoes were observed near noon at Ls=337° and 342°, respectively. The effective radius of ice particles is preliminary estimated as 1–3 μm, changing along the flanks. The corresponding visual opacity changes in the interval 0.2–0.4 above Olympus and 0.1–0.6 above Ascraeus Mons. In the case of Ascraeus Mons, the ice clouds were observed mainly above the Southern flank of the volcano with maximum opacity near the summit. In the case of Olympus, the clouds were found above both sides of the top. A different type of ice cloud is observed at latitudes above 50°N (orbit 68) in the polar hood: the effective particle radius is estimated to be 4 μm. Below the 1 mb level an inversion in the temperature profiles is found with maximum temperature at around 0.6 mb. Along orbit 68 it appears above Alba Patera, then it increases to the north and decreases above the CO 2 polar cap. Beginning from latitude 20°S above Tharsis (orbit 68), the ice clouds and dust contribute equally to the spectral shape. Further on, the ice clouds are found everywhere along orbit 68 up to the Northern polar cap, except the areas between the Northern flank of Ascraeus Mons (below 10 km) and the edge of Alba Patera. Orbit 41 is shifted from the orbit 68 by roughly 180° longitude and passes through Hellas. Ice clouds are not visible in this orbit at latitudes below 80°S. The dust opacity is anticorrelated with the surface altitude. From 70°S to 25°N latitude the vertical dust distribution follows an exponential law with a scale height of 11.5±0.5 km, which corresponds to the gaseous scale height near noon and indicates a well-mixed condition. The 9 μm dust opacity, reduced to zero surface altitude, is found to be 0.25±0.05, which corresponds to a visual opacity of 0.5–0.7 (depending on the particle size).

Condensate clouds in Titan's north polar stratosphere

Analysis of the 250–560 cm −1 spectral continuum of Titan's north polar hood just after spring equinox reveals, in addition to the ubiquitous aerosol, a tenuous but relatively uniform cloud of small particles permeating the lower stratosphere at altitudes between 58 and 90 km. Voyager 1 IRIS data suggest the particles are highly scattering, almost certainly condensed organics, with radii between 1 and 5 μm. Mole fractions for the condensed material range between 4 × 10 −8 and 4 × 10 −6 , depending upon particle size. Vapor pressure arguments imply condensed nitriles near 90 km, the most likely being HCN, with condensed hydrocarbons such as C 2H 6 restricted to regions considerably nearer the tropopause. No direct chemical identification is possible. Negligible methane supersaturation in the troposphere at 67.4° N latitude, when compared with degrees of supersaturation at other latitudes, hints at precipitation fluxes of north polar stratospheric condensates during the previous northern winter that were perhaps three orders of magnitude greater than those at low latitudes during that time. A scale height of 1.5 times the density scale height above 160 km is reaffirmed for the photochemical aerosol of the north polar hood. There appears to be a depletion of aerosol somewhere below 160 km. An aerosol mole fraction ∼ 8 × 10 −8 at 160 km is inferred, about 33% greater than the value derived in a previous study. The Cassini CIRS instrument, with its expanded spectral range and higher spectral resolution, should be able to provide highly complementary information for the time period covering most of the northern winter season.

Modeling the annual cycle of HDO in the Martian atmosphere

We present the results of the first three‐dimensional (3‐D) simulation of the water isotope HDO in the Martian atmosphere. This deuterated isotope of water has long been used on both Earth and Mars as a proxy to understand the climatic evolutions of these planets. On Mars, the current enrichment in deuterium concentration in the atmosphere is believed to be indirect evidence of a wetter climate in the past. Due to its vapor pressure being lower than that of H2O, HDO gets fractionated at condensation and therefore concentrates in the Martian water ice clouds. Our study aims at understanding the latitudinal, vertical, and temporal variations of this species under current Martian climate. Our results indicate that the globally averaged D/H ratio in the Martian atmosphere should vary modestly with season, with changes on the order of 2%. Locally, however, this same ratio exhibits large annual changes (by a factor of 2) in the high‐latitude regions. These fluctuations are controlled by the Polar Hood water ice clouds, within which HDO gets heavily fractionated. Due to the combined action of summer clouds above the north polar cap and to the cold‐trapping effect of the south residual cap, the global atmospheric deuterium concentration is predicted to be more than 15% lower than the concentration in the north permanent cap ice. We thus extrapolate by suggesting that the “true” D/H ratio of Martian water may exceed 6.5 (wrt. SMOW), rather than the 5.6 inferred from atmospheric probing. The globally and annually averaged vertical distribution of HDO exhibits a mild decline with altitude, a result in significant contrast with previous 1‐D studies. These results will help constrain more accurately the photochemical models aimed at understanding the observed low concentration of deuterium at high altitudes and thus the process of water escape to space.

Nucleation studies in the Martian atmosphere

We developed models for unary nucleation of water and carbon dioxide in the Martian atmosphere. Both homogeneous and heterogeneous nucleation on dust particles were studied. Our models are based on classical theory. We compare results of different adsorption approaches. Heterogeneous nucleation on the abundant dust particles seems to be the primary mechanism of both H2O and CO2 cloud formation in the Martian atmosphere. Heterogeneous nucleation is obtained at a saturation ratio of about 1.18 for H2O and 1.32 for CO2. Homogeneous nucleation is not likely to occur since it would require high supersaturations. We use our models to study nucleation as a function of height at different locations on Mars where ice fog or clouds have been observed. H2O ice nucleation results are in good agreement with surface fog observations and previous model studies. CO2 ice nucleation simulations in the polar hood cloud areas suggest that negative temperature perturbations caused by, e.g., adiabatic cooling in orographic waves or in convective plumes are required for the formation of CO2 clouds.

The Distribution and Nature of Titan’s Aerosols: A New Look

Titan passed through southern summer solstice in late 2002, allowing an unprecedented view of summer seasonal effects. A set of images was acquired with the Hubble Space Telescope spanning the 0.25-2 μmspectral range. Among the effects seen were a rapidly changing hemispheric asymmetry and a polar hood that is visible at short wavelengths. The north-south asymmetry has reversed, returning Titans dominant visual feature to something like its Voyager-era state. The polar hood is spectrally different from the north-south asymmetry and is due to a mode of small particles. The amount of absorbing material in the polar hood cannot be uniquely constrained, but a lower limit can be derived by assuming that the absorber is the same as that found in the main haze (i.e., it is analogous to Titan tholin) and that the absorber is entirely above the main haze. The hood spectrum is consistent with about 4xl0_6g/cm2 of tholin-like material above the main haze south of 65 S. A larger amount of material mixed lower in the atmosphere cannot be ruled out. An upper limit to the particle radius of about 0.02 μmcan be determined by the lack of a bright polar hood in near-infrared methane band images. Considering time scales for removing material from the stratosphere leads to an estimate of summer polar mass production that is within a factor of five of published estimates of annual, global aerosol mass production. More detailed modeling of the observations is proceeding. The model includes the prediction of disk-resolved polarization based on particle characteristics. Comparisons to HST polarimetric images will provide new constraints on the properties of the northern, southern, and polar aerosols. Preliminary analysis of the polarimetry images shows the polarization is radial at all wavelengths (0.25-2 μm) and the magnitude of polarization peaks near 0.75 μm. The polarization is consistent with small particles, but is less than predicted by models of Pioneer 11 and Voyager 2 polarimetry.

2003 Mars Report from Cooperative Observation Networks. I. Pre-Opposition

We have two networks for cooperative Mars observations in Japan: Nishi-Harima Astronomical Observatory Mars Cooperative Observation and Association of Lunar and Planetary Observers in Japan. This paper is a summary of 3515 images collected by the two networks during the first half of the last apparition from 2002 October 18 through 2003 August 31, corresponding to the period of $L_{\mathrm{s}} \sim {83{}^{\mathrm {\circ }}} \hbox{--} {252{}^{\mathrm {\circ }}}$ ($L_{\mathrm{s}}$ is the areocentric longitude of the Sun). During this period, many interesting phenomena were observed. (1) Two local dust storms appeared about $L_{\mathrm{s}} = {214{}^{\mathrm {\circ }}}$ and $L_{\mathrm{s}} = {231{}^{\mathrm {\circ }}}$. (2) Hellas was obscured just before the occurrence of the first dust storm. This may be relevant to the dust storm. (3) Blue clearings were observed. The phenomena in this apparition may be attributed to the optical properties of the ground for the scattering of light. (4) Dark markings in the south polar cap (SPC) developed as the SPC receded in mid-spring of the southern hemisphere. Also, some bright spots appeared on the SPC. (5) The south polar hood receded polewards and SPC began to be observed at about $L_{\mathrm{s}} \sim {185{}^{\mathrm {\circ }}}$ (mid-May). (6) The north polar hood was not detected until $L_{\mathrm{s}} \sim {180{}^{\mathrm {\circ }}}$ in this apparition. (7) The low-latitude cloud belt had been observed until mid-May. In the dissipation phase, a less cloud area appeared in longitudes of 170${{}^{\mathrm {\circ }}}$-200${{}^{\mathrm {\circ }}}$ W.

Origin and role of water ice clouds in the Martian water cycle as inferred from a general circulation model

In this paper, we present the results obtained by the general circulation model developed at the Laboratoire de Météorologie Dynamique which has been used to simulate the Martian hydrological cycle. Our model, which employs a simplified cloud scheme, reproduces the observed Martian water cycle with unprecedented agreement. The modeled seasonal evolution of cloudiness, which also compares well with data, is described in terms of the meteorological phenomena that control the Martian cloud distribution. Whereas cloud formation in the tropical region results from seasonal changes in the overturning circulation, Polar Hood clouds are mostly driven by variations of atmospheric wave activity. A sensitivity study allows us to quantify the effects of the transport of water ice clouds on the seasonal evolution of the water cycle. The residence time of cloud particles is long enough to allow cloud advection over great distances (typically thousands of kilometers). Despite the relatively low proportion of clouds (∼10%) in the total atmospheric inventory of water, their ability to be transported over large distances generally acts at the expense of the north polar cap and generates a water cycle globally wetter by a factor of 2 than a cycle produced by a model neglecting cloud transport. Around aphelion season, clouds modulate the north to south migration of water in a significant fashion and participate just as much as vapor in the cross‐equatorial transport of total water. Most of the year, atmospheric waves generate an equatorward motion of water ice clouds near the polar vortex boundaries, partially balancing the opposite poleward flux of water vapor. The combination of both effects delays the return of water to the north polar cap and allows water to build up in the Martian tropics.

A coupled dynamics-microphysics model of Titan's atmosphere

We have developed a coupled general circulation model of Titan's atmosphere in which the aerosol haze is treated with a microphysical model and is advected by the winds. The radiative transfer accounts for the non uniform haze distribution and, in turn, drives the dynamics. We analyze the GCM results, especially focusing on the difference between a uniform haze layer and a haze layer coupled to the dynamics. In the coupled simulation the aerosols tend to accumulate at the poles, at latitudes higher than ±60°. During winter, aerosols strongly radiate at thermal infrared wavelengths enhancing the cooling rate near the pole. Since this tends to increase the latitudinal gradients of temperature the direct effect of this cooling excess, in contrast to the uncoupled haze case, is to increase the strength of the meridional cells as well as the strength of the zonal winds and profile. This is a positive feedback of the haze on dynamics. The coupled model reproduces observations about the state of the atmosphere better than the uniform haze model, and in addition, the northern polar hood and the detached haze are qualitatively reproduced.

A coupled dynamics-microphysics model of Titan's atmosphere

We have developed a coupled general circulation model of Titan's atmosphere in which the aerosol haze is treated with a microphysical model and is advected by the winds. The radiative transfer accounts for the non uniform haze distribution and, in turn, drives the dynamics. We analyze the GCM results, especially focusing on the difference between a uniform haze layer and a haze layer coupled to the dynamics. In the coupled simulation the aerosols tend to accumulate at the poles, at latitudes higher than ±60°. During winter, aerosols strongly radiate at thermal infrared wavelengths enhancing the cooling rate near the pole. Since this tends to increase the latitudinal gradients of temperature the direct effect of this cooling excess, in contrast to the uncoupled haze case, is to increase the strength of the meridional cells as well as the strength of the zonal winds and profile. This is a positive feedback of the haze on dynamics. The coupled model reproduces observations about the state of the atmosphere better than the uniform haze model, and in addition, the northern polar hood and the detached haze are qualitatively reproduced.

H 2O- and OH-bearing minerals in the martian regolith: : analysis of 1997 observations from HST/NICMOS

We have analyzed observations of the Acidalia hemisphere of Mars taken by the Hubble Space Telescope's Near-Infrared Camera Multi-Object Spectrograph (HST/NICMOS) during July of 1997 ( L s =152°, northern martian summer). The data consist of images at ∼60 km/pixel resolution, using both narrow- and medium-band filters specifically selected to allow us to study the hydration state of the martian surface. Calibration was performed by comparison to Phobos-2 ISM observations of overlapping regions, and atmospheric gas correction was performed by modeling the atmosphere for each pixel using a line-by-line radiative transfer code coupled with the MOLA altimetry data. Our results indicate the presence of at least three spectrally different large-scale (>1000 km diameter) terrains corresponding to the dark regions of northern Acidalia, the southern hemisphere classical dark terrain, and the classical intermediate terrain adjacent to southern Acidalia. We also identified two other spectrally unique terrains, corresponding to the northern polar ice cap, and to the southern winter polar hood. Comparisons with mineral spectra indicate the possibility of different H 2O- or OH-bearing (i.e., hydroxides and/or hydrates) minerals existing both in northern Acidalia and in the nearby intermediate albedo terrain. Hydrated minerals do not appear to be spectrally important components of the southern hemisphere dark terrains imaged by HST in 1997.

An assessment of the global, seasonal, and interannual spacecraft record of Martian climate in the thermal infrared

Intercomparison of thermal infrared data collected by Mariner 9, Viking, and Mars Global Surveyor (MGS) is presented with a specific focus on air temperatures, dust opacities, and water ice opacities. Emphasis is placed on creating a uniform data set to most effectively reduce interinstrument biases and offsets. The annual cycle consistently shows a strong asymmetry about the equinoxes, with northern spring and summer exhibiting relatively low temperatures, very high year‐to‐year repeatability, and essentially no short‐term (tens of days) variability. The globally averaged Martian nighttime air temperatures close annually to within a Kelvin during northern spring and summer. Daytime temperatures show more variability (3–6 K). The difference in repeatability of daytime versus nighttime temperatures is not understood. Viking and MGS air temperatures are essentially indistinguishable for this period, suggesting that the Viking and MGS eras are characterized by essentially the same climatic state. Southern summer is characterized by strong dust storm activity and hence strong year‐to‐year air temperature variability. Dust opacity shows a remarkable degree of interannual variability in southern spring and summer, associated with the intermittent activity of regional and planet‐encircling dust storms, but exhibits high year‐to‐year repeatability in northern spring and summer. Specifically, late northern spring and early northern summer dust opacities appear to be completely insensitive to the occurrence (or not) of major dust storms in the previous southern spring or summer. We show that both Viking and MGS data sets exhibit significant (and similar) polar cap edge dust storm activity. The origins of the various major dust storms can be identified in the thermal infrared data from Viking and MGS, including the transport of dust from the northern autumn baroclinic zone into the southern hemisphere tropics, which has also been identified in visible imaging. We also note that the period around Ls = 225° is characterized by very high dust opacities associated with dust storm development or decay in every year thus far observed by spacecraft. Water ice opacities have been retrieved from Viking infrared data for the first time. We show that the northern spring and summer tropical cloud belt structure and evolution are essentially the same in each of the multiple years observed by Viking and MGS. Relatively subtle spatial features recur in the cloud belt from year to year, suggesting the influence of surface topography and thermophysical properties and a reasonably consistent supply of water vapor. The seasonal evolution of the tropical cloud belt through northern spring and summer is shown, with the only significant deviations between years occurring from Ls = 140° to 160°, where opacities fall in the second MGS year associated with a small dust storm. Polar hood clouds are observed in Viking and MGS observations with similar timing and extent. Interactions between dust and water ice were highlighted in the Hellas basin region during the southern spring 1977a and 2001 dust storms. The observations demonstrate that the Martian atmosphere executes a very “repeatable” annual cycle of atmospheric phenomena. However, a major part of this cycle is the occurrence of highly variable and potentially major dust storm events. After such dust storm events the atmosphere rapidly relaxes to its stable, repeatable state.

Viking‐era diurnal water‐ice clouds

Water‐ice clouds during the Viking era are mapped as a function of season, latitude, longitude, and time of day. In the springtime hemisphere, where data are available, clouds are observed to decrease from morning to midday. They are observed to increase in extent from midday to afternoon, possibly due to increased atmospheric temperatures causing uplifting of dust which then acts as cloud condensation nuclei. During the summer hemisphere clouds are observed to decrease in extent throughout the day, indicating sufficient warming to prevent water reaching the condensation level. The Hadley cell upwelling branch is seen throughout the year via cloud belts in both the northern (aphelion cloud belt) and southern hemispheres in their respective springs and summers. Clouds extending equatorward of 60°N in late northern summer and early northern autumn are seen primarily over the Acidalia Planitia longitudinal sector and may indicate the southernmost extent of the north polar hood or clouds associated with storms. Water‐ice clouds associated with topographic features are seen, including Olympus Mons, the Tharsis volcanoes, and the northern rim of Hellas basin. There is little evidence for interannual variability in the prominent cloud features (e.g., the aphelion cloud belt) since they are present in both Martian years examined, though the detailed structure does change.

Martian clouds observed by Mars Global Surveyor Mars Orbiter Camera

We have made daily global maps that cover both polar and equatorial regions of Mars for Ls 135°–360° and 0°–111° using the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) red and blue wide‐angle swaths taken from May 1999 to January 2001. We study the seasonal distribution of condensate clouds and dust clouds during roughly 1 Martian year using these daily global maps. We present the development and decay of the tropical cloud belt and the polar hoods, the spatial and temporal distributions of lee waves and spiral clouds, and an unusual “aster” cloud above the volcanoes, consisting of rays around a central disk, like the flower. The tropical cloud belt contains mostly fibrous clouds during northern spring/early summer and convective clouds during middle/late northern summer. The detailed development and decay of the tropical cloud belt is nonuniform in longitude. Two distinct stormy periods in late summer precede the formation of the north and south circumpolar hoods. The north polar storms in late summer resemble baroclinic frontal systems on Earth but contain both dust and condensate clouds. Spiral clouds occur only in the northern high latitudes and only during northern spring and summer. The north polar hood displays a stationary wave number two structure during the fall and winter. The south polar hood has fewer streak clouds and lee wave clouds than the north polar hood. During this particular year the lee wave cloud abundance in the south had two peaks (in early fall and late winter), and the lee wave cloud abundance in the north had one peak (in early fall).

A wind origin for Titan's haze structure.

Titan, the largest moon of Saturn, is the only satellite in the Solar System with a dense atmosphere. Titan's atmosphere is mainly nitrogen with a surface pressure of 1.5 atmospheres and a temperature of 95 K (ref. 1). A seasonally varying haze, which appears to be the main source of heating and cooling that drives atmospheric circulation, shrouds the moon. The haze has numerous features that have remained unexplained. There are several layers, including a 'polar hood', and a pronounced hemispheric asymmetry. The upper atmosphere rotates much faster than the surface of the moon, and there is a significant latitudinal temperature asymmetry at the equinoxes. Here we describe a numerical simulation of Titan's atmosphere, which appears to explain the observed features of the haze. The critical new factor in our model is the coupling of haze formation with atmospheric dynamics, which includes a component of strong positive feedback between the haze and the winds.

Martian dust storms: 1999 Mars Orbiter Camera observations

The Mars Orbiter Camera on board the Mars Global Surveyor from March 9, 1999 (Ls = 107°, where Ls is the areocentric longitude of the Sun measured in degrees from Mars' northern spring equinox), to December 31, 1999 (Ls = 274°), has obtained 7.5 km pixel−1 daily global maps of the Martian surface in two wavelength bands: blue (400–450 nm) and red (575–625 nm). Visual inspection of these maps during the 1999 dust storm season has resulted in the detection of 783 dust storms, ranging in size from “local” (>102 km2) to “regional” (>1.6×106 km2). No global storms were observed. The difference in the numbers of dust storms occurring in the two hemispheres (north: 343; south: 440) was not statistically significant. Fifty percent of the regional size dust storms formed as a result of the “merger” of two or more smaller local storms. The regional storms from Ls = 207° to Ls = 223° were observed to move from the northern hemisphere into the southern hemisphere between 30°W and 50°W longitude. This cross‐equatorial transport of dust suggests the location of the low‐lying southward flowing branch of the Hadley circulation. Observations show that dust storms occur in several regions on Mars: at the two polar cap edges, at the base of high elevation regions in the northern hemisphere, near the polar hood during northern fall, and at mid‐latitudes in both hemispheres. Specific regions such as Solis Planum and Hesperia, which were regions of significant dust activity in the past, showed almost no dust storm activity in 1999, suggesting that regional dust sources are variable over a 20‐year time frame. One region of exceptional activity not noted previously was the Arcadia‐Amazonis border. However, other regions, such as near Elysium Mons, Acidalia, Chryse, Hellas, Noachis, Argyre, Cimmeria, and Sirenum, again were active dust storm regions. Relative dust opacities were modeled for 117 red wavelength events and ranged from 0.54 “diffuse haze” to 2.13 “concentrated local events.” From the dust opacities we estimated the north polar sedimentation rate to be 6.0–10×10−4 g cm−2 yr−1, a factor of 40 smaller than previous estimates from the two 1977 global dust storms [Pollack et al., 1979], along with the global dust mass loading (2.6–4.0×10−4 g cm−2) and cross‐equatorial dust mass loading (3.6×10−4 g cm−2). The 1999 cross‐equatorial mass dust loading suggests that the southern hemisphere subtropical latitudes require at least 2–3 Martian years to replenish their dust sources before global storms can form.

Mars Color Imager (MARCI) on the Mars Climate Orbiter

The Mars Color Imager, or MARCI, experiment on the Mars Climate Orbiter (MCO) consists of two cameras with unique optics and identical focal plane assemblies (FPAs), Data Acquisition System (DAS) electronics, and power supplies. Each camera is characterized by small physical size and mass (∼6×6×12 cm, including baffle; <500 g), low power requirements (<2.5 W, including power supply losses), and high science performance (1000×1000 pixel, low noise). The Wide Angle (WA) camera will have the capability to map Mars in five visible and two ultraviolet spectral bands at a resolution of better than 8 km/pixel under the worst case downlink data rate. Under better downlink conditions the WA will provide kilometer‐scale global maps of atmospheric phenomena such as clouds, hazes, dust storms, and the polar hood. Limb observations will provide additional detail on atmospheric structure at 1/3 scale‐height resolution. The Medium Angle (MA) camera is designed to study selected areas of Mars at regional scale. From 400 km altitude its 6° FOV, which covers ∼40 km at 40 m/pixel, will permit all locations on the planet except the poles to be accessible for image acquisitions every two mapping cycles (roughly 52 sols). Eight spectral channels between 425 and 1000 nm provide the ability to discriminate both atmospheric and surface features on the basis of composition. The primary science objectives of MARCI are to (1) observe Martian atmospheric processes at synoptic scales and mesoscales, (2) study details of the interaction of the atmosphere with the surface at a variety of scales in both space and time, and (3) examine surface features characteristic of the evolution of the Martian climate over time. MARCI will directly address two of the three high‐level goals of the Mars Surveyor Program: Climate and Resources. Life, the third goal, will be addressed indirectly through the environmental factors associated with the other two goals.

The 1994–1995 apparition of Mars observed from Pic-du-Midi

The 1995 apparition of Mars was monitored with the Pic-du-Midi 1-m telescope from October 1994 to March 1995. Several thousand images were acquired with a CCD camera through various filters ranging from 0.4 to 1.05 μm . These images complete HST observations of the same apparition with a spatial resolution reduced by a factor of ∼4, but with a better time resolution. Low-resolution spectra were also acquired through a set of interferential filters at opposition, providing indications of mineralogical composition of surface materials. The only significant surface variations during the period are observed in the Cerberus area, which brightened at the end of winter. The whole apparition was very cloudy. The north Polar Hood dissipated in early December ( L s∼25°). Migration of polar clouds to the equator is observed between January 31 and February 6 ( L s∼50–55°). Then an Encircling Cloud Belt obscured most of the surface from February 22 to 24 at least ( L s∼62°). The north polar cap regression is similar to that observed by Mariner 9, although several internal rifts developed during early spring, in particular the dark Lowell band and Rima Tenuis. A standstill is observed in the polar cap regression curve from L s=55 to 70° at latitude ∼70°, at least in the 135–300° longitude range. The very strong spectral feature at 0.95 μm reported from HST observations of Olympia Planitia in July 1995 is not observed in the Pic-du-Midi images. The intermediate-dark region of Propontis I exhibits ferric absorption features consistent with high concentrations of crystalline hematite or other dark ferric materials.

Winter clouds over the North Martian Polar Cap

The Mars Orbiter Laser Altimeter (MOLA) experiment, carried on the Mars Global Surveyor spacecraft, has observed echoes from cloud tops on roughly 2.5% of the total data taken above 70°N over the northern winter Martian polar hood from March to June 1998. Sloping wavefronts are commonly seen at latitudes above 70°N, implying the presence of propagating buoyancy waves. Since these wavefronts frequently extend from the surface up to 10 km at a time when CO2 is known to be condensing on the polar surface, it seems likely that the laser‐scattering particles consist primarily of CO2 ice, and that the near‐surface temperature lapse rate is 0.85 K km−1, set by the vapor pressure of dry ice. From the observed wavelengths, where available, we have calculated the corresponding phase velocities; some of these waves appear to be correlated with surface discontinuities, and may represent lee waves.