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G Gloeckler - One of the best experts on this subject based on the ideXlab platform.
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neutral h density at the termination shock a consolidation of recent results
Space Science Reviews, 2009Co-Authors: M Bzowski, S. Tarnopolski, E Mobius, V V Izmodenov, G GloecklerAbstract:We discuss a consolidation of determinations of the density of neutral interstellar H at the nose of the termination shock carried out with the use of various data sets, techniques, and modeling approaches. In particular, we focus on the determination of this density based on observations of H pickup ions on Ulysses during its Aphelion passage through the ecliptic plane. We discuss in greater detail a novel method of determination of the density from these measurements and review the results from its application to actual data. The H density at TS derived from this analysis is equal to 0.087±0.022 cm−3, and when all relevant determinations are taken into account, the consolidated density is obtained at 0.09±0.022 cm−3. The density of H in CHISM based on literature values of filtration factor is then calculated at 0.16±0.04 cm−3.
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density of neutral interstellar hydrogen at the termination shock from ulysses pickup ion observations
Astronomy and Astrophysics, 2008Co-Authors: M Bzowski, S. Tarnopolski, E Mobius, V V Izmodenov, G GloecklerAbstract:Aims. By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the local interstellar cloud. Methods. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at Aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling. Results. Analysis using current heliospheric parameters yields the H density at the termination shock equal to 0.087 ± 0.022 cm −3 , including observational and modeling uncertainties.
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density of neutral interstellar hydrogen at the termination shock from ulysses pickup ion observations
arXiv: Astrophysics, 2007Co-Authors: M Bzowski, S. Tarnopolski, V V Izmodenov, E Moebius, G GloecklerAbstract:By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the LIC. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at Aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling. Analysis using current heliospheric parameters yields the H density at the termination shock equal to $0.087\pm0.022$ cm$^{-3}$, including observational and modeling uncertainties.
Jeanbaptiste Madeleine - One of the best experts on this subject based on the ideXlab platform.
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retrieval of the water ice column and physical properties of water ice clouds in the martian atmosphere using the omega imaging spectrometer
Icarus, 2021Co-Authors: Kevin Olsen, Francois Forget, Jeanbaptiste Madeleine, A Szantai, Joachim Audouard, A Geminale, F Altieri, G Bellucci, F Oliva, L MontaboneAbstract:Abstract Using spectral images recorded by the OMEGA instrument on Mars Express (Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite), we are able to 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, reff, and optical depth (at 0.67 μm), τi, 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 reff is ∼2.2 μm, with a standard deviation of 0.8 μm, and cloud formations with reff between 4.4 and 5.4 μm 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∘W and 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. μm over the Aphelion cloud belt, and 1.5–2.5 pr. μm over the polar hoods. The data set analyzed is for observations between 140°W and 90°E, and between 35∘S and 35∘N. No restriction is placed on season, but the majority of cloudy observations were during the Aphelion period from Ls 35∘ to 135∘. This work was motivated by the ability of the OMEGA instrument to observe the distribution of water-ice cloud physical properties, and by the availability of new a priori data sets, especially multi-spectral, aerosol-free surface albedo retrieved from a subset of the OMEGA data featuring a cloud-free sky. The main limitations of the retrieval algorithm are linked to the uncertainties on surface albedo, the dust opacity, and the quantity of water-ice suspended in the atmosphere, which can lead to spectral fits with lower accuracy or unrealistic results. We present distributions of each retrieved parameter, goodness of fit, ICI, and cloud mass, and our investigation of relationships between each parameter. Our approach was to maximize the amount of data analyzed, apply stringent data quality cuts and take a statistical approach to interpretation.
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the influence of radiatively active water ice clouds on the martian climate
Geophysical Research Letters, 2012Co-Authors: Francois Forget, T Navarro, Jeanbaptiste Madeleine, Ehouarn Millour, Aymeric SpigaAbstract:[1] Radiatively active water ice clouds (RAC) play a key role in shaping the thermal structure of the Martian atmosphere. In this paper, RAC are implemented in the LMD Mars Global Climate Model (GCM) and the simulated temperatures are compared to Thermal Emission Spectrometer observations over a full year. RAC change the temperature gradients and global dynamics of the atmosphere and this change in dynamics in turn implies large-scale adiabatic temperature changes. Therefore, clouds have both a direct and indirect effect on atmospheric temperatures. RAC successfully reduce major GCM temperature biases, especially in the regions of formation of the Aphelion cloud belt where a cold bias of more than 10 K is corrected. Departures from the observations are however seen in the polar regions, and highlight the need for better modeling of cloud formation and evolution.
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Aphelion water ice cloud mapping and property retrieval using the omega imaging spectrometer onboard mars express
Journal of Geophysical Research, 2012Co-Authors: Michael J. Wolff, Francois Forget, Jeanbaptiste Madeleine, Aymeric Spiga, Franck Montmessin, M Vincendon, D Jouglet, Brigitte Gondet, J P BibringAbstract:Mapping of the Aphelion clouds over Tharsis and retrieval of their particle size and visible opacity are made possible by the OMEGA imaging spectrometer aboard Mars Express. Observations cover the period from MY26 Ls=330{degree sign} to MY29 Ls=180{degree sign} and are acquired at various local times, ranging from 8AM to 6PM. Cloud maps of the Tharsis region constructed using the 3.1µm ice absorption band reveal the seasonal and diurnal evolution of Aphelion clouds. Four distinct types of clouds are identified: morning hazes, topographically controlled hazes, cumulus clouds and thick hazes. The location and time of occurrence of these clouds are analyzed and their respective formation process is discussed. An inverse method for retrieving cloud particle size and opacity is then developed and can only be applied to thick hazes. The relative error of these measurements is less than 30% for cloud particle size and 20% for opacity. Two groups of particles can be distinguished. The first group is found over flat plains and is composed of relatively small particles, ranging in size from 2 to 3.5µm. The second group is characterized by particle sizes of ~5µm which appear to be quite constant over Ls and local time. It is found west of Ascraeus and Pavonis Mons, and near Lunae Planum. These regions are preferentially exposed to anabatic winds, which may control the formation of these particles and explain their distinct properties. The water ice column is equal to 2.9pr.µm on average, and can reach 5.2pr.µm in the thickest clouds of Tharsis.
Michael J. Wolff - One of the best experts on this subject based on the ideXlab platform.
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Aphelion water ice cloud mapping and property retrieval using the omega imaging spectrometer onboard mars express
Journal of Geophysical Research, 2012Co-Authors: Michael J. Wolff, Francois Forget, Jeanbaptiste Madeleine, Aymeric Spiga, Franck Montmessin, M Vincendon, D Jouglet, Brigitte Gondet, J P BibringAbstract:Mapping of the Aphelion clouds over Tharsis and retrieval of their particle size and visible opacity are made possible by the OMEGA imaging spectrometer aboard Mars Express. Observations cover the period from MY26 Ls=330{degree sign} to MY29 Ls=180{degree sign} and are acquired at various local times, ranging from 8AM to 6PM. Cloud maps of the Tharsis region constructed using the 3.1µm ice absorption band reveal the seasonal and diurnal evolution of Aphelion clouds. Four distinct types of clouds are identified: morning hazes, topographically controlled hazes, cumulus clouds and thick hazes. The location and time of occurrence of these clouds are analyzed and their respective formation process is discussed. An inverse method for retrieving cloud particle size and opacity is then developed and can only be applied to thick hazes. The relative error of these measurements is less than 30% for cloud particle size and 20% for opacity. Two groups of particles can be distinguished. The first group is found over flat plains and is composed of relatively small particles, ranging in size from 2 to 3.5µm. The second group is characterized by particle sizes of ~5µm which appear to be quite constant over Ls and local time. It is found west of Ascraeus and Pavonis Mons, and near Lunae Planum. These regions are preferentially exposed to anabatic winds, which may control the formation of these particles and explain their distinct properties. The water ice column is equal to 2.9pr.µm on average, and can reach 5.2pr.µm in the thickest clouds of Tharsis.
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Mars aerosol studies with the MGS TES emission phase function observations: Optical depths, particle sizes, and ice cloud types versus latitude and solar longitude
Journal of Geophysical Research, 2003Co-Authors: R. Todd Clancy, Michael J. Wolff, Philip R. ChristensenAbstract:[1] Emission phase function (EPF) observations taken in 1999–2001 by Mars Global Surveyor Thermal Emission Spectrometer (MGS TES) support the broadest study of Martian aerosol properties to date. TES solar band and infrared (IR) spectral EPF sequences are analyzed to obtain first-time seasonal/latitudinal distributions of visible optical depths, particle sizes, and single scattering phase functions. This combined angular and wavelength coverage enables identification of two distinct ice cloud types over 45°S–45°N. Type 1 ice clouds exhibit small particle sizes (reff = 1–2 μm) and a distinctive backscattering increase. They are most prevalent in the southern hemisphere during Aphelion, but also appear more widely distributed in season and latitude as topographic and high-altitude (≥20 km) ice hazes. Type 2 ice clouds exhibit larger particle sizes (reff = 3–4 μm), a distinct side-scattering minimum at 90–100° phase angles (characteristic of a change in particle shape relative to the type 1), and appear most prominently in the northern subtropical Aphelion cloud belt. The majority of retrieved dust visible-to-IR optical depth ratios are indicative of reff = 1.5 ± 0.1 μm, consistent with Pathfinder and Viking/Mariner 9 reanalyses. However, increased ratios (2.7 versus 1.7) appear frequently in the northern hemisphere over LS = 50–200°, indicating substantially smaller dust particles sizes (reff = 1.0 ± 0.2 μm) at this time. In addition, larger (reff = 1.8–2.5 μm) dust particles were observed locally in the southern hemisphere during the peak of the 2001 global dust storm. Detailed spectral modeling of the TES visible band pass indicates agreement of EPF-derived dust single scattering albedos (0.92–0.94) with the spectrally resolved results from Pathfinder observations.
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minimal aerosol loading and global increases in atmospheric ozone during the 1996 1997 martian northern spring season
Icarus, 1999Co-Authors: Todd R Clancy, Michael J. Wolff, Philip B. JamesAbstract:We employed the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) to observe middle ultraviolet (220–330 nm) spectral scans of Mars for two periods during the 1996–1997 northern spring season. Analysis of these data yields quantitative measurements of atmospheric column opacities for aerosols and ozone over a wide range of latitudes (40°S–70°N) in two Mars seasons (solar longitudes,Lsof 10° and 61°). The most significant findings of the analysis are: (1) the global Mars atmosphere exhibited minimal aerosol loading during this season (variable opacities of 0.02–0.08 for clouds, or 0.05–0.2 for dust); (2) the low-to-mid latitude ozone abundance in the Mars atmosphere increased by roughly a factor of two (from 1.8 to 3.6 μm-atm) betweenLs=10° andLs=61°. These results support previous predictions (R. T. Clancy and H. Nair 1996,J. Geophys. Res.101, 12785–12790) and observations (R. T. Clancyet al.1996b,J. Geophys. Res.101, 12777–12783) of orbital (Ls) variations in Mars global ozone abundances, as driven by orbital variations in the global altitudes of water vapor saturation (R. T. Clancyet al.1996a,Icarus122, 36–62). They also support conclusions of very low dust levels in the Mars atmosphere around Aphelion (Clancyet al.1996a), with upper limits for background dust opacities which are several times lower than described for this season by the Viking-based dusty model of the Mars climate (e.g.,τ<0.2, here; versus 0.6; D. S. Colburnet al.1989,Icarus79, 159–189). Also of interest is the observation of substantially reduced cloud opacities for the “Aphelion cloud belt” in early northern spring (Ls∼ 60°) in 1997, versus 1995 or 1991.
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water vapor saturation at low altitudes around mars Aphelion a key to mars climate
Icarus, 1996Co-Authors: R T Clancy, Michael J. Wolff, Philip B. James, Brad J Sandor, A W Grossman, D J Rudy, Y N Billawala, S W Lee, D O MuhlemanAbstract: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 (Ls= 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.
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mars ozone measurements near the 1995 Aphelion hubble space telescope ultraviolet spectroscopy with the faint object spectrograph
Journal of Geophysical Research, 1996Co-Authors: Todd R Clancy, Michael J. Wolff, Philip B. James, E J Smith, Youssef N Billawala, Steven W Lee, Michael T CallanAbstract:Ultraviolet (225–330 nm) spectral scans of Mars were obtained with the Hubble space telescope (HST) faint object spectrograph (FOS) in February of 1995. These spectra yield ozone column abundances, cloud opacities (0.2–0.4 at low latitudes), and polar seasonal ice albedos from southern midlatitudes to northern high latitudes on Mars. At the time of these measurements, Mars was at a solar longitude (Ls) of 63.5°, corresponding to the late northern spring season on Mars, and very near to Mars Aphelion. The most important result of these observations is the measurement of low-latitude ozone abundances (3.1−0.5+2.1), which are significantly (≥100%) elevated relative to the northern fall (Ls = 208°, pre-perihelion) IR ozone measurements of Espenak et al. [1991] in 1988. The implied perihelion-to-Aphelion increase in the global Mars ozone column (from 1.5−1.0+0.4 to 3.1−0.5+2.1 μm atm) is quantitatively consistent with photochemical modeling analysis of Clancy and Nair [this issue], which predicts large annual variations in Mars photochemistry due to orbital forcing of the altitude of global water vapor saturation on Mars [Clancy et al., 1996]. However, the HST FOS observations are not diagnostic of the altitudes at which Mars ozone densities vary with Ls, which is a key aspect of the Clancy and Nair model prediction. Furthermore, it is the uncertain ozone density profile which leads to the large asymmetric uncertainties in the derived FOS and IR ozone columns. An ozone column of 7.3 ± 2.5 μm atm is retrieved for a high northern latitude region (71–75°N). The derived ultraviolet albedo of the north polar seasonal CO2 cap is 0.18 ± 0.07, which is roughly 10 times the ultraviolet albedo of the silicate surface of Mars, but only one quarter the visible albedo of the seasonal CO2 ice cap.
Stephen W. Bougher - One of the best experts on this subject based on the ideXlab platform.
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mgs radio science electron density profiles interannual variability and implications for the martian neutral atmosphere
Journal of Geophysical Research, 2004Co-Authors: Stephen W. Bougher, David P. Hinson, S Engel, J R MurphyAbstract:density of 7.3–8.5 � 10 4 cm � 3 is also measured during solar moderate conditions at Mars. Strong wave number 2–3 oscillations in peak heights are consistently observed as a function of longitude over the 2 Martian years. These observed ionospheric features are remarkably similar during Aphelion conditions 1 Martian year apart. This year-to-year repeatability in the thermosphere-ionosphere structure is consistent with that observed in multiyear Aphelion temperature data of the Mars lower atmosphere [Clancy et al., 2000; Smith, 2004]. Coupled Mars general circulation model (MGCM) and Mars thermospheric general circulation model (MTGCM) codes are run for Mars Aphelion conditions, yielding mean and longitude variable ionospheric peak heights that reasonably match RS observations. A tidal decomposition of MTGCM thermospheric densities shows that observed ionospheric wave number 3 features are linked to a nonmigrating tidal mode with semidiurnal period (s = 2) and zonal wave number 1 (s = � 1) characteristics. The height of this photochemically determined ionospheric peak should be monitored regularly. INDEX TERMS: 5435 Planetology: Solid Surface Planets: Ionospheres (2459); 5409 Planetology: Solid Surface Planets: Atmospheres—structure and dynamics; 6225 Planetology: Solar System Objects: Mars; KEYWORDS: ionosphere, Mars, thermosphere
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mars global surveyor radio science electron density profiles neutral atmosphere implications
Geophysical Research Letters, 2001Co-Authors: Stephen W. Bougher, David P. Hinson, S Engel, J M ForbesAbstract:The Mars Global Surveyor (MGS) Radio Sci- ence (RS) experiment permits retrieval of electron density prof iles versus height (∼90-200 km) from occultation mea- surements. An initial set of electron profiles is examined spanning high northern latitudes, early morning solar local times and high solar zenith angles (78 to 81 ◦ ) near Aphelion. Sampling for these 32-profiles is well distributed over longi- tude. The height of the photochemically driven ionospheric peak is observed to respond to the background neutral den- sity structure, with a mean height during this season at this location of ∼134.4 km. Strong wave-3 oscillations about this mean are clearly observed as a function of longitude, and correspond to neutral density variations measured by the MGS Accelerometer (ACC) experiment. The wave-3 tidal pattern implicated by both the RS and ACC datasets is consistent with a semi-diurnal wave frequency. Clearly, the height of the martian dayside ionospheric peak is a sensitive indicator of the state of the underlying Mars atmosphere. This ionospheric peak height can be used as a proxy of the longitude specific non-migrating tidal variations present in the Mars lower thermosphere.
R T Clancy - One of the best experts on this subject based on the ideXlab platform.
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an intercomparison of ground based millimeter mgs tes and viking atmospheric temperature measurements seasonal and interannual variability of temperatures and dust loading in the global mars atmosphere
Journal of Geophysical Research, 2000Co-Authors: R T Clancy, Philip R. Christensen, Brad J Sandor, M J Wolff, Michael D Smith, J C Pearl, B J Conrath, R J WilsonAbstract:During the period October 1997 to September 1999 we obtained and analyzed over 100 millimeter-wave observations of Mars atmospheric CO line absorption for atmospheric temperature profiles. These measurements extend through one full Mars year (solar longitudes LS of 190° in 1997 to 180° in 1999) and coincide with atmospheric temperature profile and dust column measurements from the Thermal Emission Spectrometer (TES) experiment on board the Mars Global Surveyor (MGS) spacecraft. A comparison of Mars atmospheric temperatures retrieved by these distinct methods provides the first opportunity to place the long-term (1982–1999) millimeter retrievals of Mars atmospheric temperatures within the context of contemporaneous, spatially mapped spacecraft observations. Profile comparisons of 0–30 km altitude atmospheric temperatures retrieved with the two techniques agree typically to within the 5 K calibration accuracy of the millimeter observations. At the 0.5 mbar pressure level (∼25 km altitude) the 30°N/30°S average for TES infrared temperatures and the disk-averaged millimeter temperatures are also well correlated in their seasonal and dust-storm-related variations over the 1997–1999 period. This period includes the Noachis Terra regional dust storm, which led to very abrupt heating (∼15 K at 0.5 mbar) of the global Mars atmosphere at LS = 224° in 1997 [Christensen et al., 1998; Conrath et al., this issue; Smith et al., this issue]. Much colder (10–20 K) global atmospheric temperatures were observed during the 1997 versus 1977 perihelion periods (LS = 200°–330°), consistent with the much (2 to 8 times) lower global dust loading of the atmosphere during the 1997 perihelion dust storm season versus the Viking period of the 1977a,b storms. The 1998–1999 Mars atmosphere revealed by both the millimeter and TES observations is also 10–15 K colder than presented by the Viking climatology during the Aphelion season (LS = 0°–180°, northern spring/summer) of Mars. We reassess the observational basis of the Viking dusty-warm climatology for this season to conclude that the global Aphelion atmosphere of Mars is colder, less dusty, and cloudier than indicated by the established Viking climatology even for the Viking period. We also conclude that Mars atmospheric temperatures exhibit their most significant interannual variations during the perihelion dust storm season (10–20 K for LS = 200°–340°) and during the post-Aphelion northern summer season (5–10 K for LS = 100°–200°).
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some characteristics of the martian Aphelion global cloud belt
The Fifth International Conference on Mars, 1999Co-Authors: M J Wolff, R T Clancy, B A Whitney, P R Christensen, J C PearlAbstract:The presence of discrete condensate clouds on Mars is certainly not a new discovery, having been observed through most of the documented history of telescopic monitoring. Furthermore, spacecraft data have been used to study discrete cloud features in the Martian atmosphere in greater detail, e.g., morphology, seasonal occurrence. Condensate clouds, specifically discrete water ice clouds, appeared to be regarded as fairly common but, with the possible exception of the polar regions, generally uninteresting from a climatological point of view. However, recent observations indicate that in addition to their large spatial scale, the water ice clouds may in fact play a more prominent role in the Martian climate. In this paper, we wish to examine the spatial and temporal variations of the cloud belt optical depth, as well as the microphysical characteristics of the water ice particles themselves.
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water vapor saturation at low altitudes around mars Aphelion a key to mars climate
Icarus, 1996Co-Authors: R T Clancy, Michael J. Wolff, Philip B. James, Brad J Sandor, A W Grossman, D J Rudy, Y N Billawala, S W Lee, D O MuhlemanAbstract: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 (Ls= 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.
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annual perihelion Aphelion cycles in the photochemical behavior of the global mars atmosphere
Journal of Geophysical Research, 1996Co-Authors: R T Clancy, H NairAbstract:Ground-based temperature and water vapor profiling of the atmosphere of Mars point to large annual variations in the water vapor saturation altitude, or hygropause, of the entire low-to middle-latitude atmosphere, as forced by the elliptical Mars orbit [Clancy et al., 1996]. We examine the effects of such an annual variation in the hygropause altitude (from 30 km) on the photochemistry of the Mars atmosphere. The one-dimensional, diffusive transport photochemical model of Nair et al. [1994] is run in a diurnally averaged mode for time-dependent calculations of the annual behavior of Mars photochemistry at low to middle latitudes. The model incorporates a specified annual variation of the water vapor profile, based on the microwave observations of Mars water vapor and temperature profiles versus season (solar longitude, Ls). Due to their long photochemical lifetimes, Mars CO and O2 are expected and found to be in significant non-equilibrium with the annually varying water vapor (and hence HOx) densities, and to present nearly constant abundances representative of the annual average water vapor profile. In contrast, Mars O3 has a photochemical lifetime of hours, and exhibits very large annual variations in response to the annual variation in the hygropause altitude. The global-scale abundance of O3 at altitudes of 20–40 km is predicted to vary from >109 cm−3 around Mars Aphelion (northern spring/summer, Ls = 71°) to ∼108 cm−3 around Mars perihelion (southern spring/summer, Ls = 251°). These model ozone variations are discussed in the context of the disparate Mars 5 (1974 [Krasnopolsky and Parshev, 1979]) and Phobos (1989 [Blamont and Chassefiere, 1993]) measurements of low-latitude ozone densities at 35–50 km altitude, as well as Hubble space telescope observations of enhanced low-latitude ozone during the 1995 Aphelion of Mars [Clancy et al., this issue].