Title: Martian surface winds: Insensitivity to orbital changes and implications for aeolian processes
Authors: Fenton, Lori K.; Richardson, Mark I.
Journal: Journal of Geophysical Research, Volume 106, Issue E12, p. 32885-32902
Publication Date: Dec 2001
Origin: AGU
Keywords: Meteorology and Atmospheric Dynamics: Planetary meteorology, Planetology: Solid Surface Planets: Atmospheres-evolution, Planetology: Solid Surface Planets: Erosion and weathering, Planetology: Solar System Objects: Mars
Abstract Copyright: (c) 2001: American Geophysical Union
DOI: http://dx.doi.org/10.1029/2000JE001407http://bit.ly/ThiNBq
Bibliographic Code: 2001JGR...10632885F
Abstract: Aeolian features observed on the surface of Mars provide insight into current, and potentially past, surface wind systems. In some cases the features are clearly transient and related to the lifting and settling of atmospheric dust. Other features, like dunefields, yardangs, and ventifacts, are more persistent and likely require significant time to form. In this study we analyze the observed directions of selected aeolian features with the aid of the Geophysical Fluid Dynamics Laboratory Mars general circulation model (GCM). Initially, we examine bright and dark streaks which have been observed to form in association with global dust storms. The ability to match these features with Mars GCM wind directions provides an important validation of the model. More important, we are able to define best fit seasons and local times for both types of features which provide the basis for extension and modification of the Veverka et al. [1981] model of bright and dark dust streak formation. In addition these best fit times correspond well with the dark streak ``wind storm'' model of Magalhães and Young [1995]. The primary focus of this paper is to provide constraints on the range of mechanisms proposed to explain inconsistencies between current wind direction patterns and long-term wind indicators (for example, the misalignment of rock tail and ventifact orientations at the Mars Pathfinder landing site). Specifically, we assess whether changes in planetary obliquity, precession, or global dust opacity could significantly alter patterns of surface wind directions. In all cases we find the seasonal and annual average wind direction patterns to be highly invariable. While changes in the dust loading (hence the partitioning of solar absorption between the surface and atmosphere) and in the surface latitude of maximum solstitial insolation cause the vigor of the large-scale circulation to increase (especially the Hadley cell), the spatial patterns of the surface wind orientations remain essentially unchanged. In the case of perihelion during northern summer (opposite of the current perihelion position), the northern summer Hadley cell remains weaker than the southern summer cell despite the strengthened heating in the northern hemisphere. Taken together, these results cast significant doubt on orbital explanations for surface wind changes. It is thus suggested that significant changes in topography (e.g., Tharsis uplift, true polar wander) or climate (e.g., the existence of a significantly thicker atmosphere or an ocean at some point in the past) are more likely explanations for long-term wind indicators such as the ventifact orientations at the Mars Pathfinder landing site.


Title: The Martian atmosphere during the Viking mission. I. Infrared measurements of atmospheric temperatures revisited.
Authors: Wilson, R. J.; Richardson, M. I.
Journal: Icarus, Vol. 145, No. 2, p. 555 - 579
Publication Date: Jun 2000
Origin: ICAR
Keywords: Mars Atmosphere: Temperatures
DOI: http://dx.doi.org/10.1006/icar.2000.6378http://bit.ly/S8T7YW
Bibliographic Code: 2000Icar..145..555W
Abstract: The Viking Infrared Thermal Mapper 15-μm channel brightness temperature observations (IRTM T15) provide extensive spatial and temporal coverage of Martian atmospheric temperatures on diurnal to seasonal time scales. The 15-μm channel was designed so that these temperatures would be representative of a deep layer of atmosphere centered at 0.5 mb (≡25 km). A re-examination of the IRTM data indicates that the 15-μm channel was additionally sensitive to surface radiance so that air temperature determinations (nominal T15) are significantly biased when the thermal contrast between the surface and atmosphere is large. This bias is suggested by the strong correlation between the diurnal variation of tropical T15 and surface temperatures for non-dust-storm conditions. The authors show that numerical modeling of the thermal tides provides a basis for distinguishing between the surface and atmospheric contributions to IRTM T15 and thus allows the atmospheric component to be estimated. The resulting bias amounts to a ≡15-K offset for midday atmospheric temperatures at subsolar latitudes during relatively clear periods and is negligible at night.
Title: Seasonal variation of aerosols in the Martian atmosphere
Authors: Toigo, Anthony D.; Richardson, Mark I.
Journal: Journal of Geophysical Research, Volume 105, Issue E2, p. 4109-4122
Publication Date: Feb 2000
Origin: AGU
Keywords: Atmospheric Composition and Structure: Aerosols and particles, Planetology: Solid Surface Planets: Meteorology, Planetology: Solar System Objects: Mars
Abstract Copyright: (c) 2000: American Geophysical Union
DOI: http://dx.doi.org/10.1029/1999JE001132http://bit.ly/TpUKni
Bibliographic Code: 2000JGR...105.4109T
Abstract: Reanalysis of Viking Lander (VL) visible and Viking Orbiter infrared optical depth measurements shows that the visible to infrared ratio of total extinction opacity varies with season. The ratio is near to its previously reported constant value, 2.5, during dust storm periods and higher during northern spring and summer. The increase in ratio is hypothesized to be due to seasonally varying water ice haze, which produces a higher optical depth in the visible than in the infrared. This differs significantly from previous analyses of VL visible opacities which have assumed that only dust contributes to the optical depth measured during the early afternoon. Consequently we suggest that the Martian atmosphere is clearer of dust, especially during northern spring and summer, than previously suggested based upon VL data. We find dust visible optical depths of 0.1-0.4 during the northern spring and summer seasons, compared to previous estimates of 0.4-0.6. We also find that water ice hazes can provide roughly 50% of the total visible opacity in these seasons. For southern spring and summer, dust optical depths are more variable, but generally >=0.4, with water ice opacity <=0.1. The data suggest water ice optical depths are slightly higher and peak earlier (Ls=80°-90°) at VL1 than at VL2 (Ls=115°-130°). We estimate average northern summer water (daytime minimum) ice masses to be roughly 0.1-0.5 precipitable microns, depending on the assumed particle size distribution and hence 1-5% of the total water column. The observation of significant and previously unrecognized amounts of water ice haze suggests a larger role for water in controlling atmospheric heating rates and the vertical distribution of dust and water vapor than has been widely accepted to date.


Title: A general circulation model study of the Mars water cycle
Authors: Richardson, Mark Ian
Journal: Thesis (PhD). UNIVERSITY OF CALIFORNIA, LOS ANGELES, Source DAI-B 60/04, p. 1652, Oct 1999, 303 pages.
Publication Date: Oct 1999
Origin: UMI
Bibliographic Code: 1999PhDT.........3R
Abstract: This study has focused on the development and application of the first fully three-dimensional model of the current Mars water cycle. Previous models of the water cycle have suggested the importance of transport processes in determining the observed variations in atmospheric water [Jakosky and Haberle (1992)]. This work addresses questions regarding the relative importance of water reservoirs, transport of water, control of global average vapour amounts, and the importance of clouds. The results of this work show that model transport out of the northern polar region during summer occurs primarily in surface forced, zonally asymmetric currents. Significantly more hemispheric transport is predicted than expected with zonal average models. Transport capacity is higher for south to north flow during southern summer than for north to south flow in northern summer. Houben et al. (1997) suggested that a model without regolith would ``flood'' with vapour. Our model does riot show this behaviour, thus the regolith may not provide sole control over global vapour amounts. The mechanism of ``equilibration'' for the model without regolith pivots on an annual average vapour flux balance across the northern high-latitude/polar latitude boundary. However, as there is always net loss to the south polar cold tap the ``equilibrium'' is only approximate. Simulations suggest that an exposed southern water cap would be unstable with respect to the northern cap. Comparison with zonal-average vapour data suggests that the residual cap provides <= 40% of the vapour observed to accrue after L S = 80°. Simulations employing seasonal ice show improved ``fits,'' but additional (regolith) sources are needed. As suggested by Kahn (1990), agreement with data requires cloud ice precipitation. Precipitation allows water to be removed from a cold atmosphere more rapidly than diffusion of ice or vapour. Simulations also suggest that cloud formation may reduce interhemispheric water transport [Clancy et al. (1996)]. The model evolution of zonal-average vapour distributions is in rough agreement with data. Cloudiness is generally overpredicted, likely due to defects in the cloud microphysics scheme. The spatial distribution of clouds compare reasonably well with the limited observations, however errors do occur in the tropics and winter hemisphere. These likely result from errors in the vertical wind field and local scale vapour transport. Differences between the true and model topography and surface temperatures are the most likely causes. Detailed modeling of vapour transport will require very close attention to the surface prescription.
Title: Comparison of microwave and infrared measurements of Martian atmospheric temperatures - Implications for short-term climate variability
Authors: Richardson, Mark I.
Affiliation: AA(California, Univ., Los Angeles)
Journal: Journal of Geophysical Research, vol. 103, p. 5911
Publication Date: Mar 1998
Origin: STI
Bibliographic Code: 1998JGR...103.5911R
Abstract: This paper presents the first comparison of simultaneous Viking infrared and ground-based microwave measurements of the Martian atmosphere. The data are examined in order to investigate a 15-20 K difference between microwave and Viking measurements of mid-level (10-40 km) air temperature. These data have been used by Clancy et al. (1990) to suggest that the Martian atmosphere is generally cooler and clearer than observed during the Viking era. This study suggests that the 15-20 K difference, which is most apparent during the non-'dust-storm' seasons, is not a real temperature difference, but instead results from a disagreement between the measurement techniques. The existence of this instrumental bias implies that the Martian climate has not substantially changed since the Viking era.
Title: New dust opacity mapping from Viking Infrared Thermal Mapper data
Authors: Martin, T. Z.; Richardson, M. I.
Affiliation: AA(JPL, Pasadena, CA), AB(Imperial College of Science, Technology, and Medicine, London, England)
Journal: Journal of Geophysical Research (ISSN 0148-0227), vol. 98, no. E6, p. 10,941-10,949.
Publication Date: Jun 1993
Origin: STI; LPI [AN-930515%J]
Bibliographic Code: 1993JGR....9810941M
Abstract: Global dust opacity mapping for Mars has been carried forward using the approach described by Martin (1986) for Viking IR Thermal Mapper data. New maps are presented for the period from the beginning of Viking observations, until Ls 210 deg in 1979 (1.36 Mars years). This range includes the second and more extensive planet-encircling dust storm observed by Viking, known as storm 1977b. Improvements in approach result in greater time resolution and smaller noise than in the earlier work. A strong local storm event filled the Hellas basin at Ls 170 deg, prior to the 1977a storm. Dust is retained in equatorial regions following the 1977b storm far longer than in mid-latitudes. Minor dust events appear to raise the opacity in northern high latitudes during northern spring. Additional mapping with high time resolution has been done for the periods of time near the major storm origins in order to search for clues to the mechanism of storm initiation. The first evidence of the start of the 1977b storm is pushed back to Ls 274.2 deg, preceding signs of the storm in images by about 15 hours.