Solis Planum Research Articles

Morphometry and kinematics correlation of wrinkle ridges on Mars: Insights from Trishear modelling

Wrinkle ridges are among the most common and controversial compressional tectonic structures on terrestrial planets. While their origins are well inferred to be related to crustal shortening driven by compressional stress, their subsurface characterization is still a matter of debate. Open questions remain about the geometry, number, structural style and kinematics of faults promoting wrinkle ridges. We use the Trishear and Fault-Parallel-Flow integrated forward kinematic modelling to model wrinkle ridges related faults. This is achieved through a series of balanced cross sections and a consequent set of narrow 3D models. We perform a detail kinematic analysis on nine wrinkle ridges: six are located in the circum-Tharsis regions of Lunae Planum and Solis Planum, while three are located in the Hellas Planitia, Hesperia Planum and Syrtis Major Planum, respectively. The applied methodology allows us to quantitatively assess wrinkle ridges geometry and kinematics, and to correlate them with morphometric parameters (i.e., width and relief). Our results indicate how wrinkle ridges tectonics can be characterized by a more complex array of faults than previously modelled. This leads to a total amount of horizontal shortening accommodated differently depending on the number and type of faults (i.e., main fault, backthrust, synthetic faults). The location and geometry of the modelled faults suggest the presence of multiple detachments at different depths and with different mechanical behaviors such as weaker and more frictional décollements, which are likely found within sedimentary interlayers. The amount of shortening, the fault geometry and spacing, as well as the upper faults tips depth are positively correlated with major morphometric parameters of wrinkle ridges topography.

Neutron emission on the surface of Mars

The neutron emission of Mars is known to be produced due to the bombardment of Martian surface by Galactic Cosmic Rays (GCRs). As evidenced by numerical simulation, the intensity of neutron emission on the planetary surface is much larger than that of the neutron emission at the orbital altitude due to multiple reflections of the neutrons emitted from soil by the atmosphere. The code developed for simulations was validated using the experimental data of neutron emission acquired in the Gale crater by the RAD and DAN instruments aboard the NASA's Curiosity rover. Neutron emission was simulated for two Martian areas: the dry Solis Planum territory and the bottom of the Valles Marineris system of canyons, which is thought to be the most wet equatorial spot on Mars. For both areas a strong effect is found, showing a considerable difference between the neutron emission on the surface and at the altitude of a typical orbit. It was also shown that surface emission in these two areas is quite different due to the difference of mass fraction of water in the shallow subsurface. Finally, the seasonal variations of neutron emission in these areas due to the atmospheric seasonal changes are also estimated.

Defining Surface Types of Mars Using Global CRISM Summary Product Maps

AbstractFor many regions on Mars, the surface composition and its geological history have been debated in the literature. Because of the limited surface coverage of in situ measurements, either new data or new processing methodologies are required to get a better understanding of the Martian geology. This paper presents the results of a multivariate, unsupervised, analysis on underutilized CRISM (Compact Reconnaissance Imaging Spectrometer) multispectral mapping mode data set for surface type analysis. The devised summary products of Pelkey et al. (2007) and Viviano‐Beck et al. (2014) are averaged for the CRISM ~5° × 5° mosaic grids and used to analyze the variability in this data set. The averaged summary product values are studied using correlation coefficients calculated between summary products, as well as the correlation with dust coverage and elevation. The degree of correlation is used to interpret the summary products for the global distribution of mafic and secondary minerals and the effect of external factors such as ice, atmosphere, and dust coverage. With unsupervised clustering, all grid pixels are classified based on the spectral variability. These clusters are plotted as global maps and interpreted for geological variations in the CRISM data. Some clusters spatially correspond with previously recognized compositional distinct regions Northern Lowlands, Southern Highlands, Meridiani Planum, Syrtis Major, and Nili Fossae. Several other clusters are described here for the first time such as Solis Planum, Ophir Planum, and Hellas Basin. These regions cover known geological units, but the interpretation of the spectral variability is uncertain whether it relates to geological or external factors.

Outline of Structural Evolution of the Thaumasian Back-Arc Trap Province of Mars, and Related Rim -Orogenic Arcs: A Speculative Hypothesis Based on Earth Geology

The Thaumasian lava plateau developed within an already deformed domain, consisting of composite orogenic belts. A crude scheme of the deformation history of the Thaumasian plateau summarizing the most significant events of its polycyclic development is presented here. It is suggested that the region evolved in four tectonism phases: (1) alpinotype regional compression, formation of basement nappes (early and middle Noachian orogen); (2) late Noachian regional, domal uplift, and denudation of the internal fold belt; (3) late Noachian-early Hesperian supracrustal deformation, as décollement nappes; and (4) Hesperian crustal foundering and great effusion of trap basalts flooding of the inner, alreadypeneplain mountain belts, balanced by uplifting of peripheral arcuate rim-highlands (swells and mountain belts). The two orogenic arcs of the Coprates and Thaumasia -Claritas belts exhibit thrusting vergence and orogenic migration, E- and SW-directed, indicating the presence of topographic doming associated with plume activity, which was probably positioned, in the late Noachian, upon the modern Solis planum lowland. There is a substantial possibility that the two arcs followed different tectonic evolutions. The lateral intrusion of hot, mafic fluids in interconnected fractures, near the crust-mantle boundary, is supposed to have been an important driving mechanism of tectonism beneath the plateau region. The thick bodies of diapir-like, moving aqueous fluid were probably related to ascending deepmantle plumes.

Well‐preserved low thermal inertia ejecta deposits surrounding young secondary impact craters on Mars

AbstractFollowing the most recent updates to the Mars Odyssey Thermal Emission Imaging System daytime and nighttime infrared global mosaics, a colorized global map was produced that combines the thermophysical information from the nighttime infrared global mosaic with the morphologic context of the daytime infrared global mosaic. During the validation of this map, large numbers of low thermal inertia ejecta deposits surrounding small young impact craters were observed. A near‐global survey (60°N–60°S) identified 4024 of these low thermal inertia ejecta deposits, which were then categorized based on their apparent state of degradation. Mapping their locations revealed that they occur almost exclusively in regions with intermediate‐to‐high thermal inertias, with distinct clusters in northern Terra Sirenum, Solis Planum, and southwestern Daedalia Planum. High‐Resolution Imaging Science Experiment images show that the thermophysically distinct facies of the deposits are well correlated with the estimated average ejecta grain sizes, which decrease with radial distance from the crater. Comparisons with recent primary impact craters and secondary impact craters surrounding Zunil Crater show that the low thermal inertia ejecta deposits very closely resemble the secondary craters, but not the primary craters. We conclude that the low thermal inertia ejecta deposits are secondary impact crater ejecta deposits, many of which originated from the rayed crater primary impact events, and are both well preserved and easily identifiable due to the absence of dust cover and aeolian modification that would otherwise reduce the thermal contrast between the ejecta facies and the surrounding terrain.

Orbital monitoring of martian surface changes

A history of martian surface changes is documented by a sequence of global mosaics made up of Mars Global Surveyor Mars Orbiter Camera daily color images from 1999 to 2006, together with a single mosaic from the Mars Reconnaissance Orbiter Mars Color Imager in 2009. These observations show that changes in the global albedo patterns of Mars take place by a combination of dust storms and strong winds. Many of the observed surface changes took place along the tracks of seasonally repeating winter dust storms cataloged by Wang and Richardson (2015). These storms tend to sweep dust towards the equator, progressively shifting albedo boundaries and continuing surface changes that began before the arrival of MGS. The largest and most conspicuous changes took place during the global dust storm of 2001 (MY 25), which blanketed Syrtis Major, stripped dust from the Tharsis region, and injected dust into Solis Planum. High wind speeds but low wind stresses are predicted in Syrtis, Tharsis and Solis by the NASA Ames GCM. Frequent changes in these regions show that dust accumulations are quickly removed by stronger winds that are not predicted by the GCM, but may result from smaller-scale influences such as unresolved topography.

An assessment of the impact of local processes on dust lifting in martian climate models

Simulation of the lifting of dust from the planetary surface is of substantially greater importance on Mars than on Earth, due to the fundamental role that atmospheric dust plays in the former’s climate, yet the dust emission parameterisations used to date in martian global climate models (MGCMs) lag, understandably, behind their terrestrial counterparts in terms of sophistication. Recent developments in estimating surface roughness length over all martian terrains and in modelling atmospheric circulations at regional to local scales (less than O(100km)) presents an opportunity to formulate an improved wind stress lifting parameterisation. We have upgraded the conventional scheme by including the spatially varying roughness length in the lifting parameterisation in a fully consistent manner (thereby correcting a possible underestimation of the true threshold level for wind stress lifting), and used a modification to account for deviations from neutral stability in the surface layer. Following these improvements, it is found that wind speeds at typical MGCM resolution never reach the lifting threshold at most gridpoints: winds fall particularly short in the southern midlatitudes, where mean roughness is large. Sub-grid scale variability, manifested in both the near-surface wind field and the surface roughness, is then considered, and is found to be a crucial means of bridging the gap between model winds and thresholds. Both forms of small-scale variability contribute to the formation of dust emission ‘hotspots’: areas within the model gridbox with particularly favourable conditions for lifting, namely a smooth surface combined with strong near-surface gusts. Such small-scale emission could in fact be particularly influential on Mars, due both to the intense positive radiative feedbacks that can drive storm growth and a strong hysteresis effect on saltation. By modelling this variability, dust lifting is predicted at the locations at which dust storms are frequently observed, including the flushing storm sources of Chryse and Utopia, and southern midlatitude areas from which larger storms tend to initiate, such as Hellas and Solis Planum. The seasonal cycle of emission, which includes a double-peaked structure in northern autumn and winter, also appears realistic. Significant increases to lifting rates are produced for any sensible choices of parameters controlling the sub-grid distributions used, but results are sensitive to the smallest scale of variability considered, which high-resolution modelling suggests should be O(1km) or less. Use of such models in future will permit the use of a diagnosed (rather than prescribed) variable gustiness intensity, which should further enhance dust lifting in the southern hemisphere in particular.

Surface mineralogy of Martian low‐albedo regions from MGS‐TES data: Implications for upper crustal evolution and surface alteration

Mars Global Surveyor Thermal Emission Spectrometer (MGS‐TES) data are used to derive the modal mineralogy of spectrally distinct Martian low‐albedo regions and to identify spatial trends in mineralogic assemblages. Results from this work are consistent with the major results of previous spectroscopic studies: (1) Plagioclase and clinopyroxene are the dominant minerals of most southern highlands regions, (2) the northern plains exhibit the lowest pyroxene abundance within Martian low‐albedo regions, and (3) the highest concentrations of high‐silica phase(s) are found in the northern plains, Solis Planum and a few southern high‐latitude regions. Low‐albedo regions may be classified into four units on the basis of relative abundances of plagioclase, pyroxene, and high‐silica phase(s). Unit distributions between ±45° latitude exhibit moderate correlation with distinct provinces (e.g., Syrtis Major, Aonium Sinus) defined by large‐scale morphology, elevation, and to some extent, surface age, suggesting that the spectral and compositional differences between these units are more strongly controlled by original bedrock mineralogy than by surface‐atmosphere interactions and alteration. Syrtis Major exhibits a difference in mineralogy from the surrounding highlands suggesting a differing degree of fractional crystallization, assimilation, or source region composition. Areas with thick crust near the Tharsis Plateau exhibit lower abundances of olivine and greater plagioclase/pyroxene ratios than surrounding highland terrains, suggesting that magmas in this region may have undergone increased olivine fractionation. Regions where surface alteration is more likely to be the primary control on observed spectral signatures are the high‐latitude areas (>45°), where globally, surfaces dominated by high‐silica phase(s) are most commonly found.

Evidence for a differentiated crust in Solis Planum, Mars, from lithospheric strength and heat flow

Two independent sets of heat flow estimates provide constraints on the Hesperian-era surface and mantle heat flows, and the thickness of the heat-producing elements (HPE)-enriched upper crust, in the Solis Planum region of Mars. The calculations, which use the concentration of uppermost crust heat sources deduced from orbital gamma ray spectroscopy and soils geochemistry, are based on the effective elastic thickness of the lithosphere and the minimum depth of faults underlying winkle ridges. We find that, for the majority of analyzed settings, the HPE-enriched crust is thinner than the whole crust thickness in this region (∼65 km). Thus, our results strongly support a differentiated martian crust.

Elastic dislocation modeling of wrinkle ridges on Mars

Wrinkle ridges are one of the most common landforms on Mars. Although it is generally agreed that they are compressional tectonic features formed by folding and thrust faulting, there is no consensus on the number of faults involved, the geometry of the faults, or the maximum fault depth. The topography of martian wrinkle ridges in Solis Planum and Lunae Planum has been studied using MOLA data. As determined in previous studies, the topography shows that most wrinkle ridges are a composite of two landforms, a broad low relief arch and a superimposed ridge. Constrained by MOLA topographic profiles, the geometry and parameters of the faults associated with wrinkle ridges have been modeled. The best fits are obtained with a blind listric thrust fault that flattens into a décollement. The listric fault geometry is approximated by a series of linear connecting segments with varying dips. The major morphologic elements of wrinkle ridges can be matched by varying the displacement on the different fault segments. Modeling of large-scale wrinkle ridges indicates that the maximum depth of faulting or depth to the décollement is about 4.5 km. This may correspond to the depth of the contact between the ridged plains volcanic sequence and the underlying megabreccia. The results suggest that wrinkle ridge thrust faults are shallow-rooted and reflect thin-skinned deformation.

Spatial inhomogeneity of the martian subsurface water distribution: implication from a global water cycle model

In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface–atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution is largely consistent with the modeled global water cycle, so a large fraction of hydrogen is likely to exist as water, at low and mid latitudes in the form of adsorbed water. Under the present climate the water content in the shallow subsurface becomes higher in the northern hemisphere than in the southern hemisphere as a result of global water cycle, regardless of the initial water distribution in the soil or adsorptive capacity. The higher annual maximum soil temperature in the south, stronger net northward transport of atmospheric water vapor, and the emission of vapor from the northern residual polar cap in northern summer contribute to this hemispheric asymmetry. The generally higher adsorptive capacity of clay minerals in the northern plains may further increase this bias. The longitudinal inhomogeneity is caused by several factors, such as thermal inertia, adsorptive capacity, and atmospheric surface pressure. The water abundance is locally high in low thermal inertia regions (e.g., Arabia Terra) and at deep places where the surface pressure is high (e.g., Hellas); it is low in soil with a low adsorptive capacity (e.g., Tharsis) and high thermal inertia regions (e.g., Solis Planum). Most of the soil humidity near the surface at low and mid latitudes may originate from the atmosphere. The model implies that the upper soil layer should be largely ice-free because otherwise an excessive sublimation and vapor emission into the atmosphere in warm seasons would violate the observational constraints. Moreover, the more uniform latitudinal variation of the observed hydrogen abundance near the surface compared to that of deeper layers is indicative of the presence of adsorbed water instead of ground ice because the adsorbed water content does not as steeply depend on latitude as the ground ice stability. Concerning the regolith mineralogy, montmorillonite can much better account for the observed water cycle than palagonite. While the presence of permanent ground ice appears likely in the polar region below a thin layer, large seasonal cycle of phase change between pore ice and adsorbed water may be possible. Regolith adsorption/desorption is neither negligible nor crucial for the seasonal atmospheric water cycle, but the surface–atmosphere coupling is a major prerequisite for the long-term evolution of subsurface water distribution.

Clues to the lithospheric structure of Mars from wrinkle ridge sets and localization instability

Wrinkle ridges are a manifestation of horizontal shortening in planetary lithospheres, for which deformation is localized on faults that underlie individual ridges. In ridged plains of Mars, such as Solis Planum or Lunae Planum, wrinkle ridges are spaced ∼40 km apart, whereas in the Martian northern lowlands, where ridges are identified only in Mars Observed Laser Altimeter (MOLA) altimetric data, the ridge spacing is at least ∼80 km. We attribute ridge spacing to an instability of the lithosphere under horizontal compression. The localization instability, which results in periodically spaced faults [Montési and Zuber, 2003a], links the difference of ridge spacing in the northern lowlands and in the highland ridged plains to the difference of crustal thickness via the depth of the brittle‐ductile transition (BDT). In Solis and Lunae Plana, where the crust is 50 to 60 km thick, the crust may be ductile at depth, limiting faulting to the BDT of crustal rocks. In the lowlands, the crust is only about 30 km thick and may be brittle throughout. Thus the depth of faulting may be controlled by the BDT of mantle rocks, which is roughly a factor of two deeper than that of crustal rocks. The geotherm can be identical in both regions, at 12 ± 3 K.km−1, although differences of a few K.km−1 can be accommodated within this model. The heat flux implied by this geotherm is similar to the heat produced by radiogenic decay 3 Gyr ago. Our analysis provides a rheological explanation for the difference in spacing between ridges in the highlands and the lowlands, in contrast to the suggestion of Head et al. [2002], who proposed that alternating lowlands ridges are buried by sediments. In addition, finite element models that use the lithospheric structure deduced from ridge spacing show that modest gradients of crustal thickness or heat flux across a ridged plains favor the formation of faults dipping toward high‐elevation areas, as may be the case in Solis Planum [Golombek et al., 2001].

Global mineral distributions on Mars

Determining the mineralogy of Mars is an essential part of revealing the conditions of the surface and subsurface. A deconvolution method was used to remove atmospheric components and determine surface mineralogy from Thermal Emission Spectrometer data at 1 pixel per degree (ppd). Minerals are grouped into categories on the basis of compositional and spectral similarity, and global concentration maps are produced. All binned pixels are fit well with RMS errors of ≤0.005 in emissivity. Higher RMS errors are attributed to short wavelength particle size effects on dust‐covered surfaces. Significant concentrations (>0.10) of plagioclase, high‐Ca pyroxene, sheet silicates/high‐Si glass, and hematite are detected and display distributions consistent with previous studies. Elevated concentrations of plagioclase and high‐Ca pyroxene are consistent with basaltic surfaces and are located in low‐albedo highlands regions north of ∼45°S. Significant concentrations of plagioclase and sheet silicates/high‐Si glass and low concentrations of high‐Ca pyroxenes are consistent with andesitic surfaces and are concentrated in both southern and northern high‐latitude, low‐albedo regions. Andesitic surfaces in the southern hemisphere have a lower spectral contrast than northern surfaces. An isolated surface located in Solis Planum is spectrally distinct but compositionally similar to other surfaces interpreted to be andesitic in composition. Concentrations of olivine below the detection limit correctly identify its presence in two of three locations. Potassium feldspar, low‐Ca pyroxene, basaltic glass, olivine, sulfate, carbonate, quartz, and amphibole are not detected with confidence at 1 ppd. The results presented here indicate a predominance of volcanic compositions within Martian dust‐free surfaces.

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.

Martian wrinkle ridge topography: Evidence for subsurface faults from MOLA

Mars Orbiter Laser Altimeter (MOLA) topography shows distinct elevation offsets across wrinkle ridges that can be explained simply by subsurface thrust faults. MOLA has provided the first reliable topographic data of wrinkle ridges to test models for their origin; although previous work has shown that wrinkle ridges result from compressional folding and faulting of near‐surface units, the role of thrust faulting and its depth penetration have been argued. Topographic profiles across wrinkle ridges in Solis Planum, Lunae Planum, Tempe Terra, Xanthe Terra, Arcadia, Terra Sirenum, Thaumasia Planum, Arabia Terra, Syrtis Major, and Hesperia Planum show characteristic features such as superposed hills, crenulations, and elevation offsets between the plains on either side of the ridge. The characteristic elevation offsets between plains surfaces on either side of the ridges shown by MOLA are easily explained by subsurface thrust faults that underlie the ridges and produce the offset. In Solis Dorsa, wrinkle ridges are 10–20 km wide (well resolved by the 300 m spaced MOLA elevations), have a total relief of 80–250 m, and have elevation offsets of 50–180 m (well above the uncertainty in the MOLA elevations). MOLA topography shows that the plains decrease in elevation toward the southeast and are deformed into folds or arches not visible in images that are parallel to and in between the ridges. The elevations of the plains on either side of the ridges in Solis and Lunae Plana are characteristically down to the southeast and east, respectively, suggesting that faults beneath the wrinkle ridges offset the plains. This offset indicates underlying thrust faults that dip to the northwest and west, respectively, and systematically lower the southeast side of the plains. The observation that the elevation offsets across the ridges are maintained to the next ridge implies that the thrust fault penetrates to depths of tens of kilometers and thus well into the mechanical lithosphere. Small foreland basement uplifts on Earth, in which slip on shallowly dipping thrust faults in the crystalline basement is accommodated by folding in the overlying sedimentary rocks, appear to be broadly analogous to these groups of wrinkle ridges and suggest that they are examples of thick‐skinned compressional deformation on Mars. These observations are not consistent with models for the formation of wrinkle ridges that involve folding of near‐surface layers only or subsurface thrust faults that flatten into decollements at shallow depths.

Indications of sulfate minerals in the Martian soil from Earth‐based spectroscopy

Telescopic measurements of Mars between 4.40 and 5.13 μm at a spectral resolution (λ/Δλ) of 300 were made on August 19, 1988, UT at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. This wavelength region contains radiation from both solar reflection and thermal emission. Additionally, the Martian atmosphere has numerous atmospheric gas absorption features, dominated by the 4.2‐μm CO2 fundamental band. The Mars spectrum rise out of the 4.2 to 4.4‐μm CO2 band on the long‐wavelength side cannot be matched solely by atmospheric gas constituents. An absorption must be added at roughly 4.5 μm in order to decrease the reflectance rise and produce the 4.5‐μm inflection which is present in the data. The location of this feature at the position of the 2v3 overtone of the SO42− anion indicates that the surface absorption is probably caused by sulfates on the Martian surface and/or in atmospheric dust. This is consistent with the known presence of sulfates on the Martian surface from the Viking results. An exact spectral match to a terrestrial sulfur mineral has not been made, but we suggest that the mineral on Mars has very weak band structure, probably due to an ion environment in the mineral with a high degree of electric field symmetry. Significant variation exists at 4.5 μm among the observed spectra for different locations on Mars. In order of strength, from strongest absorption to weakest, are Eastern Solis Planum, Argyre Basin, Eastern Tharsis, and Valles Marineris for the four regions measured at similar Mars atmospheric conditions.

Mass of dust in the Martian atmosphere

Estimates of the mass of dust suspended in the Martian atmosphere are derived from global and regional 9‐μm opacity maps produced from Viking Infrared Thermal Mapper data. During the peak of the 1977b storm, a total dust mass of approximately 4.3 × 1014 g was suspended, equivalent to 4.3 × 10−4 g/cm2, or a layer 1.4 μm thick. During a local dust storm near Solis Planum at LS 227°, approximately 1.3 × 1013 g of dust were lofted, equal to about a 6‐μm layer in that vicinity.

A simple method for planetary surface ruggedness estimation

The method for mapping the pixel-scale and larger ruggednesses of the Martian ridged plains within Solis Planum is described by assuming the Lommel-Seeliger's reflection law. This method allows surface tilt angles towards and away from the Sun to be estimated and topographic elevations to be displayed by the aid of monoscopic digital image data. The restrictions of this method are also discussed.