Topography and Geologic Characteristics of Aeolian Grooves in the South Polar Layered Deposits of Mars

The topographic and geologic characteristics of grooves and groove-like features in the south polar layered deposits near the Mars Polar Lander/Deep Space 2 landing sites are evaluated using Mariner 9 images and their derived photoclinometry, normalized using Mars Orbiter Laser Altimeter data. Although both Mariner 9 and Viking images of the south polar layered deposits were available at the time of this study, Mariner 9 images of the grooves were selected because they were generally of higher resolution than Viking images. The dimensions and slopes of the grooves, together with orientations that nearly match the strongest winds predicted in the Martian Global Circulation Model and directions inferred from other wind indicators, suggest that they formed by aeolian scour of an easily erodible surface. Most grooves are symmetric and V-shaped in transverse profile, inconsistent with an origin involving extensional brittle deformation. Although the grooves strike along slopes and terraces of the south polar layered deposits, the variable depths and lack of terracing within the grooves themselves indicate that any stratigraphy in the uppermost 100 m of the polar layered deposits is composed of layers of similar, and relatively low, resistance. The grooves do not represent landing hazards at the scale of the Mariner 9 images (72-86 m/pixel) and therefore probably would not have affected Mars Polar Lander and Deep Space 2, had they successfully reached the surface.     

North-south asymmetry of Eolian features in martian polar regions: analysis based on crater-related wind markers

A comparison of crater-related wind markers in the north and south polar (40–90° latitude) regions of Mars has been made on the basis of comprehensive mapping from Viking Orbiter and Mariner 9 Images. Wind streaks show that present wind activity is most effective in both north and south in the southern spring and summer. This asymmetry is consistent with the present asymmetry of climate. The more massive intracrater dune fields are also oriented with the presently strongest winds. This alignment may reflect a long-term asymmetry in the effectiveness of northern and southern spring flow because reorientation times far exceed the period of cycles of hemispherical climate asymmetry, ≈51, 000 years. Streaks originating from dark crater splotches indicate that windflow away from the south pole is effective over a larger latitude range than it is in the north. This difference may be partly responsible for the contrasting distribution of dune sand in the north and south polar regions.     

Mariner 9 ultraviolet spectrometer experiment: Vertical distribution of ozone on Mars

The vertical distribution of ozone in the atmosphere of Mars is computed from ultraviolet spectra obtained by the Mariner 9 spacecraft. In the Northern Hemisphere the ozone scale height is much smaller than the atmospheric scale height in midlatitudes and increases rapidly to a maximum farther north. At high latitudes (above 60°) there is no significant difference between the scale heights of ozone in the Northern (winter) Hemisphere and the Southern (summer) Hemisphere. Comparison of the ozone distribution with atmospheric temperature structure indicates that at some locations in the North, the density of water vapor increases with altitude, and the time for vertical mixing is about 3 days or more.     

An orthographic photomap of the south pole of Mars from Marina 7

Television pictures of the south polar regions of Mars obtained by the Mariner 6 and 7 spacecraft in 1969 are rectified to a standard mapping projection using computer image processing techniques. Mosaicking of these pictures produces the first photomap of the entire south polar cap.     

Mars: VLA observations of the Northern Hemisphere and the north polar region at wavelengths of 2 and 6 cm

Observations of Mars at wavelengths of 2 and 6 cm were made using the VLA in its A configuration. The season on Mars was late spring in the Northern Hemisphere (L_s = 60°). The sub-Earth latitude was 25°N, so the geometry for viewing the north polar region was optimal. Whole-disk brightness temperatures were estimated to be 193.2 ± 1.0°K at 2 cm and 191.2 ± 0.6°K at 6 cm (formal errors only). Since measurements of the polarized flux were taken at the same time, whole-disk effective dielectric constants could be estimated and from these estimates of subsurface densities could be made. The results of these calculations yielded a whole-disk effective dielectric constant of 2.34 ± 0.05, which implied a subsurface density of 1.24 ± 0.11 g cm⁻³ at 2 cm. The same calculations at 6 cm yielded an effective density of 1.45 ± 0.10 g cm⁻³ and dielectric constant of 2.70 ± 0.10. From the mapped data these parameters were also estimated as a function of latitude between latitudes of 15°S and 60°N. In addition to the effective dielectric constant and subsurface density, the radio absorption length of the subsurface was estimated. The radio absorption length for most of these latitudes was about 15 wavelengths with formal errors on the order of 5 or 10 wavelengths. The estimation of the effective dielectric constant at most latitudes was between 2 and 3.5 with only slight differences between the two different wavelengths. These estimates of the dielectric constant lead to estimation of the subsurface densities as a function of latitude. Most calculations of the subsurface density yielded results between 1 and 2 g cm⁻³ with errors on the order of 0.5 g cm⁻³. These results seem to imply that the subsurface is not much different than the surface as observed by the Viking and Mariner missions. In line with this, a comparison of the correlation of the dielectric constant at each wavelength with the thermal inertia determined from infrared measurements of the surface temperature shows that the correlation at 2 cm is slightly stronger than the correlation at 6 cm. Since the 2-cm radiation comes from a region closer to the surface than the 6-cm radiation, this decrease in correlation with depth is consistent with the idea that the physical makeup of the subsurface is varying slowly in the near subsurface region.     

Variability of the south polar cap of Mars in Mars years 28 and 29

We have used Mars Reconnaissance Orbiter data from 2007 and 2009 to compare summer behaviors of the seasonal and residual south polar caps of Mars in those two years. We find that the planet-encircling dust storm that occurred in the first of the two Mars years enhanced the loss of seasonal CO₂ deposits relative to the second year but did not have a large effect on the continuing erosion of the pits and mesas within the residual cap materials. This suggests that the increase of bright frost in some regions of the residual cap observed between Mariner 9 and Viking can be accommodated within observed martian weather variability and does not require unknown processes or climate change.     

Mars synthetic topographic mapping

Topographic contour maps of Mars are compiled by the synthesis of data acquired from various scientific experiments of the Mariner 9 mission, including S-band radio-occulation, the ultraviolet spectrometer (UVS), the infrared radiometer (IRR), the infrared interferometer spectrometer (IRIS) and television imagery, as well as Earth-based radar information collected at Goldstone, Haystack, and Arecibo Observatories. The entire planet is mapped at scales of 1:25,000,000 and 1:25,000,000 using Mercator, Lambert, and polar stereographic map projections. For the computation of map projections, a biaxial spheroid figure is adopted. The semimajor and semiminor axes are 3393.4 and 3375.7 km, respectively, with a polar flattening of 0.0052. For the computation of elevations, a topographic datum is defined by a gravity field described in terms of spherical harmonics of fourth order and fourth degree combined with a 6.1-mbar occulation pressure surface. This areoid can be approximated by a triaxial ellipsoid with semimajor axes of A = 3394.6 km and B = 3393.3 km and a semiminor axis of C = 3376.3 km. The semimajor axis A intersects the Martian surface at longitude 105°W. The dynamic flattening of Mars is 0.00525. The contour intercal of the maps is 1 km. For some prominent features where overlapping pictures from Mariner 9 are available, local contour maps at relatively larger scales were also compiled by photogrammetric methods on stereo plotters.     

A Martian yellow cloud—July 1971

A short-term yellow cloud was observed in the southern hemisphere of Mars from July 10 to 22, 1971. The initial cloud was first photographed on the morning limb at 7:45am local Martian sun time in the Serpentis-Hellespontus region (315°W; 27°S), 74 Earth days (72 Martian days) prior to the onset of the September major yellow storm. Historical observations from the Lowell Observatory archives indicate that prestorm yellow clouds of limited extent are not uncommon.Positional measurement and photographic differential photometry of the July yellow cloud were possible throughout its entire evolution because Mars was favorably situated and photographic coverage by the International Planetary Patrol was adequate. Maps showing the cloud's initial location, hourly and diurnal behavior, apparent motion, and areal coverage by haze are presented. The similarities between the July cloud and other southern hemisphere yellow clouds are summarized. Its relationships to classical albedo features and Mariner 9 topographic data are discussed. Photographic photometry indicates that the enhanced contrast between the yellow cloud and its surroundings is probably due more to the brightening of the cloud-covered areas than to any darkening of contiguous areas.     

Secular acceleration of Phobos and Q of Mars

Reductions of Mariner 9 TV data of Phobos and Deimos tend to corroborate the existence of a secular acceleration of Phobos commensurate with two recently reported values based on a reprocessing of Earth-based data. These values of secular acceleration have been used together with Mariner 9 data on the physical size of Phobos and Earth-based photoelectric observations which infer a carbonaceous composition for Phobos to place bounds of 50 < Q < 150 on the tidal dissipation function of Mars. The corresponding bounds on the tidal lag angle are 0.19° < Φ < 0.57°.     

Mariner 9 mission profile and project history

Mariner 9 was inserted into orbit about Mars on November 14, 1971, to study that planet for a period of 90 days. Observations and measurements made by Mariner 9 continued beyond the planned 90 days, providing data to meet all science objectives. The new knowledge of Mars gained from this mission has made obsolete all previous concepts of Mars.A general background of the Mariner Mars 1971 Project and the significant events of the Mariner 9 mission are presented.     

On the nature and visibility of crater-associated streaks on Mars

R. O. Kuzmin has proposed that all crater-associated wind streaks on Mars are depositional and consist of unresolved barchan-like dunes. He claims that any streak can appear either bright or dark relative to its surroundings depending on the azimuth of the Sun relative to the streak axis and on the elevation of the Sun above the horizon. Our studies of the entire Mariner 9 picture collection as well as of available Viking data lend no support to these ideas. We find that the conditions for visibility of bright and dark streaks are identical. In Mariner 9 images both types of streaks are visible for viewing angles ? ? 60°, illumination angles of 15° ? i ? 75°, and over the whole range of phase angles covered (about 15 to 85°). There are numerous examples of dark and light streaks visible at the same azimuth angle of the Sun, contrary to Kuzmin's claim. There is much evidence to indicate that bright and dark streaks differ both in morphology and in character. The common ragged dark streaks are probably erosion scars, while most bright streaks probably represent accumulations of bright dust-storm fallout. There is no evidence at present that these accumulations have a barchan-like texture.     

Mariner 9 photometric observations of Mars from November 1971 through March 1972

The large quantity of Mariner 9 television pictures taken at phase angles ranging up to twice those accessible from Earth has been used to describe intergrated photometric properties of Mars. Although frame-to-frame variations emphasize the shortcomings of vidicon cameras when used in a photometric mode, statistical trends have yielded data comparable with Earth-based observations.Analysis of atmospheric parameters over a period of changing opacity, November 1971 through March 1972, has provided time-varying optical depths and light-scattering information. Linear function characterization of the Mars reflectance is clearly discrepant at large incidence or emission angles and at phase angles greater than 40°.     

Mariner 9 ultraviolet spectrometeer experiment: 1971 Mars' dust storm

The Mariner 9 ultraviolet spectrometer observed the brightness of a region on the south polar cap centered at approximately ?87°S, 10°W. Measurements taken at various incidence and emission angles (i and ?) show that the brightness increased with decreasing air mass, ≈(sec i + sec ?). The observed intensity consists primarily of a component reflected from the cap and twice-attenuated by the atmosphere and a component diffusely reflected from the atmosphere. The diffusely reflected component was determined from nearby observations of non-polar regions at the same incidence and emission angles and was substrated from the total intensity. Inversion of the intensity difference using a formula analogous to the Bouger-Langley law yielded the optical thickness of the atmosphere. The dust cloud over the polar cap was moderately thick between November 26 and December 2, 1971. At this time the optical thickness was near unity, and it decreased approximately linearly with time, reaching a value close to that of a Rayleigh atmosphere by mid-February. The optical thickness showed little dependence on the wavelength during the early orbital observations. As the dust storm cleared, the atmospheric optical thickness exhibited increasingly strong inverse wavelength dependence. Particles large compared with the wavelength dominated the Martian dust storm. These particles are estimated to have a mean radius of about 2 μm.     

Gravity field of Mars from Mariner 9 tracking data

Further reduction of Doppler tracking data from Mariner 9 confirms our earlier conclusion that the gravity field of Mars is considerably rougher than the fields of either the Earth or the Moon. The largest positive gravity anomaly uncovered is in the Tharsis region which is also topographically high and geologically unusual. The best determined coefficients of the harmonic expansion of the gravitational potential are: J₂ = (1.96 ± 10.01) × 10^?3 ; C₂₂ = ?(5.1 ± 0.2) × 10^?5; and S₂₂ = (3.4 ± 0.2) × 10^?5. The other coefficients have not been well determined on an individual basis, but the ensemble yields a useful model for the gravity field for all longitudes in the vicinity of 23° South latitude which corresponds to the periapse position for the orbiter.The value obtained for the inverse mass of Mars (3 098 720 ± 70 M⊙^?1) is in good agreement with prior determinations from Mariner flyby trajectories. The direction found for the rotational pole of Mars, referred to the mean equinox and equator of 1950.0, is characterized by α = 317°.3 ± 0°.2, δ = 52°.7 ± 0°.2. This result is in excellent agreement with Sinclair's recent value, determined from earth-based observations of Mars' satellites, but differs by about 0°.5 from the previously accepted value. Other important physical constants that have either been refined or confirmed by the Mariner 9 data include: (i) the dynamical flattening, f = (5.24 ± 0.02) × 10^?3; (ii) the maximum principal moment of inertia, C = (0.375 ± 0.006) MR²; and (iii) the period of precession of Mars' pole, P ? (1.73 ± 0.03) × 10⁵ yr, corresponding to a rate of 7.4 sec of arc per yr.     

Mean thermal and albedo behavior of the Mars surface and atmosphere over a Martian year

A Mars average data set (MADS) has been constructed from thermal and albedo measurements of the Viking Infrared Thermal Mapper; by merging information from all longitudes; and, ensuring reasonably complete longitudinal sampling, a representation of mean Mars behavior is obtained. Brightness temperatures at 7, 9, 11, 15, and 20 μm and albedo information in the band 0.3–3.0 μm have been binned using 2° latitude strips, 24 times of day, 3 emission angle intervals, and 23 nonoverlapping L_s periods covering 1.43 Mars years starting at L_s = 84°. The MADS is ideally suited to parametric study of latitudinal, diurnal, angular, and seasonal dependences. Data are presented for surface thermal and albedo behavior in clear and dusty atmospheric conditions; the thermal response of the atmospheric temperature to a major dust storm is found to be consistent with Mariner 9 data from the 1971 storm. Examples of use of the MADS, which is available through the Mars Consortium, indicate how averaged data reveal specific surface and atmospheric phenomena.     

Early Mars climate near the Noachian–Hesperian boundary: Independent evidence for cold conditions from basal melting of the south polar ice sheet (Dorsa Argentea Formation) and implications for valley network formation

Currently, and throughout much of the Amazonian, the mean annual surface temperatures of Mars are so cold that basal melting does not occur in ice sheets and glaciers and they are cold-based. The documented evidence for extensive and well-developed eskers (sediment-filled former sub-glacial meltwater channels) in the south circumpolar Dorsa Argentea Formation is an indication that basal melting and wet-based glaciation occurred at the South Pole near the Noachian–Hesperian boundary. We employ glacial accumulation and ice-flow models to distinguish between basal melting from bottom-up heat sources (elevated geothermal fluxes) and top-down induced basal melting (elevated atmospheric temperatures warming the ice). We show that under mean annual south polar atmospheric temperatures (?100 °C) simulated in typical Amazonian climate experiments and typical Noachian–Hesperian geothermal heat fluxes (45–65 mW/m²), south polar ice accumulations remain cold-based. In order to produce significant basal melting with these typical geothermal heat fluxes, the mean annual south polar atmospheric temperatures must be raised from today’s temperature at the surface (?100 °C) to the range of ?50 to ?75 °C. This mean annual polar surface atmospheric temperature range implies lower latitude mean annual temperatures that are likely to be below the melting point of water, and thus does not favor a “warm and wet” early Mars. Seasonal temperatures at lower latitudes, however, could range above the melting point of water, perhaps explaining the concurrent development of valley networks and open basin lakes in these areas. This treatment provides an independent estimate of the polar (and non-polar) surface temperatures near the Noachian–Hesperian boundary of Mars history and implies a cold and relatively dry Mars climate, similar to the Antarctic Dry Valleys, where seasonal melting forms transient streams and permanent ice-covered lakes in an otherwise hyperarid, hypothermal climate.     

Evidence for the nonuniform distribution of microwave attentuating clouds in the atmosphere of Venus: Mariner 2

Using the data from Veneras 4–8 and Mariners 5 and 10 related to the composition and structure of the atmosphere of Venus, the three scans obtained with the microwave radiometer on Mariner 2 at a wavelength of 1.9 cm have been reanalyzed. In the previous analysis of the microwave data, both the percentage of Co₂ and the surface pressures were considerably lower than the in situ measurements and the assumed longitudinal temperature gradient was much larger than indicated by more recent measurements. Using these more recent data, it has not been possible to match the measured scan ratios with or without any spherically symmetric distribution of microwave cloud absorber. The scan ratios, therefore, require the existence of different average values of microwave cloud opacity for each scan. In addition, the anomalous temperature drop observed in the south polar region of the terminator scan has been found to require a very opaque microwave cloud in the local zenith angle range of 40°–70°. This type of distribution is consistent with the trend seen in the Mariner 2 infrared terminator scan suggesting some degree of coupling between the infrared and microwave clouds. It is suggested that some of the variability seen in the earth-based interferometer data may be a result of changes in the distribution of the microwave clouds over the disc of the planet.     

The infrared photometric function of Mars and its bolometric albedo

The photometric properties of local areas on Mars are studied using Minnaert's rule of surface scattering to analyze Mariner 6 and 7 Infrared Spectrometer data. Several bright deserts, Hellas, and the south polar cap are found to obey Minnaert's function well. The coefficients B₀(α, λ) and k(α, λ) are obtained at α = 39, 48, 56, 84° and λ = 1.85, 2.23, 3.50 μm. Observed bright regions all have similar values of k, except for Hellas and the south polar cap. The lower k of Hellas is apparently caused by microscopic effects rather than by large-scale roughness due to cratering. The higher k of the cap is similar to terrestrial snows in the visual at the same phase angle. Using existing Earth-based observations, at smaller α and λ, a bolometric Bond albedo of $\text{A}{}^1\text{= 0.24 ± 0.05}$ is calculated.     

South polar residual cap of Mars: Features, stratigraphy, and changes

The south residual polar cap of Mars, rich in CO₂ ice, is compositionally distinct from the north residual cap which is dominantly H₂O ice. The south cap is also morphologically distinct, displaying a bewildering variety of depressions formed in thin layered deposits, which have been observed to change by scarp retreat over an interval of one Mars year (Malin et al., 2001, Science 294, 2146-2148). The climatically sensitive locale of the residual caps suggests that their behavior may help in the interpretation of recent fluctuations or repeatability of the Mars climate. We have used Mars Global Surveyor Mars Orbiter Camera (MOC) images obtained in three southern summers to map the variety of features in the south residual cap and to evaluate changes over two Mars years (Mars y). The images show that there are two distinct layered units which were deposited at different times separated by a period of degradation. The older unit, ∼10 m thick, has layers approximately 2 m thick. The younger unit has variable numbers of layers, each ∼1 m thick. The older unit is eroding by scarp retreat averaging 3.6 m/Mars y, a rate greater than the retreat of 2.2 m/Mars y observed for the younger unit. The rates of scarp retreat and sizes of the different types of depressions indicate that the history of the residual cap has been short periods of deposition interspersed with longer erosional periods. Erosion of the older unit probably occupied ∼100-150 Mars y. One layer may have been deposited after the Mariner 9 observations in 1972. Residual cap layers appear to differ from normal annual winter deposits by having a higher albedo and perhaps by having higher porosities. These properties might be produced by differences in the depositional meteorology that affect the fraction of high porosity snow included in the winter deposition.