Pole-facing Slopes Research Articles

Near‐tropical subsurface ice on Mars

Near‐surface perennial water ice on Mars has been previously inferred down to latitudes of about 45° and could result from either water vapor diffusion through the regolith under current conditions or previous ice ages precipitations. In this paper we show that at latitudes as low as 25° in the southern hemisphere buried water ice in the shallow (<1 m) subsurface is required to explain the observed surface distribution of seasonal CO2 frost on pole facing slopes. This result shows that possible remnants of the last ice age, as well as water that will be needed for the future exploration of Mars, are accessible significantly closer to the equator than previously thought, where mild conditions for both robotic and human exploration lie.

A recent ice age on Mars: Evidence for climate oscillations from regional layering in mid‐latitude mantling deposits

Two end‐member hypotheses have been proposed to account for the emplacement and distribution of ice in the near‐subsurface of Mars at mid to high latitudes during recent spin‐axis/orbital variation‐induced climate change. In the first, diffusion of atmospheric water vapor into and out of a porous regolith forms ice‐cemented soils whose latitudinal stability migrates as a function of orbitally controlled climatic conditions. In the alternative hypothesis, atmospheric deposition of ice, snow, and dust produces dusty ice‐rich layers during periods of higher obliquity. New image data reveal meters‐thick layered deposits exposed on mid‐latitude pole‐facing slopes supporting the latter hypothesis. These observations suggest that the near surface ice detected by the GRS instrument suite and the Phoenix lander at high latitudes is linked to thick, buried ice that was atmospherically deposited during recent ice ages and that significant amounts of subsurface ice may remain today in the 30–50° mid‐latitude regions where fresh craters, imaged by HiRISE, expose abundant ice.

Distribution and orientation of northern-hemisphere gullies on Mars from the evaluation of HRSC and MOC-NA data

Martian gullies show morphologic characteristics that indicate the presence of a flowing liquid during their formation process. However, the current climate in the northern hemisphere of Mars does not allow the existence of substantial amounts of liquid water and, therefore, an assessment of the regional distribution and characteristics of gullies is required in order to understand the most recent climatic development of Mars. We examined High Resolution Stereo Camera (HRSC) and Mars Orbiter Camera—Narrow Angle (MOC-NA) data covering the northern hemisphere in order to determine the distribution, geological setting and orientation of gullied slopes. We used these results to test the consistency of formation models proposed in earlier works and to define the climate dependence. Gully orientations were analyzed only at impact crater walls, because craters have an azimuthally uniform distribution of slopes. This is ideal when investigating the influence of insolation on the distribution and development of landforms. We examined 230 HRSC and ~ 21,000 MOC-NA images and identified more than 2200 gullies on HRSC and more than 3100 gullies in MOC-NA data. Both surveys show the highest densities of gullies between 35°N and 55°N and a shift of gully orientation at 40°N from pole-facing slopes to slopes facing the equator. Orientation trends as well as the latitudinal distribution of identified gullies strongly support the view of gully formation caused by insolation/climatic conditions. This result suggests that formation mechanisms based on the assumption of atmospheric water-ice deposition proposed in earlier work are more likely than processes related to groundwater flow. Further support for a theory related to deposition is gained through observation of the high occurrence of gullies at isolated knobs and hills, which cannot be associated with formation models based upon groundwater flow.

Periods of active permafrost layer formation during the geological history of Mars: Implications for circum-polar and mid-latitude surface processes

Permafrost is ground remaining frozen (temperatures are below the freezing point of water) for more than two consecutive years. An active layer in permafrost regions is defined as a near-surface layer that undergoes freeze–thaw cycles due to day-average surface and soil temperatures oscillating about the freezing point of water. A “dry” active layer may occur in parched soils without free water or ice but significant geomorphic change through cryoturbation is not produced in these environments. A wet active layer is currently absent on Mars. We use recent calculations on the astronomical forcing of climate change to assess the conditions under which an extensive active layer could form on Mars during past climate history. Our examination of insolation patterns and surface topography predicts that an active layer should form on Mars in the geological past at high latitudes as well as on pole-facing slopes at mid-latitudes during repetitive periods of high obliquity. We examine global high-resolution MOLA topography and geological features on Mars and find that a distinctive latitudinal zonality of the occurrence of steep slopes and an asymmetry of steep slopes at mid-latitudes can be attributed to the effect of active layer processes. We conclude that the formation of an active layer during periods of enhanced obliquity throughout the most recent period of the history of Mars (the Amazonian) has led to significant degradation of impact craters, rapidly decreasing the steep slopes characterizing pristine landforms. Our analysis suggests that an active layer has not been present on Mars in the last ∼5 Ma, and that conditions favoring the formation of an active layer were reached in only about 20% of the obliquity excursions between 5 and 10 Ma ago. Conditions favoring an active layer are not predicted to be common in the next 10 Ma. The much higher obliquity excursions predicted for the earlier Amazonian appear to be responsible for the significant reduction in magnitude of crater interior slopes observed at higher latitudes on Mars. The observed slope asymmetry at mid-latitudes suggests direct insolation control, and hence low atmospheric pressure, during the high obliquity periods throughout the Amazonian. We formulate predictions on the nature and distribution of candidate active layer features that could be revealed by higher resolution imaging data.

Martian gullies in the southern mid-latitudes of Mars: Evidence for climate-controlled formation of young fluvial features based upon local and global topography

A new survey of Mars Orbiter Camera (MOC) narrow-angle images of gullies in the 30°–45° S latitude band includes their distribution, morphology, local topographic setting, orientation, elevation, and slopes. These new data show that gully formation is favored over a specific range of conditions: elevation (−5000 to +3000 m), slope (>10°), and orientation (83.8% on pole-facing slopes). These data, and the frequent occurrence of gullies on isolated topographic highs, lead us to support the conclusion that climatic-related processes of volatile accumulation and melting driven by orbital variations are the most likely candidate for processes responsible for the geologically recent formation of martian gullies.

Seasonal surface frost at low latitudes on Mars

During the past 4 Mars years, Mars Orbiter Camera imaging capabilities have been used to document occurrence of seasonal patches of frost at latitudes as low as 33° S, and even 24° S. Monitoring reveals bright patches on pole-facing slopes; these appear in early southern winter and disappear in mid winter. The frost forms annually. Thermal Emission Spectrometer and daytime Thermal Emission Imaging System observations show surface temperatures on and near pole facing slopes reach the condensation temperature of CO 2, indicating the patches consist of carbon dioxide rather than water frost. For several months, temperatures on pole-facing crater walls are so low that even carbon dioxide condenses on them, although the slopes are illuminated by the Sun every day. Thermal model calculations show slopes accumulate a several centimeter thick layer of CO 2 frost. The frost becomes visible only months after it has begun to form, and has an orientational preference which is due to illumination bias at the time of observation. H 2O condenses at higher temperatures and water frost must therefore also be present. Potential opportunities to observe seasonal water frost at low latitudes are also described.

Formation of recent martian gullies through melting of extensive water-rich snow deposits.

The observation of gullies on Mars indicates the presence of liquid water near the surface in recent times, which is difficult to reconcile with the current cold climate. Gullies have been proposed to form through surface runoff from subsurface aquifers or through melting of near-surface ice under warmer conditions. But these gullies are observed to occur preferentially in cold mid-latitudes, where the presence of liquid water is less likely, and on isolated surfaces where groundwater seepage would not be expected, making both potential explanations unsatisfactory. Here I show that gullies can form by the melting of water-rich snow that has been transported from the poles to mid-latitudes during periods of high obliquity within the past 10(5) to 10(6) years (refs 5, 6). Melting within this snow can generate sufficient water to erode gullies in about 5,000 years. My proposed model for gully formation is consistent with the age and location of the gullies, and it explains the occurrence of liquid water in the cold mid-latitudes as well as on isolated surfaces. Remnants of the snowpacks are still present on mid-latitude, pole-facing slopes, and the recent or current occurrence of liquid water within them provides a potential abode for life.

Seasonal Water Stress in Some Chilean Matorral Shrubs

Environmental and plant characteristics relating to plant water stress were measured in seven shrub species, Satureja gilliesii, Cryptocarya alba, Quillaja saponaria, Lithraea caustica, Colliguaya odorifera, Retanilla ephedra, and Proustia cinerea, growing on equator- (EFS) and pole-facing (PFS) slopes in the mediterranean region of central Chile. Midday air temperatures and vapor pressure deficits were greater on the EFS than on the PFS. Soil moisture was greater during the winter but less during the summer on the EFS. Midday xylem potentials in all species decreased during the summer, increased after the first rain, and were lower on the EFS. Mean leaf size was smaller on the EFS. Stomatal frequency differed only for Q. saponaria and C. odorifera. Species with shallow root systems had lower xylem potentials.