摘要 :
Polygonal ridge networks, also known as boxwork or reticulate ridges, are found in numerous locations and geological contexts across Mars. Distinguishing the morphologies and geological context of the ridge networks sheds light on...
展开
Polygonal ridge networks, also known as boxwork or reticulate ridges, are found in numerous locations and geological contexts across Mars. Distinguishing the morphologies and geological context of the ridge networks sheds light on their potential as astrobiological and mineral resource sites of interest. The most widespread type of ridge morphology is characteristic of the Nili Fossae and Nilosyrtis region and consists of thin, criss-crossing ridges with a variety of heights, widths, and intersection angles. They are found in ancient Noachian terrains at a variety of altitudes (between -2500 and 2200 m) and geographic locations and are likely to be chemically altered fracture planes or mineral veins. They occur in the same general areas as valley networks and ancient lake basins, but they are not more numerous where these water-related features are concentrated, and can appear in places where th morphologies are absent. Similarly, some of the ridge networks are located near hydrated mineral detections, but there is not a one-to-one correlation. Smaller, light-toned ridges of variable widths have been found in Gale Crater and other rover sites and are interpreted to be smaller versions of the Nili-like ridges, mostly formed by the mineralization of fractures. This type of ridge is likely to be found in many other places on Mars as more high-resolution data become available. Sinus Meridiani contains many flat-topped ridges arranged into quasi-circular patterns. The ridges are eroding from a clay-rich unit, and could be formed by a similar process as the Nili-type ridges, but at a much larger scale and controlled by fractures made through a different process. Hellas Basin is host to a fourth type of ridge morphology consisting of large, thick, light-toned ridges forming regular polygons at several superimposed scales. While still enigmatic, these are most likely to be the result of sediment-filled fractures. The Eastern Medusae Fossae Formation contains large swaths of a fifth, previously undocumented, ridge network type. The dark ridges, reaching up to 50m in height, enclose regular polygons and erode into dark boulders. These ridge networks are interpreted to form as a result of lava flow embayment of deeply fractured Medusae Fossae Formation outcrops. (C) 2016 Published by Elsevier Inc.
收起
摘要 :
The geomorphic record indicates that alpine glaciers in the McMurdo Dry Valleys of southern Victoria Land, Antarctica, appear to advance during interglacial periods in response to ice-free conditions in the Ross Sea. Few records o...
展开
The geomorphic record indicates that alpine glaciers in the McMurdo Dry Valleys of southern Victoria Land, Antarctica, appear to advance during interglacial periods in response to ice-free conditions in the Ross Sea. Few records of these advances are preserved and/or subaerially exposed, complicating the interpretations of regional glacier response to climate changes. Here, we present geophysical and geochemical analyses of a rock glacier that originates from icefalls fed by alpine Doran Glacier in central Taylor Valley. The rock glacier exhibits a trend of increased weathering of granitic clasts via ventifaction and grussification down-flow. Meltwater ponds on the rock glacier exhibit variable salinity that ranges from freshwater to higher than seawater, with the highest salinity pond near the rock glacier toe. Ground-penetrating radar analyses reveal the feature to possess a primarily clean ice interior, with layers of englacial debris. Stable isotopic data from three ice cores support a glacial origin for the ice within the rock glacier. These data suggest that the current morphology of the rock glacier is the result of multiple events of increased ice contribution caused by advances of Doran Glacier, which is the main source of ice to the rock glacier. We therefore demonstrate the potential of ice-cored rock glaciers to record multiple advances and retreats of Dry Valley glaciers, permitting the interpretation of glacial responses to Pleistocene and Holocene climate change even where direct records are not present.
收起
摘要 :
Steep channel-like landforms-referred to here as bedrock chutes-line the rocky walls of some craters on the Moon and Mars. The role of volatiles, such as H2O or CO2, or dry rockfall in the formation of bedrock chutes is unknown on...
展开
Steep channel-like landforms-referred to here as bedrock chutes-line the rocky walls of some craters on the Moon and Mars. The role of volatiles, such as H2O or CO2, or dry rockfall in the formation of bedrock chutes is unknown on either planetary body. To test whether bedrock chute formation on Mars involved volatile activity, we used digital elevation models of Mars generated from HiRISE and CTX stereo-imagery to survey 4-9 km diameter craters globally and measure chute morphology as a function of latitude and orientation-properties that might co-vary with volatile activity. We also analyzed bedrock chutes on the Moon, which is presumably devoid of significant erosion due to volatile activity, using LROC NAC data, to serve as a volatile-free endmember for comparison with Mars. Martian bedrock chutes occur at all latitudes and have median values of chute spacing (wavelength) of similar to 300 m, relief of similar to 15 m, and slope of similar to 33 degrees. Chutes on the Moon are less common and are generally steeper (similar to 41 degrees) with less relief (similar to 5 m) as compared to Mars. While dry rockfall might have formed bedrock chutes on both the Moon and Mars, martian chutes are systematically deeper on pole-facing slopes between 10 degrees S-30 degrees S indicating a likely role for volatile activity in chute formation. Bedrock chutes are also deeper where they co-occur with well-incised martian gully channels. The latitude-dependence for deeper bedrock chutes on pole-facing slopes extends to lower latitudes than gullies-within the contemporary tropics-indicating the potential for volatile-related activity closer to the equator than documented for gullies or other ice-related features on Mars. Chutes carved into bedrock likely form slowly compared to gully channels, which are incised into more erodible ice-cemented sediment, and therefore might provide a longer record of environmental conditions over larger swaths of Mars.
收起
摘要 :
Reconstructions of the orbital parameters of Mars spanning the last similar to 20 Myr, combined with global circulation models, predict multiple cycles of accumulation and degradation of an ice-rich mantle in the mid-latitudes, dr...
展开
Reconstructions of the orbital parameters of Mars spanning the last similar to 20 Myr, combined with global circulation models, predict multiple cycles of accumulation and degradation of an ice-rich mantle in the mid-latitudes, driven primarily by insolation at the poles during periods when obliquity was more than ten degrees greater than it is today (i.e., >similar to 35 degrees). While evidence of an ice-rich "latitude dependent mantle" (LDM) consistent with these predictions is abundant, features indicative of cycles of emplacement and degradation of this unit are isolated and rare. In addition, fundamental physical properties of the LDM, such as paleo-thickness maxima, have not been determined. Gullies, which are sinuous channels found on steep slopes in mid- and high-latitudes, interact with the LDM and provide a stratigraphic feature useful for documenting both cyclical emplacement/removal and thickness estimates in past climate regimes. In the southern hemisphere, where gullies are most common, we present extensive evidence of (1) cyclical degradation and removal of gullies in the lower mid-latitudes (30-40 degrees S), and (2) burial and exhumation of inverted gully channels in the transitional latitude band between dissected and preserved LDM (40-50 degrees S), which can only be accounted for if an additional tens of meters of LDM were present at these locations during channel formation. These relationships support a model in which end-to-end gully evolution is controlled by the behavior of the LDM: at lower latitudes, gullies incise an ice-rich substrate and are removed when that ice becomes unstable, and at higher latitudes gullies are buried by successive emplacement of LDM where ice remains stable near the surface. Further, the presence of dormant buried gullies implies that present-day activity within gullies, likely to be controlled by the behavior of CO2 frost, is insufficient to explain the entire gully population, and that conditions conducive to increased gully activity preceded the most recent phase of LDM emplacement. (C) 2015 Elsevier Inc. All rights reserved.
收起
摘要 :
Channel-like forms are ubiquitous on steep hillslopes on Earth, Mars, and other planetary bodies. On Earth and Mars, these landforms are commonly attributed to water activity, especially for slopes below the angle of repose (simil...
展开
Channel-like forms are ubiquitous on steep hillslopes on Earth, Mars, and other planetary bodies. On Earth and Mars, these landforms are commonly attributed to water activity, especially for slopes below the angle of repose (similar to 30 degrees) where dry granular flows are considered ineffective. While the angle of repose characterizes loose sediment stability, it is unclear whether dry rockfall can traverse and erode channels in bedrock or cemented substrates. We used a large-scale experiment to show that bedrock chutes can form spontaneously at low gradients from dry rockfall. Our results, combined with observations of rocky outcrops and boulders on Mars, indicate that rockfall can be an important bedrock degradation process that can produce low-gradient channels in the absence of water.
收起
原文获取