Thermokarst is a land surface lowered and disrupted by melting ground ice. Thermokarst is a major driver of landscape change in the Arctic, but has been considered to be a minor process in Antarctica. Here, we use ground-based and airborne LiDAR coupled with timelapse imaging and meteorological data to show that 1) thermokarst formation has accelerated in Garwood Valley, Antarctica; 2) the rate of thermokarst erosion is presently ~ 10 times the average Holocene rate; and 3) the increased rate of thermokarst formation is driven most strongly by increasing insolation and sediment/albedo feedbacks. This suggests that sediment enhancement of insolation-driven melting may act similarly to expected increases in Antarctic air temperature (presently occurring along the Antarctic Peninsula) and may serve as a leading indicator of imminent landscape change in Antarctica that will generate thermokarst landforms similar to those in Arctic periglacial terrains.
ABSTRACT Infrared and high contrast panoramic photographs of Devonian nonmarine sedimentary rocks exposed along the Catskill Front, southeastern New York, reveal details of sandstone body continuity. The laterally persistent Kaaterskill sandstones in the upper part of the sequence may mark a transition from meandering stream to braided stream deposition.
The observed differences between complex craters and multi‐ringed basins are described and a combined “transient cavity/displaced zone/melt cavity” model is explored to account for the observations. In this “nested melt‐cavity model”, the increasing influence of the percentage of the target undergoing impact melting at the sub‐impact point with increasing size (the differential melt scaling of Cintala and Grieve [1998]) causes fundamental changes in the nature of the transient cavity, its relationship to the displaced zone, and its short‐term collapse behavior. The transition from complex craters to two‐ring basins involves expansion of the melting front into the displaced zone, formation of a two‐component excavation cavity, and a concomitant radial expansion outward of the zone of maximum elastic rebound. The short‐term modification stage is then dominated by the strength differences between the fluid melt in the inner cavity and rocks of the displaced zone; the highly shocked rocks at the outer margin of the expanded parabolic melting front rebound to form the expanded peak ring, moving upward and laterally inward, easily displacing fluid melt filling the inner depression. At larger sizes, differential melt scaling causes the peak ring diameter to expand relatively more rapidly than the basin rim, and upon collapse, the increased volume of melt ponds in the rebounded melt cavity inside the expanding peak ring. As the transient melt cavity further increases proportionally in size, it penetrates through the base of the displaced zone with significant consequences. The resulting modification stage now incorporates into the collapse process inward and upward movement along the base of the displaced zone; listric failure occurs inward into the fluid melt cavity beginning at the edge of the melt cavity and extending out along the base of the displaced zone up to the base of the rim structural uplift (at ∼1.5 crater radii). This forms an additional outer ring and a resulting megaterrace, modifying the radial ejecta on the collapsed rim to form a domical facies. At multi‐ring basin scales, the significantly deeper penetration that occurs in the expanding melt cavity accounts for the maximum crustal thickness decrease that occurs inside the peak ring in the final basin. Early onset and higher density of peak ring basins on Mercury is predicted by higher mean impact velocity and differential melt scaling.
Abstract Over the extreme temperature variations experienced in a single lunar day (∆ T ≈ 300 K), particular minerals common to the lunar surface show spectral changes at discrete near‐infrared wavelengths in laboratory settings (Roush & Singer, 1986, 1987, https://10.1111/10.1029/JB091iB10p10301 , https://10.1111/10.1016/0019-1035(87)90026-1 ; Singer & Roush, 1985, https://10.1111/10.1029/JB090iB14p12434 ). Variations in temperature can cause variations in the size and shape of crystallographic sites, which control the position, shape, and depth of crystal field absorptions. At an observation wavelength of 1,064 nm, the Lunar Orbiter Laser Altimeter (LOLA) should be highly sensitive to temperature‐dependent changes of orthopyroxene. Here we analyze temperature‐dependent spectral changes of the lunar surface as measured from orbit by LOLA. We couple LOLA measurements of normal albedo with measurements of surface temperature from the Diviner Lunar Radiometer Experiment, analyzing the maria and highlands between ±50°. We provide the first evidence of temperature‐dependent spectral changes on the lunar surface from orbital observations, finding that the majority of the surface between ±50° demonstrates a small, yet measurable, negative change in 1,064‐nm albedo with temperature (−∆ R /∆ T ). The measurable effect is on the order of a few percent change in reflectance per ~80 K, indicating that temperature changes do not have a large effect on measurements of albedo at the sensitivity of the LOLA instrument. Stronger −∆ R /∆ T values tend to be associated with regions with elevated orthopyroxene, and the maria typically have higher levels of orthopyroxene and stronger −∆ R /∆ T values than the highlands. Our results suggest that single‐wavelength lasers may be powerful tools for understanding the distribution of particular minerals on the lunar surface.
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