Ninth International Conference on Permafrost ... - IARC Research

HiRISE Observations of Fractured Mounds in the Martian Mid-LatitudesColin M. DundasThe University of Arizona, Department of Planetary Sciences, Tucson, AZ, USAAlfred S. McEwenThe University of Arizona, Department of Planetary Sciences, Tucson, AZ, USAIntroductionThe planet Mars has widespread permafrost, muchof it ice-rich. Some ground ice features such as thermalcontraction cracks have been confidently identified basedon orbital imagery (e.g., Mellon 1997). The possibility ofother periglacial features has been considered; in particular,a number of authors have suggested various Martian featuresas pingos (Judson & Rossbacher 1979, Parker et al. 1993,Cabrol et al. 2000, Soare et al. 2005, Burr et al. 2005,Page & Murray 2006). However, low resolution of orbitalimagery made it difficult to assess detailed morphologies ofsuch features, and in several cases, later work has indicateddifferent formation mechanisms (Farrand et al. 2005,Martinez-Alonso et al. 2005, Jaeger et al. 2007). This hasleft the possibility of pingos on Mars an open question.The issue is of particular importance, since pingos wouldprovide information about the state and history of water onMars, with implications for the origins of other water-relatedfeatures such as young gullies (Malin et al. 2000).The High Resolution Imaging Science Experiment(HiRISE) camera on the Mars Reconnaissance Orbiter(MRO) spacecraft has returned several thousand images ofthe Martian surface with scale as small as 26 cm/pixel. Thisresolution allows assessment of fine-scale morphologies inmuch more detail than previously possible. Early imagesrevealed fractured mounds, similar to pingos on Earth insome respects, in both northern and southern mid-latitudes(Dundas et al. 2008). Dundas et al. (2008) also used lowerresolutionimages to examine the distribution of flat-toppedmounds (matching the approximate shape of some of theobserved features) and found that in the northern plainsregion of Utopia Planitia, the mounds occurred in thelatitudinal band where gullies are most common. This studyuses HiRISE images to examine the planet-wide distributionand range of morphologies in order to better compare thesemounds with pingos on Earth.polygonal cracks occurring as part of a widespread network(e.g., thermal contraction cracks).To date, 1350 HiRISE images have been incorporated intothis survey. All images obtained through orbit 2150 wereused; among images from subsequent orbits, only those fromlatitudes 20°–60° of each hemisphere were examined.Fractured mound morphologiesA wide range of morphologies fitting the loose definitionof fractured mound have been observed. Some are isolated,while others, typically smaller, are clustered. In severalinstances, roughly radial fracture patterns are observed,but in other cases the fractures are irregular. Mounds arefound in pits or small impact craters at several sites, oftenin association with other fractured features. The breadth ofthe definition also includes some anomalous forms, suchas irregular, few-hundred-meter scale raised plateaus withdistinct fracturing.This breadth prevents any feature from being truly typical,but several broad categories are observed. Fractured moundsin the Southern Hemisphere are generally found on thefloors of multi-kilometer diameter impact craters, often withgullied walls. This group includes both isolated mounds(e.g., Fig. 1) and clusters of fractured features. A groupof flat-topped mounds in Utopia Planitia displays distinctsummit fracturing, often radially oriented, and is often foundHiRISE ObservationsHiRISE surveyHiRISE images from across Mars were examined inorder to assess the distribution of fractured mounds. Thisis a non-genetic definition that could encompass a rangeof morphologies and origins, used here in order to ensurea comprehensive survey of possible pingos. We searchfor features where fracturing occurs only on the moundor is distinctly enhanced there, since much of the Martiansurface has fractures of various origins. We specificallyexclude outcrops of jointed rock and knobs with superposedFigure 1. Fractured mound in HiRISE image PSP_007522_1480.The mound is approximately 100 m across and lies on the floor ofan impact crater with gullied walls. Illumination is from the rightand north down in this non-map-projected image.67