Mapping Blue-Ice Areas and Crevasses in West Antarctica Using ASTER Images, GPS, and Radar Measurements
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CHAPTER 31 Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements Andre´s Rivera, Fiona Cawkwell, Anja Wendt, and Rodrigo Zamora ABSTRACT from ASTER imagery, with the aim of identifying other landing sites for aircraft, as well as providing Before the satellite era, relatively little was known a detailed map for meteorite seekers. ASTER com- about the interior of the West Antarctic Ice Sheet posite images have also been used to map safe (WAIS). Of special interest are the rock outcrops routes for terrestrial traverses through crevasse associated with blue-ice areas (BIAs), which have zones. High-pass filters enhanced crevasse features, been exploited for logistical purposes as well as but visual analysis proved to be the most reliable being the subject of scientific research. The blue method of identifying all crevasses. ASTER images ice consists of relatively snow-free glacier ice that were superior to microwave imagery for crevasse is undergoing ablation. detection, as the latter can lack sufficient contrast; One of these BIAs is Patriot Hills (80180S, however, only Radarsat imagery provided coverage 81220W) where aircraft with conventional landing of higher latitude regions. Information gleaned gear have been landing for more than 20 years. This from visible imagery can be combined with that is now the main hub supporting large terrestrial of field-based radio-echo sounding and ground- expeditions conducted by Chilean scientists within penetrating radar profiles through the ice to map Antarctica. Kinematic GPS has been used to map internal layers and bedrock topography with the BIAs since 1996, with areas delineated on ASTER objective of enhancing our knowledge of this images since 2001 using both manual and auto- remote region. mated approaches. The GPS method typically delimits the largest area, and supervised classifica- tion of the images by an algorithm demarcates the 31.1 INTRODUCTION smallest area due to thin patchy snow cover over- lying blue ice. These areas do not display a unique Blue-ice areas (BIAs) are a rare feature in Antarc- spectral response when mostly snow covered, so tica (occupying between 0.8 and 1.6% of the con- that they can only be visually discriminated. This tinent; Winther et al., 2001) and are commonly, but detailed record of BIA extent shows no significant not exclusively, associated with nunataks. These areal change with time, but does display interannual rock outcrops represent a barrier to the strong variability, which most likely is connected to pre- katabatic winds that flow constantly from the inter- vailing meteorological conditions. BIAs around ior of the ice sheet, with the resulting turbulent air other nunataks in the region have been mapped flow responsible for removing surface snow, leaving 744 Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements a bare ice face. BIAs have no net annual accumula- Thiel Mountains (Choi et al. 2007). One of the tion, and ablation occurs mainly through sublima- reasons commonly given to explain the scarcity of tion, which can be much higher than over adjacent meteorites in these BIAs, especially Patriot Hills, is snowfields (Bintanja and Reijmer 2001), and the occasional occurrence of warm events, which through wind erosion, resulting in a local negative melt the ice surface, causing surface materials to surface mass balance. BIAs tend to be smooth, sink into the ice (Lee et al. 1998). One of these warm although they may be rippled, which facilitates their events took place in December 1997, when air use for aircraft with conventional landing gear temperatures reached 2.5C (Carrasco et al. rather than aircraft with skis. Other BIAs are asso- 2000), resulting in a pond at the margin of the ciated with steep slopes, glacial valleys, or the lower BIA (Casassa et al. 2004). Very little is known, parts of glacial basins, where accelerated katabatic however, about the frequency of such warm events, winds can effectively remove snow leaving a smooth or whether there is any longer term trend of change ice surface. For more information on BIAs see in the areal extents of BIAs. Bintanja (1999). Analysis of satellite images has been shown to Under steady state conditions, ice flows converge be especially useful for mapping remote areas in horizontally in the vicinity of BIAs, with upward ice Antarctica (Bindschadler 1999), not only to identify flows at the margins of nunataks balanced by ice potential meteorite sites (Choi et al. 2007), but also mass losses mainly due to sublimation at the surface to monitor fluctuations in the extent of BIAs (Bintanja 1999). These ice flows transport englacial (Casassa et al. 2004), to detect possible crevasse material to the surface, with unusually large vol- fields near terrestrial traverse routes (Bindschadler umes of surface deposits found along the margins and Vornberger 2003), to determine ice velocities of BIAs adjacent to many nunataks and mountain (Stearns and Hamilton 2006), and to detect the pos- ranges. Thus BIAs are also of interest because they ition and variations of grounding and hinge lines can be a concentrated source of meteorites that (Rignot 1998). In this work we combine ASTER have fallen over a wider region over many millen- satellite images with GPS and radar data collected nia, been trapped and transported within the ice, on the ground to map BIAs and crevasse fields in and covered by snow, before being exhumed on the West Antarctica. These studies have proven to be surface (Corti et al. 2003). As of 1999, more than an important precursor to more detailed analyses 20,000 meteorites had been discovered in Antarctic related to the age and origin of Antarctic ice feat- BIAs (Bintanja 1999). ures, especially with respect to glaciers flowing into Patriot Hills, with a maximum altitude of 1,246 m ice shelves that may be susceptible to future col- asl (USGS 1966), are located at the southeastern tip lapse, as observed farther north in the Antarctic of Ellsworth Mountains. They comprise one of Peninsula (Rignot et al. 2005). Union Glacier and many Antarctic nunataks, which act as obstacles ice in Horseshoe Valley, where the Patriot Hills’ to katabatic winds (Figs. 31.1 and 31.2). Increased BIA is located, flow into the Ronne Ice Shelf; local wind speeds on the leeward side of Patriot Hills grounding lines are only a few tens of kilometers limits the accumulation of snow, and as a conse- downstream from the surveyed areas. These glaciers quence has led to the generation of a BIA with an have subglacial topographies well below present sea area of approximately 12 km2. This BIA is located level, so upstream migration of grounding lines in at the southern edge of Horseshoe Valley and has the future could affect the stability of these glaciers, slopes of less than 1 degree and surface topography most likely inducing an acceleration of flow and varying in altitude between 1,100 m asl on the west- dynamic thinning. ern side to 700 m asl on the eastern side. Ice flows from west to east, with very low velocities at the BIA, and a maximum velocity of 14 m yrÀ1 at the 31.2 BLUE-ICE AREAS center of Horseshoe Valley (Wendt et al. 2009). No meteorites were found in the BIA of Patriot 31.2.1 Mapping BIA extent in the field Hills during an initial expedition in 1997/1998 (Lee and on imagery et al. 1998). However, in 2000 a meteorite was found in a nearby moraine band (Grossman and The extent of the Patriot Hills’ BIA was first Zipfel 2001). As for other BIAs in the region, surveyed in 1996/1997, using topographic quality meteorites have been found at Martin Hill and Trimble Geoexplorer II GPS receivers (single fre- Pirrit Hill (Lee et al. 1999), and more recently at quency). Differential correction procedures were Blue-ice areas 745 Figure 31.1. Map of Antarctica showing the nunataks studied in this chapter and some of the main stations in the area. Figure can also be viewed as Online Supplement 31.1. applied to the GPS data, giving a horizontal pre- mapped in the field because of the proximity of cision of between 5 and 10 m. In 2005, 2006, and the ice margin to the lateral moraine; therefore, this 2008 geodetic-quality Javad Lexon GD GPS re- margin was defined from satellite imagery (Fig. ceivers (dual frequency) were used to outline the 31.3). BIA. After correction, submeter vertical and hori- BIAs are easily distinguishable on visible imagery zontal accuracies were obtained. These GPS surveys as a result of their color and unique spectral signa- attempted to follow the snow/blue-ice interface. ture. Thus they can be readily delineated either by However, in many places intermittent patches of manual digitization or supervised classification. By thin snow covering the surface made the margin manually defining the margin, the same maximum difficult to distinguish. Consequently, a maximum extent criterion as that used in the field was applied extent criterion was applied which joined up ice to estimate the greatest extent of blue ice likely areas separated by patches of snow, as specified covered by thin snow. Applying this same rule to in the first survey by Casassa et al. (2004). The a supervised classification procedure, however, is southernmost extent of the BIA could not be more complex as the surface expression of blue 746 Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements Figure 31.2. Radarsat mosaic from October 20, 1997 showing the main features discussed in the text and the GPS tracks of the 2004 and 2007 traverses to the South Pole, the 2006 traverse to subglacial Lake Ellsworth, and the 2008 traverse to Union Glacier.