Potential for a Time-sequenced 100,000-year Record of Micrometeorites at South Pole

Introduction: A neutrino observatory planned for South Pole offers an opportunity to collect a timesequenced 100,000-yr record of micrometeorites and terrestrial dust deposited in Antarctic ice with 20–100yr resolution. This unprecedented record bears on numerous studies including the evolution of near-Earth cosmic-dust and the provenance of micrometeorites, the relationship between ET influx and global climate change, and the provenance of biogenic materials in Antarctic ice. We describe here a proposed collection system and selected science plans for the project. Collection System Context IceCube is a planned $200M astrophysics project consisting of strings of downward-looking photo-multipliers, frozen into the ice at South Pole, to observe the passage of high-energy neutrinos (www.ssec.wisc.edu/a3ri/). The University of Wisconsin has been awarded $15M to build an Enhanced HotWater Drill (EHWD) to install the strings. A heating plant will deliver hot water through a continuous hose to a nozzle in the drill head to melt ice ahead of the drill. Cool water will return up the melted hole to a pump near the surface and thence to the heating plant for re-injection (Fig. 1). Each of the 80 holes drilled will measure 2,450-m-deep x 0.6-m-diameter and will terminate in ice that is ~ 100,000 years old [1]. Preliminary Collector Design A major requirement is compatibility with routine drilling operations, allowing particle-collection from all 80 drill holes. The collector must operate at the drill’s 100-m/hr descent rate, preserve vertical alignment, and permit holechangeover times of 2.5 days. Other requirements include high collection efficiency (>90%), fine time resolution, minimum contamination, and efficient postcollection particle handling. Essentially, the collector will replace the EHWD drill head. Its main features are a shaped body to flatten the hole bottom (to improve time resolution) and an internal scrolling filter (to extract and sequence the particles). Hot water will flow through the 2-m-long collector body and discharge horizontally. The body will confine flow and entrained particles to a 1-cm annulus adjacent to the ice wall for high heat-transfer rates. A seal at the top of the collector body (0.3-mdia.) will direct flow inside (minimum flow velocity 1.4 m/s or ~ 3 times the fall velocity of a 2,000-μm iron spherule). The flow will manifold into single pipe that spirals downward. A 30-m-long x 150-mm-dia. tube filter will be bunched up over this pipe. As the collector descends, spiked wheels engaged against ice walls will rotate a spool that pulls the filter, at a rate of 25 mm per 2 m of drill descent, through a sleeve and over the discharge end of the pipe. A second sleeve will limit the open length of the filter to 25 mm and flatten the filter as it winds onto the spool. Entrained particles will deposit on the inside of the tube filter, with each 25-mm of filter length preserving a 2-m depositional interval. The flow will then exit the collector above the seal and continue its ascent. After the drill completes a hole, we will open the collector body, retrieve the spooled filter and install a new one. Heat-transfer analyses indicate that a shaped collector body can achieve a 0.3-m hole diameter within 2 m of the drill tip (versus > 8 m for the EHWD drill head) at 100 m/hr drilling rate. This 2-m interval represents uncertainty in particle depositional ages of 20 yr near the surface and 100 yr near the bottom of the hole [1]. We measured pressure losses for 1-μm to 53-μm membrane filters and conducted tests to confirm that a scrolling tube filter will preserve the time sequence of entrained particles. Pressure loss through a 1-μm tube filter will be less than the losses developed by the EHWD components that the collector replaces.