Sub-ice Exploration of West Lake Bonney: Endurance 2008 Mission

ENDURANCE (Environmentally Non-Disturbing Under-ice Robotic Antarctic Explorer) is a highly maneuverable, hovering autonomous underwater science platform descended from the DEPTHX vehicle, both of which were developed under NASA ASTEP funding. ENDURANCE had the specifi c mission of descending through a 5 meter deep melt hole in the ice cap of West Lake Bonney, Taylor Valley, Antarctica and conducting three autonomous science tasks: 1) measuring the 3D water chemistry of the lake; 2) mapping the underwater face of Taylor Glacier where it enters the lake; 3) charting the bathymetry of the lake bottom; and then 4) returning safely on its own to the melt hole – barely 0.25 m larger in diameter than the vehicle – from more than 700 m radial range and rising up the hole to be retrieved for data download and servicing for the next mission. Many features of ENDURANCE represented signfi cant changes and improvements over its highly successful predecessor craft. Included among these was the development of an automated sub-sea servo winch and sonde payload with nine water chemistry probes, high defi nition imaging system, and bottom ranging altimeter. A specialized ice-picking behavior was developed to maximize cast initial proximity to the underside of the ice sheet and to reduce power consumption during casts. The navigation system was comprised of a three layer fi lter utilizing high grade dead reckoning, ultrashort baseline localization, and machine vision. Also new were web-based glacier imaging systems and a 120-degree swath high resolution multi-beam mapping sonar system – used for both lake bottom and glacier face mapping. All of these systems, following several days of initial shakedown in the -4C water, worked fl awlessly for the duration of the expedition, producing 108 sonde casts spread over 80% of the lake area (2.5 km length x 1.25 km width x 40 m depth); greater than 100 million 3D sonar hits across the Taylor glacier; and more than 400 million bathymetric 3D hits on the lake bottom. A total of 95 hours of operation beneath the ice were logged along with 19 kilometers of cumulative traversed distance under the ice cap. The vehicle successfully demonstrated autonomous melt hole location, position lock and auto recovery on a routine, daily basis. A custom magnetic beacon tracking system was developed that enabled real-time surface position fi xes to be established on the vehicle – a powerful new feature that allowed for precisely geo-referencing all water chemistry samples. Many of the characteristics and capabilities of ENDURANCE – now successfully demonstrated in complex under-ice settings beneath West Lake Bonney -are the types of behaviors that will be needed for sub-ice autonomous probes to Europa, Enceladas, and other outer planet watery moons.