Exploring Beneath an Antarctic Ice Shelf with the Autosub3 AUV Stephen McPhail, Maaten Furlong, Miles Pebody, James Perrett, Peter Stevenson, Andy Webb, Dave White (All with the National Marine Facilities Division, NOC, Southampton, UK.) The most recent report of the Intergovernmental Panel on Climate Change (IPCC) noted that because so little is understood about ice-sheet behavior it is difficult to predict how ice sheets will contribute to sea level rise in a warming world. The Pine Island Glacier (PIG) ice shelf in the Western Antarctic has been thinning and accelerating over the past decades. It is vitally important to understand why this is happening, so in January 2009 a team of scientists and engineers set sail from Punta Arenas, Chile, on the US icebreaking ship, the Nathaniel B. Palmer. The collaborative effort involved the National Science Foundation, who provided the ship time, (cruise chief scientist and co PI Dr Stanley Jacobs of Lamont Doherty Earth Observatory), the British Antarctic Survey (PI Dr Adrian Jenkins) provided the scientific rationale, and the National Oceanography Centre, Southampton, provided the technology for the investigation with the Autosub3 Autonomous Underwater Vehicle. The Autosub3 Autonomous Underwater Vehicle was developed at the Underwater Systems Laboratory (USL) within the National Marine Facilities Division at NOC, Southampton. Here it is ready for launch at the start of its first of 6 missions under the Pine Island Glacier (the ice front can be seen on the horizon). Autosub3 is 7 m long, weighs 2.5 tonnes in air, and is powered by over 5000 primary D cell batteries, giving a maximum endurance of 400 km at a speed of 1.5 ms-1. It is capable of safely diving to 1600 m. Objectives were the mapping of the ice shelf, the seabed beneath and the study of the flow of water in the cavity. Sensors on board the AUV included high precision conductivity, temperature, and turbidity sensors, plus a high resolution (2 m pixel) multibeam mapping sonar, configured to either point upwards at the ice, or downwards at the seabed below. PIG The ice (grey in the figure) is 500 m thick at the ice front, and 1000 m thick at 70 km in, where it contacts the seabed (the grounding line). In the example mission plan, the AUV runs into the ice cavity at constant depth (blue trace). When it cannot safely continue it turns and flies a constant distance from the ice above for a distance of 20 km, whereupon it continues towards the ice front in profiling mode. Under the ice shelf, the AUV self navigates relative to either the sea floor or the ice above, using a combination of upwards and downwards looking Doppler sonars, and inertial navigation system. Typically it achieved accuracies of 1 m error in every 1000m travelled, ensuring the AUV returned close to its rendezvous position with the ship. To avoid collision with either the ice above or the seabed, several pencil beam sonars are used together with the bespoke (USL developed) control and collision avoidance software. The upward looking multibeam sonar system provided never before seen detail of the underside of an ice shelf. The image shown shows two 300 m wide swaths of multibeam data, one collected as the AUV headed in towards the grounding line, the other as the AUV headed out towards the ice front. The AUV was set to fly at 100 m below the ice – not a easy task, given the fissured nature of the under ice surface. 100 km In total, over a period of two weeks, 6 missions were run under the ice shelf (shown over a satellite image of the PIG – the sea is black), the longest lasting 30 hours and covering 170 km.