Responsive Autonomous Rovers to Enable Polar Science

PI Institute/Department Email
Ray, Laura
Dartmouth College, Thayer School of Engineering
Award#(s)
NNX15AM80A
Funding Agency
US\Federal\NASA
Program Manager Funding Agency Email
Wagner, Dr. Thomas
NASA
Discipline(s)
Cryosphere
Instrument Development\Glaciology
Science Summary

In order to determine the mass balance, and thus contribution to sea level, of the Earth’s ice sheets, researchers must understand the spatiotemporal variations in snow accumulation and firn compaction rates. The amount of snow that falls and accumulates from year-to-year across the ice sheets acts as a hydrological sink, lowering sea level, whereas ablation or melt that runs off acts to increase sea level. The total ice-sheet surface mass balance is primarily composed of the total mass of snow that accumulatedminus the mass of runoff from ice melt Subtracting the total ice mass lost through discharge at glacier calving fronts, known as the input-output method provides the ice-sheet mass balance. This project focuses on the positive component of mass balance: snow accumulation. The ice-sheet mass balance can also be measured by monitoring changes in surface elevation from satellite borne altimeters (e.g., NASA’s ICESat and upcoming ICESat-2, ESA’s CryoSat-2). The measured elevation change is used to determine the volumetric change, which must be converted to mass through knowledge of the density of the material gained or lost. Along the periphery of Greenland in the ablation zone, the change is easily attributed to solid ice, but in the interior, the conversion from volume to mass is less straightforward. The presence of a firn layer in the accumulation zone complicates conversion to mass for a few reasons. Firstly, firn is less dense than ice. The observed elevation change is the summation of change to the firn thickness and to the underlying ice column, which must be properly partitioned to adequately assign a density to the observed change. Secondly, variations in the firn compaction rate result in surface elevation fluctuations that are not the result of a mass change. Therefore, determining the ice-sheet mass balance requires a precise understanding of the spatiotemporal variations in snow accumulation and firn compaction rates, both of which are challenging to measure frequently and at large spatial scales. Snow accumulation varies substantially in time and space. While records of accumulation exist from ice cores and stake lines, their spatial coverage is sparse suggesting that they are inadequate for mass balance studies when used alone. Instead the core records have been used to calibrate climate model output, providing an important spatially and temporally complete accumulation history. Additionally, regional climate model surface mass balance products have been used in ice sheet-wide mass balance studies due to their complete spatiotemporal coverage. While these models are extremely useful for current and future mass balance assessment, field observations of snow accumulation are required to evaluate model ability over the ice sheets. While spatiotemporal measurements of snow accumulation are difficult and few, measurements of firn compaction are even rarer. For example, Arthern et al. (2010) measured vertical strain in the firn at hourly intervals over several years at three sites in Antarctica to investigate the controls on compaction rates as well as to evaluate different firn densification models. In Greenland, tracking the displacements of features observed using borehole optical stratigraphy over a single year at Summit provided measurements of the vertical compaction profile of the firn. Finally, repeat measurements of the firn density profile, derived using a neutron scattering technique, along a ~500 km traverse allowed Morris and Wingham (2011; 2014) to measure strain rates as a function of depth and develop a new densification equation based on their findings. The models developed by the aforementioned studies are tuned by data ranging from a few data points to a few 10s of sites, which limits their use outside of the surveyed areas. Thus, some ground-truthing of the temporal and spatial variability of compaction rates derived from firn densification models exists, but their evaluation would benefit from a method that can easily measure compaction rates over large areas. This project aims to expand our understanding of the spatiotemporal variations in snow accumulation and firn compaction rates by using GPR towed by an autonomous rover.

Logistics Summary

This project focuses on the positive component of mass balance: snow accumulation. Goals are to support efforts to build and validate existing models through targeted, spatiotemporal ground-based measurements using autonomous robots; to develop infrastructure enabling New Hampshire scientists and engineers to build collaborations by providing such measurements efficiently and reliably in polar regions; and to train the next generation of scientists and engineers through student research in robotics and polar science. In 2017 researchers will conduct a pilot deployment to Summit in advance of a larger effort in 2018. The Summit campaign will deploy two people and a robot to Summit for ~3 weeks to initiate planned experiments: 1) repeated measurement of firn layering using robot-towed GPR in regions of undisturbed snow within the 7 km camp radius (if possible), 2) autonomous mapping of snow specific surface area, and 3) development of robotic-towed albedo measurement instrumentation. The two personnel will also be part of the Dartmouth JSEP team (NSF grant # 1506155). In 2017 the project will deploy two field team members (also JSEP fellows) to conduct some initial field tests with the rover and to make some associated measurements. They will require some initial heated assembly space for 1-2 days to assemble the rover and have also requested access to a snow machine in the event that the robot needs to be recovered away from Summit. The team will coordinate with on-site technicians and the project manager to ensure their rover does not operate in a scientifically sensitive area. They have also requested to accompany the technicians during an ICESat traverse to tow a GPR and SSA measuring instrument and to make measurements with a penetrometer. That activity will be coordinated with the technicians and the NASA ICESat field team if schedule allows. The team will operate at Summit for approximately 3.5 weeks and then transfer to assist with the JSEP grant (Virginia, NSF grant 1506155) from July 21- 25. They will then head south via ANG with the rest of the group.

In 2017 CPS will provide Air National Guard (ANG) coordination for passengers and cargo, user days and access to Summit infrastructure including assembly and storage space, provision of one snow machine and fuel in the event of a robot failure that requires retrieval, and communication and safety equipment if leaving the Summit area. Via interagency transfer NASA will provide funds for shipment of the robot via ANG NY><Summit and Summit user days for the two JSEP fellows. All other logistics, including lodging in Kanger, will either be paid for by the NASA award or the Virginia JSEP grant.

Season Field Site Date In Date Out #People
2017
Greenland - Summit
2
2018
Greenland - Summit
2