*5.1. Progress*

Scientific investigation is one of the most important tasks for Snow Eagle 601 aviation operations in Antarctica. However, the aircraft must also be available for emergency response and assisting with transportation logistics between Antarctic stations. In the past five austral seasons, Snow Eagle 601 has averaged about 120 flight hours for scientific survey out of a total of about 320 flight hours allocated for each season. Approximately 130 h are used to move the aircraft in and out of Antarctica from Calgary, Canada. Among the ~120 h of scientific flights each season, about 40 h are reserved for international collaborations. To date, China has collaborated with Australia, France, Russia, South Korea and the USA for scientific or logistical duties, sharing critical aviation capabilities in Antarctica. International collaborations improve the usability and coverage of Snow Eagle 601 in Antarctica and also benefit airborne data interpretation and scientific research. In China, scientists from many universities and institutions including Tongji University, Beijing University of Technology, Wuhan University, Beijing Normal University, Zhejiang University and the First Institute of Oceanography of the Ministry of Natural Resources (MNR) have been involved in airborne surveys in the field or through post-season data processing, interpretation and scientific research. Radioglaciological and aerogeophysical studies of Antarctica in China are significantly extended and improved through the scientific operations of Snow Eagle 601.

In its first five austral seasons, Snow Eagle 601 has acquired data along more than 175,000 line kilometers. The flight lines cover many critical areas in East Antarctica including Princess Elizabeth Land, Grove Mountains, the Amery Ice Shelf, the South Prince Charles Mountains, Ridge B, the West Ice Shelf, the Shackleton Ice Shelf, the Titan Dome, the George V Coast and the David Glacier Catchment, as shown in Figure 5.

**Figure 5.** Flight line coverage of airborne surveys in the past five austral seasons by Snow Eagle 601. Profiles along A-A', B-B' and C-C' are shown in Figure 6.

The initial scientific motivations for the scientific operations of Snow Eagle 601 were to map the ice sheet geometry, bedrock topography and infer the subglacial conditions and geological conditions of PEL, the largest data gap in Antarctica prior to these efforts. The Bedmap 2 [15], ADMAP 2 [16] and AntGG [17] datasets reflected the state of coverage from ice bottom elevation, magnetics and gravity field mapping before the beginning of Snow Eagle 601 operations, highlighting the large data gap in PEL. Moreover, ice surface features in the satellite data potentially showed a large subglacial lake in PEL and an extensive canyon system beneath the ice that may link the lake with the grounding zone of the West Ice Shelf [19]. However, these have not ye<sup>t</sup> been identified and characterized by geophysical observations and their impacts on the dynamics and stability of the interior ice sheet have not been well studied, leading to our poor understanding of future mass balance in this sector and its potential contribution to the sea level. Data collected by Snow Eagle 601 over the last five years in PEL have filled this gap and ice thickness and bed topography datasets have been released by Cui et al. [20]. Airborne radar data along a transect (LSE/GCX0e/Y87a) also validate the existence of the subglacial lake (a relatively light and flat reflector in Figure 6a). Airborne survey data will continue to provide key parameters and boundary conditions for numerical models to study past and future ice sheet dynamics in PEL.

**Figure 6.** Data profiles along project/set/transects (PSTs) in Princess Elizabeth Land (PEL), Ridge B and the Amery Ice Shelf, respectively. (**a**) Radargram along LSE/GCX0e/Y87a, LSE = Lake of Snow Eagle; (**b**) radargram along TSH/GCX0g/R41a; (**c**) radargram, free air gravity anomaly and magnetic anomaly along AMY/GCX0e/Y184a, AMY = Amery.

Further airborne surveys have been extended to investigate the Amery Ice Shelf, the West Ice Shelf and the Shackleton Ice Shelf with the motivation of inferring bathymetry using gravity data [12], identifying properties of basal ice and grounding lines or zones and mapping ice shelf geometry with a more modern radar system relative to previous data. Along a transect near the ice front of the Amery Ice Shelf, airborne radar data have revealed a clear water interface beneath the ice shelf and unfocused hyperbolae caused by outcroppings and surface crevasses in the coastal region (Figure 6c). Gravity and magnetic anomalies along the transect show obvious relationships, which is a joint result from the deep geological structure and lithosphere (Figure 6c). Ice shelves are the most vulnerable components of ice sheets due to their direct contact with warm ocean water and play a vital role in the stability of interior ice sheets [21]. Studies show that ice shelves over most of the West Antarctic Ice Sheet (WAIS) are experiencing rapid dynamic thinning and grounding line retreat [22,23]. Ice shelves in the East Antarctic Ice Sheet (EAIS) are also beginning to earn increased attention to recently observed changes; for example, recent studies of the Totten Glacier Ice Shelf revealed rapid changes and ocean conditions similar to what was found in the coastal WAIS [12,24,25] indicating the possible instability of sectors of the EAIS. However, most ice shelves in the EAIS have not been adequately surveyed. Airborne surveys in these regions are ongoing by both the ICECAP/EAGLE (East Antarctic Grounding Lines Experiments) and ICECAP/PEL campaigns through international collaborations in Australia, China, France, Italy, South Korea, the UK and the USA. These programs have particularly emphasized coastal areas, such as the George V Coast, which is under-surveyed but is critically important for understanding the fate of the interior Wilkes Subglacial Basin.

In the austral season of 2018/19, the first e ffort was made to survey Ridge B. Ridge B is located in the deep interior of the EAIS. Modeling results indicated that Ridge B is one of the most likely places to host the oldest ice on Earth [26] and, therefore, the longest high resolution climate record but there are no su fficient observations there to support and verify this. Two critical flights were successfully achieved using a temporary skiway located at the Chinese Taishan Station as a transit and refueling stop. Due to the location of the interior of Antarctica, the radargram along the PST of TSH/GCX0g/R41a in Ridge B showed remarkable internal layers and continuous, clear, frequently varied bedrock topography (Figure 6b). These are essential for extracting ice thickness, bedrock elevation, past snow accumulation rate and basal conditions and have a significant value for estimating the bottom ice age in this region. The airborne data in Ridge B can improve our understanding of ice sheet geometry and improve the modeling accuracy of ice age estimation. In addition, two short surveys with similar research motivations were completed over the Titan Dome, another "old ice" target, by Snow Eagle 601 during the 2015/2016 and 2016/2017 austral seasons when the aircraft was ferrying out of Antarctica [27].

Aerogeophysical surveys in the Grove Mountains and the South Prince Charles Mountains have mainly focused on geological studies. Measurements in the regions o ffer a valuable extension for geological studies based on outcrops [28] and could have significant implications for the assembly and breakup of the Gondwana Supercontinent [16].

#### *5.2. Future Work*

In the past five austral seasons, Snow Eagle 601 has successfully surveyed PEL, the Amery Ice Shelf and the West Ice Shelf with moderately high spatial resolution. Data processing and publication should be accelerated to reveal the ice thickness, bedrock topography, internal layers and basal conditions of the ice sheets and ice shelves as well as analyzing the gravimetric and magnetic anomalies of the continent to an acceptable data density and accuracy. These results will significantly improve our knowledge of the ice sheet and its changes and of the geological settings of the regions. Flight lines over the South Prince Charles Mountains, Ridge B and other interior areas are still sparse. More attention should be given to these areas in future years. It is di fficult to survey these areas from a coastal skiway because they require a very long transit; therefore, inland skiways such as Taishan Station, Kunlun Station, Vostok Station and South Pole Station should be considered through international

collaborations and partnerships. International collaborations should also be strengthened to survey fast changing areas around the continent such as grounding zones, coastal outlet glaciers and ice shelves in the Ross Sea and Amundsen Sea sectors with powerful IPRs and aerogeophysical instruments to better characterize and infer the ice geometry, bedrock topography, basal conditions and bathymetry. Complex surface conditions and the strong attenuation of radar signals in these small-scale areas are a substantial challenge for airborne surveys. Moreover, to survey these areas, nearby landing areas are also needed. The continent–ocean transition zones in the sector of the Southern Indian Ocean are another target for the scientific operations of Snow Eagle 601 in future years. Surveys there can help to build knowledge on the connection of the geological structure between the continent and ocean there, which has significant implications for the joint interpretation of the geological evolution of Antarctica and the surrounding continents.

Updated and improved airborne instruments are also needed in the near future. The existing instruments have a strong capability for aerogeophysical investigation but are insu fficient to map snow and shallow ice layers and are not su fficient to capture ice surface properties and their changes with high e fficiency. Airborne shallow ice and snow radars are needed to map snow and shallow ice stratigraphy in detail, which is important to study the mass balance of Antarctica. The airborne laser altimeter and camera are not su fficient to measure surface elevation e fficiently and characterize surface features in good quality. An advanced airborne optical camera or photogrammetry system as well as airborne LiDAR (light detection and ranging) and SAR (synthetic aperture radar) systems are priority choices for updating the Snow Eagle 601 airborne platform.

Snow Eagle 601 will continue operating in Antarctica for the foreseeable future. Airborne surveys remain an important part of the annual tasking; however, flight hours dedicated to scientific operations may be reduced with increased logistical requirements. Hopefully, more than 100 flight hours (~30,000 km flight lines) can be achieved in the upcoming seasons. In addition to the continuous and long-term data acquisition by Snow Eagle 601 in Antarctica, both national and international collaborations for data interpretation and research should be strengthened to extend the influence of Snow Eagle 601 scientific operations and promote scientific achievements.
