About 40% of the World’s population lives within 100 km of the coast (United Nations 2017). Sea levels are on the rise around the World and the trend is accelerating every year. The UN, countless international organizations and national agencies are working hard every year to support the efforts of climate scientists to accurately model our changing climate. The role of ice in shaping the Earth’s seas is indisputable. As continental ice melts and as ice sheets break off from continental shelves and fall into the sea, more and more coastal communities are threatened. Rising sea levels pose enormous challenges for the future.
Having accurate climate models allows policy-makers to make informed decisions about climate change mitigation efforts. One of the greatest sources of uncertainty in our climate models is the contribution of continental ice shelves to rising sea levels (Visser et al. 2000). Their dynamics are unpredictable. Some continental ice shelves seem to grow while others shrink. The motion of ice sheets is complex and yet essential to understanding how continental ice shelves change over time. Understanding the contribution of continental ice shelves to sea level rise is essential in informing good climate policy. Radioglaciology can offer scientists valuable data to refine and adjust their climate models.
Radioglaciology is the study of glaciers and ice sheets using radar. Radioglaciology uses ground penetrating radar operating in the Medium, High and Very High Frequency ranges to study the structure of continental ice shelves, the dynamics of ice sheet motion and the basal conditions under the ice sheets. In its simplest form, a radioglaciology payload emits a radio signal towards an ice mass and records the return signal. The approach focuses on the remote polar areas of greatest scientific interest like Antarctica and Greenland. Historically, airborne radar has been used to perform short-term studies of polar ice, but airborne platforms suffer from weather limitations and require pilots to perform risky flights in remote areas. An example of such a mission was the effort led by the Alfred Wegener Institute to find the oldest stable ice formation on Earth to extract an ancient ice core. The mission was called “Oldest Ice Reconnaissance Dome Fuji” (OIR). An article on The European Facility for Airborne Research (EUFAR)’s website notes: “To carry out the airborne survey in such a remote area, complex logistics were necessary to set up a temporary camp and to supply it with fuel to enable multiple flights, and supply engines and electricity” (Guy and Binder 2017). There is, however, another way to access these remote areas beyond the reach of terrestrial weather while having frequent revisit capabilities and mission endurances measured in years – space-based radioglaciology.
Past missions like Cryosat-2 and the RadarSat missions have demonstrated that the approach is feasible and that satellites can obtain useful data (Wingham et al 2006) (Jezek 1999). Satellite missions tailored specifically for radioglaciology could give climate scientists a better idea of the below-sea-level environment on which continental ice shelves rest which is important in predicting the motion of ice shelves. Basal Roughness of the seabed determines how quickly the ice sheets can move once they get going. As the ice near the bed begins to flow, deformations within the ice sheet build up, thus restraining the flow of the ice. These lateral shear margins can be characterized using radioglaciology despite being deep within the internal structure of ice sheets. Understanding how these shear margins evolve in real time can help scientists refine their predictions on how ice sheets will flow (Jordan et al 2017). There are many other variables which help characterize ice flows including the presence of melt-water, voids within ice sheets and basal conditions – all of these are within the reach of space-based radioglaciology missions.
The data itself can be interpreted in many ways. One of the most common are radar sounding data maps like figure 1. Radar sounding can reveal the internal structure of glaciers through englacial radio stratigraphy by measuring the refracted and reflected radio signals returned from ice sheets. Radioglaciology however is not without limitations.
Some of the major challenges facing radio glaciology are unknown temperatures within ice sheets and unknown ice chemistry making the attenuation calibration essential to receiving good data challenging (Schroeder et al 2016). Data analysis at scale is also a challenge. These challenges are also compounded with all the challenges of operating spacecraft when the radioglaciology experiment is hosted aboard a satellite.
Innovative techniques from other fields like laser communication can downlink the enormous amounts of data and neural networks can help sift through the data returned by radioglaciology satellites. Core samples, pattern analysis and other measurements can offer insights into the temperatures inside ice sheets and their chemical composition. These insights, in turn, make it easier to accurately interpret radioglaciology data.
A clear advantage of space-based radio glaciology is that it can be applied to other celestial bodies. Of particular interest are the Gas Giant moons Europa, Enceladus and Ganymede. Data from other celestial bodies can refine the models we use for ice flows on Earth. The major challenges associated with radio glaciology for other celestial bodies are the power constraints (solar energy flux at Jupiter is 4% of the flux at Earth) for the satellite and payload and the constrained data downlink rate (Dawson et al 2012). The lessons learned from optimizing radio glaciology payloads for deep space applications can be applied to terrestrial systems to improve the on-board data analysis techniques and increase useful data throughput to ground stations while operating more powerful and efficient radars for Earth Observation Missions.
Ongoing deep space radioglaciology missions focus on Mars. The Italian Space Agency (ASI) has led the development of the MARSIS and SHARAD payloads for Mars Express (ESA) and the Mars Reconnaissance Orbiter (NASA) (Flamini et al 2007). MARSIS and SHARAD are meant to work together. MARSIS offers excellent penetration to examine the deeper structures of Martian ice formations while SHARAD has much better resolution at shallower depths up to 1 km. Side by side images of the two radars working together are shown in Figure 2 below. These radars have improved our understanding of radioglaciology for planetary research applications and have yielded discoveries about the history of Mars’ hypothesized water cycle before the planet was stripped of liquid water on its surface. These discoveries offer scientific insights into the dynamics affecting terrestrial ice.
Future space-based radioglaciology experiments are planned both for terrestrial studies and for extreme deep-space applications. ESA’s JUICE (Jupiter Icy Moons Explorer) will carry the RIME (Radar for Icy Moons Exploration) instrument which is a single-frequency radioglaciology payload operating in the 9 MHz band (HF) (Bruzzone et al 2015) while NASA’s Europa Clipper mission is planned to carry REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) which is a radioglaciology payload that will have two frequencies to probe Europa (one HF like RIME and one VHF) (Pappalardo et al 2017). These missions are both slated to launch in the 2020s and have very ambitious scientific objectives. The discovery of liquid water beneath the protective icy crusts of the Gas Giant moons could revolutionize the way we see the Space - Water relationship. If the oceans are discovered to be lifeless, it could offer scientists a sterile environment in which to study macroscopic hydrospheres without organic variables at play. If the oceans are discovered to have life, then a whole host of new scientific questions will be posed.
As the climate crisis worsens over the coming years, climate models that inform public policy will become more and more integral to managing the fallout of climate change. Superior climate models enabled by space-based radioglaciology have the potential to save lives, protect communities and secure valuable assets while enabling future projects to mitigate and perhaps reduce the impact of climate change. Coupled with other technologies, radioglaciology will give us a better idea of the state of the water domain on Earth.