Alberto Behar

Efficient meltwater drainage through supraglacial streams and rivers on the southwest Greenland ice sheet

Significance Meltwater runoff from the Greenland ice sheet is a key contributor to global sea level rise and is expected to increase in the future, but it has received little observational study. We used satellite and in situ technologies to assess surface drainage conditions on the southwestern ablation surface after an extreme 2012 melting event. We conclude that the ice sheet surface is efficiently drained under optimal conditions, that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater release from the ice sheet. Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km2 of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54–2.81 cm⋅d−1) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41–98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056–0.112 km3⋅d−1 vs. ∼0.103 km3⋅d−1), and when integrated over the melt season, totaled just 37–75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean.

NASA/JPL Tumbleweed polar rover

The Tumbleweed rover, currently under development at the Jet Propulsion Laboratory (JPL) in Pasadena, California, is a large, windblown, inflated ball, which carries an instrument payload in its interior. Such rovers offer an effective and simple means of gathering data over large spatial extents of Earth, Mars, and other solar system bodies. Tumbleweeds could prove to be a safe and economical way of deploying instruments such as a ground penetrating radar or a magnetometer in numerous hostile environments. The latest version of the rover was recently deployed in Greenland, where it completed a more than 130km autonomous traverse across an ice sheet. Communicating via the Iridium satellite network, the rover in question successfully and reliably relayed live GPS, temperature, and pressure data to a ground station at JPL for nearly ten days. The follow-on rover is currently being readied for a traverse from the South Pole to the coast of Antarctica some 2000km away. The Antarctic test is set to take place in February of 2004 and will serve to verify Tumbleweed as an effective means of harvesting data in extreme and remote settings.

An Overview of Wind-Driven Rovers for Planetary Exploration

The use of in-situ propulsion is considered enabling technology for long duration planetary surface missions. Most studies have focused on stored energy from chemicals extracted from the soil or the use of soil chemicals to produce photovoltaic arrays. An older form of in-situ propulsion is the use of wind power. Recent studies have shown potential for wind driven craft for exploration of Mars, Titan and Venus. The power of the wind, used for centuries to power wind mills and sailing ships, is now being applied to modern land craft. Efforts are now underway to use the wind to push exploration vehicles on other planets and moons in extended survey missions. Tumbleweed rovers are emerging as a new type of wind-driven science platform concept. Recent investigations by the National Aeronautics and Space Administration (NASA) and Jet Propulsion Laboratory (JPL) indicate that these light-weight, mostly spherical or quasi-spherical devices have potential for long distance surface exploration missions. As a power boat has unique capabilities, but relies on stored energy (fuel) to move the vessel, the Tumbleweed, like the sailing ships of the early explorers on earth, uses an unlimited resource the wind to move around the surface of Mars. This has the potential to reduce the major mass drivers of robotic rovers as well as the power generation and storage systems. Jacques Blamont of JPL and the University of Paris conceived the first documented Mars wind-blown ball in 1977, shortly after the Viking landers discovered that Mars has a thin CO2 atmosphere with relatively strong winds. In 1995, Jack Jones, et al, of JPL conceived of a large wind-blown inflated ball for Mars that could also be driven and steered by means of a motorized mass hanging beneath the rolling axis of the ball. A team at NASA Langley Research Center started a biomimetic Tumbleweed design study in 1998. Wind tunnel and CFD analysis were applied to a variety of concepts to optimize the aerodynamic characteristics of the Tumbleweed Rovers. Bare structures, structures carrying sails and a tumbleweed plant (of the Salsola genus) were tested in Langleys wind tunnels. Thomas Estier of the Swiss Federal Institute of Technology developed a memory metal collapsible structure, the Windball. Numerous other researchers have also suggested spherical rovers.

Direct measurements of meltwater runoff on the Greenland ice sheet surface

Significance Meltwater runoff is an important hydrological process operating on the Greenland ice sheet surface that is rarely studied directly. By combining satellite and drone remote sensing with continuous field measurements of discharge in a large supraglacial river, we obtained 72 h of runoff observations suitable for comparison with climate model predictions. The field observations quantify how a large, fluvial supraglacial catchment attenuates the magnitude and timing of runoff delivered to its terminal moulin and hence the bed. The data are used to calibrate classical fluvial hydrology equations to improve meltwater runoff models and to demonstrate that broad-scale surface water drainage patterns that form on the ice surface powerfully alter the timing, magnitude, and locations of meltwater penetrating into the ice sheet. Meltwater runoff from the Greenland ice sheet surface influences surface mass balance (SMB), ice dynamics, and global sea level rise, but is estimated with climate models and thus difficult to validate. We present a way to measure ice surface runoff directly, from hourly in situ supraglacial river discharge measurements and simultaneous high-resolution satellite/drone remote sensing of upstream fluvial catchment area. A first 72-h trial for a 63.1-km2 moulin-terminating internally drained catchment (IDC) on Greenland’s midelevation (1,207–1,381 m above sea level) ablation zone is compared with melt and runoff simulations from HIRHAM5, MAR3.6, RACMO2.3, MERRA-2, and SEB climate/SMB models. Current models cannot reproduce peak discharges or timing of runoff entering moulins but are improved using synthetic unit hydrograph (SUH) theory. Retroactive SUH applications to two older field studies reproduce their findings, signifying that remotely sensed IDC area, shape, and supraglacial river length are useful for predicting delays in peak runoff delivery to moulins. Applying SUH to HIRHAM5, MAR3.6, and RACMO2.3 gridded melt products for 799 surrounding IDCs suggests their terminal moulins receive lower peak discharges, less diurnal variability, and asynchronous runoff timing relative to climate/SMB model output alone. Conversely, large IDCs produce high moulin discharges, even at high elevations where melt rates are low. During this particular field experiment, models overestimated runoff by +21 to +58%, linked to overestimated surface ablation and possible meltwater retention in bare, porous, low-density ice. Direct measurements of ice surface runoff will improve climate/SMB models, and incorporating remotely sensed IDCs will aid coupling of SMB with ice dynamics and subglacial systems.

Polar traverse rover development for Mars, Europa and Earth

The history of Antarctic exploration is filled with traverses for scientific and operational purposes, and they continue to be conducted today. Similarly, traverses are a crucial part of planned planetary science and operations. This paper addresses the proposal for development of an autonomous rover capable of making such extreme environment traverses while accomplishing complex tasks.

Tumbleweed: A New Paradigm for Surveying the Surface of Mars for In-Situ Resources

Mars missions to date have interrogated the planet at very large scales using orbital platforms or at very small scales intensively studying relatively small patches of terrain. In order to facilitate discovery and eventual utilization of Martian resources for future missions, a strategy that will bridge these scales and allow assessment of large areas of Mars in pursuit of a resource base will be essential. Long-range surveys of in-situ resources on the surface of Mars could be readily accomplished with a fleet of Tumbleweeds - vehicles capable of using the readily available Martian wind to traverse the surface of Mars with minimal power, while optimizing their capabilities to perform a variety of measurements over relatively large swaths of terrain. These low-cost vehicles fill the niche between orbital reconnaissance and landed rovers, which are capable of much more localized study. Fleets of Tumbleweed vehicles could be used to conduct long-range, randomized surveys with simple, low-cost instrumentation functionally equivalent to conventional coordinate grid sampling. Gradients of many potential volatile resources (e.g. H2O, CH4, etc.) will also tend to follow wind-borne trajectories thus making the mobility mode of the vehicles well matched to the possible target resources. These vehicles can be suitably instrumented for surface and near-surface interrogation and released to roam for the duration of a season or longer, possibly on the residual ice cap or anywhere orbital surveillance indicates that usable resources may exist. Specific instrument selections can service the exact exploration goals of particular survey missions. Many of the desired instruments for resource discovery are currently under development for in-situ applications, but have not yet been miniaturized to the point where they can be integrated into Tumbleweeds. It is anticipated that within a few years, instruments such as gas chromatograph mass spectrometers (GC-MS) and ground-penetrating radar (GPR) will be deployable on Tumbleweed vehicles. The wind-driven strategy conforms to potential natural gradients of moisture and potentially relevant resource gases that also respond to wind vectors. This approach is also useful for characterizing other resources and performing a variety of basic science missions. Inflatable and deployable structure Tumbleweeds are wind-propelled long-range vehicles based on well-developed and field tested technology (Antol et al., 2005; Behar et al., 2004; Carsey et al., 2004; Jones and Yavrouian, 1997; Wilson et al., 2008). Different Tumbleweed configurations can provide the capability to operate in varying terrains and accommodate a wide range of instrument packages making them suitable for autonomous surveys for in-situ natural resources. Tumbleweeds are lightweight and relatively inexpensive, making them very attractive for multiple deployments or piggybacking on larger missions.

The JPL PAUSE aerobot

The PAUSE (Picosat and Uninhabited Aerial Vehicle Systems Engineering) project, currently under development at the Jet Propulsion Laboratory in Pasadena, California and the University of California at Los Angeles, is a high-altitude balloon-based aerobot which carries a gondola outfitted with various instruments. An aerobot is a robot designed to fly in the atmosphere of planets and moons, providing extensive regional access while gathering high resolution data. Aerobots have a multitude of applications such as mapping a terrestrial surface, ground surveillance, and in-situ atmospheric composition surveying. The latest version of PAUSE was deployed from Oregon where it successfully relayed live GPS, temperature, altitude, velocity, battery status, images, and magnetometer data to multiple ground stations. The goals of PAUSE are to demonstrate that existing technologies can be used for Aero-Robotic exploration and to develop new technology where existing options prove unsatisfactory.

Simple sensors for performing useful tasks autonomously in complex outdoor terrain

This paper describes the control system for Rocky IV, a prototype microrover designed to demonstrate proof-of-concept for a low-cost scientific mission to Mars. Rocky IV uses a behavior-based control architecture which implements a large variety of functions displaying various degrees of autonomy, from completely autonomous long-duration conditional sequences of actions to very precisely described actions resembling classical AI operators. The control system integrates information from infrared proximity sensors, proprioceptive encoders which report on the state of the articulation of the rovers suspension system and other mechanics, a homing beacon, a magnetic compass, and contact sensors. In addition, significant functionality is implemented as virtual sensors, computed values which are presented to the system as if they were sensors values. The robot is able to perform a variety of useful tasks, including soil sample collection, removal of surface weathering layers from rocks, spectral imaging, instrument deployment, and sample return, under realistic mission- like conditions in Mars-like terrain.

Correction to Supporting Information for Smith et al., Direct measurements of meltwater runoff on the Greenland ice sheet surface

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Correction to Supporting Information for “Direct measurements of meltwater runoff on the Greenland ice sheet surface,” by Laurence C. Smith, Kang Yang, Lincoln H Pitcher, Brandon T. Overstreet, Vena W. Chu, Åsa K. Rennermalm, Jonathan C. Ryan, Matthew G. Cooper, Colin J. Gleason, Marco Tedesco, Jeyavinoth Jeyaratnam, Dirk van As, Michiel R. van den Broeke, Willem Jan van de Berg, Brice Noël, Peter L. Langen, Richard I. Cullather, Bin Zhao, Michael J. Willis, Alun Hubbard, Jason E. Box, Brittany A. Jenner, and Alberto E. Behar, which was first published December 5, 2017; 10.1073/pnas.1707743114 (Proc Natl Acad Sci USA 114:E10622–E10631). The authors note that in the SI Appendix, page 23, line 570, “tp = Ct(Lc) 0.3 ” should instead appear as “tp = Ct(LLc) .” Additionally, on page 24 of the SI Appendix, line 613, Eq. 1 should instead appear as:

Moball: An intelligent wind-opportunistic mobile sensor to monitor the polar regions

Sensors capable of operating in harsh environments for long periods of time and having wireless readout capability are needed for mapping environmental conditions in remote areas, such as polar regions. In this paper, we report the first measured results of an innovative in-situ system of controllable and wind-opportunistic spherical mobile sensors (called Moballs), used to monitor and map various environmental factors in polar areas. To have self-powered controlled motion, Moballs exploit the abundance of wind and a novel dual-functioning mechanical control and linear induction system. Moballs have peer-to-peer and Moball-to-base (e.g. satellite) communication capability, reporting the sensory data back to the base station or other peer Moballs for task sharing decisions, improving area coverage, optimizing system performance, etc.