arXiv subjects

Robert Law

Publications and source records attributed to Robert Law.

4 recordsLinked to original sources

arXiv2024

Exploring the conditions conducive to convection within the Greenland Ice Sheet

Large plume-like features within the Greenland Ice Sheet disrupt radiostratigraphy and complicate the use of isochrones in reconstructions of past ice dynamics. Here we use numerical modeling to test the hypothesis that convection is a viable mechanism for the formation of the large (>1/3 ice thickness) englacial plume-like features observed in north Greenland. Greater horizontal shear and snow accumulation impede formation of convection plumes, while stable and softer ice encourages them. These results potentially explain the dearth of basal plumes in the younger and higher-accumulation southern ice sheet. Leveraging this mechanism to place bounds on ice rheology suggests that -- for north Greenland -- ice viscosity may be ~9-15 times lower than commonly assumed. Softer-than-assumed ice there implies significantly reduced basal sliding compared to standard models. Implementing a softer basal ice rheology in numerical models may help reduce uncertainty in projections of future ice-sheet mass balance.

physics.geo-ph↗

arXiv2024

What is glacier sliding

Glacier and ice-sheet motion is fundamental to glaciology. However, we still lack a consensus for the optimal way to relate basal velocity to basal traction for large-scale glacier and ice-sheet models (the 'sliding relationship'). Typically, a single tunable coefficient loosely connected to one or a limited number of physical processes is varied spatially to reconcile model output with observations. Yet, process-agnostic studies indicate that the suitability of a given sliding relationship depends on the setting. Here, we suggest that this arises from myriad overlapping setting- and scale-dependent sliding sub-processes, including complicated near-basal stress states not captured by large-scale models, reviewed here as comprising a basal 'sliding layer'. A corresponding 'bulk layer' then accounts for ice deformation only minimally influenced by bed properties. We provide a framework for incorporating arbitrarily many sub-processes within a given region -- separated into normal ('form drag') and tangential ('slip') resistance at the ice-bed interface, stressing that the maximum scale of cavitation is an important contributor to the division between the two. Under reasonable assumptions, our framework implies that sliding relationships should fall within a sum of regularised-Coulomb and power-law components, with a rough-smooth distinction proving more consequential in dictating sliding behaviour than a traditional hard-soft transition.

physics.geo-ph↗

arXiv2024

Rapid hydrofracture of icy moon shells: insights from glaciology

Europa's surface exhibits many regions of complex topography termed 'chaos terrains'. One set of hypotheses for chaos terrain formation requires upward migration of liquid water from perched water bodies within the icy shell formed by convection and tidal heating. However, consideration of the behavior of terrestrial ice sheets suggests the upwards movement of water from englacial water bodies is uncommon. Instead, rapid downwards hydrofracture from supraglacial lakes - unbounded given a sufficient volume of water - can occur in relatively low tensile stress states given a sufficiently deep initial fracture due to the negative relative buoyancy of water. I suggest that downwards, not upwards, fracture may be more reasonable for perched water bodies but show that full hydrofracture is unlikely if the perched water body is located beneath a mechanically strong icy lid. However, full hydrofracture is possible in the event of lid break up over a perched water body and likely in the event of a meteor impact that generates sufficient meltwater and a tensile shock. This provides a possible mechanism for the transfer of biologically important nutrients to the subsurface ocean and the formation of chaos terrains.

astro-ph.EP↗

arXiv2024

Cosmic dust fertilization of glacial prebiotic chemistry on early Earth

Earth's surface is deficient in available forms of many elements considered limiting for prebiotic chemistry. In contrast, many extraterrestrial rocky objects are rich in these same elements. Limiting prebiotic ingredients may, therefore, have been delivered by exogenous material; however, the mechanisms by which exogeneous material may be reliably and non-destructively supplied to a planetary surface remains unclear. Today, the flux of extraterrestrial matter to Earth is dominated by fine-grained cosmic dust. Although this material is rarely discussed in a prebiotic context due to its delivery over a large surface area, concentrated cosmic dust deposits are known to form on Earth today due to the action of sedimentary processes. Here we combine empirical constraints on dust sedimentation with dynamical simulations of dust formation and planetary accretion to show that localized sedimentary deposits of cosmic dust could have accumulated in arid environments on early Earth, in particular glacial settings that today produce cryoconite sediments. Our results challenge the widely held assumption that cosmic dust is incapable of fertilizing prebiotic chemistry. Cosmic dust deposits may have plausibly formed on early Earth and acted to fertilize prebiotic chemistry.

astro-ph.EP↗