Needle ice

Abstract: Frost heave is the process by which the freezing of water-saturated soil causes the deformation and upward thrust of the ground surface. We describe the fundamental interactions between phase change and fluid flow in partially frozen, saturated porous media (soils) that are responsible for frost heave. Water remains only partially frozen in a porous medium at temperatures below C owing both to the depression of the freezing temperature at curved phase boundaries and to interfacial premelting caused by long-range intermolecular forces. We show that while the former contributes to the geometry of fluid pathways, it is solely the latter effect that generates the forces necessary for frost heave. We develop a simple model describing the formation and evolution of the ice lenses (layers of ice devoid of soil particles) that drive heave, based on integral force balances. We determine conditions under which either (i) a single ice lens propagates with no leading frozen fringe, or (ii) a single, propagating ice lens is separated from unfrozen soil by a partially frozen fringe, or (iii) multiple ice lenses form.

Abstract: [1] A primary objective of the Phoenix mission was to examine the characteristics of high latitude ground ice on Mars. We report observations of ground ice, its depth distribution and stability characteristics, and examine its origins and history. High latitude ground ice was explored through a dozen trench complexes and landing thruster pits, over a range of polygon morphological provinces. Shallow ground ice was found to be abundant under a layer of relatively loose ice-free soil with a mean depth of 4.6 cm, which varied by more than 10x from trench to trench. These variations can be attributed mainly to slope effects and thermal inertia variations in the overburden soil affecting ground temperatures. The presence of ice at this depth is consistent with vapor-diffusive equilibrium with respect to a mean atmospheric water content of 3.4 × 1019 m−3, consistent with the present-day climate. Significant ice heterogeneity was observed, with two major forms: ice-cemented soil and relatively pure light toned ice. Ice-cemented soils, which comprised about 90% of the icy material exposed by trenching, are best explained as vapor deposited pore ice in a matrix supported porous soil. Light toned ice deposits represent a minority of the subsurface and are thought to consist of relatively thin near surface deposits. The origin of these relatively pure ice deposits appears most consistent with the formation of excess ice by soil ice segregation, such as would occur by thin film migration and the formation of ice lenses, needle ice, or similar ice structures.

Abstract: ANALYSIS OF THE BEHAVIOR OF AIR--WATER, ICE--WATER, AND AIR--ICE INTERFACES SHOWS THAT THE APPARENT CONTACT ANGLE BETWEEN AN AIR--ICE INTERFACE AND THE WALL OF A SOIL PORE OUGHT TO VARY RAPIDLY WITH CHANGES IN PORE WATER PRESSURE AND ICE PRESSURE. THIS PAPER GIVES THE EXPECTED RELATIONSHIP AND SHOWS THAT COOLING OF FAIRLY DRY SOIL, WITH FREEZING NUCLEATED AT ONE SPOT, SHOULD CAUSE CERTAIN PORES TO FILL ABRUPTLY WITH ICE, THUS DEPLETING THE WATER CONTENT OF THE SURROUNDING SOIL. THIS CONCLUSION AGREES WITH AVAILABLE DATA. IT IS SUGGESTED THAT THE ICE PRESSURE IS SLIGHTLY GREATER THAN ATMOSPHERIC PRESSURE WHEN PORES FILL. LIMITED DATA AVAILABLE IN THE LITERATURE INDICATE THAT A GIVEN MOIST SOIL MAY OR MAY NOT HEAVE; WHEN IT DOES, THE ICE LENSES APPARENTLY FORM SOME DISTANCE BEHIND THE FREEZING FRONT. TO EXPLAIN THIS, THE CONCEPTS OF PRIMARY AND SECONDARY HEAVING ARE PROPOSED FOR SATURATED SOIL. PRIMARY HEAVING OCCURS WHEN THE BASE OF THE GROWING ICE LENS COINCIDES WITH THE LIMIT OF FREEZING, AND THE RATE OF HEAVING IS LIMITED BY THE RATE OF HEAT EXTRACTION. SECONDARY HEAVING IS BELIEVED TO OCCUR WHEN THE FREEZING EXTENDS BELOW THE NOMINAL BASE OF THE (VISIBLE) ICE LENS. ICE IN THE FROZEN PORES CAN MOVE, RELATIVE TO THE PARTICLES, AS AN INTEGRAL PART OF THE GROWING LENS. A SOLUTION MODEL OF SECONDARY HEAVING IS USED TO ILLUSTRATE AN INCREASE OF ICE PRESSURE FROM THE FREEZING FRONT TO THE BASE OF THE ICE LENS, WHICH WILL OCCUR AT A POINT WHERE ICE PRESSURE IS EQUAL TO THE OVERBURDEN PRESSURE. IT IS CONTENTED THAT SECONDARY HEAVING PRODUCES LARGER HEAVING PRESSURES THAN PRIMARY HEAVING AND EXPLAINS WHY PREVIOUS THEORIES UNDERSTIMATED THE MAXIMUM HEAVING PRESSURE OF SATURATED SOIL. IT IS INFERRED THAT WHENEVER HEAVING OCCURS AS THE FREEZING FRONT IS DESCENDING THROUGH THE SOIL, THE PROCESS MUST BE SECONDARY HEAVING. IT IS SUGGESTED THAT THE DEVELOPMENT OF SIGNIFICANT HEAVING PRESSURES BY UNSATURATED SOIL WILL ORDINARILY INVOLVE SECONDARY HEAVING AND THAT THE PRESSURE DEVELOPED IS LESS THAN THAT DEVELOPED BY THE SAME SOIL WHEN IT IS SATURATED. /AUTHOR/