Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p43b..07d&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P43B-07
Other
5114 Permeability And Porosity, 5430 Interiors (8147), 6020 Ice, 6062 Satellites, 6280 Saturnian Satellites
Scientific paper
A series of experiments was carried out to measure the hydrostatic compaction of granular ice I as a function of pressure at low temperatures. Most runs were conducted on the same initial size distribution of ice granules (0.18 - 0.25 mm) and most were conducted at the same temperature of 77 K. For one run the size range of the granules was wider (0.25 - 2.0 mm), and for one run the temperature was 120 K. Granulated ice was sealed in an indium metal jacket, the sample placed into a cyrogenic pressure vessel, and hydrostatic pressure applied. Initial and final porosity were calculated from mass and volume measurements. Porosity during the experiments, when the sample was inside the pressure vessel undergoing compaction, was calculated from the length of the sample (measured in situ) assuming that porosity change and length change were proportional. The starting and ending shapes of the capsule were always right cylindrical, suggesting that the compaction was uniform throughout the samples. In the experiments, the rate of change of volume with pressure was highest at low pressure and decreased monotonically with increasing pressure. Surprisingly, the samples continued to compact with increasing pressure even at the highest pressures achieved (150 MPa). This observation is consistent with the existence of substantial residual porosity (10-15%) measured in the samples and confirmed by SEM observation after testing. The sample tested at 120 K had a compaction curve indistinguishable from the those of the other three samples of 0.18-0.25-mm ice, all tested at 77 K. Because creep is generally a very temperature-sensitive phenomenon, we infer that creep was not an important process in these tests. The sample with the wider range of granule size started with lower porosity (as expected), and ended with lower porosity. We conclude that over the interior pressures found in smaller midsize icy satellites and Kuiper Belt objects (KBOs), significant porosity can be sustained over solar system history in the absence of significant heating and sintering. Phoebe's Cassini-derived density of 1.6 g cm-3 is consistent with a solid, non-porous density of 2.0 (from Pluto and Triton, the largest KBOs), mechanical domination by ice I, and an average porosity of 20%. Interior pressures for Phoebe reach ˜5 MPa; the porosity of the experimental sample with the wider granule size range was ˜25-30% in this modest pressure range. Thermal evolution models for porous ice-rock bodies of Phoebe's size (Proc. ACM 2002, ESA-SP-500, 29-38) indicate peak interior temperatures <150 K after 50 Myr, so in the absence of creep-driven densification, we hypothesize that Phoebe's "modest" porosity is due to its being built from a wide size spectrum of ice-rock fragments.
Durham William B.
McKinnon William B.
Stern Laura A.
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