Lunar and Planetary Science XLVIII (2017)
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THERMOMECHANICAL BEHAVIOR OF ICE AND ICE-ROCK MIXTURES AT THE MINERAL
GRAIN SCALE. J. L. Molaro and C. B. Phillips, NASA-Jet Propulsion Laboratory, California Institute of Technology (MS 183-205, 4800 Oak Grove drive, Pasadena, CA 91109; [email protected])
Introduction: A growing body of research suggests
that thermally induced rock breakdown is an active
process in the solar system, operating on the Moon,
Mercury, Mars, Earth, and near-Earth (and possibly
main belt) asteroids [1-8]. Diurnal, thermal stresses
induced at macro- and microscopic scales serve to
propagate microcracks and fractures in rocks on these
surfaces, driving granular disintegration and boulder
disaggregation over time. Models provide valuable
insight into the amplitude of these induced stresses, as
well as the spatiotemporal nature of stress fields. At
macroscopic scales, stresses of varying magnitude are
induced in different locations and at different times of
day [e.g., 8, 9] within boulders, controlled by their size
and shape. While these stresses contribute to the location and extent of induced damage, crack propagation
itself is driven at microscopic scales.
Molaro et al. (2015) [1] modeled heterogeneous
mineral-grain microstructures composed of pyroxene
and plagioclase (typical of basalt) on airless body surfaces, and found that the amplitude of stresses (Figure
1) is controlled by the diurnal temperature range and
the elastic properties of the mineral constituents. Stress
fields develop at heterogeneous grain boundaries that
experience differential behavior during expansion and
contraction. Stresses are also amplified in regions
where these fields interact, suggesting that the path that
a crack will take through the microstructure in order to
relieve stress will be controlled by grain distribution.
Stresses on the order of 100s of MPa are induced
in basaltic microstructures on the Moon, increasing
Figure 1. Peak tensile stress in a lunar microstructure
of 75% pyroxene (lighter grains) and 25% plagioclase
(darker grains), data from Molaro et al. (2015).
with increased day length and decreased solar distance.
The strength of materials at the grain scale and in vacuum environments is not well constrained [e.g., 1, 8],
making it challenging to predict how thermally induced breakdown compares with other weathering
processes (e.g., micrometeorite bombardment). However, research [1, 3, 8] suggests that even if only a
small amount of crack propagation occurs each day,
asteroids with rapid cycling rates may be particularly
susceptible to this process.
The work of Molaro et al. (2015) focused on rocky
airless bodies, however their work also suggests thermally induced stresses may play a role on icy surfaces.
In spite of their lower diurnal temperature range, surfaces composed of ice-rock mixtures can sustain significant grain-scale stresses (Figure 2) due to the orderof-magnitude difference in their coefficients of thermal
expansion. Additionally, the thermal expansion coefficient of ice is strongly temperature dependent, and is
negative below ~70 K [19], which may have important
implications for the thermomechanical response of
objects such as comets to changing indicent solar flux
throughout their orbits.
This is consistent with the findings of previous
models, which suggest that thermal stresses drive the
development cometary features, such as the surface
fractures and loose regolith on comet 67P/ChuryumovGerasimenko [e.g., 10], and contribute to larger scale
cometary splitting [e.g., 11, 12]. Recent work also suggests that cometary outbursts are the result of landslides rather than sublimation pressure [13], which
may be triggered by the response of the landscape to
thermal cycling.
Observations from the Dawn mission have also revealed that the crust of Ceres is composed of an icerock mixture, leading to the formation of ice-related
geomorphic features [e.g., 14], and fractures dominate
the surfaces of icy moons such as Europa. Thermal
stresses may also contribute to microcrack propagation
and regolith production on these surfaces, and/or affect
the bulk elastic properties of the near-surface, interacting with global and landscape-scale processes in the
formation of larger scale features. Understanding the
difference in thermomechanical behavior and bulk
properties of ice, rock, and ice-rock mixtures is crucial
to understanding the surface evolution of rocky and icy
objects throughout the solar system.
We present preliminary results of simulated temperatures and stresses in a microstructure composed of
Lunar and Planetary Science XLVIII (2017)
a mixture of ice and rock at the surface of Ceres. We
compare the behavior of rock and ice-rock microstructures, and the amplitude of their stresses.
Model: Following Molaro et al. (2015) [1], we
used a 2-D finite-element modeling program (OOF2)
[15] to model the diurnal temperatures and stresses of
microstructures. We imposed solar and conductive
fluxes (calculated from a 1D thermal model) on a microstructure over one solar day, and solved the heat
and displacement equations. The boundary conditions
are defined to simulate a microstructure embedded in
an infinite half-space. Thus this model focuses on
grain-scale processes only, and is suitable for investigating the thermoelastic behavior of large rock faces
where the effects of size, shape, and surface curvature
do not come into play. The microstructures are grids
of hexagonal grains (360 µm in diameter) that are assigned properties of water ice and pyroxene.
Preliminary Results: Figure 2 shows the maximum principal stress (where positive stresses are tensile) induced a heterogeneous microstructure composed of 75% water ice and 25% pyroxene. Results
show that ice-rock microstructures on Ceres experience a peak tensile stress of ~9.5 MPa, an order of
magnitude lower than in the rock microstructure composed of pyroxene and plagioclase (Figure 1). Regions
where ice grains are contacting multiple pyroxene
grains along surface-parallel boundaries (black arrows)
show the highest tensile stresses. Additionally, regions
where ice grains are contacting multiple pyroxene
grains along surface-perpendicular boundaries (white
arrows) show compressional stresses, as the strong
contraction of the ice grains induces compression within pyroxene grains or grain clusters. A similar effect
occurs during compressional states, leading to both
tensile and compressional stresses within the microstructure at all times of day. In contrast, the rock microstructure experiences variations in either tension or
Figure 2. Peak tensile stress in a microstructure of 75%
water ice and 25% pyroxene, using the same hexagon
distribution as in Figure 1.
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compression, but not both simultaneously. In both microstructures, the highest stresses concentrate within
the grains or regions that have the largest Young’s
modulus (and thus are the “stiffest”).
During the state of peak tension, the maximum
stresses is in the rock microstructure ~2x the mean
stress. However, the ice-rock microstructure has a
much less uniform stress field, with a maximum stress
that is ~7x the mean stress. The concentration of the
highest stresses in fewer locations suggests that the
thermomechanical behavior of ice-rock mixtures will
strongly depend on relative grain volume and grain
distribution. It also suggests that while the more uniform stresses in rock microstructures may lead to crack
propagation throughout, ice-rock microstructures are
more likely to concentrate propagation in specific locations forming fewer, larger-scale features. This has
important implications for the rate and nature of
breakdown on these surfaces, as well as the properties
of disaggregated material.
We will present results of simulated temperatures
and stresses in microstructures with varying volumes
of ice and rock grains, on the surface of Ceres,
67P/Churyumov-Gerasimenko, and Europa. We will
compare the thermomechanical behavior of rocky and
icy microstructures, and discuss implications for fracture propagation, boulder and landscape breakdown,
and regolith production on their surfaces.
Acknowledgement: This work was supported by
an appointment to the NASA Postdoctoral Program at
the Jet Propulsion Laboratory, California Institute of
Technology, under contract with NASA, administered
by the Universities Space Research Association
through a contract with the NASA.
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