Journal of Geophysical Research: Planets
Experimental Constraints on the Fatigue of Icy Satellite
Lithospheres by Tidal Forces
Noah P. Hammond
, Amy C. Barr
, Reid F. Cooper
, Tess E. Caswell
, and Greg Hirth
Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA,
Institute, Tucson, AZ, USA
Fatigue can cause materials that undergo cyclic loading to experience brittle failure at much
lower stresses than under monotonic loading. We propose that the lithospheres of icy satellites could
become fatigued and thus weakened by cyclical tidal stresses. To test this hypothesis, we performed
a series of laboratory experiments to measure the fatigue of water ice at temperatures of 198 K and 233 K
and at a loading frequency of 1 Hz. We ﬁnd that ice is not susceptible to fatigue at our experimental
conditions and that the brittle failure stress does not decrease with increasing number of loading cycles.
Even though fatigue was not observed at our experimental conditions, colder temperatures, lower loading
frequencies, and impurities in the ice shells of icy satellites may increase the likelihood of fatigue crack
growth. We also explore other mechanisms that may explain the weak behavior of the lithospheres
of some icy satellites.
Plain Language Summary
Fatigue is an important phenomenon in engineering in which
materials can weaken during cyclic loading, causing fractures to form at much lower stresses than normal.
Many icy satellites on eccentric orbits about their parent planets experience cyclic loading from gravitational
tides. We propose that the surfaces of icy satellites could become fatigued by tidal forces and that billions
of cycles of tidal stress could potentially weaken their surfaces and increase tectonic activity. To test this
hypothesis we perform cyclic loading experiments on water ice in the laboratory at temperatures of
75 degrees Celsius. We ﬁnd that ice does not weaken or fatigue at our experimental conditions. We
argue, however, that colder temperatures on the surface icy satellites could make fatigue more likely.
Fatigue is an extensively studied phenomenon in engineering, where under certain conditions cyclic loading
can cause ﬂaws and microcracks in a material to slowly propagate (Paris & Erdogan, 1963; Rice, 1967; Suresh,
1998). As ﬂaws grow, they create stress concentrations in the material that can reduce the brittle failure stress.
Icy satellites on eccentric orbits about their parent planet experience cyclic loading from tides (Burns &
Matthews, 1986). We propose that fatigue is a process that could potentially occur in the tidally ﬂexed litho-
spheres of icy satellites. Cyclic tidal stresses may act upon preexisting ﬂaws, causing them to slowly propagate,
creating stress concentrations near the surface and gradually reducing the brittle failure stress at the surface.
The weakening of icy lithospheres by this process would have major implications for the geologic evolution
of icy satellites, as the brittle yield stress can control what types of geological process are capable of deform-
ing the surface. A gradual reduction in the strength of the lithosphere through accumulated tidal cycles may
allow icy satellites to become more geologically active over time.
It is unknown whether ice is susceptible to fatigue under tidal loading conditions. Not all materials are
susceptible to fatigue, and in materials that are, the rate of weakening can depend on many factors, includ-
ing temperature, microstructure, chemical environment, and loading frequency (cf. Pelloux, 1969; Stephens,
1985). Laboratory experiments are required to measure the rate of weakening in materials at speciﬁc condi-
tions. In this work, we perform fatigue experiments on fresh water ice in order to understand whether fatigue
might be occurring in the ice shells of icy satellites.
Geologic features on icy satellites can provide clues about the strength of their surfaces. Eruptions of water
vapor from the south pole of Saturn’s satellite Enceladus appear to be regulated by diurnal tidal stresses,
• Cyclic tidal stresses may weaken and
fatigue the surface of icy satellites
• Our experiments show that ice does
not weaken with cyclic loading when
loaded at 1 Hz and temperatures of
198 to 233 K
• Other mechanisms may cause the
lithospheres of icy satellites to appear
weak though fatigue may still occur at
icy satellite conditions
• Supporting Information S1
N. P. Hammond,
Hammond, N. P., Barr, A. C.,
Cooper, R. F., Caswell, T. E., &
Hirth, G. (2018). Experimental
constraints on the fatigue of icy
satellite lithospheres by tidal
forces. Journal of Geophysical
Research: Planets, 123, 390–404.
Received 11 OCT 2017
Accepted 18 JAN 2018
Accepted article online 6 FEB 2018
Published online 10 FEB 2018
©2018. American Geophysical Union.
All Rights Reserved.
HAMMOND ET AL. FATIGUE OF ICY SATELLITES