Enigmatically, some landslides flow farther than normal frictional resistance allows. Cassini images of Saturn’s icy

moon Iapetus reveal a multitude of long-runout landslides that may have been enabled by flash heating along the

sliding surface

A Lucas

Antoine Lucas

AZ Heim

CM Malin

E Lajeunesse

F Legros

FC Chuang

FV De Blasio

GS Collins

I Vardoulakis

KN Singer

L Goren

M Beeman

O Hungr

RJ Rice

English

Lucas, Antoine

Hal-Diderot

Slippery sliding on icy Iapetus

Crossref

Nature Geoscience

Caltech Authors - Main

524 NATURE GEOSCIENCE | VOL 5 | AUGUST 2012 | www.nature.com/naturegeoscience
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unusual event within the past 10,000 years3–5. 
The recent rapid movement of the eccentric 
dipole towards the eastern hemisphere is 
associated with a gathering of magnetic 
field concentrations at high latitude in this 
hemisphere3, and the appearance of a weak 
field anomaly in the south Atlantic region 
that has grown and moved towards the 
west. According to the numerical dynamo 
simulations of Olson and Deguen, similar 
rapid changes in the eccentric dipole position 
often occur when there is a drop in dipole 
intensity, particularly before significant 
directional changes such as full reversals of 
polarity or temporary excursions.
Olson and Deguen5 use a rather simple 
numerical dynamo model to show how 
asymmetric growth of Earth’s inner core may 
contribute to the observed eccentricity of 
the geomagnetic dipole. Extrapolation of the 
details of numerical dynamo calculations to 
the conditions of Earth’s core remains 
controversial, but the prospect of fresh 
insights into the mechanism by which Earth’s 
magnetic field operates is tantalizing. ❐
Christopher C. Finlay is at the National Space 
Institute, Technical University of Denmark, 
Kongens Lyngby, DK-2800, Denmark. 
e-mail: cfinlay@space.dtu.dk
References
1. Schmidt, A. Gerl. Beitr. Geophys. 41, 346–358 (1934).
2. Bartels, J. Terrest. Magn. Atmos. Electr.  
41, 225–250 (1936).
3. Gallet, Y., Hulot, G., Chulliat, A. & Genevey, A. Earth Planet. Sci. 
Lett. 284, 179–186 (2009).
4. Korte, M., Constable, C. G., Donadini, F. & Holme, R. Earth 
Planet. Sci. Lett. 312, 497–505 (2011).
5. Olson, P. & Deguen, R. Nature Geosci. 5, 565–569 (2012).
6. Hide, R. Science 157, 55–56 (1967).
7. Bloxham, J. & Gubbins, D. Nature 325, 511–513 (1987).
8. Monnereau, M., Calvet, M., Margerin, L. & Souriau, A. Science 
238, 1014–1017 (2010).
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466, 744–747 (2010).
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155, 337–370 (2010).
Published online: 1 July 2012
Friction is an unavoidable force of nature, on Earth and beyond. Some landslides, however, travel longer 
horizontal distances over shallow slopes 
than would be expected under the normal 
friction conditions of sliding rock. Several 
mechanisms have been proposed to 
temporarily reduce the friction for these 
long-runout landslides, such as lubrication by 
water or air, thermal pressurization, acoustic 
fluidization or flash heating1–5. An anomalous 
reduction in friction is not limited to Earth 
environments; mass wasting processes are 
common on other planetary bodies, and 
long-runout landslides have been observed 
on terrestrial planets and the icy satellites6–8. 
Combined, these different planetary 
environments make up a laboratory for 
testing hypotheses of landslide emplacement. 
Long-runout avalanches on cold and airless 
icy satellites challenge existing explanations 
for reduced friction. Writing in Nature 
Geoscience, Singer and colleagues9 present 
analyses of long-runout landslides on 
Saturn’s moon Iapetus and propose that 
frictional heating of icy avalanche rubble 
makes the interface between avalanche and 
ground slippery.
Landslides are often characterized by the 
ratio of drop height to runout length. This 
ratio has been frequently used to approximate 
the friction coefficient for terrestrial and 
martian landslides6,10. For landslides on Earth 
and Mars, the height–length ratio decreases 
with increasing landslide volume6, starting 
from a value of 0.6 for a purely frictional 
sliding mass with a small volume of less 
than 100,000 m3, and dropping to values 
lower than 0.07 for large volumes of more 
than 16 km3.
Understanding these events is important 
for landslide disaster mitigation and 
management on Earth. However, the 
underlying mechanisms that control these 
landslides are the subject of an active debate, 
in part because of the limited available data. 
Planetary exploration has revealed that long-
runout landslides are ubiquitous throughout 
the Solar System, and these far-flung 
landslides have much to teach us about their 
underlying causes.
Singer and colleagues9 catalogued mass 
wasting deposits on icy Iapetus. They 
use data from the Cassini mission and 
photogrammetric techniques to map Iapetian 
landslides. In doing so, they assembled the 
largest data set of landslides beyond Earth 
and Mars. They found that, like Earth and 
Mars, Iapetus is rife with mass movements, 
including long-runout landslides. The 
conditions on Iapetus are particularly 
favourable for landslide triggering, both 
because topographic relief is great relative 
to the moon’s small size and because the 
moon’s surface is ancient. Therefore, there are 
many precarious slopes that are vulnerable 
to collapse. As a laboratory of mass wasting 
investigations, Iapetus is a rare gem in having 
a large number of long-runout landslides that 
formed in similar environmental conditions, 
and readily available spacecraft data to 
study them.
According to Singer and colleagues’ 
measurements, typical height–length ratios 
of landslides on Iapetus lie between 0.1 and 
0.3. On the lower end, this is analogous 
to terrestrial submarine landslides and 
PLANETARY SCIENCE
Slippery sliding on icy Iapetus
Enigmatically, some landslides flow farther than normal frictional resistance allows. Cassini images of Saturn’s icy 
moon Iapetus reveal a multitude of long-runout landslides that may have been enabled by flash heating along the 
sliding surface.
Antoine Lucas
L H
Figure 1 | Images from the Cassini ISS probe 
reveal numerous landslides across the surface of 
Iapetus. Originating from unstable slopes such 
as steep crater walls, these landslides often flow 
greater distances (L) than expected for their fall 
heights (H) under the normal frictional properties 
of ice. Singer et al.9 propose that such long-runout 
landslides can be explained by a slippery sliding 
surface caused by frictional heating during 
the landslide.
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© 2012 Macmillan Publishers Limited. All rights reserved
NATURE GEOSCIENCE | VOL 5 | AUGUST 2012 | www.nature.com/naturegeoscience 525
news & views
mudflows; the upper end is comparable to 
small subaerial rock avalanches on Earth 
or large landslides on Mars. Unlike those 
on Earth and Mars, the Iapetian landslide 
ratio does not show a dependence on the 
size of the landslide. However, laboratory 
measurements11 indicate friction coefficients 
of low-temperature water ice (around 0.7) 
that would prohibit such long-runout 
landslides on the icy moon’s surface. A 
frictional weakening mechanism must 
therefore be at work on Iapetus.
Singer et al.9 propose that the most 
likely cause of the long-runout landslides 
on Iapetus is frictional heating of the icy 
rubble by shear along sliding surfaces until 
they become slippery. Although similar 
mechanisms employing basal melting — 
whether involving the melting of ice or rock 
— have also been proposed for long-runout 
landslides on Earth and Mars2,3,12, further 
studies are needed to test this explanation. 
It has been shown that friction is not always 
well approximated by the ratio of landslide 
height to length because of geometric 
biases and topographic effects13,14. Further 
constraints on topography and landslide 
volume are therefore needed for better 
estimates of the frictional weakening 
behaviour during landslide emplacement.
Numerical modelling has shown 
that friction for long-runout landslides 
on Mars is very low15 and, according to 
Singer et al.9, similar values are expected 
on Iapetus. Moreover, further work with 
the data set could differentiate between 
landslides and rock falls, which are likely 
to have different formation mechanisms 
(for example, rock avalanche as opposed to 
cliff collapse). Despite these caveats, flash 
heating is a reasonable hypothesis to explain 
long-runout avalanches on Iapetus. The 
upcoming European Space Agency’s JUICE 
(Jupiter Icy Moon Explorer) mission, set to 
launch in 2022, might provide data to test 
this hypothesis on the other icy satellites, 
for long-runout landslides as well as faults 
cutting the icy crusts, which might show 
similar frictional weakening.
Singer et al.9 show that, with its many 
landslides and relatively simple surface 
conditions (as compared with warmer, 
wetter planets), Iapetus is a useful 
playground for landslide science. At first 
glance, Earth and a small icy satellite such 
as Iapetus may seem nothing alike, but 
many of the same geomorphic processes are 
observed. Understanding the long landslides 
on Iapetus may help us to understand the 
causes of similar destructive events on our 
own planet. ❐
Antoine Lucas is at the Division of Geological 
and Planetary Sciences, California Institute of 
Technology, 1200 East California Boulevard, 
Pasadena, California 91125, USA. 
e-mail: alucas@caltech.edu
References
1. Hungr, O. & Evans, S. G. Geol. Soc. Am. Bull. 116, 1240–1252, (2004).
2. De Blasio, F. V. Planet. Space Sci. 59, 1384–1392 (2011).
3. Goren, L. & Aharonov, E. Geophys. Res. Lett. 34, L07301 (2007).
4. Rice, R. J. J. Geophys. Res. 111, B05311 (2006).
5. Collins, G. S. & Melosh, H. J. J. Geophys. Res. 108, 2473 (2003).
6. Legros, F. Eng. Geol. 63, 301–331 (2002).
7. Chuang, F. C. & Greeley, R. J. Geophys. Res. 105, 20227–20244 (2000).
8. Malin, C. M. J. Geophys. Res. 97, 16337–16352, (1992).
9. Singer, K. N., McKinnon, W. B., Schenk, P. M. & Moore, J. M. 
Nature Geosci. 5, 574–578 (2012).
10. Heim, A. Z. Dtsch. Geol. Ges. 34, 74–115 (1882).
11. Beeman, M., Durham, W. B. & Kirby, S. H. J. Geophys. Res.  
93, 7625–7633 (1988).
12. Vardoulakis, I. Mech. Coh. Frict. Mat. 5, 443–467 (2000).
13. Lajeunesse, E., Quantin, C., Allemand, P. & Delacourt C. Geophys. 
Res. Lett. 33, L04403 (2006).
14. Lucas, A. & Mangeney A. Geophys. Res. Lett. 34, L10201 (2007).
15. Lucas, A., Mangeney, A., Mège, D. & Bouchut, F. J. Geophys. Res. 
116, E10001 (2011).
© 2012 Macmillan Publishers Limited. All rights reserved


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Slippery sliding on icy Iapetus

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