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M
ercury is a planet of extremes.
The innermost planet of the Solar
System, Mercury is subject to
strong temperature fluctuations as it slowly
rotates, its solar day twice as long as its year.
At the equator, Mercury’s airless surface
reaches daytime highs of 700 K (ref. 1).
At the poles, the Sun remains half hidden
below the horizon year-round, and polar
impact craters cast permanent shadows
that harbour deposits of water ice and other
frozen volatiles2
. Mercury is extremely
dense owing to a large iron core, which is
estimated to be 2,020 km in radius3
, leaving
only about 420 km for mantle and crust.
And Mercury has also been shrinking.
Writing in Nature Geoscience, Byrne et al.4
report that Mercury’s faulted and wrinkled
surface, as imaged by the MESSENGER
spacecraft, accommodates far more surface
contraction than previously thought.
That Mercury’s surface area has
decreased over geologic time is well known.
Mariner 10, the only previous mission to
Mercury, flew past the planet three times
between 1974 and 1975, and imaged 45%
of the surface. Among the ubiquitous
impact craters and scattered smooth plains,
subsequently determined to be volcanic
in origin5
, Mariner 10 imaged lobate
scarps — sinuous surface features that seem
to be caused by thrust faulting (Fig. 1) —
scattered across the imaged surface at all
stratigraphic levels1,6
.
According to this structural
interpretation, crustal rocks on one side of
a scarp have been pushed up and over those
on the other side, shortening the crust in the
process. Similar to thrust faults on Earth,
the deformation is thought to have occurred
along fault planes that dip shallowly at about
30 degrees and extend tens of kilometres
deep into Mercury’s crust6,7
. Given that
the vertical offset of the land surface
across the lobate scarps can reach up to
3 km, the horizontal displacements are
correspondingly larger. And, given that these
scarps can extend laterally for hundreds of
kilometres, the inference is that Mercury
has lost quite a bit of surface area over its
history. Mariner 10 scientists estimated that
the shortening across the observed lobate
scarps was equivalent to a loss of 1 to 2 km
in global radius6
.
This estimate of global contraction
clashes, however, with thermal evolution
models for Mercury. All planets and
satellites, including the Earth8
, must
ultimately cool and shrink over time,
unless their internal engines are somehow
renewed, for example by tidal heating.
Mercury is again an extreme case because
of its enormous iron core. A portion of the
core must be liquid and convecting in order
to explain Mercury’s dipole magnetic field3
.
The cooling of Mercury should not only
lead to the simple thermal contraction of
the entire planet owing to the temperature
change, but to the gradual freezing of the
molten iron in its core. Notably, the phase
transition from liquid to solid iron will
reduce Mercury’s overall volume much
further. As such, estimates of Mercury’s
shrinking based on numerical thermal
evolution models of the planet’s interior
yield a substantial loss in volume, equivalent
to 5 to 10 km of contraction radially over
4 billion years9
. This is much larger than
what is consistent with the lobate scarps
observed by Mariner 10.
Byrne et al.4
took advantage of a global
imaging campaign by the MESSENGER
spacecraft that is currently in orbit
around Mercury, and carefully mapped
compressional structures across the planet’s
surface, including the 55% that was missed
by Mariner 10. They find that lobate scarps
are nearly everywhere, and are more or less
randomly arranged. These characteristics
are consistent with a planet that is cooling
and contracting evenly in all directions,
as opposed to alternative hypotheses from
Mariner 10 days — such as a despinning
planet. In the latter case, the structures
would show preferred orientations, because
Mercury’s shape would have changed as it
slowed down from an initially faster spin
rate due to solar tides7
.
PLANETARY SCIENCE
Shrinking wrinkling Mercury
As Mercury’s interior cools and its massive iron core freezes, its surface feels the squeeze. A comprehensive global
census of compressional deformation features indicates that Mercury has shrunk by at least 5 km in radius over the
past 4 billion years.
William B. McKinnon
a b
–2.5 1.7Elevation (km)
50 500 km
60° N
310°E
Figure 1 | Mercury’s surface contraction. a, MESSENGER image of Carnegie Rupes, a lobate scarp on
Mercury. b, A stereo-derived digital elevation model of the scene. Cross-cutting and offsetting a large,
100-km-wide crater, the vertical offset exceeds 2 km, which implies a horizontal shortening to the
southwest of about 3.5 km. Thousands of similar structures have been mapped by Byrne et al.4
across the
surface of Mercury, implying a loss of at least 0.4% of the planet’s surface area over geologic time. The
contraction has been attributed to the solidification of the planet’s large iron core.
©NASA/JOHNSHOPKINSUNIVAPPLIEDPHYSICSLAB/
CARNEGIEINSTWASHINGTON
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news & views
Byrne and colleagues4
also find that
the dominant compressional landform on
Mercury’s volcanic plains is not the lobate
scarp, but the wrinkle ridge. Wrinkle ridges
are well known on the Moon, Mars, Venus
and Earth, and are formed by smaller
hidden thrust faults and surface folding of
layered terrain7
. Although relatively modest
in elevation and displacement compared
to the lobate scarps, there are a lot more
wrinkle ridges per unit area in the regions
where they are found. Together, the lobate
scarps and wrinkle ridges accommodate a
substantial amount of surface area loss that
is equivalent to a reduction in Mercury’s
radius of 5 to 7 km. Taking into account
observational biases of the MESSENGER
images — for example, east–west-trending
structures at lower latitudes are difficult to
identify — the total shrinkage of Mercury
over the past 4 billion years could be
even greater.
The presence of vast volcanic plains in
Mercury’s north is, however, seemingly
inconsistent with such strong contraction.
A reduction in the planet’s radius of the
magnitude proposed by Byrne et al.4
should
lead to strong compression in Mercury’s
lithosphere, squeezing volcanic conduits
shut and preventing the eruption of buoyant
magma. So, perhaps the lavas erupted
sufficiently long ago that contraction and
compression were not yet important. Or
perhaps residual extensional stresses from
an even earlier time, when Mercury’s spin
was slowing, offset the compressive stresses
at the poles7
. Stresses from despinning could
have had the required effect, but they would
have been symmetric about the equator, and
a southern equivalent to Mercury’s northern
volcanic plains is not seen.
The survey of compressional structures
by Byrne et al.4
suggests that Mercury is
7 km smaller today than it was after its
crust solidified. The findings provide a
global framework for investigations into
Mercury’s surface and interior evolution.
It is fascinating to recall that a nineteenth-
century notion for the origin of the Earth’s
mountains also invoked a cooling, shrinking
globe. The foremost geologist of the time,
Sir Charles Lyell, was, however, quite
critical of what he termed the ‘the secular
refrigeration of the entire planet’ as the
cause10
. Lyell instead argued that volcanic
action, and the ‘reiteration of ordinary
earthquakes’ driven ultimately by the escape
of the Earth’s internal heat, would suffice.
The shrinking Earth hypothesis is, of course,
long obsolete, and in fact was abandoned
even before the advent of modern plate
tectonics. We now know that the Earth’s
lithosphere is broken into plates, and the
lateral motions of these plates give rise to
mountain chains. But Mercury’s lithosphere
forms a single shell, and thus Mercury
provides an example of what may really
happen to a planet that is shrinking. ❐
William B. McKinnon is in the Department of Earth
and Planetary Sciences and McDonnell Center for
the Space Sciences, Washington University in Saint
Louis, Saint Louis, Missouri 63130, USA.
e-mail: mckinnon@wustl.edu
References
1. Strom, R. J. Mercury: The Elusive Planet (Cambridge Univ.
Press, 1987).
2. Neumann, G. A. et al. Science 339, 296–300 (2013).
3. Hauck, S. A. et al. J. Geophys. Res. Planets 118, 1204–1220 (2013).
4. Byrne, P. K. et al. Nature Geosci. http://dx.doi.org/10.1038/
ngeo2097 (2014).
5. Denevi, B. W. et al. J. Geophys. Res. Planets 118, 891–907 (2013).
6. Strom, R. G., Trask, J. J. Guest, J. E. J. Geophys. Res.
80, 2478–2507 (1975).
7. Watters, T. R. Nimmo, F. in Planetary Tectonics (eds Watters, T. R.
Schultz, R. A.) 15–80 (Cambridge Univ. Press, 2010).
8. Solomon, S. C. Earth Planet. Sci. Lett. 83, 153–158 (1987).
9. Hauck, S. A. et al. Earth Planet. Sci. Lett. 222, 713–728 (2004).
10. Lyell, C. Principles of Geology, 11th Edition (D. Appleton and
Co., 1872).
Published online: 16 March 2014
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