DOI: 10.1126/science.1254960
, 318 (2014);346Science
et al.Ian A. McKenzie
Motor skill learning requires active central myelination
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RESEARCH ARTICLE
◥
SKILL DEVELOPMENT
Motor skill learning requires active
central myelination
Ian A. McKenzie,1
* David Ohayon,1
* Huiliang Li,1
Joana Paes de Faria,1
† Ben Emery,2
Koujiro Tohyama,3
William D. Richardson1
‡
Myelin-forming oligodendrocytes (OLs) are formed continuously in the healthy adult brain.
In this work, we study the function of these late-forming cells and the myelin they produce.
Learning a new motor skill (such as juggling) alters the structure of the brain’s white
matter, which contains many OLs, suggesting that late-born OLs might contribute to motor
learning. Consistent with this idea, we show that production of newly formed OLs is briefly
accelerated in mice that learn a new skill (running on a “complex wheel” with irregularly
spaced rungs). By genetically manipulating the transcription factor myelin regulatory
factor in OL precursors, we blocked production of new OLs during adulthood without
affecting preexisting OLs or myelin. This prevented the mice from mastering the complex
wheel. Thus, generation of new OLs and myelin is important for learning motor skills.
M
yelin is the spirally wrapped cell membrane
that surrounds and insulates axons
in the central and peripheral nervous systems
(CNS and PNS, respectively). Myelin
greatly increases the speed of electrical
communication among neurons and, hence, the
brain’s computational power. CNS myelin is synthesized
by oligodendrocytes (OLs), the majority
of which develop in the first 6 postnatal weeks in
rodents, from proliferating OL precursors [(OPs),
also known as NG2 glia] (1, 2). However, many
OPs persist in the adult mouse CNS (~5% of all
neural cells) and continue to divide and differentiate
into myelinating OLs throughout life (1–3).
For example, nearly 30% of OLs in the 8-monthold
corpus callosum are formed after 8 weeks of
age (2). What is the function of adult-born OLs
and myelin? Magnetic resonance imaging (MRI)
has detected changes in the structure of white
matter in people trained in complex sensorimotor
tasks such as playing the piano, juggling, or abacus
use (4–6). Analogous MRI changes are observed
in rats during motor training (7). The
histological basis of the MRI change is not known,
but one possibility is that newly generated myelin
is laid down preferentially in circuits that are engaged
during motor learning. Here we show that
active myelination during adulthood is required
for motor skill learning and that motor learning
increases OL production.
Preventing adult myelination by
conditional deletion of myelin
regulatory factor
Myelin regulatory factor (MyRF) is a transcription
factor required in OLs to initiate and maintain
their myelination program (8–10). It is not
expressed in OPs, in otherCNScells, orin Schwann
cells, which myelinate PNS axons. We have a
mouse line that carries a “floxed” allele of Myrf
(10). By breeding (see supplementary materials
and methods), we obtained Myrf (+/flox)
and
Myrf (flox/flox)
littermates on a Pdgfra-CreERT2
:
Rosa-YFP background (2, 11); we refer to these as
P-Myrf (+/flox)
and P-Myrf (flox/flox)
, respectively.
Administering tamoxifen induces Cre-mediated
recombination, inactivating one or both alleles
of Myrf in Pdgfra-expressing OPs while simultaneously
labeling the OPs with yellow fluorescent
protein (YFP) (see supplementary materials
and methods). We refer to the tamoxifen-treated
mice as P-Myrf (+/−)
and P-Myrf (−/−)
. Recombination
at the Myrf locus was confirmed by
reverse transcription polymerase chain reaction
(fig. S1).
We inactivated Myrf in OPs by tamoxifen administration
on postnatal day 60 (P60) or P90.
Subsequently, we identified YFP+
OPs and newly
differentiated YFP+
OLs by triple-immunolabeling
with anti-YFP, anti-Pdgfra (for OPs), and the CC1
monoclonal antibody (for OLs). In P-Myrf (+/−)
mice, YFP+
,CC1+
,Pdgfra–
OLs accumulated in the
anterior corpus callosum (beneath the motor cortex)
after the administration of tamoxifen (posttamoxifen)
(arrows in Fig. 1A). In P-Myrf (−/−)
mice,
production of YFP+
,CC1+
OLs was decreased
to ~10% of control (Fig. 1, A and B); at 1 month
post-tamoxifen, we counted 301 T 59 YFP+
,CC1+
cells/mm2
in 20-mm sections of P-Myrf (+/−)
corpus
RESEARCH
318 17 OCTOBER 2014 • VOL 346 ISSUE 6207 sciencemag.org SCIENCE
1
The Wolfson Institute for Biomedical Research, University
College London, Gower Street, London WC1E 6BT, UK.
2
Department of Anatomy and Neuroscience and the Florey
Institute for Neuroscience and Mental Health, University of
Melbourne, Melbourne, Victoria 3010, Australia. 3
The Center for
Electron Microscopy and Bio-Imaging Research, Iwate Medical
University, 19-1 Uchimuru, Morioka, Iwate 020-8505, Japan.
*These authors contributed equally to this work. †Present address:
Instituto de Biologia Molecular e Celular, Rua do Campo Alegre
823, 4150-180 Porto, Portugal. ‡Corresponding author.
E-mail: w.richardson@ucl.ac.uk
Fig. 1. Deleting Myrf in OPs
prevents new myelination.
(A) Many YFP+
(newly
formed) cells accumulated
1 month after tamoxifen
treatment in the P-Myrf(+/−)
corpus callosum, including
Pdgfra+
,CC1–
OPs
(arrowheads) and CC1+
,
Pdgfra–
OLs (arrows). In
contrast, the P-Myrf(−/−)
corpus callosum contained
few YFP+
cells, mainly
Pdgfra+
OPs. Some
YFP+
,CC1+
cells appeared
fragmented, presumably
because they are apoptotic
(yellow arrow). (B) Numbers
of YFP+
,CC1+
OLs in the
P-Myrf(−/−)
versus P-Myrf(+/−)
corpus callosum (****P <
0.0001). Error bars indicate
SEM. (C) Very few GFP+
(newly formed) myelin
sheaths are present in the
P-Myrf (−/−)
:Tau-mGFP
corpus callosum 1 month
post-tamoxifen relative to
P-Myrf (+/−)
siblings. Asterisk
indicates third ventricle.
(D) The number densities of
Pdgfra+
OPs or CC1+
,YFP–
(preexisting) OLs did not change between P60 and P150. Error bars indicate SEM. Scale bars: 50 mm,
(A) and (C).
callosum but only 33 T 7 cells/mm2
in P-Myrf (−/−)
(means T SEM; six fields of view in three sections
of three mice of each genotype). There
was a comparable reduction in other regions
of the P-Myrf (−/−)
brain, including the cerebral
cortex, striatum, midbrain, and cerebellum.
In the motor cortex, for example, we counted
123 T 15 YFP+
,CC1+
cells/mm2
in P-Myrf (+/−)
and 10 T 3 cells/mm2
in P-Myrf (−/−)
. There
was no recovery of OL production over at least
3 months (Fig. 1B). Loss of newly formed OLs
was confirmed visually using a different reporter
line, Tau-mGFP (GFP, green fluorescent protein),
that expresses a membrane-bound green fluorescent
protein, revealing whole-cell morphology
including the myelin sheaths (3, 12). One
month post-tamoxifen, P-Myrf (−/−)
:Tau-mGFP
corpus callosum was almost devoid of GFPpositive
myelin sheaths, in contrast to their
Myrf (+/−)
littermates, which had many (Fig. 1C).
To quantify new OL production in P-Myrf (−/−)
versus P-Myrf (+/−)
mice, we administered 5-ethynyl-
2′-deoxyuridine (EdU) to P60 mice for 1 week,
after tamoxifen treatment. One month later, 5.7% T
0.7% of CC1+
OLs in P-Myrf (+/−)
corpus callosum
were positive for EdU (i.e., recently formed from
cycling OPs), compared with only 0.20% T 0.07%
in P-Myrf (−/−)
littermates (means T SEM; six fields,
>250 OLs per field in three sections of three mice
of each genotype) (fig. S2). Therefore, Myrf was
deleted in >90% of all OPs, whether or not they
recombined the Rosa-YFP reporter. Over the same
period, we detected no significant changes in the
number density of Pdgfra+
OPs or CC1+
,YFP–
(i.e.,
preexisting) OLs in P-Myrf (−/−)
versus P-Myrf (+/−)
corpus callosum (Fig. 1D). Therefore, our strategy
prevents the formation of new OLs without affecting
preexisting OLs.
Preventing new OL production does not
trigger demyelination
Myelin histochemistry with Eriochrome cyanine
(Life Technologies) showed that P-Myrf(−/−)
mice had
normal-appearing white matter (Fig. 2, A and B),
indistinguishable from their P-Myrf(+/−)
littermates
(Fig. 2, C and D). In contrast, when Myrf (flox/flox)
was deleted conditionally in both OLs and OPs
using Sox10-CreERT2
mice [S10-Myrf (−/−)
] (see
supplementary materials and methods and fig.
S3), there was dramatic loss of myelin (Fig. 2, E
and F). Electron microscopy (EM) revealed compact
myelin sheaths in P-Myrf (−/−)
mice (Fig. 2,
G and H) that were indistinguishable from
P-Myrf (+/−)
controls (Fig. 2, I and J), whereas
S10-Myrf (−/−)
mice were severely demyelinated
(Fig. 2, K and L). Phagocytic cells (macrophages
or activated microglia) containing cell debris
were observed by EM in S10-Myrf (−/−)
corpus
callosum (34 cells counted in four fields from
two P90 mice 5 weeks post-tamoxifen; mean cell
density ~220 cells/mm2
) (Fig. 2M); such cells
were much less frequent in P-Myrf (−/−)
(7 cells;
~44 cells/mm2
) or in P-Myrf (+/−)
controls (10 cells;
~55 cells/mm2
). For comparison, the density of
OPs in the healthy CNS is ~150 cells/mm2
. There
was no evidence of inflammation or blood-brain
barrier breakdown marked by invasion of immune
cells (e.g., neutrophils or T cells), loss of
tight junctions between endothelial cells, or retraction
of astrocyte processes from blood vessels,
even in the severely demyelinated S10-Myrf (−/−)
brain.
Consistent with the myelin histology, P-Myrf (–/–)
mice showed no outward signs of demyelination
(e.g., tremors) and were indistinguishable from
their P-Myrf (+/−)
littermates on an accelerating
rotarod, a test for motor coordination and balance
(Fig. 2N). In contrast, S10-Myrf (−/−)
mice
developed severe tremors around 1 month posttamoxifen
(movie S1), and their performance on
the rotarod was seriously impaired (Fig. 2O),
similar to when Myrf deletion was targeted to
mature OLs using Plp-CreER (9).
The complex running wheel
We assessed motor learning ability using a running
wheel with irregularly spaced rungs (“complex
wheel”) (Fig. 3) (13, 14). Mice run on the wheel
spontaneously and, when skilled, can run the
equivalent of 5 to 7 km per night. When wildtype
(WT) (C57B6/CBA hybrid) mice accustomed
SCIENCE sciencemag.org 17 OCTOBER 2014 • VOL 346 ISSUE 6207 319
Fig. 2. Deleting Myrf in OPs does not trigger demyelination.Tamoxifen was administered to P60 mice
and, 5 weeks later, their brains were examined by Eri-C histochemistry (A to F) or electron microscopy
(G to M).There was no visible loss of myelin in P-Myrf (−/−)
[(A), (B), (G), and (H)] or P-Myrf (+/−)
[(C),
(D), (I), and (J)] brains, but there was marked demyelination in S10-Myrf (−/−)
[(E), (F), (K), and (L)]. In
S10-Myrf (−/−)
white matter, phagocytic cells containing membrane debris were present (M). Performance
on an accelerating rotarod was not impaired in P-Myrf (−/−)
mice for at least 8 weeks post-tamoxifen
compared with their P-Myrf (+/−)
littermates (N),whereas S10-Myrf (−/−)
mice were seriously impaired after
4 to 5 weeks (O). Error bars indicate SEM. *P < 0.05; ***P < 0.001; ****P < 0.0001. Scale bars: 2 mm, (A),
(C), and (E); 1 mm, (B), (D), and (F); 5 mm, (G), (I), and (K); 1 mm, (H), (J), and (L); and 2 mm (M).
RESEARCH | RESEARCH ARTICLE
to a regular wheel with equally spaced rungs are
switched to the complex wheel, they experience
great difficulty at first but persevere and after
about a week can run as fast on the complex
wheel as they can on the regular wheel (movies
S2 and S3). High-speed filming reveals that on
the regular wheel mice adopt a symmetrical
“running walk” with an eight-gap stride, out of
phase by four gaps left to right (15) (Fig. 3A). They
bring their hindpaws up to the rung immediately
behind their forepaws. On the complex wheel,
they break step, adopting an asymmetrical gait
with six- to nine-gap strides out of phase by two
to six gaps between sides. A critical adaptation is
that the mice bring their hindpaws forward to
grasp the same rung as their forepaws (Fig. 3, B
to E). Thus, their hindpaws always find a rung,
whatever the pattern of gaps. They also prefer
rungs preceded by a one- or two-rung gap (Fig. 3,
B to D); presumably, they reach forward into a
gap and “pull back” to grasp the nearest rung, a
second adaptation that is transferable to other
rung patterns. Therefore, mice do not memorize
specific stepping patterns but develop general
strategies for running on wheels with unequal
gaps; mastering one rung pattern primes them
to master a different pattern more easily (fig. S4).
Active myelination is required for
motor skill learning
Learning to run on the complex wheel presumably
engages motor control circuits in addition
to those required for normal symmetrical gait,
involving the basal ganglia, cerebellum, motor
cortex, and connecting pathways including the
corpus callosum, but independent of the hippocampus
(16–18). We introduced P-Myrf (−/−)
and
P-Myrf (+/−)
littermates (mixed C57B6/CBA/129
background, predominantly C57B6; see supplementary
materials and methods) to the complex
wheel 3 weeks after tamoxifentreatment beginning
on P60 (four experiments) or P90 (one experiment)
(Fig. 4, A and B). The P90 experiment is
320 17 OCTOBER 2014 • VOL 346 ISSUE 6207 sciencemag.org SCIENCE
Fig. 3. Mice learn general strategies for coping with even rung spacing. (A) On the regular running
wheel, WT mice place fore- and hindpaws on consecutive rungs while reaching forward with the contralateral
forepaw. (B to E) On the complex wheel, they grasp the same rung with fore- and hindpaws (red
dots), selecting rungs preceded by a one- or two-rung gap [e.g., (B) and (D)]. These strategies are transferable
to other rung patterns. (B) and (C) and (D) and (E) are consecutive video frames (240 frames/s).
Fig. 4. Active CNS
myelination is required
for motor skill learning.
(A) The complex wheel
pattern. (B) Experimental
design. P-Myrf (–/-)
and
P-Myrf (+/−)
mice were
housed singly, and
tamoxifen was
administered from P60 or
P90. Three weeks later,
they were introduced to
the complex wheel (CW).
(C and D) On the wheel,
P-Myrf (−/−)
mice were
impaired relative to
P-Myrf (+/−)
[tamoxifen on
P90; means T SEM (error
bars), n = 7 and 5,
respectively]. (E) Time on
wheel at >1 m/min was
not different between
cohorts. Error bars
denote SEM. (F) Individual
performances, distance
versus time.
(G to J) Five pooled
experiments confirm
divergence between
P-Myrf (+/−)
and P-Myrf (−/−)
mice [P = 0.0063 for
accumulated distances,
P = 0.0003 for average
speeds; K-S test, n = 36
(20 males) and 32 (17
males), respectively]. Error
bars in (G) denote SEM.
(K and L) P-Myrf (+/flox)
and P-Myrf (flox/flox)
mice were introduced to the complex wheel before tamoxifen exposure and reintroduced 3 weeks after treatment. Both before and
after, there was no difference between cohorts (n = 7 and 8, respectively). Error bars in (L) denote SEM. *P < 0.05; **P < 0.01; ***P < 10−3
; ****P < 10−4
.
Also see fig. S6.
RESEARCH | RESEARCH ARTICLE
shown (Fig. 4, C to F). Both cohorts improved
their performance during the first week on the
complex wheel, but the daily average and maximum
speeds attained by the P-Myrf (−/−)
group
were always less than their P-Myrf (+/−)
siblings
(Fig. 4, C and D). Time spent turning the wheel
>1 m/min was the same for both genotypes,
arguing against a difference in motivation (Fig.
4E). Individual performances varied widely, and
there was substantial overlap between genotypes
(Fig. 4F). Pooled data for all five experiments
confirmed significant differences in the
average speeds attained by P-Myrf (−/−)
versus
P-Myrf (+/−)
animals (Fig. 4G), as well as their
individual performances (Fig. 4H) [P = 0.0063,
Kolgomorov-Smirnov (K-S) nonparametric test;
n = 32 and 36 mice, respectively]. One-third (12
of 36) of P-Myrf (+/−)
mice ran further in 1 week
than the best-performing of their P-Myrf (−/−)
counterparts (Fig. 4H). High-speed filming showed
that P-Myrf (−/−)
mice ran less rhythmically and
sometimes appeared to propel the wheel with
their rear ankle or lower leg rather than their
hindpaw (movies S4 and S5). The average speeds
of P-Myrf (+/−)
mice (on the seventh day) had an
approximately normal distribution (P = 0.2, K-S
test), whereas the average speeds of P-Myrf (−/−)
mice were bimodal and skewed toward lower
speeds (different distributions, P = 0.007) (Fig. 4I),
possibly reflecting multistage learning (Fig. 3).
The maximum speed distribution of P-Myrf (−/−)
mice was also shifted to lower speeds (P < 0.0001)
(Fig. 4J). When retested 1 month later, the difference
between P-Myrf (−/−)
and P-Myrf (+/−)
animals persisted (fig. S4). There were no significant
differences between males and females
(fig. S5).
Active myelination is not required to
recall a prelearned skill
Despite the lack of a general locomotor defect on
the rotarod, Myrf deletion could have caused
some subtle neural or physical impairment unrelated
to learning. To control for this, we introduced
P60 P-Myrf (flox/flox)
and P-Myrf (+/flox)
littermates to the complex wheel before tamoxifen
treatment (Fig. 4K). As expected, the two
cohorts were indistinguishable before tamoxifen
(Fig. 4L and fig. S6). We administered tamoxifen,
housed the mice singly for 3 weeks without a
wheel, and then reintroduced them to the
complex wheel (Fig. 4K). Both P-Myrf (−/−)
and
P-Myrf (+/−)
groups were immediately able to run
at speed (Fig. 4L and fig. S6). We conclude that
(i) P-Myrf (−/−)
mice are inherently able to run at
speed on the complex wheel (i.e., they are physically
capable) and (ii) myelin formation is not
required to perform a prelearned skill.
Running stimulates OP proliferation
and OL production
To relate motor learning to cell dynamics, we
introduced P60 WT mice to the complex wheel
while administering EdU via their drinking
water, and we counted EdU+
cells in the corpus
callosum after various periods. At 4 days, there
was a transient increase (~40%, P = 0.006, n = 4)
in the fraction of Pdgfra+
OPs that was EdUlabeled
(“labeling index”) in runners relative to
control mice housed without a wheel, indicating
that running had accelerated the G1-to-S transition
(Fig. 5A). This was followed 2 days later by
a spike in the absolute number of Pdgfra+
OPs
as they completed the cell cycle (~40% increase,
P = 0.04, n = 3) (Fig. 5B), then 5 days after that
(11 days running) by an increase (~40%, P = 0.003,
n = 3) in the number of EdU+
,Pdgfra–
cells—a
mixture of CC1+
and CC1–
OLs (Fig. 5C). At 11 days,
almost all (94% T 4%) EdU+
cells were Sox10+
OL lineage cells. After 3 weeks, there were many
newly formed EdU+
,CC1+
OLs in control animals
housed without a wheel, as expected (2, 3), but
a ~50% greater number of those cells in runners
(Fig. 5D).
The transient increase in EdU labeling index
was not observed a second time when the complex
wheel was removed from the cage and reintroduced
1 week later (Fig. 5, E and F), suggesting
that it was triggered by novel experience (e.g.,
learning), not exercise per se. OP division and
differentiation was increased by running on the
regular wheel (fig. S7) as well as the complex
wheel, suggesting that the region of corpus
callosum we examined is involved in skills common
to both (e.g., grasping or general bilateral
coordination).
The cellular events described above occurred
on a similar time scale as the improvement in
running performance and, together with our data
from Myrf knockout mice, support an important
role for newly formed OLs in motor skill
acquisition. How might new myelinating cells
contribute to skill learning? It is likely that new
neuronal connections are formed, or existing
connections strengthened, in response to repetitive
firing of neural circuits that elicit a particular
sequence of movements (18). The increased
activity in these circuits might then stimulate
myelination of their axons, or myelin remodeling,
making the circuit more efficient. There might
even be a reserve of preformed, parallel circuits
in the brain, and motor training selects the best
of these by stimulating myelination in the most
active circuits. The fact that most axons in the
mouse corpus callosum and cerebral cortex remain
incompletely myelinated into adulthood
could be consistent with this idea (19, 20). The
existence of an activity-driven myelination mechanism
has been postulated, based on the fact that
OPs express various neurotransmitter receptors,
form synapses with naked axons, and display
transmembrane ion currents in response to action
potentials in the axons that they contact
(21–25). There is evidence that activity or experience
can regulate OP division and differentiation
in vivo [(26–32) and this paper] and also MRI
evidence of structural changes in the white matter
of individuals learning sensorimotor skills (4–7),
undertaking working memory training (33), or
learning a second language (34). We have now
provided experimental evidence that OL genesis
and myelin formation are important for motor
learning and, therefore, are likely to contribute
SCIENCE sciencemag.org 17 OCTOBER 2014 • VOL 346 ISSUE 6207 321
Fig. 5. Running stimulates
OP proliferation and OL
production. Running on the
complex wheel (CW) caused (A)
a transient increase in the EdU
labeling index of Pdgfra+
OPs in
the corpus callosum after 2 days,
(B) an increase in the number
density of OPs at 6 days, and (C)
increased production of OLs
(EdU+
,Pdgfra–
) by 11 days. The
latter were a mixture of mature
CC1+
and immature CC1–
OLs.
The numbers of both cell types
were greater in runners than
nonrunners at 11 days, although
individually the increases did not
reach significance (P = 0.15 and
0.06, respectively). (D) After
3 weeks running, there were ~50%
more EdU+
,CC1+
OLs in runners
than nonrunners. (E) Experimental
design. EdU was administered in
the drinking water for 4 days
during running, as indicated
(arrows 1 and 2). (F) The EdU
labeling index was increased by
the first but not the second
encounter with the wheel. Each
data point represents multiple
fields from at least three sections
from three or more mice. Error
bars in (A) to (D) and (F) represent SEM. Also see fig. S7.
RESEARCH | RESEARCH ARTICLE
to the changes observed by MRI. Future experiments
can assess whether new myelinating cells
are required for other types of learning as well.
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ACKNOWLEDGMENTS
We thank the staff of the Center for Electron Microscopy and
Bio-Imaging Research, Iwate Medical University, and U. Dennehy
and M. Grist (University College London) for technical help and our
colleagues in the Wolfson Institute for Biomedical Research for
encouragement and suggestions. W.D.R. thanks T. Richardson for
stimulating discussions. The study was supported by the European
Research Council (grant agreement 293544), the UK Medical
Research Council, the Wellcome Trust, and Grants-in-Aid from the
Japanese Ministry of Education, Culture, Sports, Science and
Technology. W.D.R. and K.T. acknowledge an Invitation Fellowship
from the Japan Society for Promotion of Science. I.M. was
supported by a Royal Society USA/Canada Exchange Fellowship,
J.P.d.F. by a fellowship from the Portuguese Fundação para a
Ciência e a Tecnologia, and B.E. by a Career Development
Fellowship from the Australian National Health and Medical
Research Council. Pdgfra-CreERT2
and Sox10-CreERT2
mice can be
obtained by request through www.e-lucid.com/ with a material
transfer agreement. Myrf(flox) mice can be obtained from Jackson
Laboratories, strain 010607. The supplementary materials
include additional data.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/346/6207/318/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S7
References (35, 36)
Movies S1 to S5
17 April 2014; accepted 14 August 2014
10.1126/science.1254960
REPORTS
◥
PLANETARY GEOLOGY
Constraints on Mimas’ interior from
Cassini ISS libration measurements
R. Tajeddine,1,2,3
* N. Rambaux,2,3
V. Lainey,2
S. Charnoz,4
A. Richard,2
A. Rivoldini,5
B. Noyelles6
Like our Moon, the majority of the solar system’s satellites are locked in a 1:1 spin-orbit
resonance; on average, these satellites show the same face toward the planet at a
constant rotation rate equal to the satellite’s orbital rate. In addition to the uniform rotational
motion, physical librations (oscillations about an equilibrium) also occur. The librations may
contain signatures of the satellite’s internal properties. Using stereophotogrammetry on
Cassini Image Science Subsystem (ISS) images, we measured longitudinal physical forced
librations of Saturn’s moon Mimas. Our measurements confirm all the libration amplitudes
calculated from the orbital dynamics, with one exception. This amplitude depends mainly on
Mimas’ internal structure and has an observed value of twice the predicted one, assuming
hydrostatic equilibrium. After considering various possible interior models of Mimas, we argue
that the satellite has either a large nonhydrostatic interior, or a hydrostatic one with an
internal ocean beneath a thick icy shell.
A
mong Saturn’s inner main moons, Mimas
is the smallest (radius ~198 km) and closest
to the planet (semimajor axis ~189,000 km).
Along with Enceladus, Tethys, Dione, and
Rhea, it is classified as a mid-sized icy moon;
the origin of these moons is still being debated.
The classical model describes their formation by
accretion in the protoplanetary subnebula (1–6)
or by collision between two large satellites and
reaccretion of the impact ejecta (7, 8), but does
not explain the satellites’ masses, sizes, and radial
locations. A new model reconciles these parameters
by forming the satellites in the rings
(9–11). However, this model assumes that the
primordial rings were massive and contained
silicate; furthermore, Saturn must have been
tidally very dissipative (12) to move all the midsized
moons to their current locations.
We measured Mimas’ forced librations using
Cassini’s ISS Narrow Angle Camera (NAC) images,
using methods previously applied to Phobos
(13, 14), to gain insights into Mimas’ interior. The
measurements of rotational parameters have been
proven to be a powerful tool to investigate the
interior state of celestial bodies (15). The absence
of evident geological activity on Mimas’ surface
(16) suggests that it may have a cold interior that
may have helped in conserving a “fossil” record
of structures within its interior. This encouraged
us to investigate Mimas’ rotational variations,
which are directly linked to its internal structure
and may inform us about its origin.
First, we developed a control point network
across Mimas’ surface by applying the method of
stereophotogrammetry, where (X, Y, Z) coordinates
of a surface point in the satellite’s frame are
projected in an image as sample (x) and line (y)
coordinates in pixels. For the 3D reconstruction,
each point has been observed at least twice and
from two different viewing angles (17). After selecting
recognizable landmarks from Mimas’ map
(18), a least-squares method was applied comparing
the (X, Y, Z) coordinates of each projected
point in the image to the observed ones. From
this, a topographic map of 260 surface points
was built (Fig. 1A), based on 2135 point measurements
from 40 Cassini images with resolutions
ranging from 360 to 1450 m per pixel (see table
S2 for a full list of images). The mean uncertainties
on a point’s coordinates are estimated as
T599 m, T731 m, and T395 m on X, Y, and Z coordinates,
respectively. To test our method, we
rebuilt and confirmed the satellite’s triaxial shape
using these points (17, 19).
In the photogrammetric reconstruction method
described above, a rotational model of Mimas
is used (17). The better this model describes
Mimas’ rotation, the smaller the c2
errors from
the topographic reconstruction. Hence, we repeatedly
built Mimas’ control point network by
varying its forced libration amplitudes and phases
until the total value of c2
was minimized. The
measurements (Table 1) (17) confirm all the theoretical
values (20) except for the amplitude corresponding
to the 0.945-day period, which is
almost twice the predicted one (Fig. 1B). The
uncertainty on this libration amplitude has been
322 17 OCTOBER 2014 • VOL 346 ISSUE 6207 sciencemag.org SCIENCE
1
Department of Astronomy, Cornell University, Ithaca, NY
14853, USA. 2
IMCCE–Observatoire de Paris, UMR 8028 du
CNRS, UPMC, Université Lille 1, 77 Av. Denfert-Rochereau,
75014 Paris, France. 3
Université Pierre et Marie Curie,
UPMC – Paris VI, 4 Place Jussieu, 75005 Paris, France.
4
Laboratoire AIM, UMR 7158, Université Paris Diderot/CEA
IRFU/CNRS, Centre de l’Orme les Merisiers, 91191
Gif-sur-Yvette Cedex, France. 5
Royal Observatory of Belgium,
Avenue Circulaire 3, B-1180 Brussels, Belgium. 6
Department
of Mathematics and Namur Center for Complex Systems,
Université de Namur, 5000 Namur, Belgium.
*Corresponding author. E-mail: tajeddine@astro.cornell.edu
RESEARCH