Speed metal Tim Elliott Science 344, 1086 (2014); DOI: 10.1126/science.1254943
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 5, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6188/1086.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6188/1086.full.html#related This article cites 12 articles, 3 of which can be accessed free: http://www.sciencemag.org/content/344/6188/1086.full.html#ref-list-1
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
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INSIGHTS | P E R S P E C T I V E S
PLANETARY SCIENCE
Speed metal Meteorite dating reveals that planetary core formation is a relatively fast process
A
s in many building booms, planets were put together pretty rapidly. Transforming nebular dust to fully formed planets took less than ~100 million years of the ~4.5 billion years of solar system history. Accurate determination of the rates of planetary growth is key for understanding these tumultuous beginnings of the solar system, but obtaining high-precision ages on short-lived events that happened so long ago is a formidable challenge. On page 1150 of this issue, Kruijer et al. (1) determine with remarkable accuracy that planetary core formation began less than 1 million years after the first solids condensed—extraordinarily fast on geological time scales. Planets are believed to grow by progressive accretion of smaller bodies (see the figure). Planetesimals are an intermediate step. They have radii up to a few hundred kilometers, which makes them sufficiently large to retain enough heat to melt or differentiate. During differentiation, metallic melts sink to form a core and silicate melts rise to form a crust. The larger asteroids, such as 4 Vesta, represent differentiated planetesimals. Asteroids are held in orbit between Mars and Jupiter, having been thwarted from further growth by Jupiter’s gravitational influence, and provide a frozen snapshot of activity that has long since run to completion elsewhere. Some meteorites—ejected fragments of asteroids that land on Earth—are derived from such differentiated bodies, and they provide samples that allow this early period of planetary growth to be studied. Kruijer et al. focus on the iron meteorites, which represent fragments of disrupted planetesimal cores. The chronometer of choice is the radioactive decay of hafnium isotope 182Hf to tungsten isotope 182W, which has been of great value elsewhere in dating early solar system processes (2). The half-life of 182Hf is only 9 million years, so it gives a high-resolution chronology over a period of ~50 million years at the beginning of the solar system when 182Hf was extant. Although precise, such “extinct” isotope systems only provide School of Earth Science, University of Bristol, Queen’s Road, Clifton BS8 1RJ, UK. E-mail: [email protected]
1086
ages relative to other dated objects (3). The oldest objects in the solar system, millimeter-sized calcium- and aluminum-rich inclusions (CAIs) from primitive meteorites, are often used as a reference. Over early solar system history, the decay of 182Hf leads to changing 182W/184W. During planetary melting, W is strongly partitioned into the coreforming metal phase, while Hf is left behind in the outer silicate portion of the planet. This process effectively freezes the 182W/184W
Planetary core formation. Planets grow by accreting smaller objects. At some point, differentiation occurs where metals sink to form a core, and silicates rise to form a crust. Accurate dating of meteorites reveals that the core forms very fast, within 1 million years of the first solids condensing in the early solar system (1).
ratio at the time of core formation and enables the core to be dated relative to evolving planetary 182W/184W (expressed as ε182W or the parts per 10,000 deviation of 182W/184W relative to a reference silicate Earth value). Thus, low (unradiogenic) ε182W in iron meteorites means an early-formed core. Kruijer et al. are not the first to note that extremely unradiogenic W in iron meteorites implies planetesimal core formation within the first few million years of solar system history (4–6). However, they comprehensively address what has been a major problem to the accuracy of all previous studies, namely the pernicious influence of cosmic rays in perturbing W isotope ratios at the required level of precision. The W isotope chronometer is sensitive to its past exposure to cosmic rays. Kruijer et al. use an elegant means to overcome this problem by making simultaneous platinum (Pt) isotope
the condensation of the first dated objects (CAIs) in the solar system, presenting an appreciable constraint for numerical accretion models. Furthermore, Kruijer et al. are able to discern differences in the timing of core formation between different planetesimals, represented by the different iron meteorite groups. Such differences were previously unresolvable. Kruijer et al. discuss an interesting model, which relates the timing of core formation to the variable amounts of sulfur (S) in the parent bodies. This approach makes good conceptual sense because the contents of volatile elements, such as S, vary widely in meteorites and planetary bodies. More S should lead to a larger amount of core formation at lower temperatures, earlier on in a planetesimal’s history. In Kruijer et al. ’s highly simplified thermal model, the time difference between initial segregation sciencemag.org SCIENCE
6 JUNE 2014 • VOL 344 ISSUE 6188
Published by AAAS
ILLUSTRATION: FAHAD SULEHRIA/WWW.NOVACELESTIA.COM AND SANTIAGO BILLY
By Tim Elliott
measurements to monitor the received dose of cosmic rays in each meteorite. Pt isotope ratios are more sensitive than W to the influence of cosmic rays but should be constant in all meteorites. Primary ε182W can therefore be obtained by back-correcting to a common, unperturbed Pt isotope ratio. This is more easily said than done, and the coupled, high-precision Pt and W data sets presented in this study are an isotopic tour de force. With the effects of cosmic rays removed, Kruijer et al. can date robustly the timing of planetesimal core formation. This endeavor is further helped by the recent reassessment of initial ε182W of the CAI datum (7), against which ages are measured. Thus, Kruijer et al. demonstrate that the sinking of metal in planetesimals is indeed speedy, occurring in as little as 0.6 ± 0.3 million years after
of a S-rich metallic melt and final formation of S-free metallic melt is ~0.4 million years, a time scale sufficient to account for the spread in their W isotope data. To put these ages of core formation into perspective, it is worth comparing them with ages of chondrules. These quenched melt droplets are the principal constituent of most primitive, undifferentiated meteorites, traditionally taken to be the building blocks of planets. Much recent effort has been expended in dating chondrules, and although some are as old as CAIs (8), most are ~1 million years younger (9, 10). Thus, chondrules were being formed, before their accretion into chondrites, at the same time as fully formed planetesimals were segregating cores. Presumably the respective parent bodies of iron meteorites and chondrites formed in different parts of the nebular disk to allow these very different accretion rates. The refined, rapid time scales of core formation presented by Kruijer et al. underline the notion that chondritic meteorites themselves did not accrete to form planets, even if their compositions do provide a valuable guide to bulk planetary composition. Finally, it is worth noting that the apparently youngest core formation age occurs in iron meteorites (IVB), which are also distinct in other aspects of their isotopic composition (11, 12). Although Kruijer et al. have carefully corrected the ε182W for estimated effects of W isotope heterogeneity, possible differences in Pt isotopes (and thus an influence on the cosmic ray correction) and even the initial amount of 182Hf remain to be assessed. This reemphasizes an age-old problem in distinguishing between time and initial isotopic composition in high-precision chronology. Nevertheless, for the majority of the samples studied here, the new data yield unprecedentedly accurate timing of the earliest stages of planetary formation. ■
ILLUSTRATION:C. BICKEL/SCIENCE
REFERENCES
1. T. S. Kruijer et al., Science 344, 1150 (2014). 2. T. Kleine et al., Geochim. Cosmochim. Acta 73, 5150 (2009). 3. N. Dauphas, M. Chaussidon, Annu. Rev. Earth Planet. Sci. 39, 351 (2011). 4. T. Kleine, K. Mezger, H. Palme, E. Scherer, C. Münker, Geochim. Cosmochim. Acta 69, 5805 (2005). 5. A. Scherstén, T. Elliott, C. Hawkesworth, S. Russell, J. Masarik, Earth Planet. Sci. Lett. 241, 530 (2006). 6. A. Markowski, G. Quitte, A. N. Halliday, T. Kleine, Earth Planet. Sci. Lett. 242, 1 (2006). 7. C. Burkhardt, T. Kleine, N. Dauphas, R. Wieler, Astrophys. J. 753, L6 (2012). 8. J. N. Connelly et al., Science 338, 651 (2012). 9. Y. Amelin, A. N. Krot, I. D. Hutcheon, A. A. Ulyanov, Science 297, 1678 (2002). 10. N. T. Kita, H. Nagahara, S. Togashi, Y. Morishita, Geochim. Cosmochim. Acta 64, 3913 (2000). 11. M. Regelous, T. Elliott, C. D. Coath, Earth Planet. Sci. Lett. 272, 330 (2008). 12. L. P. Qin et al., Astrophys. J. 674, 1234 (2008).
10.1126/science.1254943
NEUROSCIENCE
Memories—getting wired during sleep Sleep gives dendritic spines staying power By David R. Euston and Hendrik W. Steenland
T
he idea that sleep enhances memory has a long history, but only in the last 20 years has it gained solid empirical support. Many studies have shown that sleep deprivation impairs skill learning (1). Moreover, learning enhances oscillations in the brain’s electrical activity known as “slow waves” which occur during deeper stages of sleep. The strength of these oscillations also predicts future memory-based performance (2). But how exactly does sleep benefit memory? On page 1173 of this issue, Yang et al. (3) show that sleep influences changes in neuronal connectivity after learning. Studies in invertebrates and mammals have suggested that learning increases the strength of the connections, or “synapses,” between neurons (4). Given that skill learning is often enhanced during sleep, one would expect to see concomitant increases in synaptic strength. However, in many studies, sleep actually decreases synaptic strength. In mice, the number of dendritic spines on neurons, which correlates with the number of synapses, increased during wakefulness and decreased after a period of sleep (5). This led to the idea that sleep is a time for reducing the number of synaptic connections to enhance the information storage capacity of the brain (6). That sleep strengthens learning but weakens synapses presents a seeming paradox. Another outstanding issue is how sleep leads to synaptic change. Theory suggests that during sleep, the brain replays or “reactivates” neural activity patterns corresponding to recently learned experiences, thus enabling the modification of synaptic connections necessary to stabilize memory Canadian Centre for Behavioral Neuroscience, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada. E-mail: [email protected]
SCIENCE sciencemag.org
(7). This replay of recent experiences during sleep has indeed been observed in several areas of the brain in both rodents and monkeys (8–10). What has been missing is direct evidence that this reactivation is actually tied to learning rather than being just an epiphenomenon. To address whether synaptic strength increases or decreases during sleep, Yang et al. used a powerful technique to visual-
ize dendritic spines in the motor cortex of live mice. The mice were genetically engineered to express a fluorescent protein in a subset of cortical cells. A small window was created in the skull, allowing microscopic imaging of dendritic spines repeatedly over the course of hours or even days. This technique was previously used to show that training mice to stay atop a rotating rod— an acquired skill—induced the formation of new dendritic spines in the motor cortex (11). Further, the rate of new spine formation was correlated with the degree of task improvement. These findings provided direct evidence that synaptic change in the mammalian cortex underlies learning. Yang et al. extend these findings, showing that learning-induced spine changes are segregated on specific dendritic branches. After learning, when two branches on the same dendritic arbor were examined, one typically showed many more new spines than the other. If mice were subsequently trained on a different skill (i.e., running backward on the spinning rod), the new 6 JUNE 2014 • VOL 344 ISSUE 6188
Published by AAAS
1087
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 5, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6188/1086.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6188/1086.full.html#related This article cites 12 articles, 3 of which can be accessed free: http://www.sciencemag.org/content/344/6188/1086.full.html#ref-list-1
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on June 6, 2014
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
INSIGHTS | P E R S P E C T I V E S
PLANETARY SCIENCE
Speed metal Meteorite dating reveals that planetary core formation is a relatively fast process
A
s in many building booms, planets were put together pretty rapidly. Transforming nebular dust to fully formed planets took less than ~100 million years of the ~4.5 billion years of solar system history. Accurate determination of the rates of planetary growth is key for understanding these tumultuous beginnings of the solar system, but obtaining high-precision ages on short-lived events that happened so long ago is a formidable challenge. On page 1150 of this issue, Kruijer et al. (1) determine with remarkable accuracy that planetary core formation began less than 1 million years after the first solids condensed—extraordinarily fast on geological time scales. Planets are believed to grow by progressive accretion of smaller bodies (see the figure). Planetesimals are an intermediate step. They have radii up to a few hundred kilometers, which makes them sufficiently large to retain enough heat to melt or differentiate. During differentiation, metallic melts sink to form a core and silicate melts rise to form a crust. The larger asteroids, such as 4 Vesta, represent differentiated planetesimals. Asteroids are held in orbit between Mars and Jupiter, having been thwarted from further growth by Jupiter’s gravitational influence, and provide a frozen snapshot of activity that has long since run to completion elsewhere. Some meteorites—ejected fragments of asteroids that land on Earth—are derived from such differentiated bodies, and they provide samples that allow this early period of planetary growth to be studied. Kruijer et al. focus on the iron meteorites, which represent fragments of disrupted planetesimal cores. The chronometer of choice is the radioactive decay of hafnium isotope 182Hf to tungsten isotope 182W, which has been of great value elsewhere in dating early solar system processes (2). The half-life of 182Hf is only 9 million years, so it gives a high-resolution chronology over a period of ~50 million years at the beginning of the solar system when 182Hf was extant. Although precise, such “extinct” isotope systems only provide School of Earth Science, University of Bristol, Queen’s Road, Clifton BS8 1RJ, UK. E-mail: [email protected]
1086
ages relative to other dated objects (3). The oldest objects in the solar system, millimeter-sized calcium- and aluminum-rich inclusions (CAIs) from primitive meteorites, are often used as a reference. Over early solar system history, the decay of 182Hf leads to changing 182W/184W. During planetary melting, W is strongly partitioned into the coreforming metal phase, while Hf is left behind in the outer silicate portion of the planet. This process effectively freezes the 182W/184W
Planetary core formation. Planets grow by accreting smaller objects. At some point, differentiation occurs where metals sink to form a core, and silicates rise to form a crust. Accurate dating of meteorites reveals that the core forms very fast, within 1 million years of the first solids condensing in the early solar system (1).
ratio at the time of core formation and enables the core to be dated relative to evolving planetary 182W/184W (expressed as ε182W or the parts per 10,000 deviation of 182W/184W relative to a reference silicate Earth value). Thus, low (unradiogenic) ε182W in iron meteorites means an early-formed core. Kruijer et al. are not the first to note that extremely unradiogenic W in iron meteorites implies planetesimal core formation within the first few million years of solar system history (4–6). However, they comprehensively address what has been a major problem to the accuracy of all previous studies, namely the pernicious influence of cosmic rays in perturbing W isotope ratios at the required level of precision. The W isotope chronometer is sensitive to its past exposure to cosmic rays. Kruijer et al. use an elegant means to overcome this problem by making simultaneous platinum (Pt) isotope
the condensation of the first dated objects (CAIs) in the solar system, presenting an appreciable constraint for numerical accretion models. Furthermore, Kruijer et al. are able to discern differences in the timing of core formation between different planetesimals, represented by the different iron meteorite groups. Such differences were previously unresolvable. Kruijer et al. discuss an interesting model, which relates the timing of core formation to the variable amounts of sulfur (S) in the parent bodies. This approach makes good conceptual sense because the contents of volatile elements, such as S, vary widely in meteorites and planetary bodies. More S should lead to a larger amount of core formation at lower temperatures, earlier on in a planetesimal’s history. In Kruijer et al. ’s highly simplified thermal model, the time difference between initial segregation sciencemag.org SCIENCE
6 JUNE 2014 • VOL 344 ISSUE 6188
Published by AAAS
ILLUSTRATION: FAHAD SULEHRIA/WWW.NOVACELESTIA.COM AND SANTIAGO BILLY
By Tim Elliott
measurements to monitor the received dose of cosmic rays in each meteorite. Pt isotope ratios are more sensitive than W to the influence of cosmic rays but should be constant in all meteorites. Primary ε182W can therefore be obtained by back-correcting to a common, unperturbed Pt isotope ratio. This is more easily said than done, and the coupled, high-precision Pt and W data sets presented in this study are an isotopic tour de force. With the effects of cosmic rays removed, Kruijer et al. can date robustly the timing of planetesimal core formation. This endeavor is further helped by the recent reassessment of initial ε182W of the CAI datum (7), against which ages are measured. Thus, Kruijer et al. demonstrate that the sinking of metal in planetesimals is indeed speedy, occurring in as little as 0.6 ± 0.3 million years after
of a S-rich metallic melt and final formation of S-free metallic melt is ~0.4 million years, a time scale sufficient to account for the spread in their W isotope data. To put these ages of core formation into perspective, it is worth comparing them with ages of chondrules. These quenched melt droplets are the principal constituent of most primitive, undifferentiated meteorites, traditionally taken to be the building blocks of planets. Much recent effort has been expended in dating chondrules, and although some are as old as CAIs (8), most are ~1 million years younger (9, 10). Thus, chondrules were being formed, before their accretion into chondrites, at the same time as fully formed planetesimals were segregating cores. Presumably the respective parent bodies of iron meteorites and chondrites formed in different parts of the nebular disk to allow these very different accretion rates. The refined, rapid time scales of core formation presented by Kruijer et al. underline the notion that chondritic meteorites themselves did not accrete to form planets, even if their compositions do provide a valuable guide to bulk planetary composition. Finally, it is worth noting that the apparently youngest core formation age occurs in iron meteorites (IVB), which are also distinct in other aspects of their isotopic composition (11, 12). Although Kruijer et al. have carefully corrected the ε182W for estimated effects of W isotope heterogeneity, possible differences in Pt isotopes (and thus an influence on the cosmic ray correction) and even the initial amount of 182Hf remain to be assessed. This reemphasizes an age-old problem in distinguishing between time and initial isotopic composition in high-precision chronology. Nevertheless, for the majority of the samples studied here, the new data yield unprecedentedly accurate timing of the earliest stages of planetary formation. ■
ILLUSTRATION:C. BICKEL/SCIENCE
REFERENCES
1. T. S. Kruijer et al., Science 344, 1150 (2014). 2. T. Kleine et al., Geochim. Cosmochim. Acta 73, 5150 (2009). 3. N. Dauphas, M. Chaussidon, Annu. Rev. Earth Planet. Sci. 39, 351 (2011). 4. T. Kleine, K. Mezger, H. Palme, E. Scherer, C. Münker, Geochim. Cosmochim. Acta 69, 5805 (2005). 5. A. Scherstén, T. Elliott, C. Hawkesworth, S. Russell, J. Masarik, Earth Planet. Sci. Lett. 241, 530 (2006). 6. A. Markowski, G. Quitte, A. N. Halliday, T. Kleine, Earth Planet. Sci. Lett. 242, 1 (2006). 7. C. Burkhardt, T. Kleine, N. Dauphas, R. Wieler, Astrophys. J. 753, L6 (2012). 8. J. N. Connelly et al., Science 338, 651 (2012). 9. Y. Amelin, A. N. Krot, I. D. Hutcheon, A. A. Ulyanov, Science 297, 1678 (2002). 10. N. T. Kita, H. Nagahara, S. Togashi, Y. Morishita, Geochim. Cosmochim. Acta 64, 3913 (2000). 11. M. Regelous, T. Elliott, C. D. Coath, Earth Planet. Sci. Lett. 272, 330 (2008). 12. L. P. Qin et al., Astrophys. J. 674, 1234 (2008).
10.1126/science.1254943
NEUROSCIENCE
Memories—getting wired during sleep Sleep gives dendritic spines staying power By David R. Euston and Hendrik W. Steenland
T
he idea that sleep enhances memory has a long history, but only in the last 20 years has it gained solid empirical support. Many studies have shown that sleep deprivation impairs skill learning (1). Moreover, learning enhances oscillations in the brain’s electrical activity known as “slow waves” which occur during deeper stages of sleep. The strength of these oscillations also predicts future memory-based performance (2). But how exactly does sleep benefit memory? On page 1173 of this issue, Yang et al. (3) show that sleep influences changes in neuronal connectivity after learning. Studies in invertebrates and mammals have suggested that learning increases the strength of the connections, or “synapses,” between neurons (4). Given that skill learning is often enhanced during sleep, one would expect to see concomitant increases in synaptic strength. However, in many studies, sleep actually decreases synaptic strength. In mice, the number of dendritic spines on neurons, which correlates with the number of synapses, increased during wakefulness and decreased after a period of sleep (5). This led to the idea that sleep is a time for reducing the number of synaptic connections to enhance the information storage capacity of the brain (6). That sleep strengthens learning but weakens synapses presents a seeming paradox. Another outstanding issue is how sleep leads to synaptic change. Theory suggests that during sleep, the brain replays or “reactivates” neural activity patterns corresponding to recently learned experiences, thus enabling the modification of synaptic connections necessary to stabilize memory Canadian Centre for Behavioral Neuroscience, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada. E-mail: [email protected]
SCIENCE sciencemag.org
(7). This replay of recent experiences during sleep has indeed been observed in several areas of the brain in both rodents and monkeys (8–10). What has been missing is direct evidence that this reactivation is actually tied to learning rather than being just an epiphenomenon. To address whether synaptic strength increases or decreases during sleep, Yang et al. used a powerful technique to visual-
ize dendritic spines in the motor cortex of live mice. The mice were genetically engineered to express a fluorescent protein in a subset of cortical cells. A small window was created in the skull, allowing microscopic imaging of dendritic spines repeatedly over the course of hours or even days. This technique was previously used to show that training mice to stay atop a rotating rod— an acquired skill—induced the formation of new dendritic spines in the motor cortex (11). Further, the rate of new spine formation was correlated with the degree of task improvement. These findings provided direct evidence that synaptic change in the mammalian cortex underlies learning. Yang et al. extend these findings, showing that learning-induced spine changes are segregated on specific dendritic branches. After learning, when two branches on the same dendritic arbor were examined, one typically showed many more new spines than the other. If mice were subsequently trained on a different skill (i.e., running backward on the spinning rod), the new 6 JUNE 2014 • VOL 344 ISSUE 6188
Published by AAAS
1087
