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Sputnik Planitia is a nitrogen-ice-filled basin on Pluto.sup.1. Its polygonal surface patterns.sup.2 have been previously explained as a result of solid-state convection with either an imposed heat flow.sup.3 or a temperature difference within the 10-km-thick ice layer.sup.4. Neither explanation is satisfactory, because they do not exhibit surface topography with the observed pattern: flat polygons delimited by narrow troughs.sup.5. Internal heating produces the observed patterns.sup.6, but the heating source in such a setup remains enigmatic. Here we report the results of modelling the effects of sublimation at the surface. We find that sublimation-driven convection readily produces the observed polygonal structures if we assume a smaller heat flux (~0.3 mW m.sup.-2) at the base of the ice layer than the commonly accepted value of 2-3 mW m.sup.-2 (ref. .sup.7). Sustaining this regime with the latter value is also possible, but would require a stronger viscosity contrast (~3,000) than the nominal value (~100) considered in this study. A modelling study describing the formation of the polygonal surface structures in Sputnik Planitia on Pluto shows that convection driven by ice sublimation can generate planetary-scale surface patterns., Author(s): Adrien Morison [sup.1] , Stéphane Labrosse [sup.2] , Gaël Choblet [sup.3] Author Affiliations: (1) Physics and Astronomy, University of Exeter, Exeter, UK (2) LGL-TPE, Université de Lyon, ENSL, Univ [...]

Models of terrestrial planet formation predict that the final stages of planetary assembly--lasting tens of millions of years beyond the dispersal of young protoplanetary disks--are dominated by planetary collisions. It is through these giant impacts that planets like the young Earth grow to their final mass and achieve long-term stable orbital configurations.sup.1. A key prediction is that these impacts produce debris. So far, the most compelling observational evidence for post-impact debris comes from the planetary system around the nearby 23-million-year-old A-type star HD 172555. This system shows large amounts of fine dust with an unusually steep size distribution and atypical dust composition, previously attributed to either a hypervelocity impact.sup.2,3 or a massive asteroid belt.sup.4. Here we report the spectrally resolved detection of a carbon monoxide gas ring co-orbiting with dusty debris around HD 172555 between about six and nine astronomical units--a region analogous to the outer terrestrial planet region of our Solar System. Taken together, the dust and carbon monoxide detections favour a giant impact between large, volatile-rich bodies. This suggests that planetary-scale collisions, analogous to the Moon-forming impact, can release large amounts of gas as well as debris, and that this gas is observable, providing a window into the composition of young planets. A carbon monoxide gas ring co-orbiting with dusty debris is observed in the outer terrestrial planet region of the star HD 172555, which indicates that a planetary-scale impact took place., Author(s): Tajana Schneiderman [sup.1] , Luca Matrà [sup.2] , Alan P. Jackson [sup.3] [sup.4] , Grant M. Kennedy [sup.5] [sup.6] , Quentin Kral [sup.7] , Sebastián Marino [sup.8] [sup.9] , [...]

The event rate, energy distribution and time-domain behaviour of repeating fast radio bursts (FRBs) contain essential information regarding their physical nature and central engine, which are as yet unknown.sup.1,2. As the first precisely localized source, FRB 121102 (refs. .sup.3-5) has been extensively observed and shows non-Poisson clustering of bursts over time and a power-law energy distribution.sup.6-8. However, the extent of the energy distribution towards the fainter end was not known. Here we report the detection of 1,652 independent bursts with a peak burst rate of 122 h.sup.-1, in 59.5 hours spanning 47 days. A peak in the isotropic equivalent energy distribution is found to be approximately 4.8 × 10.sup.37 erg at 1.25 GHz, below which the detection of bursts is suppressed. The burst energy distribution is bimodal, and well characterized by a combination of a log-normal function and a generalized Cauchy function. The large number of bursts in hour-long spans allows sensitive periodicity searches between 1 ms and 1,000 s. The non-detection of any periodicity or quasi-periodicity poses challenges for models involving a single rotating compact object. The high burst rate also implies that FRBs must be generated with a high radiative efficiency, disfavouring emission mechanisms with large energy requirements or contrived triggering conditions. For FRB 121102, 1,652 burst events are detected over 47 days, with a peak burst rate of 122 per hour, a bimodal burst rate energy distribution, and no periodicity or quasi-periodicity., Author(s): D. Li [sup.1] [sup.2] , P. Wang [sup.1] , W. W. Zhu [sup.1] , B. Zhang [sup.3] , X. X. Zhang [sup.1] , R. Duan [sup.1] , Y. K. [...]

Studies.sup.1,2 have shown that the remnants of destroyed planets and debris-disk planetesimals can survive the volatile evolution of their host stars into white dwarfs.sup.3,4, but few intact planetary bodies around white dwarfs have been detected.sup.5-8. Simulations predict.sup.9-11 that planets in Jupiter-like orbits around stars of [less-than or equivalent to]8 M.sub.â (solar mass) avoid being destroyed by the strong tidal forces of their stellar host, but as yet, there has been no observational confirmation of such a survivor. Here we report the non-detection of a main-sequence lens star in the microlensing event MOA-2010-BLG-477Lb.sup.12 using near-infrared observations from the Keck Observatory. We determine that this system contains a 0.53 [plus or minus] 0.11 M.sub.â white-dwarf host orbited by a 1.4 [plus or minus] 0.3 Jupiter-mass planet with a separation on the plane of the sky of 2.8 [plus or minus] 0.5 astronomical units, which implies a semi-major axis larger than this. This system is evidence that planets around white dwarfs can survive the giant and asymptotic giant phases of their host's evolution, and supports the prediction that more than half of white dwarfs have Jovian planetary companions.sup.13. Located at approximately 2.0 kiloparsecs towards the centre of our Galaxy, it is likely to represent an analogue to the end stages of the Sun and Jupiter in our own Solar System. The authors show not only that planetary bodies around white dwarfs can survive but also that more than half of white dwarfs might have Jovian planetary companions., Author(s): J. W. Blackman [sup.1] [sup.2] , J. P. Beaulieu [sup.1] [sup.2] , D. P. Bennett [sup.3] [sup.4] , C. Danielski [sup.2] [sup.5] [sup.6] , C. Alard [sup.2] , A. [...]

Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs. .sup.1-3), during the so-called epoch of reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs.sup.4-7, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies.sup.8-12 and companions of rare quasars.sup.13,14. These studies conclude that the contribution of dust-obscured galaxies to the cosmic star formation rate density at z > 6 is sub-dominant. Recent ALMA and Spitzer observations have identified a more abundant, less extreme population of obscured galaxies at z = 3-6 (refs. .sup.15,16). However, this population has not been confirmed in the reionization epoch so far. Here, we report the discovery of two dust-obscured star-forming galaxies at z = 6.6813 [plus or minus] 0.0005 and z = 7.3521 [plus or minus] 0.0005. These objects are not detected in existing rest-frame UV data and were discovered only through their far-infrared [C ii] lines and dust continuum emission as companions to typical UV-luminous galaxies at the same redshift. The two galaxies exhibit lower infrared luminosities and star-formation rates than extreme starbursts, in line with typical star-forming galaxies at z [almost equal to] 7. This population of heavily dust-obscured galaxies appears to contribute 10-25% to the z > 6 cosmic star formation rate density. Two serendipitously detected dust-obscured galaxies are reported at z = 6.7 and 7.4, with estimates that such galaxies provide an additional 10-25% contribution to the total star formation rate density at z > 6., Author(s): Y. Fudamoto [sup.1] [sup.2] [sup.3] , P. A. Oesch [sup.1] [sup.4] , S. Schouws [sup.5] , M. Stefanon [sup.5] , R. Smit [sup.6] , R. J. Bouwens [sup.5] , [...]

Star formation in half of massive galaxies was quenched by the time the Universe was 3 billion years old.sup.1. Very low amounts of molecular gas seem to be responsible for this, at least in some cases.sup.2-7, although morphological gas stabilization, shock heating or activity associated with accretion onto a central supermassive black hole are invoked in other cases.sup.8-11. Recent studies of quenching by gas depletion have been based on upper limits that are insufficiently sensitive to determine this robustly.sup.2-7, or stacked emission with its problems of averaging.sup.8,9. Here we report 1.3 mm observations of dust emission from 6 strongly lensed galaxies where star formation has been quenched, with magnifications of up to a factor of 30. Four of the six galaxies are undetected in dust emission, with an estimated upper limit on the dust mass of 0.0001 times the stellar mass, and by proxy (assuming a Milky Way molecular gas-to-dust ratio) 0.01 times the stellar mass in molecular gas. This is two orders of magnitude less molecular gas per unit stellar mass than seen in star forming galaxies at similar redshifts.sup.12-14. It remains difficult to extrapolate from these small samples, but these observations establish that gas depletion is responsible for a cessation of star formation in some fraction of high-redshift galaxies. The authors report 1.3 mm observations of dust emission from strongly lensed galaxies where star formation is quenched, demonstrating that gas depletion is responsible for the cessation of star formation in some high-redshift galaxies., Author(s): Katherine E. Whitaker [sup.1] [sup.2] , Christina C. Williams [sup.3] , Lamiya Mowla [sup.4] , Justin S. Spilker [sup.5] , Sune Toft [sup.2] [sup.6] , Desika Narayanan [sup.2] [sup.7] [...]

Fast radio bursts (FRBs) are extragalactic astrophysical transients.sup.1 whose brightness requires emitters that are highly energetic yet compact enough to produce the short, millisecond-duration bursts. FRBs have thus far been detected at frequencies from 8 gigahertz (ref. .sup.2) down to 300 megahertz (ref. .sup.3), but lower-frequency emission has remained elusive. Some FRBs repeat.sup.4-6, and one of the most frequently detected, FRB 20180916B.sup.7, has a periodicity cycle of 16.35 days (ref. .sup.8). Using simultaneous radio data spanning a wide range of wavelengths (a factor of more than 10), here we show that FRB 20180916B emits down to 120 megahertz, and that its activity window is frequency dependent (that is, chromatic). The window is both narrower and earlier at higher frequencies. Binary wind interaction models predict a wider window at higher frequencies, the opposite of our observations. Our full-cycle coverage shows that the 16.3-day periodicity is not aliased. We establish that low-frequency FRB emission can escape the local medium. For bursts of the same fluence, FRB 20180916B is more active below 200 megahertz than at 1.4 gigahertz. Combining our results with previous upper limits on the all-sky FRB rate at 150 megahertz, we find there are 3-450 FRBs in the sky per day above 50 Jy ms. Our chromatic results strongly disfavour scenarios in which absorption from strong stellar winds causes FRB periodicity. We demonstrate that some FRBs are found in 'clean' environments that do not absorb or scatter low-frequency radiation. The fast radio burst FRB 20180916B repeats with a periodicity of 16 days, and is now found to emit down to a frequency of 120 MHz, much lower than previously observed., Author(s): Inés Pastor-Marazuela [sup.1] [sup.2] , Liam Connor [sup.1] [sup.2] [sup.3] , Joeri van Leeuwen [sup.1] [sup.2] , Yogesh Maan [sup.2] , Sander ter Veen [sup.2] , Anna Bilous [sup.2] [...]

Recent multi-dimensional simulations suggest that high-entropy buoyant plumes help massive stars to explode.sup.1,2. Outwardly protruding iron (Fe)-rich fingers of gas in the galactic supernova remnant.sup.3,4 Cassiopeia A seem to match this picture. Detecting the signatures of specific elements synthesized in the high-entropy nuclear burning regime (that is, [alpha]-rich freeze out) would constitute strong substantiating evidence. Here we report observations of such elements--stable titanium (Ti) and chromium (Cr)--at a confidence level greater than 5 standard deviations in the shocked high-velocity Fe-rich ejecta of Cassiopeia A. We found that the observed Ti/Fe and Cr/Fe mass ratios require [alpha]-rich freeze out, providing evidence of the existence of the high-entropy ejecta plumes that boosted the shock wave at explosion. The metal composition of the plumes agrees well with predictions for strongly neutrino-processed proton-rich ejecta.sup.2,5,6. These results support the operation of the convective supernova engine via neutrino heating in the supernova that produced Cassiopeia A. The abundances of stable Ti and Cr relative to Fe observed in the Cassiopeia A core-collapse supernova remnant reveal that Ti and Cr must have formed in neutrino-driven plumes that helped to drive the explosion., Author(s): Toshiki Sato [sup.1] [sup.2] [sup.3] [sup.4] , Keiichi Maeda [sup.5] , Shigehiro Nagataki [sup.6] [sup.7] , Takashi Yoshida [sup.8] [sup.9] , Brian Grefenstette [sup.10] , Brian J. Williams [sup.2] [...]

Our current knowledge of cosmic star-formation history during the first two billion years (corresponding to redshift z > 3) is mainly based on galaxies identified in rest-frame ultraviolet light.sup.1. However, this population of galaxies is known to under-represent the most massive galaxies, which have rich dust content and/or old stellar populations. This raises the questions of the true abundance of massive galaxies and the star-formation-rate density in the early Universe. Although several massive galaxies that are invisible in the ultraviolet have recently been confirmed at early epochs.sup.2-4, most of them are extreme starburst galaxies with star-formation rates exceeding 1,000 solar masses per year, suggesting that they are unlikely to represent the bulk population of massive galaxies. Here we report submillimetre (wavelength 870 micrometres) detections of 39 massive star-forming galaxies at z > 3, which are unseen in the spectral region from the deepest ultraviolet to the near-infrared. With a space density of about 2 × 10.sup.-5 per cubic megaparsec (two orders of magnitude higher than extreme starbursts.sup.5) and star-formation rates of 200 solar masses per year, these galaxies represent the bulk population of massive galaxies that has been missed from previous surveys. They contribute a total star-formation-rate density ten times larger than that of equivalently massive ultraviolet-bright galaxies at z > 3. Residing in the most massive dark matter haloes at their redshifts, they are probably the progenitors of the largest present-day galaxies in massive groups and clusters. Such a high abundance of massive and dusty galaxies in the early Universe challenges our understanding of massive-galaxy formation. Submillimetre-wavelength observations reveal a sample of galaxies that have no detectable emission in the ultraviolet-to-near-infrared region, and are probably the progenitors of the largest present-day galaxies in clusters., Author(s): T. Wang [sup.1] [sup.2] [sup.3] , C. Schreiber [sup.2] [sup.4] [sup.5] , D. Elbaz [sup.2] , Y. Yoshimura [sup.1] , K. Kohno [sup.1] [sup.6] , X. Shu [sup.7] , [...]

Astronomers peer inside planetary nurseries for clues about how our Solar System and others came to be. Astronomers peer inside planetary nurseries for clues about how our Solar System and others came to be., Author(s): Rebecca Boyle Author Affiliations: These dusty young stars are changing the rules of planet-building An artist's impression of a planetary nursery, in which growing planets etch rings in the [...]

Author(s): Judd D. Bowman (corresponding author) [1]; Alan E. E. Rogers [2]; Raul A. Monsalve [1, 3, 4]; Thomas J. Mozdzen [1]; Nivedita Mahesh [1] After stars formed in the [...]

Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black holes.sup.1. However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars.sup.2,3 and others showing no such association.sup.4,5. Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated.sup.6-8 (5 to 40 kiloparsecs). Merger simulations further suggest that such obscuration and black-hole accretion peaks in the final merger stage, when the two galactic nuclei are closely separated.sup.9 (less than 3 kiloparsecs). Resolving this final stage requires a combination of high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray observations to detect highly obscured sources. However, large numbers of obscured luminous accreting supermassive black holes have been recently detected nearby (distances below 250 megaparsecs) in X-ray observations.sup.10. Here we report high-resolution infrared observations of hard-X-ray-selected black holes and the discovery of obscured nuclear mergers, the parent populations of supermassive-black-hole mergers. We find that obscured luminous black holes (bolometric luminosity higher than 2 × 10.sup.44 ergs per second) show a significant (P < 0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a sample of inactive galaxies with matching stellar masses and star formation rates (1.1 per cent), in agreement with theoretical predictions. Using hydrodynamic simulations, we confirm that the excess of nuclear mergers is indeed strongest for gas-rich major-merger hosts of obscured luminous black holes in this final stage.High-resolution infrared observations of hard-X-ray-selected black holes show an excess of late-stage mergers in obscured luminous black holes compared with inactive galaxies of similar stellar masses and star formation rates., Author(s): Michael J. Koss [sup.1] [sup.2] , Laura Blecha [sup.3] , Phillip Bernhard [sup.2] , Chao-Ling Hung [sup.4] , Jessica R. Lu [sup.5] , Benny Trakthenbrot [sup.6] [sup.7] , Ezequiel [...]

Author(s): Will M. Farr (corresponding author) [1]; Simon Stevenson [1, 2]; M. Coleman Miller [3]; Ilya Mandel [1, 2]; Ben Farr [4]; Alberto Vecchio [1] The direct detection of gravitational [...]

A sophisticated climate model suggests that liquid-water oceans never formed on Venus, and that some planets outside the Solar System that were thought to be habitable might not be. Climate model suggests that liquid-water oceans didn't form on Venus., Author(s): James F. Kasting, Chester E. Harman Author Affiliations: Venus might never have been habitable Today, Earth's sister planet Venus is more like a distant relative than a sibling. Its [...]

A renewed focus on our planetary neighbour could help to answer major questions about its atmosphere and geology. A renewed focus on our planetary neighbour could help to answer major questions about its atmosphere and geology., Author(s): Jonathan O'Callaghan Author Affiliations: How three missions to Venus could solve the planet's biggest mysteries In three newly-approved missions to Venus, scientists will probe the planet's atmosphere and geology. [...]

The solid, central part of a comet--its nucleus--is subject to destructive processes (1,2), which cause nuclei to split at a rate of about 0.01 per year per comet (3). These destructive events are due to a range of possible thermophysical effects (4); however, the geophysical expressions of these effects are unknown. Separately, over two-thirds of comet nuclei that have been imaged at high resolution show bilobate shapes (5), including the nucleus of comet 67P/Churyumov-Gerasimenko (67P), visited by the Rosetta spacecraft. Analysis of the Rosetta observations suggests that 67P's components were brought together at low speed after their separate formation (6). Here, we study the structure and dynamics of 67P's nucleus. We find that sublimation torques have caused the nucleus to spin up in the past to form the large cracks observed on its neck. However, the chaotic evolution of its spin state has so far forestalled its splitting, although it should eventually reach a rapid enough spin rate to do so. Once this occurs, the separated components will be unable to escape each other; they will orbit each other for a time, ultimately undergoing a low-speed merger that will result in a new bilobate configuration. The components of four other imaged bilobate nuclei have volume ratios that are consistent with a similar reconfiguration cycle, pointing to such cycles as a fundamental process in the evolution of short-period comet nuclei. It has been shown (7,8) that comets were not strong contributors to the so-called late heavy bombardment about 4 billion years ago. The reconfiguration process suggested here would preferentially decimate comet nuclei during migration to the inner solar system, perhaps explaining this lack of a substantial cometary flux., Along the neck of 67P's nucleus (the Hapi region) are two straight cracks, each a few hundred metres long and separated from each other by 750 metres (Fig. 1a, b) [...]

In order for quiescent galaxies to maintain their low-to-non-existent star formation, there must be a mechanism to remove or heat gas that would otherwise cool to form stars; now supermassive black hole winds that are sufficient to suppress star formation in such galaxies are reported. Red geysers keep a cap on star formation Edmond Cheung et al. describe a previously unrecognized class of galaxy that they call 'red geysers': the prototypical example, Akira, is a quiescent 'red and dead' elliptical galaxy driving an ionized outflowing wind. Quiescent galaxies with little or no ongoing star formation are widespread, their featureless spheroid structure contrasting to the more varied star-forming structures seen in spiral galaxies. Some unknown mechanism must remove or heat gas from stellar mass loss or mergers that would otherwise cool to form stars. Edmond Cheung et al. report the presence of centrally driven winds in typical quiescent galaxies that host low-luminosity active nuclei. These galaxies are surprisingly common, accounting for as much as ten per cent of the population of galaxies of 20 billion solar masses or greater. The authors calculate that, for Akira, the energy input is sufficient to suppress star formation, and suggest that such red geysers may represent how typical quiescent galaxies maintain their quiescence. Quiescent galaxies with little or no ongoing star formation dominate the population of galaxies with masses above 2 × 10.sup.10 times that of the Sun; the number of quiescent galaxies has increased by a factor of about 25 over the past ten billion years (refs 1, 2, 3, 4). Once star formation has been shut down, perhaps during the quasar phase of rapid accretion onto a supermassive black hole.sup.5,6,7, an unknown mechanism must remove or heat the gas that is subsequently accreted from either stellar mass loss.sup.8 or mergers and that would otherwise cool to form stars.sup.9,10. Energy output from a black hole accreting at a low rate has been proposed.sup.11,12,13, but observational evidence for this in the form of expanding hot gas shells is indirect and limited to radio galaxies at the centres of clusters.sup.14,15, which are too rare to explain the vast majority of the quiescent population.sup.16. Here we report bisymmetric emission features co-aligned with strong ionized-gas velocity gradients from which we infer the presence of centrally driven winds in typical quiescent galaxies that host low-luminosity active nuclei. These galaxies are surprisingly common, accounting for as much as ten per cent of the quiescent population with masses around 2 × 10.sup.10 times that of the Sun. In a prototypical example, we calculate that the energy input from the galaxy's low-level active supermassive black hole is capable of driving the observed wind, which contains sufficient mechanical energy to heat ambient, cooler gas (also detected) and thereby suppress star formation., Author(s): Edmond Cheung [sup.1] , Kevin Bundy [sup.1] , Michele Cappellari [sup.2] , Sébastien Peirani [sup.1] [sup.3] , Wiphu Rujopakarn [sup.1] [sup.4] , Kyle Westfall [sup.5] , Renbin Yan [sup.6] [...]

Quasars are associated with and powered by the accretion of material onto massive black holes; the detection of highly luminous quasars with redshifts greater than z = 6 suggests that [...]

The signature of [sup.60]Fe in deep-sea crusts indicates that one or more supernovae exploded in the solar neighbourhood about 2.2 million years ago (1-4). Recent isotopic analysis is consistent with a core-collapse or electron-capture supernova that occurred 60 to 130 parsecs from the Sun (5). Moreover, peculiarities in the cosmic ray spectrum point to a nearby supernova about two million years ago (6). The Local Bubble of hot, diffuse plasma, in which the Solar System is embedded, originated from 14 to 20 supernovae within a moving group, whose surviving members are now in the Scorpius-Centaurus stellar association (7,8). Here we report calculations of the most probable trajectories and masses of the supernova progenitors, and hence their explosion times and sites. The [sup.60]Fe signal arises from two supernovae at distances between 90 and 100 parsecs. The closest occurred 2.3 million years ago at present-day galactic coordinates l = 327°, b = 11°, and the second-closest exploded about 1.5 million years ago at l = 343°, b = 25°, with masses of 9.2 and 8.8 times the solar mass, respectively. The remaining supernovae, which formed the Local Bubble, contribute to a smaller extent because they happened at larger distances and longer ago ([sup.60]Fe has a half-life of 2.6 million years (9,10)). There are uncertainties relating to the nucleosynthesis yields and the loss of [sup.60]Fe during transport, but they do not influence the relative distribution of [sup.60]Fe in the crust layers, and therefore our model reproduces the measured relative abundances very well., It has been shown that supernovae from a moving stellar group, whose surviving members are now in the Sco-Cen association, were able to generate the Local Bubble (7,11). This hypothesis [...]

Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes (1,2). The main site of production of the r-process elements (such as europium) has been [...]

Polar hydrogen deposits on the Moon provide evidence that its spin axis has shifted; analysis of the locations of these deposits and of the lunar figure suggests that the shift occurred as a result of changes in the Moon's moments of inertia caused by a low-density thermal anomaly beneath the Procellarum region. Polar wandering on the Moon Matthew Siegler and co-authors show that the polar hydrogen deposits detected on the Moon by orbital neutron spectroscopy pose something of a mystery: the spatial distribution of hydrogen, which is thought to arise from the presence of water ice, does not match that expected from present-day lunar temperatures. The explanation may lie in the phenomenon known as 'true polar wander', in which a reference point on a solid body rotates with respect to its spin axis. Based on an analysis of polar deposit locations and of the lunar figure, the authors suggest that the shift occurred as a result of changes in the Moon's moment of inertia caused by a low-density thermal anomaly beneath the Procellarum region. Procellarum was most geologically active early in lunar history, which would imply that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. The earliest dynamic and thermal history of the Moon is not well understood. The hydrogen content of deposits near the lunar poles may yield insight into this history, because these deposits (which are probably composed of water ice) survive only if they remain in permanent shadow. If the orientation of the Moon has changed, then the locations of the shadowed regions will also have changed. The polar hydrogen deposits have been mapped by orbiting neutron spectrometers.sup.1,2,3, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures.sup.4,5. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury's poles.sup.6. Here we show that polar hydrogen preserves evidence that the spin axis of the Moon has shifted: the hydrogen deposits are antipodal and displaced equally from each pole along opposite longitudes. From the direction and magnitude of the inferred reorientation, and from analysis of the moments of inertia of the Moon, we hypothesize that this change in the spin axis, known as true polar wander, was caused by a low-density thermal anomaly beneath the Procellarum region. Radiogenic heating within this region resulted in the bulk of lunar mare volcanism.sup.7,8,9,10,11 and altered the density structure of the Moon, changing its moments of inertia. This resulted in true polar wander consistent with the observed remnant polar hydrogen. This thermal anomaly still exists and, in part, controls the current orientation of the Moon. The Procellarum region was most geologically active early in lunar history.sup.7,8,9, which implies that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. Our hypothesis provides an explanation for the antipodal distribution of lunar polar hydrogen, and connects polar volatiles to the geologic and geophysical evolution of the Moon and the bombardment history of the early Solar System., Author(s): M. A. Siegler [sup.1] [sup.2] , R. S. Miller [sup.3] , J. T. Keane [sup.4] , M. Laneuville [sup.5] , D. A. Paige [sup.6] , I. Matsuyama [sup.4] , [...]

A momentous signal from space has confirmed decades of theorizing on black holes -- and launched a new era of gravitational-wave astronomy., Author(s): Davide Castelvecchi Author Affiliations: Binary black holes radiate a huge amount of orbital energy as gravitational waves. Illustration by Mark Garlick The event was catastrophic on a cosmic scale [...]

Massive stars inject mechanical and radiative energy into the surrounding environment, which stirs it up, heats the gas, produces cloud and intercloud phases in the interstellar medium, and disrupts molecular clouds (the birth sites of new stars.sup.1,2). Stellar winds, supernova explosions and ionization by ultraviolet photons control the lifetimes of molecular clouds.sup.3-7. Theoretical studies predict that momentum injection by radiation should dominate that by stellar winds.sup.8, but this has been difficult to assess observationally. Velocity-resolved large-scale images in the fine-structure line of ionized carbon ([C ii]) provide an observational diagnostic for the radiative energy input and the dynamics of the interstellar medium around massive stars. Here we report observations of a one-square-degree region (about 7 parsecs in diameter) of Orion molecular core 1--the region nearest to Earth that exhibits massive-star formation--at a resolution of 16 arcseconds (0.03 parsecs) in the [C ii] line at 1.9 terahertz (158 micrometres). The results reveal that the stellar wind originating from the massive star [theta].sup.1 Orionis C has swept up the surrounding material to create a 'bubble' roughly four parsecs in diameter with a 2,600-solar-mass shell, which is expanding at 13 kilometres per second. This finding demonstrates that the mechanical energy from the stellar wind is converted very efficiently into kinetic energy of the shell and causes more disruption of the Orion molecular core 1 than do photo-ionization and evaporation or future supernova explosions.Wind from the most massive star in the Trapezium cluster in Orion has carved out a large and expanding cavity around the cluster, bounded by a thin, 2,600-solar-mass shell., Author(s): C. Pabst [sup.1] , R. Higgins [sup.2] , J. R. Goicoechea [sup.3] , D. Teyssier [sup.4] , O. Berne [sup.5] , E. Chambers [sup.6] , M. Wolfire [sup.7] , [...]

Solar X-ray jets are thought to be made by a burst of reconnection of closed magnetic field at the base of a jet with ambient open field (1,2). In the accepted version of the 'emerging-flux' model, such a reconnection occurs at a plasma current sheet between the open field and the emerging closed field, and also forms a localized X-ray brightening that is usually observed at the edge of the jet's base (1,3). Here we report high-resolution X-ray and extreme-ultraviolet observations of 20 randomly selected X-ray jets that form in coronal holes at the Sun's poles. In each jet, contrary to the emerging-flux model, a miniature version of the filament eruptions that initiate coronal mass ejections (4-7) drives the jet-producing reconnection. The X-ray bright point occurs by reconnection of the 'legs' of the minifilament-carrying erupting closed field, analogous to the formation of solar flares in larger-scale eruptions. Previous observations have found that some jets are driven by base-field eruptions (8-11), but only one such study, of only one jet, provisionally questioned the emerging-flux model (12). Our observations support the view that solar filament eruptions are formed by a fundamental explosive magnetic process that occurs on a vast range of scales, from the biggest mass ejections and flare eruptions down to X-ray jets, and perhaps even down to smaller jets that may power coronal heating (10,13,14). A similar scenario has previously been suggested, but was inferred from different observations and based on a different origin of the erupting minifilament (15)., Solar X-ray jets are imaged by satellite-borne telescopes in space in the ~0.2-2.0-keV range. They are dynamic (with upward velocities of around 200 km [s.sup.-1]), long (about 5 X [10.sup.4] [...]

Most of the properties of the Earth-Moon system can be explained by a collision between a planetary embryo (giant impactor) and the growing Earth late in the accretion process (1-3). Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor (1,4,5). However, analysis of the terrestrial and lunar isotopic compositions show them to be highly similar (6-11). In contrast, the compositions of other Solar System bodies are significantly different from those of the Earth and Moon (12-14), suggesting that different Solar System bodies have distinct compositions. This challenges the giant impact scenario, because the Moon-forming impactor must then also be thought to have a composition different from that of the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion (15), to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are statistically more similar to the planets they impact. A large fraction of planet-impactor pairs have almost identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact., Successful models for Moon formation typically require a relatively low-velocity, oblique impact (1) between the proto-Earth and a planetary embryo of up to a few tenths of an Earth-mass ([M.sub.[direct [...]

Observations of an ultraluminous quasar, SDSS J010013.02+280225.8, at redshift z = 6.30 show that the object has an optical and near-infrared luminosity a few times greater than those of previously known quasars at z > 6; the black hole that drives the quasar has a mass about 12 billion times that of the Sun. Ultraluminous quasar illuminates epoch of cosmic reionization Cosmic redshifts of between 6 and 7 represent a time when the intergalactic medium was in transition from a neutral state to being completely ionized. Here Xue-Bing Wu et al. report the discovery of an ultraluminous quasar at redshift z = 6.30 that has optical and near-infrared luminosity several times greater than previously known quasars at redshifts beyond 6. Based on near-infrared spectral data, the authors estimate a mass of approximately twelve-billion solar-masses for the associated black hole, consistent with the thirteen-billion solar masses derived by assuming an Eddington-limited accretion rate, where the force of radiation acting outwards and the gravitational force acting inwards are in balance. As the most luminous quasar known to date at z = 6, this object will be a useful resource for the study of galaxy formation around massive black holes at the end of the epoch of cosmic reionization. So far, roughly 40 quasars with redshifts greater than z = 6 have been discovered.sup.1,2,3,4,5,6,7,8. Each quasar contains a black hole with a mass of about one billion solar masses (10.sup.9[Formula omitted]).sup.2,6,7,9,10,11,12,13. The existence of such black holes when the Universe was less than one billion years old presents substantial challenges to theories of the formation and growth of black holes and the coevolution of black holes and galaxies.sup.14. Here we report the discovery of an ultraluminous quasar, SDSS J010013.02+280225.8, at redshift z = 6.30. It has an optical and near-infrared luminosity a few times greater than those of previously known z > 6 quasars. On the basis of the deep absorption trough.sup.15 on the blue side of the Lyman-[alpha] emission line in the spectrum, we estimate the proper size of the ionized proximity zone associated with the quasar to be about 26 million light years, larger than found with other z > 6.1 quasars with lower luminosities.sup.16. We estimate (on the basis of a near-infrared spectrum) that the black hole has a mass of ~1.2 × 10.sup.10[Formula omitted], which is consistent with the 1.3 × 10.sup.10[Formula omitted] derived by assuming an Eddington-limited accretion rate., Author(s): Xue-Bing Wu [sup.1] [sup.2] , Feige Wang [sup.1] [sup.2] , Xiaohui Fan [sup.2] [sup.3] , Weimin Yi [sup.4] [sup.5] [sup.6] , Wenwen Zuo [sup.7] , Fuyan Bian [sup.8] , [...]

Examination of amateur observations of Mars shows atmospheric plumes 200 to 250 kilometres high that are observed in the morning but not in the evening over a period of more than a week; our current understanding of Martian atmospheric dynamics and plume formation cannot account for the creation of such enormous plumes. Mars's mystery plume In March and April 2012, amateur astronomers observing Mars recorded what seemed like an eruption of some kind, a protrusion from the planet's disk that took about 10 hours to develop and lasted for more than a week. Agustin Sánchez-Lavega and colleagues have now analysed these observations in detail and describe the highest-reaching plume-like feature ever detected on Mars. It reached an altitude of 250 kilometres -- previous high-altitude features reached no higher than 100 kilometres -- and extended more than 500 kilometres in both the north-south and east-west directions. Suggested explanations for this mysterious plume include clouds of CO.sub.2 or H.sub.2O ice or a phenomenon related to auroral emissions, but our current understanding of Martian atmospheric dynamics and plume formation cannot account for the creation of such enormous plumes. The Martian limb (that is, the observed 'edge' of the planet) represents a unique window into the complex atmospheric phenomena occurring there. Clouds of ice crystals (CO.sub.2 ice or H.sub.2O ice) have been observed numerous times by spacecraft and ground-based telescopes, showing that clouds are typically layered and always confined below an altitude of 100 kilometres; suspended dust has also been detected at altitudes up to 60 kilometres during major dust storms.sup.1,2,3,4,5,6. Highly concentrated and localized patches of auroral emission controlled by magnetic field anomalies in the crust have been observed at an altitude of 130 kilometres.sup.7. Here we report the occurrence in March and April 2012 of two bright, extremely high-altitude plumes at the Martian terminator (the day-night boundary) at 200 to 250 kilometres or more above the surface, and thus well into the ionosphere and the exosphere.sup.8,9. They were spotted at a longitude of about 195° west, a latitude of about -45° (at Terra Cimmeria), extended about 500 to 1,000 kilometres in both the north-south and east-west directions, and lasted for about 10 days. The features exhibited day-to-day variability, and were seen at the morning terminator but not at the evening limb, which indicates rapid evolution in less than 10 hours and a cyclic behaviour. We used photometric measurements to explore two possible scenarios and investigate their nature. For particles reflecting solar radiation, clouds of CO.sub.2-ice or H.sub.2O-ice particles with an effective radius of 0.1 micrometres are favoured over dust. Alternatively, the plume could arise from auroral emission, of a brightness more than 1,000 times that of the Earth's aurora, over a region with a strong magnetic anomaly where aurorae have previously been detected.sup.7. Importantly, both explanations defy our current understanding of Mars' upper atmosphere., Author(s): A. Sánchez-Lavega [sup.1] [sup.2] , A. García Muñoz [sup.3] , E. García-Melendo [sup.1] [sup.4] , S. Pérez-Hoyos [sup.1] [sup.2] , J. M. Gómez-Forrellad [sup.4] , C. Pellier [sup.5] , [...]

The age of dense interstellar cloud cores, where stars and planets form, is a crucial parameter in star formation and difficult to measure. Some models predict rapid collapse (1,2), whereas others predict timescales of more than one million years (ref. 3). One possible approach to determining the age is through chemical changes as cloud contraction occurs, in particular through indirect measurements of the ratio of the two spin isomers (ortho/para) of molecular hydrogen, [H.sub.2], which decreases monotonically with age (4-6). This has been done for the dense cloud core L183, for which the deuterium fractionation of diazenylium ([N.sub.2][H.sup.+]) was used as a chemical clock to infer (7) that the core has contracted rapidly (on a timescale of less than 700,000 years). Among astronomically observable molecules, the spin isomers of the deuterated trihydrogen cation, ortho-[H.sub.2][D.sup.+] and para-[H.sub.2][D.sup.+], have the most direct chemical connections to [H.sub.2] (refs 8-12) and their abundance ratio provides a chemical clock that is sensitive to greater cloud core ages. So far this ratio has not been determined because para-[H.sub.2][D.sup.+] is very difficult to observe. The detection of its rotational ground-state line has only now become possible thanks to accurate measurements of its transition frequency in the laboratory (13), and recent progress in instrumentation technology (14,15). Here we report observations of ortho- and para-[H.sub.2][D.sup.+] emission and absorption, respectively, from the dense cloud core hosting IRAS 16293-2422 A/B, a group of nascent solar-type stars (with ages of less than 100,000 years). Using the ortho/para ratio in conjunction with chemical models, we find that the dense core has been chemically processed for at least one million years. The apparent discrepancy with the earlier [N.sub.2][H.sup.+] work (7) arises because that chemical clock turns off sooner than the [H.sub.2][D.sup.+] clock, but both results imply that star-forming dense cores have ages of about one million years, rather than 100,000 years., We detected the ground-state rotational transition of the para spin isomer of the deuterated trihydrogen cation (para-[H.sub.2][D.sup.+]) at 1.370085 THz (ref. 13) (wavelength λ = 219 µm) towards IRAS 16293-2422 [...]

Magnetic fields in accretion disks play a dominant part during the star formation process (1,2) but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri [...]

The formation of planets around binary stars maybe more difficult than around single stars (1-3). In a close binary star (with a separation of less than a hundred astronomical units), [...]

Analysis of archival quasar data reveals that two quantities -- the Eddington ratio and orientation -- explain most of the diverse characteristics of quasars. Sources of quasar variation identified Quasars are powered by the accretion of material onto supermassive black holes in the centres of galaxies. Here, Yue Shen and Luis Ho tackle a long-standing question, the physical basis for the remarkable spectroscopic diversity displayed by quasars. Based on data from a large uniform sample of quasars, the authors demonstrate that the variations in the observed quasars' properties can be attributed to two basic parameters -- the accretion rate onto the central black hole (measured as the Eddington ratio, luminosity divided by black hole mass) and the orientation of a disk-like distribution of the gas clouds orbiting close to the hole. Quasars are rapidly accreting supermassive black holes at the centres of massive galaxies. They display a broad range of properties across all wavelengths, reflecting the diversity in the physical conditions of the regions close to the central engine. These properties, however, are not random, but form well-defined trends. The dominant trend is known as 'Eigenvector 1', in which many properties correlate with the strength of optical iron and [O iii] emission.sup.1,2,3. The main physical driver of Eigenvector 1 has long been suspected.sup.4 to be the quasar luminosity normalized by the mass of the hole (the 'Eddington ratio'), which is an important parameter of the black hole accretion process. But a definitive proof has been missing. Here we report an analysis of archival data that reveals that the Eddington ratio indeed drives Eigenvector 1. We also find that orientation plays a significant role in determining the observed kinematics of the gas in the broad-line region, implying a flattened, disk-like geometry for the fast-moving clouds close to the black hole. Our results show that most of the diversity of quasar phenomenology can be unified using two simple quantities: Eddington ratio and orientation., Author(s): Yue Shen [sup.1] [sup.2] , Luis C. Ho [sup.2] [sup.3] Author Affiliations: (1) Carnegie Observatories, 813 Santa Barbara Street, Pasadena, USA (2) Kavli Institute for Astronomy and Astrophysics, Peking [...]

M82 X-1, the brightest X-ray source in the galaxy M82, has been thought to be an intermediate-mass black hole (100 to 10,000 solar masses) because of its extremely high luminosity [...]

Galaxies congregate in clusters and along filaments, and are missing from large regions referred to as voids. These structures are seen in maps derived from spectroscopic surveys (1,2) that reveal networks of structure that are interconnected with no clear boundaries. Extended regions with a high concentration of galaxies are called 'superclusters', although this term is not precise. There is, however, another way to analyse the structure. If the distance to each galaxy from Earth is directly measured, then the peculiar velocity can be derived from the subtraction of the mean cosmic expansion, the product of distance times the Hubble constant, from observed velocity. The peculiar velocity is the line-of-sight departure from the cosmic expansion and arises from gravitational perturbations; a map of peculiar velocities can be translated into a map of the distribution of matter (3). Here we report a map of structure made using a catalogue of peculiar velocities. We find locations where peculiar velocity flows diverge, as water does at watershed divides, and we trace the surface of divergent points that surrounds us. Within the volume enclosed by this surface, the motions of galaxies are inward after removal of the mean cosmic expansion and long range flows. We define a supercluster to be the volume within such a surface, and so we are defining the extent of our home supercluster, which we call Laniakea., The distribution of matter can be determined by two independent methods: either based on surveys of the distribution of galaxies in projection and redshift, or from the motions of galaxies. [...]

A model in which the stellar wind of the fast-moving red supergiant Betelgeuse is photoionized by radiation from external sources can explain the dense, almost static shell recently discovered around the star, and predicts both that debris from Betelgeuse's eventual supernova explosion will violently collide with the shell and that other red supergiants should have similar, but much more massive, shells. Inside the Betelgeuse shell The discovery in 2012 of a static, dense gaseous shell around the nearby red supergiant Betelgeuse raised doubts about the assumption that it was a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings. These two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Hilding Neilson and colleagues describe a model in which Betelgeuse's wind is photoionized by radiation from external sources, where pressure induced by photoionization generates a standing shock in the neutral part of the wind. This forms an almost-static photoionization-confined shell, confining gas close to the star, where it can interact with ejecta from a future supernova explosion. This provides a natural explanation for the many supernovae with the signatures of circumstellar interaction. Betelgeuse, a nearby red supergiant, is a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings.sup.1,2,3,4. This picture has been challenged by the discovery of a dense and almost static shell.sup.5 that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse's wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind.sup.6 and forms an almost static, photoionization-confined shell. Other red supergiants should have much more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova light curve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction., Author(s): Jonathan Mackey [sup.1] , Shazrene Mohamed [sup.2] , Vasilii V. Gvaramadze [sup.3] [sup.4] [sup.5] , Rubina Kotak [sup.6] , Norbert Langer [sup.1] , Dominique M.-A. Meyer [sup.1] , Takashi [...]

Type Iax supernovae are stellar explosions that are spectroscopically similar to some type Ia supernovae at the time of maximum light emission, except with lower ejecta velocities (1,2). They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae (3-6), but their composition (dominated by iron-group and intermediate-mass elements (1,7)) suggests a physical connection to normal type Ia supernovae. Supernovae of type Iax are not rare; they occur at a rate between 5 and 30 per cent of the normal type Ia rate (1). The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon-oxygen white dwarfs that do not completely unbind the star (8-10),implying that they are 'less successful' versions of normal type Ia supernovae, where complete stellar disruption is observed. Here we report the detection of the luminous, blue progenitor system of the type Iax SN 2012Z in deep pre-explosion imaging. The progenitor system's luminosity, colours, environment and similarity to the progenitor of the Galactic helium nova V445 Puppis (11-13) suggest that SN 2012Z was the explosion of a white dwarf accreting material from a helium-star companion. Observations over the next few years, after SN 2012Z has faded, will either confirm this hypothesis or perhaps show that this supernova was actually the explosive death of a massive star (14,15)., SN 2012Z was discovered (16) in the Lick Observatory Supernova Search on 2012 January 29.15 UT. It had an optical spectrum similar to the type Iax (previously called SN 2002cx-like) [...]

Flaring and warping of the disk of the Milky Way have been inferred from observations of atomic hydrogen (1,2) but stars associated with flaring have not hitherto been reported. In [...]

Gamma-ray bursts (GRBs) are most probably powered by collimated relativistic outflows (jets) from accreting black holes at cosmological distances. Bright afterglows are produced when the outflow collides with the ambient medium. Afterglow polarization directly probes the magnetic properties of the jet when measured minutes after the burst, and it probes the geometric properties of the jet and the ambient medium when measured hours to days after the burst (1-5). High values of optical polarization detected minutes after the burst of GRB 120308A indicate the presence of large-scale ordered magnetic fields originating from the central engine (5) (the power source of the GRB). Theoretical models predict low degrees of linear polarization and no circular polarization at late times (6-8), when the energy in the original ejecta is quickly transferred to the ambient medium and propagates farther into the medium as a blast wave. Here we report the detection of circularly polarized light in the afterglow of GRB 121024A, measured 0.15 days after the burst. We show that the circular polarization is intrinsic to the afterglow and unlikely to be produced by dust scattering or plasma propagation effects. A possible explanation is to invoke anisotropic (rather than the commonly assumed isotropic) electron pitch-angle distributions, and we suggest that new models are required to produce the complex microphysics of realistic shocks in relativistic jets (9-11)., Magnetic fields play a crucial role in the physics of relativistic jets--for example, in their formation, acceleration and colliniation (1,12,13). On smaller spatial scales, there is a strong connection between [...]

Isotopic analyses of 40 Martian meteorites indicate that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history and that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues. Martian sulphur chemistry Many questions remain regarding the surface processes operating on Mars and the magmatic assimilation of crustal material, and sulphur chemistry is crucial to these and other phenomena on the planet. A new sulphur isotope analysis of 40 Martian meteorites now suggests an atmospheric sulphur chemistry, notably the photochemical processing preserved in meteoritic sulphide and sulphate, very different from that seen on Earth. The data provide evidence to suggest that the assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history. The authors conclude that sulphur isotope systematics, used in conjunction with data for other isotopic systems and trace element characteristics, is a powerful tool for reconstructing the geological history of Mars. The geochemistry of Martian meteorites provides a wealth of information about the solid planet and the surface and atmospheric processes that occurred on Mars. The degree to which Martian magmas may have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear.sup.1. This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts.sup.1,2 or in mixing between enriched and depleted mantle reservoirs.sup.3. Sulphur isotope systematics offer insight into some aspects of crustal assimilation. The presence of igneous sulphides in Martian meteorites with sulphur isotope signatures indicative of mass-independent fractionation suggests the assimilation of sulphur both during passage of magmas through the crust of Mars and at sites of emplacement. Here we report isotopic analyses of 40 Martian meteorites that represent more than half of the distinct known Martian meteorites, including 30 shergottites (28 plus 2 pairs, where pairs are separate fragments of a single meteorite), 8 nakhlites (5 plus 3 pairs), Allan Hills 84001 and Chassigny. Our data provide strong evidence that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history. The signature of mass-independent fractionation observed also indicates that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues, suggesting fundamental differences between the dominant sulphur chemistry in the atmosphere of Mars and that in the atmosphere of Earth.sup.4., Author(s): Heather B. Franz [sup.1] [sup.2] , Sang-Tae Kim [sup.3] , James Farquhar [sup.2] , James M. D. Day [sup.4] , Rita C. Economos [sup.5] , Kevin D. McKeegan [sup.5] [...]

The co-evolution of a supermassive black hole with its host galaxy (1) through cosmic time is encoded in its spin (2-4). At z > 2, supermassive black holes are thought [...]

The element abundance ratios of four low-mass stars with extremely low metallicities (abundances of elements heavier than helium) indicate that the gas out of which the stars formed was enriched in each case by at most a few- and potentially only one--low-energy supernova (1-4). Such supernovae yield large quantities of light elements such as carbon but very little iron. The dominance of low-energy supernovae seems surprising, because it had been expected that the first stars were extremely massive, and that they disintegrated in pair-instability explosions that would rapidly enrich galaxies in iron (5). What has remained unclear is the yield of iron from the first supernovae, because hitherto no star has been unambiguously interpreted as encapsulating the yield of a single supernova. Here we report the optical spectrum of SMSS J031300.36-670839.3, which shows no evidence of iron (with an upper limit of [10.sup.-7.1] times solar abundance). Based on a comparison of its abundance pattern with those of models, we conclude that the star was seeded with material from a single supernova with an original mass about 60 times that of the Sun (and that the supernova left behind a black hole). Taken together with the four previously mentioned low-metallicity stars, we conclude that low-energy supernovae were common in the early Universe, and that such supernovae yielded light-element enrichment with insignificant iron. Reduced stellar feedback both chemically and mechanically from low-energy supernovae would have enabled first-generation stars to form over an extended period. We speculate that such stars may perhaps have had an important role in the epoch of cosmic reionization and the chemical evolution of early galaxies., Whereas the solar spectrum contains many thousands of spectral lines due to iron and other elements, the high-resolution (R = 28,000) optical spectrum of SMSS J031300.36-670839.3 (hereafter SMSS 0313 - [...]

Models and simulations (1-4) of the epoch of reionization predict that spectra of the 21-centimetre transition of atomic hydrogen will show a clear fluctuation peak, at a redshift and scale, [...]

A principal discovery in modern cosmology is that standard model particles comprise only 5 per cent of the mass-energy budget of the Universe. In the ACDM paradigm, the remaining 95 per cent consists of dark energy (Λ) and cold dark matter. ΛCDM is being challenged by its apparent inability to explain the low-density 'cores' of dark matter measured at the centre of galaxies, where centrally concentrated high-density 'cusps' were predicted. But before drawing conclusions, it is necessary to include the effect of gas and stars, historically seen as passive components of galaxies. We now understand that these can inject heat energy into the cold dark matter through a coupling based on rapid gravitational potential fluctuations, explaining the observed low central densities., Despite the unknown nature of the dominant components, the ΛCDM paradigm successfully describes the evolution of the Universe from its near-uniform early state, as measured by the cosmic microwave background [...]

Simulations of structure formation in the Universe predict that galaxies are embedded in a 'cosmic web'1, where most baryons reside as rarefied and highly ionized gas (2). This material has been studied for decades in absorption against background sources (3), but the sparseness of these inherently one-dimensional probes preclude direct constraints on the three-dimensional morphology of the underlying web. Here we report observations of a cosmic web filament in Lyman-α emission, discovered during a survey for cosmic gas fluorescently illuminated by bright quasars (4,5) at redshift z ≅ 2.3. With a linear projected size of approximately 460 physical kiloparsecs, the Lyman-α emission surrounding the radio-quiet quasar UM 287 extends well beyond the virial radius of any plausible associated dark-matter halo and therefore traces intergalactic gas. The estimated cold gas mass of the filament from the observed emission--about [10.sup.12.0 ± 0.5]/[C.sup.1/2] solar masses, where cis the gas clumping factor--is more than ten times larger than what is typically found in cosmological simulations (5,6), suggesting that a population of intergalactic gas clumps with subkiloparsec sizes may be missing in current numerical models., A recent pilot survey (5) using a custom-built, narrow-band filter on the Very Large Telescope demonstrated that bright quasars can, like a flashlight, 'illuminate' the densest knots in the surrounding [...]

Brown dwarfs--substellar bodies more massive than planets but not massive enough to initiate the sustained hydrogen fusion that powers self-luminous stars (1, 2)--are born hot and slowly cool as they [...]

Advances in the discovery and characterization of asteroids over the past decade have revealed an unanticipated underlying structure that points to a dramatic early history of the inner Solar System. The asteroids in the main asteroid belt have been discovered to be more compositionally diverse with size and distance from the Sun than had previously been known. This implies substantial mixing through processes such as planetary migration and the subsequent dynamical processes., Although studies of exoplanetary systems have the advantage of numbers (1) to answer the question of how planetary systems are built, our Solar System has the advantage of detail. For [...]

Gravitationally bound three-body systems have been studied for hundreds of years (1,2) and are common in our Galaxy (3,4). They show complex orbital interactions, which can constrain the compositions, masses and interior structures of the bodies (5) and test theories of gravity (6), if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 (refs (7,8); with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in ahierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13)[M.sub.[dot in a circle]], where [M.sub.[dot in a circle]] is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15)[M.sub.[dot in a circle]] and 0.4101(3)[M.sub.[dot in a circle]]), as well as the inclinations of the orbits (both about 39.2°). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity., Millisecond pulsars (MSPs) are neutron stars that rotate hundreds of times per second and emit beams of radio waves much as a lighthouse emits light. They are thought to form [...]

Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions (this gas is either stripped from low-mass stars or supplied as winds from massive ones). Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars (1-3) (stars of the Be type are hot blue irregular variables showing characteristic spectral emission lines of hydrogen). Of the approximately 80 Be X-ray binaries known in the Galaxy, however, only pulsating neutron stars have been found as companions (2-4). A black hole was formally allowed as a solution for the companion to the Be star MWC 656 (ref. 5; also known as HD 215227), although that conclusion was based on a single radial velocity curve of the Be star, a mistaken spectral classification (6) and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC 656. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe ii profiles from the equatorial disk, and a refined Be classification (to that of a B1.5-B2 III star), indicates that ablackhole of 3.8 to 6.9 solar masses orbits MWC 656, the candidate counterpart of the γ-ray source AGLJ2241+4454 (refs 5, 6). The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow giving a luminosity less than 1.6 X [10.sup.-7] times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys., The majority of Be X-ray binaries (7) (BeXBs) contain proven neutron stars and are characterized by their transient changes in X-ray luminosity, when episodes of increased accretion onto the compact [...]

Galaxies observed at redshift z > 6, when the Universe was less than a billion years old, thus far very rarely show evidence (1-3) of the cold dust that accompanies [...]

GJ 436b is a warm--approximately800 kelvin--exoplanet that periodically eclipses its low-mass (half the mass of the Sun) host star, and is one of the few Neptune-mass planets that is amenable to detailed characterization. Previous observations (1-3) have indicated that its atmosphere has a ratio of methane to carbon monoxide that is 105 times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures (4,5). A recent study proposed that this unusual chemistry could be explained if the planet's atmosphere is significantly enhanced in elements heavier than hydrogen and helium (6). Here we report observations of GJ 436b's atmosphere obtained during transit. The data indicate that the planet's transmission spectrum is featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48σ. The measured spectrum is consistent with either a layer of high cloud located at a pressure level of approximately one millibar or with a relatively hydrogen-poor (three per cent hydrogen and helium mass fraction) atmospheric composition (7-9)., We observed four transits of the Neptune-mass planet GJ 436b on 26 October 2012 Universal Time (UT), 29 November 2012, 10 December 2012 and 2 January 2013 using the red [...]

The temperature in the crust of an accreting neutron star, which comprises its outermost kilometre, is set by heating from nuclear reactions at large densities (1-4), neutrino cooling (5,6) and heat transport from the interior (7-11). The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope (10,11). Here we report that cycles of electron capture and its inverse, [β.sub.-] decay, involving neutron-rich nuclei at a typical depth of about 150 metres, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This 'Urca' mechanism (12) has been studied in the context of white dwarfs (13) and type Ia supernovae (14,15), but hitherto was not considered in neutron stars, because previous models (1,2) computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. The thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts (16) are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface., Continual accretion onto a neutron star pushes the ashes of surface thermonuclear burning, which is often observed as type I X-ray bursts (17,18), to greater pressures and densities, at which [...]