Discovery of five irregular moons of Neptune

MATTHEW J. HOLMAN1, J. J. KAVELAARS2, TOMMY GRAV1,3, BRETT J. GLADMAN4, WESLEY C. FRASER5, DAN MILISAVLJEVIC5, PHILIP D. NICHOLSON6, JOSEPH A. BURNS6, VALERIO CARRUBA6, JEAN-MARC PETIT7, PHILIPPE ROUSSELOT7, OLIVER MOUSIS7, BRIAN G. MARSDEN1 & ROBERT A. JACOBSON8

1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
2 National Research Council of Canada, 5071 West Saanich Road, Victoria, British Columbia V9E ZE7, Canada
3 University of Oslo, Institute of Theoretical Astrophysics, Postbox 1029 Blindern, 0315 Oslo, Norway
4 Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
5 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
6 Department of Astronomy, Cornell University, Ithaca, New York 14853, USA
7 Obervatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
8 Jet Propulsion Laboratory, MS 301-150, 4800 Oak Grove Drive, Pasadena, California 91109, USA

Correspondence and requests for materials should be addressed to M.J.H. (mholman@cfa.harvard.edu).

Each giant planet of the Solar System has two main types of moons. 'Regular' moons are typically larger satellites with prograde, nearly circular orbits in the equatorial plane of their host planets at distances of several to tens of planetary radii. The 'irregular' satellites (which are typically smaller) have larger orbits with significant eccentricities and inclinations. Despite these common features, Neptune's irregular satellite system, hitherto thought to consist of Triton and Nereid, has appeared unusual. Triton is as large as Pluto and is postulated to have been captured from heliocentric orbit; it traces a circular but retrograde orbit at 14 planetary radii from Neptune. Nereid, which exhibits one of the largest satellite eccentricities, is believed to have been scattered from a regular satellite orbit to its present orbit during Triton's capture. Here we report the discovery of five irregular moons of Neptune, two with prograde and three with retrograde orbits. These exceedingly faint (apparent red magnitude mR = 24.2–25.4) moons, with diameters of 30 to 50 km, were presumably captured by Neptune.

Nature 430, 326 - 329 (15 July 2004)

A substantial amount of hidden magnetic energy in the quiet Sun

J. TRUJILLO BUENO1,2, N. SHCHUKINA3 & A. ASENSIO RAMOS1

1 Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
2 Consejo Superior de Investigaciones Científicas, E-28006 Madrid, Spain
3 Main Astronomical Observatory, National Academy of Sciences, Zabolotnogo 27, 03680 Kyiv, Ukraine

Correspondence and requests for materials should be addressed to J.T.B. (jtb@iac.es).

Deciphering and understanding the small-scale magnetic activity of the quiet solar photosphere should help to solve many of the key problems of solar and stellar physics, such as the magnetic coupling to the outer atmosphere and the coronal heating. At present, we can see only 1 per cent of the complex magnetism of the quiet Sun, which highlights the need to develop a reliable way to investigate the remaining 99 per cent. Here we report three-dimensional radiative transfer modelling of scattering polarization in atomic and molecular lines that indicates the presence of hidden, mixed-polarity fields on subresolution scales. Combining this modelling with recent observational data, we find a ubiquitous tangled magnetic field with an average strength of 130 G, which is much stronger in the intergranular regions of solar surface convection than in the granular regions. So the average magnetic energy density in the quiet solar photosphere is at least two orders of magnitude greater than that derived from simplistic one-dimensional investigations, and sufficient to balance radiative energy losses from the solar chromosphere.

Nature 430, 323 - 325 (15 July 2004)

Fast delivery of meteorites to Earth after a major asteroid collision

PHILIPP R. HECK1, BIRGER SCHMITZ2, HEINRICH BAUR1, ALEX N. HALLIDAY1 & RAINER WIELER1

1 ETH Zürich, Isotope Geology, NO C61, CH-8092 Zürich, Switzerland
2 University of Lund, Department of Geology, Sölvegatan 12, SE-22362 Lund, Sweden

Correspondence and requests for materials should be addressed to Ph.R.H. (heck@erdw.ethz.ch).


Very large collisions in the asteroid belt could lead temporarily to a substantial increase in the rate of impacts of meteorites on Earth. Orbital simulations predict that fragments from such events may arrive considerably faster than the typical transit times of meteorites falling today, because in some large impacts part of the debris is transferred directly into a resonant orbit with Jupiter. Such an efficient meteorite delivery track, however, has not been verified. Here we report high-sensitivity measurements of noble gases produced by cosmic rays in chromite grains from a unique suite of fossil meteorites preserved in 480 million year old sediments. The transfer times deduced from the noble gases are as short as 105 years, and they increase with stratigraphic height in agreement with the estimated duration of sedimentation. These data provide powerful evidence that this unusual meteorite occurrence was the result of a long-lasting rain of meteorites following the destruction of an asteroid, and show that at least one strong resonance in the main asteroid belt can deliver material into the inner Solar System within the short timescales suggested by dynamical models.

Nature 430, 184 - 187 (08 July 2004)

Old galaxies in the young Universe

A. CIMATTI1, E. DADDI2, A. RENZINI2, P. CASSATA3, E. VANZELLA3, L. POZZETTI4, S. CRISTIANI5, A. FONTANA6, G. RODIGHIERO3, M. MIGNOLI4 & G. ZAMORANI4

1 INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125, Firenze, Italy
2 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748, Garching, Germany
3 Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio, 2, I-35122 Padova, Italy
4 INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127, Bologna, Italy
5 INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34131 Trieste, Italy
6 INAF - Osservatorio Astronomico di Roma, via dell'Osservatorio 2, Monteporzio, Italy

Correspondence and requests for materials should be addressed to A.C. (cimatti@arcetri.astro.it).


More than half of all stars in the local Universe are found in massive spheroidal galaxies, which are characterized by old stellar populations with little or no current star formation. In present models, such galaxies appear rather late in the history of the Universe as the culmination of a hierarchical merging process, in which larger galaxies are assembled through mergers of smaller precursor galaxies. But observations have not yet established how, or even when, the massive spheroidals formed, nor if their seemingly sudden appearance when the Universe was about half its present age (at redshift z 1) results from a real evolutionary effect (such as a peak of mergers) or from the observational difficulty of identifying them at earlier epochs. Here we report the spectroscopic and morphological identification of four old, fully assembled, massive (1011 solar masses) spheroidal galaxies at l.6 < z < 1.9, the most distant such objects currently known. The existence of such systems when the Universe was only about one-quarter of its present age shows that the build-up of massive early-type galaxies was much faster in the early Universe than has been expected from theoretical simulations.

Nature 430, 181 - 184 (08 July 2004)

A high abundance of massive galaxies 3–6 billion years after the Big Bang

KARL GLAZEBROOK1, ROBERTO G. ABRAHAM2, PATRICK J. MCCARTHY3, SANDRA SAVAGLIO1, HSIAO-WEN CHEN4, DAVID CRAMPTON5, RICK MUROWINSKI5, INGER JØRGENSEN6, KATHY ROTH6, ISOBEL HOOK7, RONALD O. MARZKE8 & R. G. CARLBERG2

1 Department of Physics & Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218-2686, USA
2 Department of Astronomy & Astrophysics, University of Toronto, 60 St George Street, Toronto, Ontario M5S 3H8, Canada
3 Observatories of the Carnegie Institute of Washington, Santa Barbara Street, Pasadena, California 9110, USA
4 Center for Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA
5 Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, British Columbia, V9E 2E7, Canada
6 Gemini Observatory, Hilo, Hawaii 96720, USA
7 Department of Astrophysics, Nuclear & Astrophysics Laboratory, Oxford University, Keble Road, Oxford OX1 3RH, UK
8 Department of Physics and Astronomy, San Francisco State University, 1600 Holloway Avenue, San Francisco, California 94132, USA

Correspondence and requests for materials should be addressed to K.G. (kgb@pha.jhu.edu).


Hierarchical galaxy formation is the model whereby massive galaxies form from an assembly of smaller units. The most massive objects therefore form last. The model succeeds in describing the clustering of galaxies, but the evolutionary history of massive galaxies, as revealed by their visible stars and gas, is not accurately predicted. Near-infrared observations (which allow us to measure the stellar masses of high-redshift galaxies) and deep multi-colour images indicate that a large fraction of the stars in massive galaxies form in the first 5 Gyr (refs 4–7), but uncertainties remain owing to the lack of spectra to confirm the redshifts (which are estimated from the colours) and the role of obscuration by dust. Here we report the results of a spectroscopic redshift survey that probes the most massive and quiescent galaxies back to an era only 3 Gyr after the Big Bang. We find that at least two-thirds of massive galaxies have appeared since this era, but also that a significant fraction of them are already in place in the early Universe.




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