Planetary science: Lunar conspiracies
Nature 504, 7478 (2013). doi:10.1038/504027a
Author: Robin Canup
Current theories on the formation of the Moon owe too much to cosmic coincidences, says Robin Canup. She calls for better models and a mission to Venus.
Astrophysics: Magnetic fields in γ-ray bursts
Nature 504, 7478 (2013). doi:10.1038/504092a
Authors: Maxim Lyutikov
Observations of a high degree of polarization in the immediate optical afterglow of a γ-ray burst indicate that these powerful cosmic explosions carry large-scale, ordered magnetic fields. See Letter p.119
Planetary science: Shadows cast on Moon's origin
Nature 504, 7478 (2013). doi:10.1038/504090a
Authors: Tim Elliott & Sarah T. Stewart
Our knowledge of how Earth's natural satellite formed is increasingly being challenged by observations and computer simulations. Two scientists outline our current understanding from the point of view of the satellite's geochemistry and its early dynamical history.
Highly polarized light from stable ordered magnetic fields in GRB 120308A
Nature 504, 7478 (2013). doi:10.1038/nature12814
Authors: C. G. Mundell, D. Kopač, D. M. Arnold, I. A. Steele, A. Gomboc, S. Kobayashi, R. M. Harrison, R. J. Smith, C. Guidorzi, F. J. Virgili, A. Melandri & J. Japelj
After the initial burst of γ-rays that defines a γ-ray burst (GRB), expanding ejecta collide with the circumburst medium and begin to decelerate at the onset of the afterglow, during which a forward shock travels outwards and a reverse shock propagates backwards into the oncoming collimated flow, or ‘jet’. Light from the reverse shock should be highly polarized if the jet’s magnetic field is globally ordered and advected from the central engine, with a position angle that is predicted to remain stable in magnetized baryonic jet models or vary randomly with time if the field is produced locally by plasma or magnetohydrodynamic instabilities. Degrees of linear polarization of P ≈ 10 per cent in the optical band have previously been detected in the early afterglow, but the lack of temporal measurements prevented definitive tests of competing jet models. Hours to days after the γ-ray burst, polarization levels are low (P < 4 per cent), when emission from the shocked ambient medium dominates. Here we report the detection of P = per cent in the immediate afterglow of Swift γ-ray burst GRB 120308A, four minutes after its discovery in the γ-ray band, decreasing to P = per cent over the subsequent ten minutes. The polarization position angle remains stable, changing by no more than 15 degrees over this time, with a possible trend suggesting gradual rotation and ruling out plasma or magnetohydrodynamic instabilities. Instead, the polarization properties show that GRBs contain magnetized baryonic jets with large-scale uniform fields that can survive long after the initial explosion.
Astronomers continue to tally up how many planets are orbiting other stars. But finding out what their atmospheres are made of is another story. Two teams of scientists using the Hubble Space Telescope have found faint signatures of water in the atmospheres of five distant exoplanets. The planets are not the size of Earth, but rather massive worlds known as hot Jupiters because they orbit so close to their stars. Hubble's instruments can deduce the types of gases in the atmospheres of these monsters by determining which colors of a star's light are transmitted and which are partially absorbed as the planet passes in front of its star. The observations demonstrate Hubble's continuing exemplary performance in exoplanet research.