Lunar true polar wander inferred from polar hydrogen
Nature 531, 7595 (2016). doi:10.1038/nature17166
Authors: M. A. Siegler, R. S. Miller, J. T. Keane, M. Laneuville, D. A. Paige, I. Matsuyama, D. J. Lawrence, A. Crotts & M. J. Poston
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, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury’s poles. 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 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, 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.
Acceleration of petaelectronvolt protons in the Galactic Centre
Nature 531, 7595 (2016). doi:10.1038/nature17147
Galactic cosmic rays reach energies of at least a few petaelectronvolts (of the order of 1015 electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators (‘PeVatrons’), but all proposed models of Galactic cosmic-ray accelerators encounter difficulties at exactly these energies. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of teraelectronvolts (of the order of 1013 electronvolts) were inferred from recent γ-ray observations. However, none of the currently known accelerators—not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays—has shown the characteristic tracers of petaelectronvolt particles, namely, power-law spectra of γ-rays extending without a cut-off or a spectral break to tens of teraelectronvolts. Here we report deep γ-ray observations with arcminute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of petaelectronvolt protons within the central 10 parsecs of the Galaxy. We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outburstsand an outflow from the Galactic Centre. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 106–107 years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays.
Planetary science: Signs of a wandering Moon
Nature 531, 7595 (2016). doi:10.1038/531455a
Authors: Ian Garrick-Bethell
The presence of ice at two positions on opposite sides of the Moon suggests that the satellite's orientation was once shifted away from its present spin axis — a finding that has implications for the Moon's volcanic history. See Letter p.480
The black-hole collision that reshaped physics
Nature 531, 7595 (2016). http://www.nature.com/doifinder/10.1038/531428a
Author: Davide Castelvecchi
A momentous signal from space has confirmed decades of theorizing on black holes — and launched a new era of gravitational-wave astronomy.
Planetary science: Cassini aids hunt for Planet Nine
Nature 531, 7595 (2016). doi:10.1038/531417e
Researchers using the Cassini spacecraft have narrowed down the search for the Solar System's hypothetical ninth planet.Planet Nine is thought to be orbiting in the far outer Solar System, but has not yet been found. If it exists, its gravity should tug slightly on
Planetary science: A peek at Pluto's rich landscapes
Nature 531, 7595 (2016). doi:10.1038/531416d
Data collected by NASA's New Horizons probe during its Pluto fly-by last year has revealed just how geologically active Pluto is, and that its moon Charon was once active but is now dead.Jeffrey Moore at the NASA Ames Research Center in Moffett Field, California,
An international team of astronomers using the ultraviolet capabilities of NASA's Hubble Space Telescope has identified nine monster stars with masses over 100 times the mass of the sun in the star cluster R136. This makes for the largest sample of very massive stars identified to date. The results, which will be published in the Monthly Notices of the Royal Astronomical Society, raise many new questions about the formation of massive stars. R136 is only a few light-years across and is located in the Tarantula Nebula within the Large Magellanic Cloud, about 170,000 light-years away from Earth. The young cluster hosts many extremely massive, hot, and luminous stars whose energy is mostly radiated in the ultraviolet.
NASA Mars woes could delay other planetary missions
Nature 531, 7594 (2016). http://www.nature.com/doifinder/10.1038/nature.2016.19549
Author: Devin Powell
Plan to postpone launch of InSight probe will cost agency an extra US$150 million.
On the hunt for a mystery planet
Nature 531, 7594 (2016). http://www.nature.com/doifinder/10.1038/531290a
Author: Alexandra Witze
Scientists are searching for an unseen world at the fringes of the solar system.