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Lunar true polar wander inferred from polar hydrogen

Astronomy News - 24 March 2016 - 10:30am

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

Astronomy News - 24 March 2016 - 10:30am

Acceleration of petaelectronvolt protons in the Galactic Centre

Nature 531, 7595 (2016). doi:10.1038/nature17147

Authors:

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

Astronomy News - 24 March 2016 - 10:30am

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

Astronomy News - 24 March 2016 - 10:29am

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

Astronomy News - 24 March 2016 - 10:28am

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

Astronomy News - 24 March 2016 - 10:28am

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,

The Wilds of the Local Group

Astronomy News - 24 March 2016 - 10:26am
This scene, captured by ESO’s OmegaCAM on the VLT Survey Telescope, shows a lonely galaxy known as Wolf-Lundmark-Melotte, or WLM for short. Although considered part of our Local Group of dozens of galaxies, WLM stands alone at the group’s outer edges as one of its most remote members. In fact, the galaxy is so small and secluded that it may never have interacted with any other Local Group galaxy — or perhaps even any other galaxy in the history of the Universe.

Moon’s lack of water down to ancient shift in its spin axis

Astronomy News - 24 March 2016 - 10:12am

Molten rock flowing beneath the moon's crust billions of years ago shifted the moon's spin axis, hinting that lunar water stores date back to the very early solar system









New detail in Ceres' bight spots

Astronomy News - 23 March 2016 - 10:25am

The US space agency's Dawn satellite continues to return remarkable images from the dwarf planet Ceres, in particular from its collection of bright spots in Occator Crater.

Ghostly galaxies are light on stars but heavy on dark matter

Astronomy News - 23 March 2016 - 10:08am

The first measurement of an ultra-diffuse galaxy's mass finds that dark matter makes up more than 99.96 per cent of its weight - a startling figure









LIGO could catch dark matter made of black holes

Astronomy News - 23 March 2016 - 10:07am

The black holes that kicked off the first detection of gravitational waves seem to be the right size and frequency to be long-sought primordial black holes









Ceres surprises with water ice and colourful bright spots

Astronomy News - 23 March 2016 - 10:07am

NASA's Dawn spacecraft zoomed in on dwarf planet Ceres and found unexpected treasures hidden in its craters









Pluto may have hosted lakes and rivers of liquid nitrogen

Astronomy News - 22 March 2016 - 10:08am

Studying how the dwarf planet's climate changed over time shows it once had the right conditions for liquid to flow on its surface - and may still have buried lakes today









Hubble unveils monster stars [heic1605]

Astronomy News - 18 March 2016 - 10:24am

Astronomers using the unique ultraviolet capabilities of the NASA/ESA Hubble Space Telescope have identified nine monster stars with masses over 100 times the mass of the Sun in the star cluster R136. This makes it 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.

Hubble Unveils Monster Stars

Astronomy News - 18 March 2016 - 10:23am

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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.

Pluto gives up its icy secrets as New Horizons data pours in

Astronomy News - 18 March 2016 - 10:22am

Eight months after NASA's New Horizons spacecraft's historic fly-by of Pluto, new data reveals startling links between Pluto, Charon, their four smaller satellites, and the space environment that surrounds them









Hubble telescope spies 'land of giants'

Astronomy News - 18 March 2016 - 10:21am

Astronomers use the Hubble Space Telescope to investigate a clutch of monster stars on the edge of our Milky Way Galaxy.

Meteor sighting reports across Britain

Astronomy News - 18 March 2016 - 10:21am
A meteor has been sighted above Britain in the early hours with witnesses describing seeing a green flash.

NASA Mars woes could delay other planetary missions

Astronomy News - 17 March 2016 - 10:55am

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

Astronomy News - 17 March 2016 - 10:55am

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.