Potassium isotopic evidence for a high-energy giant impact origin of the Moon
Nature 538, 7626 (2016). doi:10.1038/nature19341
Authors: Kun Wang & Stein B. Jacobsen
The Earth–Moon system has unique chemical and isotopic signatures compared with other planetary bodies; any successful model for the origin of this system therefore has to satisfy these chemical and isotopic constraints. The Moon is substantially depleted in volatile elements such as potassium compared with the Earth and the bulk solar composition, and it has long been thought to be the result of a catastrophic Moon-forming giant impact event. Volatile-element-depleted bodies such as the Moon were expected to be enriched in heavy potassium isotopes during the loss of volatiles; however such enrichment was never found. Here we report new high-precision potassium isotope data for the Earth, the Moon and chondritic meteorites. We found that the lunar rocks are significantly (>2σ) enriched in the heavy isotopes of potassium compared to the Earth and chondrites (by around 0.4 parts per thousand). The enrichment of the heavy isotope of potassium in lunar rocks compared with those of the Earth and chondrites can be best explained as the result of the incomplete condensation of a bulk silicate Earth vapour at an ambient pressure that is higher than 10 bar. We used these coupled constraints of the chemical loss and isotopic fractionation of K to compare two recent dynamic models that were used to explain the identical non-mass-dependent isotope composition of the Earth and the Moon. Our K isotope result is inconsistent with the low-energy disk equilibration model, but supports the high-energy, high-angular-momentum giant impact model for the origin of the Moon. High-precision potassium isotope data can also be used as a ‘palaeo-barometer’ to reveal the physical conditions during the Moon-forming event.
Icy heart could be key to Pluto’s strange geology
Nature 538, 7626 (2016). http://www.nature.com/doifinder/10.1038/nature.2016.20856
Author: Alexandra Witze
NASA’s New Horizons mission plumbs complex interplay between the dwarf planet's surface and its sky.
Astrophysics: Birth of stellar siblings
Nature 538, 7626 (2016). doi:10.1038/538466a
Authors: Adele Plunkett
Binary and multiple star systems result from the fragmentation of dense material in young molecular clouds. Observations reveal that this can occur on small scales, supporting a previous model of star formation. See Letter p.483
A triple protostar system formed via fragmentation of a gravitationally unstable disk
Nature 538, 7626 (2016). doi:10.1038/nature20094
Authors: John J. Tobin, Kaitlin M. Kratter, Magnus V. Persson, Leslie W. Looney, Michael M. Dunham, Dominique Segura-Cox, Zhi-Yun Li, Claire J. Chandler, Sarah I. Sadavoy, Robert J. Harris, Carl Melis & Laura M. Pérez
Binary and multiple star systems are a frequent outcome of the star formation process and as a result almost half of all stars with masses similar to that of the Sun have at least one companion star. Theoretical studies indicate that there are two main pathways that can operate concurrently to form binary/multiple star systems: large-scale fragmentation of turbulent gas cores and filaments or smaller-scale fragmentation of a massive protostellar disk due to gravitational instability. Observational evidence for turbulent fragmentation on scales of more than 1,000 astronomical units has recently emerged. Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems. The triple protostar system L1448 IRS3B is an ideal system with which to search for evidence of disk fragmentation as it is in an early phase of the star formation process, it is likely to be less than 150,000 years old and all of the protostars in the system are separated by less than 200 astronomical units. Here we report observations of dust and molecular gas emission that reveal a disk with a spiral structure surrounding the three protostars. Two protostars near the centre of the disk are separated by 61 astronomical units and a tertiary protostar is coincident with a spiral arm in the outer disk at a separation of 183 astronomical units. The inferred mass of the central pair of protostellar objects is approximately one solar mass, while the disk surrounding the three protostars has a total mass of around 0.30 solar masses. The tertiary protostar itself has a minimum mass of about 0.085 solar masses. We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 and 320 astronomical units, overlapping with the location of the tertiary protostar. This is consistent with models for a protostellar disk that has recently undergone gravitational instability, spawning one or two companion stars.
Celebrating 170th anniversary of the discovery of Neptune, I review the story of the discovery that startled the world. The story is an interplay of scientific triumph and human weakness and an example of how science works in a socio-political context.
Astrobiology: Martian dance of fiction and fact
Nature 538, 7625 (2016). doi:10.1038/538317e
Author: Jonathan Cowie
In marking the H. G. Wells anniversary, you highlight what Carl Sagan dubbed the “dance” between science fiction and science fact (see www.nature.com/scifispecial).Wells's The War of the Worlds saw the Martian invasion stopped in its tracks by Earth pathogens (S. J.
Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies
Nature 538, 7625 (2016). doi:10.1038/nature19822
Authors: Jimmy A. Irwin, W. Peter Maksym, Gregory R. Sivakoff, Aaron J. Romanowsky, Dacheng Lin, Tyler Speegle, Ian Prado, David Mildebrath, Jay Strader, Jifeng Liu & Jon M. Miller
A flaring X-ray source was found near the galaxy NGC 4697 (ref. 1). Two brief flares were seen, separated by four years. During each flare, the flux increased by a factor of 90 on a timescale of about one minute. There is no associated optical source at the position of the flares, but if the source was at the distance of NGC 4697, then the luminosities of the flares were greater than 1039 erg per second. Here we report the results of a search of archival X-ray data for 70 nearby galaxies looking for similar flares. We found two ultraluminous flaring sources in globular clusters or ultracompact dwarf companions of parent elliptical galaxies. One source flared once to a peak luminosity of 9 × 1040 erg per second; the other flared five times to 1040 erg per second. The rise times of all of the flares were less than one minute, and the flares then decayed over about an hour. When not flaring, the sources appear to be normal accreting neutron-star or black-hole X-ray binaries, but they are located in old stellar populations, unlike the magnetars, anomalous X-ray pulsars or soft γ repeaters that have repetitive flares of similar luminosities.
Astrophysics: Unexpected X-ray flares
Nature 538, 7625 (2016). doi:10.1038/538321a
Authors: Sergio Campana
Two sources of highly energetic flares have been discovered in archival X-ray data of 70 nearby galaxies. These flares have an undetermined origin and might represent previously unknown astrophysical phenomena. See Letter p.356
Astronomy: Two stars have three disks
Nature 538, 7625 (2016). doi:10.1038/538292b
Young stars are surrounded by a rotating disk of gas and dust, from which planets are born — but astronomers have discovered that one pair of young stars orbiting around each other has three disks, not just two.Christian Brinch at the University of Copenhagen