Pebble accretion helps explain origin of gas giants in Solar System

It is widely held that the first step in forming gas-giant planets, such as Jupiter and Saturn, was the production of solid cores each with a mass roughly ten times that of the Earth. Getting the cores to form before the solar nebula dissipates (in about one to ten million years has been a major challenge for planet formation models. Recently models have emerged in which “pebbles” (centimetre-to-metre-sized objects) are first concentrated by aerodynamic drag and then gravitationally collapse to form objects 100 to 1,000 kilometres in size. These “planetesimals” can then efficiently accrete left-over pebbles and directly form the cores of giant planets. This model is known as “pebble accretion” theoretically, it can produce cores of ten Earth masses in only a few thousand years. Unfortunately, full simulations of this process show that, rather than creating a few such cores, it produces a population of hundreds of Earth-mass objects that are inconsistent with the structure of the Solar System. Here we report that this difficulty can be overcome if pebbles form slowly enough to allow the planetesimals to gravitationally interact with one another. In this situation, the largest planetesimals have time to scatter their smaller siblings out of the disk of pebbles, thereby stifling their growth. Our models show that, for a large and physically reasonable region of parameter space, this typically leads to the formation of one to four gas giants between 5 and 15 astronomical units from the Sun, in agreement with the observed structure of the Solar System.


Did Asteroid Impact or Volcanic Eruption Kill the Dinosaurs?

Compelling new evidence supports the hypothesis that the Chicxulub asteroid off the coast of the Yucatan Peninsula in Mexico 66 million years ago ignited volcanoes around the globe, most catastrophically in India, and that, together, these two planet-wide catastrophes caused the extinction of many land and marine animals, including the dinosaurs.

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Primordial O2 perhaps incorporated into comet during formation

The European Space Agency (ESA) announced today that its Rosetta spacecraft has made the first in situ detection of oxygen molecules outgassing from a comet, a surprising observation that suggests they were incorporated into the comet during its formation.

“We weren’t really expecting to detect O2 at the comet – and in such high abundance – because it is so chemically reactive, so it was quite a surprise,” says Kathrin Altwegg of the University of Bern, and principal investigator of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA.

Photolysis and radiolysis of water ice rejected

Her team explored various possibilities to explain the presence and consistently high abundance of Oand its relationship to water, as well as the lack of ozone. They first considering whether processes called photolysis and radiolysis of water ice could have converted ice into oxygen. They seemed the most likely sources, but finally they rejected these mechanisms.

“The instantaneous generation of O2 also seems unlikely, as that should lead to variable O2 ratios under different illumination conditions. Instead, it seems more likely that primordial O2 was somehow incorporated into the comet’s ices during its formation, and is being released with the water vapour today.”

Oxygen dissolved in water ice rejected

In one scenario, gaseous O2 would first be incorporated into water ice in the early protosolar nebula stage of our Solar System. Chemical models of protoplanetary discs predict that high abundances of gaseous O2 could be available in the comet forming zone, but rapid cooling from temperatures above –173ºC to less than –243ºC would be required to form water ice with O2 trapped on dust grains. The grains would then have to be incorporated into the comet without being chemically altered.

Possible warm molecular cloud

“Other possibilities include the Solar System being formed in an unusually warm part of a dense molecular cloud, at temperatures of 10–20ºC above the –263ºC or so typically expected for such clouds,” says Ewine van Dishoeck of Leiden Observatory in the Netherlands, co-author of the paper cited below.

“This is still consistent with estimates for the comet formation conditions in the outer solar nebula, and also with previous findings at Rosetta’s comet regarding the low abundance of N2.”

Radiolysis on dust grains also possible

Alternatively, radiolysis of icy dust grains could have taken place prior to the comet’s accretion into a larger body. In this case, the O2 would remain trapped in the voids of the water ice on the grains while the hydrogen diffused out, preventing the reformation of O2 to water, and resulting in an increased and stable level of O2 in the solid ice. Incorporation of such icy grains into the nucleus could explain the observed strong correlation with H2O observed at the comet today.

“Regardless of how it was made, the O2 was also somehow protected during the accretion stage of the comet: this must have happened gently to avoid the O2 being destroyed by further chemical reactions,” adds Kathrin.

“This is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of Solar System evolution,” says Matt Taylor, ESA’s Rosetta project scientist.


Abundant molecular oxygen in the coma of 67P/Churyumov–Gerasimenko,” by A. Bieler et al is published in the 29 October 2015 issue of the journal Nature.

Hadza people – Last living hunter-gatherers

The Hadza, or Hadzabe are an indigenous ethnic group in north-central Tanzania, living around Lake Eyasi in the central Rift Valley and in the neighbouring Serengeti Plateau. The Hadza number just under 1,000. Some 300–400 Hadza live as hunter-gatherers, much as their ancestors have for tens of thousands of years. They are among the last hunter-gatherers in the world. The Hadza are not closely genetically related to any other people. While traditionally classified with the Khoisan languages,

Source: Hadza people – Wikipedia, the free encyclopedia

Charting the slow death of the universe — ScienceDaily

Charting the slow death of the universeDate:August 10, 2015Source:European Southern Observatory – ESOSummary:Astronomers studying more than 200,000 galaxies have measured the energy generated within a large portion of space more precisely than ever before. This represents the most comprehensive assessment of the energy output of the nearby Universe. They confirm that the energy produced in a section of the Universe today is only about half what it was two billion years ago and find that this fading is occurring across all wavelengths from the ultraviolet to the far infrared. The Universe is slowly dying.

Source: Charting the slow death of the universe — ScienceDaily

DESY publishes new precise picture of the proton

Phe proton not only consists of three quarks (green) being held together by gluons (springs), but is a sizzling place of gluons and pairs of quarks and antiquarks (orange) interacting with each other.

After 15 years of measurement and another eight years of scrutinizing and calculation, the particle physics collaborations H1 and ZEUS have published the most precise results about the innermost structure and behaviour of the proton. The two experiments which took data at DESY´s particle accelerator HERA from 1992 to 2007 have combined their data of over a billion collisions of protons with electrons or positrons, antiparticles of the electrons. About 300 authors of 70 institutions have contributed to the analysis.

“This publication is the culmination of HERA´s scientific programme and will be the most precise picture of the proton for a long time,” says DESY research director Joachim Mnich. “This legacy is not only important for the understanding of the very basic properties of matter but also an essential basis for experiments at proton colliders like the LHC at CERN in Geneva.”
Protons are in the core of each single atomic nucleus in the universe. Their composition of three quarks – two up and one down quark – which are held together by so-called gluons, carrier particles of the strong force, is well known since decades and taught in schools. However, the real picture of the proton is much more complicated: the proton is a sizzling soup where gluons can produce more gluons and can also split into pairs of quarks and antiquarks – the so-called sea quarks – all of them interacting again very quickly.

Source: DESY News: The most precise picture of the proton – Deutsches Elektronen-Synchrotron DESY