1,442 research outputs found
A hit-and-run Giant Impact scenario
The formation of the Moon from the debris of a slow and grazing giant impact
of a Mars-sized impactor on the proto-Earth (Cameron & Ward 1976, Canup &
Asphaug 2001) is widely accepted today. We present an alternative scenario with
a hit-and-run collision (Asphaug 2010) with a fractionally increased impact
velocity and a steeper impact angle.Comment: 11 pages, 2 figures, in press in ICARUS note
Cosmic-ray exposure ages of fossil micrometeorites from mid-Ordovician sediments at Lynna River, Russia
We measured the He and Ne concentrations of 50 individual extraterrestrial
chromite grains recovered from mid-Ordovician (lower Darriwilian) sediments
from the Lynna River section near St. Petersburg, Russia. High concentrations
of solar wind-like He and Ne found in most grains indicate that they were
delivered to Earth as micrometeoritic dust, while their abundance,
stratigraphic position and major element composition indicate an origin related
to the L chondrite parent body (LCPB) break-up event, 470 Ma ago. Compared to
sediment-dispersed extraterrestrial chromite (SEC) grains extracted from coeval
sediments at other localities, the grains from Lynna River are both highly
concentrated and well preserved. As in previous work, in most grains from Lynna
River, high concentrations of solar wind-derived He and Ne impede a clear
quantification of cosmic-ray produced He and Ne. However, we have found several
SEC grains poor in solar wind Ne, showing a resolvable contribution of
cosmogenic 21Ne. This makes it possible, for the first time, to determine
robust cosmic-ray exposure (CRE) ages in these fossil micrometeorites, on the
order of a few hundred-thousand years. These ages are similar to the CRE ages
measured in chromite grains from cm-sized fossil meteorites recovered from
coeval sediments in Sweden. As the CRE ages are shorter than the orbital decay
time of grains of this size by Poynting-Robertson drag, this suggests that the
grains were delivered to Earth through direct injection into an orbital
resonance. We demonstrate how CRE ages of fossil micrometeorites can be used,
in principle, to determine sedimentation rates, and to correlate the sediments
at Lynna River with the fossil meteorite-bearing sediment layers in Sweden.Comment: 25 pages, 4 figures, 2 table
Evaluating On-Farm Biodiversity: A Comparison of Assessment Methods
Strategies to stop the loss of biodiversity in agriculture areas will be more successful if farmers have the means to understand changes in biodiversity on their farms and to assess the effectiveness of biodiversity promoting measures. There are several methods to assess on-farm biodiversity but it may be difficult to select the most appropriate method for a farmer’s individual circumstances. This study aims to evaluate the usability and usefulness of four biodiversity assessment methods that are available to farmers in Switzerland. All four methods were applied to five case study farms, which were ranked according to the results. None of the methods were able to provide an exact statement on the current biodiversity status of the farms, but each method could provide an indication, or approximation, of one or more aspects of biodiversity. However, the results also showed that it is possible to generate different statements on the state of biodiversity on the same farms by using different biodiversity assessment methods. All methods showed strengths and weaknesses so, when choosing a method, the purpose of the biodiversity assessment should be kept in the foreground and the limitations of the chosen methods should be considered when interpreting the outcomes
Mercury (Hg) in meteorites: variations in abundance, thermal release profile, mass-dependent and mass-independent isotopic fractionation
We have measured the concentration, isotopic composition and thermal release
profiles of Mercury (Hg) in a suite of meteorites, including both chondrites
and achondrites. We find large variations in Hg concentration between different
meteorites (ca. 10 ppb to 14'000 ppb), with the highest concentration orders of
magnitude above the expected bulk solar system silicates value. From the
presence of several different Hg carrier phases in thermal release profiles
(150 to 650 {\deg}C), we argue that these variations are unlikely to be mainly
due to terrestrial contamination. The Hg abundance of meteorites shows no
correlation with petrographic type, or mass-dependent fractionation of Hg
isotopes. Most carbonaceous chondrites show mass-independent enrichments in the
odd-numbered isotopes 199Hg and 201Hg. We show that the enrichments are not
nucleosynthetic, as we do not find corresponding nucleosynthetic deficits of
196Hg. Instead, they can partially be explained by Hg evaporation and
redeposition during heating of asteroids from primordial radionuclides and
late-stage impact heating. Non-carbonaceous chondrites, most achondrites and
the Earth do not show these enrichments in vapor-phase Hg. All meteorites
studied here have however isotopically light Hg ({\delta}202Hg = ~-7 to -1)
relative to the Earth's average crustal values, which could suggest that the
Earth has lost a significant fraction of its primordial Hg. However, the late
accretion of carbonaceous chondritic material on the order of ~2%, which has
been suggested to account for the water, carbon, nitrogen and noble gas
inventories of the Earth, can also contribute most or all of the Earth's
current Hg budget. In this case, the isotopically heavy Hg of the Earth's crust
would have to be the result of isotopic fractionation between surface and
deep-Earth reservoirs.Comment: 43 Pages, 9 Figures. Accepted for publication in Geochimica et
Cosmochimica Act
On the origin and composition of Theia: Constraints from new models of the Giant Impact
Knowing the isotopic composition of Theia, the proto-planet which collided
with the Earth in the Giant Impact that formed the Moon, could provide
interesting insights on the state of homogenization of the inner solar system
at the late stages of terrestrial planet formation. We use the known isotopic
and modeled chemical compositions of the bulk silicate mantles of Earth and
Moon and combine them with different Giant Impact models, to calculate the
possible ranges of isotopic composition of Theia in O, Si, Ti, Cr, Zr and W in
each model. We compare these ranges to the isotopic composition of carbonaceous
chondrites, Mars, and other solar system materials. In the absence of
post-impact isotopic re-equilibration, the recently proposed high angular
momentum models of the Giant Impact ("impact-fission", Cuk & Stewart, 2012; and
"merger", Canup, 2012) allow - by a narrow margin - for a Theia similar to
CI-chondrites, and Mars. The "hit-and-run" model (Reufer et al., 2012) allows
for a Theia similar to enstatite-chondrites and other Earth-like materials. If
the Earth and Moon inherited their different mantle FeO contents from the bulk
mantles of the proto-Earth and Theia, the high angular momentum models cannot
explain the observed difference. However, both the hit-and-run as well as the
classical or "canonical" Giant Impact model naturally explain this difference
as the consequence of a simple mixture of two mantles with different FeO.
Therefore, the simplest way to reconcile the isotopic similarity, and FeO
dissimilarity, of Earth and Moon is a Theia with an Earth-like isotopic
composition and a higher (~20%) mantle FeO content.Comment: 53 Pages, 10 Figures, 1 Table, 3 Supplementary Table
Primordial Earth mantle heterogeneity caused by the Moon-forming giant impact
The giant impact hypothesis for Moon formation successfully explains the
dynamic properties of the Earth-Moon system but remains challenged by the
similarity of isotopic fingerprints of the terrestrial and lunar mantles.
Moreover, recent geochemical evidence suggests that the Earth's mantle
preserves ancient (or "primordial") heterogeneity that predates the
Moon-forming giant impact. Using a new hydrodynamical method, we here show that
Moon-forming giant impacts lead to a stratified starting condition for the
evolution of the terrestrial mantle. The upper layer of the Earth is
compositionally similar to the disk, out of which the Moon evolves, whereas the
lower layer preserves proto-Earth characteristics. As long as this predicted
compositional stratification can at least partially be preserved over the
subsequent billions of years of Earth mantle convection, the compositional
similarity between the Moon and the accessible Earth's mantle is a natural
outcome of realistic and high-probability Moon-forming impact scenarios. The
preservation of primordial heterogeneity in the modern Earth not only
reconciles geochemical constraints but is also consistent with recent
geophysical observations. Furthermore, for significant preservation of a
proto-Earth reservoir, the bulk composition of the Earth-Moon system may be
systematically shifted towards chondritic values.Comment: Comments are welcom
Detection of Tropheryma whippelii DNA in a patient with AIDS
A case of an AIDS patient infected with the Whipple's disease bacterium, Tropheryma whippelii, is reported. A DNA fragment with sequence specificity for the 16S rRNA gene of the bacterium was detected by PCR in a duodenal biopsy specimen from a 55-year-old male patient with AIDS and diarrhea. The biopsy specimen contained periodic acid-Schiff stain-positive macrophages which did not, however, resemble the sickleform-particle-containing cells characteristic of Whipple's disease. This observation raises two possibilities: either the patient had a coincidence of AIDS and Whipple's disease or Tropheryma whippelii acted as an opportunistic pathogen in this immunodeficient patient. The latter explanation is of interest in light of the ongoing discussion of immunologic abnormalities as predisposing factors for Whipple's disease
GENGA. II. GPU Planetary N-body Simulations with Non-Newtonian Forces and High Number of Particles
We present recent updates and improvements of the graphical processing unit (GPU) N-body code GENGA. Modern state-of-the-art simulations of planet formation require the use of a very high number of particles to accurately resolve planetary growth and to quantify the effect of dynamical friction. At present the practical upper limit is in the range of 30,000–60,000 fully interactive particles; possibly a little more on the latest GPU devices. While the original hybrid symplectic integration method has difficulties to scale up to these numbers, we have improved the integration method by (i) introducing higher level changeover functions and (ii) code improvements to better use the most recent GPU hardware efficiently for such large simulations. We added treatments of non-Newtonian forces such as general relativity, tidal interaction, rotational deformation, the Yarkovsky effect, and Poynting–Robertson drag, as well as a new model to treat virtual collisions of small bodies in the solar system. We added new tools to GENGA, such as semi-active test particles that feel more massive bodies but not each other, a more accurate collision handling and a real-time openGL visualization. We present example simulations, including a 1.5 billion year terrestrial planet formation simulation that initially started with 65,536 particles, a 3.5 billion year simulation without gas giants starting with 32,768 particles, the evolution of asteroid fragments in the solar system, and the planetesimal accretion of a growing Jupiter simulation. GENGA runs on modern NVIDIA and AMD GPUs
A comprehensive study of noble gases and nitrogen in Hypatia, a diamond-rich pebble from SW Egypt
This is a follow-up study of a work by Kramers et al. (2013) on an unusual
diamond-rich rock found in the SW side of the Libyan Desert Glass strewn field.
This pebble, called Hypatia, is composed of almost pure carbon. Transmission
Electron Microscopy and X-ray diffraction results reveal that Hypatia is made
of defect-rich diamond containing lonsdaleite and deformation bands. These
characteristics are compatible with an impact origin on Earth and/or in space.
We analyzed concentrations and isotopic compositions of all five noble gases
and nitrogen in several mg sized Hypatia samples. These data confirm that
Hypatia is extra-terrestrial. The sample is rich in trapped noble gases with an
isotopic composition close to the meteoritic Q component. 40Ar/36Ar ratios in
individual steps are as low as 0.4. Concentrations of cosmic-ray produced 21Ne
correspond to a nominal cosmic-ray exposure age of ca. 0.1 Myr if produced in a
typical m-sized meteoroid. Such an atypically low nominal exposure age suggests
high shielding in a considerably larger body. In addition to the Xe-Q
composition, an excess of radiogenic 129Xe (from the decay of extinct 129I) is
observed (129Xe/132Xe = 1.18 +/- 0.03). Two N components are present, an
isotopically heavy component ({\delta}15N = +20 permil) released at low temp.
and a major light component ({\delta}15N = -110 permil) at higher temp. This
disequilibrium in N suggests that the diamonds in Hypatia were formed in space.
Our data are broadly consistent with concentrations and isotopic compositions
of noble gases in at least three different types of carbon-rich meteoritic
materials. However, Hypatia does not seem to be related to any of these
materials, but may have sampled a similar cosmochemical reservoir. Our study
does not confirm the presence of exotic noble gases that led Kramers et al. to
propose that Hypatia is a remnant of a comet that impacted the Earth
The solar particle event on 10-13 September 2017 – Spectral reconstruction and calculation of the radiation exposure in aviation and space
The solar energetic particle event on 10 September 2017 and on the following
days was the strongest event in recent years. It was recorded as Ground Level
Enhancement 72 by Neutron Monitors Stations on the Earth and measured by a
number of instruments in space. One aspect of such a space weather event is
the potentially increased radiation exposure in aviation and space. Numerical
simulations can help estimate the elevated dose rates during the event; a
critical aspect in these simulations is the description of the primary particle
spectrum. In this work, we present 1 hour averaged proton spectra during the
event derived from GOES measurements and described by two different
analytic functions. The derived proton spectra are used to calculate the
radiation exposure in aviation and different space scenarios: low-Earth orbit,
interplanetary space, and Mars surface and the results are discussed in the
context of available experimental data. While the results indicate that in most
of these scenarios in aviation and space the event was of little relevance
compared to the total exposure from galactic cosmic radiation, the skin dose in
a lightly shielded environment in interplanetary space may have reached about
30% to 60% of the NASA 30-day dose limit
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