1,172 research outputs found
Recommended from our members
Value clashes, power competition and community trust: why an NGO's earthquake recovery program faltered in rural China
Water Delivery and Giant Impacts in the 'Grand Tack' Scenario
A new model for terrestrial planet formation (Hansen 2009, Walsh et al. 2011)
has explored accretion in a truncated protoplanetary disk, and found that such
a configuration is able to reproduce the distribution of mass among the planets
in the Solar System, especially the Earth/Mars mass ratio, which earlier
simulations have generally not been able to match. Walsh et al. tested a
possible mechanism to truncate the disk--a two-stage, inward-then-outward
migration of Jupiter and Saturn, as found in numerous hydrodynamical
simulations of giant planet formation. In addition to truncating the disk and
producing a more realistic Earth/Mars mass ratio, the migration of the giant
planets also populates the asteroid belt with two distinct populations of
bodies--the inner belt is filled by bodies originating inside of 3 AU, and the
outer belt is filled with bodies originating from between and beyond the giant
planets (which are hereafter referred to as `primitive' bodies).
We find here that the planets will accrete on order 1-2% of their total mass
from primitive planetesimals scattered onto planet-crossing orbits during the
formation of the planets. For an assumed value of 10% for the water mass
fraction of the primitive planetesimals, this model delivers a total amount of
water comparable to that estimated to be on the Earth today. While the radial
distribution of the planetary masses and the dynamical excitation of their
orbits are a good match to the observed system, we find that the last giant
impact is typically earlier than 20 Myr, and a substantial amount of mass is
accreted after that event. However, 5 of the 27 planets larger than half an
Earth mass formed in all simulations do experience large late impacts and
subsequent accretion consistent with the dating of the Moon-forming impact and
the estimated amount of mass accreted by Earth following that event
Studying Chinese Politics in an Age of Specialization
The relationship between area studies and political science is fraught with tradeoffs. In particular, a danger exists that the field of Chinese politics is being hollowed out because (a) there are many islands of highly specialized research with few bridges between them; and (b) more and more Chinese politics scholars are engaged in debates in which the 'other side' is no longer a China scholar but instead a colleague in the discipline. At a time when China's economic growth and prominence in world affairs have generated remarkable interest inside and outside the academy, few scholars are willing to take a stab at characterizing the polity or addressing other, equally large questions. Further thought is needed about the 'terms of enlistment' for China scholars in political science, in an era when ever more-focused studies and greater participation in disciplinary debates have become the norm. © 2011 Taylor & Francis
Thin films of tin(II) sulphide (SnS) by aerosol-assisted chemical vapour deposition (AACVD) using tin(II) dithiocarbamates as single-source precursors
A low mass for Mars from Jupiter's early gas-driven migration
Jupiter and Saturn formed in a few million years (Haisch et al. 2001) from a
gas-dominated protoplanetary disk, and were susceptible to gas-driven migration
of their orbits on timescales of only ~100,000 years (Armitage 2007).
Hydrodynamic simulations show that these giant planets can undergo a two-stage,
inward-then-outward, migration (Masset & Snellgrove 2001, Morbidelli & Crida
2007, Pierens & Nelson 2008). The terrestrial planets finished accreting much
later (Klein et al. 2009), and their characteristics, including Mars' small
mass, are best reproduced by starting from a planetesimal disk with an outer
edge at about one astronomical unit from the Sun (Wetherill 1978, Hansen 2009)
(1 AU is the Earth-Sun distance). Here we report simulations of the early Solar
System that show how the inward migration of Jupiter to 1.5 AU, and its
subsequent outward migration, lead to a planetesimal disk truncated at 1 AU;
the terrestrial planets then form from this disk over the next 30-50 million
years, with an Earth/Mars mass ratio consistent with observations. Scattering
by Jupiter initially empties but then repopulates the asteroid belt, with
inner-belt bodies originating between 1 and 3 AU and outer-belt bodies
originating between and beyond the giant planets. This explains the significant
compositional differences across the asteroid belt. The key aspect missing from
previous models of terrestrial planet formation is the substantial radial
migration of the giant planets, which suggests that their behaviour is more
similar to that inferred for extrasolar planets than previously thought.Comment: 12 pages, 4 figures + Supplementary Material 46 pages, 10 figure
From Centroided to Profile Mode: Machine Learning for Prediction of Peak Width in HRMS Data
Centroiding is one of the major approaches used for size reduction of the data generated by high-resolution mass spectrometry. During centroiding, performed either during acquisition or as a pre-processing step, the mass profiles are represented by a single value (i.e., the centroid). While being effective in reducing the data size, centroiding also reduces the level of information density present in the mass peak profile. Moreover, each step of the centroiding process and their consequences on the final results may not be completely clear. Here, we present Cent2Prof, a package containing two algorithms that enables the conversion of the centroided data to mass peak profile data and vice versa. The centroiding algorithm uses the resolution-based mass peak width parameter as the first guess and self-adjusts to fit the data. In addition to the m/z values, the centroiding algorithm also generates the measured mass peak widths at half-height, which can be used during the feature detection and identification. The mass peak profile prediction algorithm employs a random-forest model for the prediction of mass peak widths, which is consequently used for mass profile reconstruction. The centroiding results were compared to the outputs of the MZmine-implemented centroiding algorithm. Our algorithm resulted in rates of false detection ≤5% while the MZmine algorithm resulted in 30% rate of false positive and 3% rate of false negative. The error in profile prediction was ≤56% independent of the mass, ionization mode, and intensity, which was 6 times more accurate than the resolution-based estimated values.publishedVersio
Forest floor depth mediates understory vigor in xeric Pinus palustris ecosystems
Longleaf pine (Pinus palustris) woodlands and savannas are among the most frequently burned ecosystems in the world with fire return intervals of 1-10 years. This fire regime has maintained high levels of biodiversity in terms of both species richness and endemism. Land use changes have reduced the area of this ecosystem by >95%, and inadequate fire frequencies threaten many of the remnants today. In the absence of frequent fire, rapid colonization of hardwoods and shrubs occurs, and a broad-leaved midstory develops. This midstory encroachment has been the focus of much research and management concern, largely based on the assumption that the midstory reduces understory plant diversity through direction competition via light interception. The general application of this mechanism of degradation is questionable, however, because midstory density, leaf area, and hardwood species composition vary substantially along a soil moisture gradient from mesic to extremely xeric sites. Reanalysis of recently reported data from xeric longleaf pine communities suggests that the development of the forest floor, a less conspicuous change in forest structure, might cause a decline in plant biodiversity when forests remain unburned. We report here a test of the interactions among fire, litter accumulation, forest floor development, and midstory canopy density on understory plant diversity. Structural equation modeling showed that within xeric sites, forest floor development was the primary factor explaining decreased biodiversity. The only effects of midstory development on biodiversity were those mediated through forest floor development. Boundary line analysis of functional guilds of understory plants showed sensitivity to even minor development of the forest floor in the absence of fire. These results challenge the prevailing management paradigm and suggest that within xeric longleaf pine communities, the primary focus of managed fire regime should be directed toward the restoration of forest floor characteristics rather than the introduction of high-intensity fires used to regulate midstory structure.Peer reviewedForestr
- …