5,552 research outputs found
Triton's surface age and impactor population revisited in light of Kuiper Belt fluxes: Evidence for small Kuiper Belt objects and recent geological activity
Neptune's largest satellite, Triton, is one of the most fascinating and
enigmatic bodies in the solar system. Among its numerous interesting traits,
Triton appears to have far fewer craters than would be expected if its surface
was primordial. Here we combine the best available crater count data for Triton
with improved estimates of impact rates by including the Kuiper Belt as a
source of impactors. We find that the population of impactors creating the
smallest observed craters on Triton must be sub-km in scale, and that this
small-impactor population can be best fit by a differential power-law size
index near -3. Such results provide interesting, indirect probes of the unseen
small body population of the Kuiper Belt. Based on the modern, Kuiper Belt and
Oort Cloud impactor flux estimates, we also recalculate estimated ages for
several regions of Triton's surface imaged by Voyager 2, and find that Triton
was probably active on a time scale no greater than 0.1-0.3 Gyr ago (indicating
Triton was still active after some 90% to 98% of the age of the solar system),
and perhaps even more recently. The time-averaged volumetric resurfacing rate
on Triton implied by these results, 0.01 km yr or more, is likely
second only to Io and Europa in the outer solar system, and is within an order
of magnitude of estimates for Venus and for the Earth's intraplate zones. This
finding indicates that Triton likely remains a highly geologically active world
at present, some 4.5 Gyr after its formation. We briefly speculate on how such
a situation might obtain.Comment: 14 pages (TeX), plus 2 postscript figures Stern & McKinnon, 2000, AJ,
in pres
Spatially embedded random networks
Many real-world networks analyzed in modern network theory have a natural spatial element; e.g., the Internet, social networks, neural networks, etc. Yet, aside from a comparatively small number of somewhat specialized and domain-specific studies, the spatial element is mostly ignored and, in particular, its relation to network structure disregarded. In this paper we introduce a model framework to analyze the mediation of network structure by spatial embedding; specifically, we model connectivity as dependent on the distance between network nodes. Our spatially embedded random networks construction is not primarily intended as an accurate model of any specific class of real-world networks, but rather to gain intuition for the effects of spatial embedding on network structure; nevertheless we are able to demonstrate, in a quite general setting, some constraints of spatial embedding on connectivity such as the effects of spatial symmetry, conditions for scale free degree distributions and the existence of small-world spatial networks. We also derive some standard structural statistics for spatially embedded networks and illustrate the application of our model framework with concrete examples
Looking for the Charged Higgs Boson
This review article starts with a brief introduction to the charged Higgs
boson (H^\pm) in the Minimal Supersymmetric Standard Model (MSSM). It then
discusses the prospects of a relatively light H^\pm boson search via top quark
decay at Tevatron/LHC, and finally a heavy H^\pm boson search at LHC. The
viable channels for H^\pm search are identified in both the cases, with
particular emphasis on the H^\pm --> tau + nu decay channel. The effects of NLO
QCD correction in the SM as well as the MSSM are discussed briefly.Comment: 17 pages with 8 eps figures, Invited review, Reference adde
Polarization of lepton from scalar tau decay as a probe of neutralino mixing
The lepton arising from the scalar tau (\st) decay is naturally
polarized. \ptau depends on the left--right mixing of the \st and the
gaugino--higgsino mixing of the neutralino. The polarization \ptau could be
measured from the energy distribution of the decay products of at future
\epem colliders. A measurement of \ptauand of the \st production cross
section allows to determine both these mixing angles.Comment: 20 pages Latex, 5 figures(not included). compressed ps file of the
figures available at ftp://ftp.kek.jp/kek/preprints/TH/TH-425/fig.ps.g
Venus climate stability and volcanic resurfacing rates
The climate of Venus is to a large degree controlled by the radiative properties of its massive atmosphere. In addition, outgassing due to volcanic activity, exospheric escape processes, and surface/atmosphere interactions may all be important in moderating the abundances of atmospheric CO2 and other volatiles. We have developed an evolutionary climate model for Venus using a systems approach that emphasizes feedbacks between elements in the climate system. Modules for atmospheric radiative transfer, surface/atmosphere interactions, tropospheric chemistry, and exospheric escape processes have so far been developed. Climate feedback loops result from interconnections between modules, in the form of the environmental parameters pressure, temperature, and atmospheric mixing ratios. The radiative transfer module has been implemented by using Rosseland mean opacities in a one dimensional grey radiative-convective model. The model has been solved for the static (time independent) case to determine climate equilibrium points. The dynamics of the model have also been explored by employing reaction/diffusion kinetics for possible surface atmosphere heterogeneous reactions over geologic timescales. It was found that under current conditions, the model predicts that the climate of Venus is at or near an unstable equilibrium point. The effects of constant rate volcanism and corresponding exsolution of volatiles on the stability of the climate model were also explored
Dust from collisions: A way to probe the composition of exo-planets?
In order to link infrared observations of dust formed during planet formation
in debris disks to mid-infrared spectroscopic data of planetary materials from
differentiated terrestrial and asteroidal bodies, we obtained absorption
spectra of a representative suite of terrestrial crustal and mantle materials,
and of typical Martian meteorites. A series of debris disk spectra
characterized by a strong feature in the 9.0-9.5 micron range (HD23514,
HD15407a, HD172555 and HD165014), is comparable to materials that underwent
shock, collision or high temperature events. These are amorphous materials such
as tektites, SiO2-glass, obsidian, and highly shocked shergottites as well as
inclusions from mesosiderites (Group A). A second group (BD+20307, Beta
Pictoris, HD145263, ID8, HD113766, HD69830, P1121, and Eta Corvi) have strong
pyroxene and olivine bands in the 9-12 micron range and is very similar to
ultramafic rocks (e.g. harzburgite, dunite)(Group B). This could indicate the
occurrence of differentiated materials similar to those in our Solar System in
these other systems. However, mixing of projectile and target material, as well
as that of crustal and mantle material has to be taken into account in large
scale events like hit-and-run and giant collisions or even large-scale
planetary impacts. This could explain the olivine-dominated dust of group B.
The crustal-type material of group A would possibly require the stripping of
upper layers by grazing-style hit-and run encounters or high energy events like
evaporation/condensation in giant collisions. In tidal disruptions or the
involvement of predominantly icy/water bodies the resulting mineral dust would
originate mainly in one of the involved planetesimals. This could allow
attributing the observed composition to a specific body (such as e.g. Eta
Corvi)
DockoMatic - Automated Ligand Creation and Docking
Background: The application of computational modeling to rationally design drugs and characterize macro biomolecular receptors has proven increasingly useful due to the accessibility of computing clusters and clouds. AutoDock is a well-known and powerful software program used to model ligand to receptor binding interactions. In its current version, AutoDock requires significant amounts of user time to setup and run jobs, and collect results. This paper presents DockoMatic, a user friendly Graphical User Interface (GUI) application that eases and automates the creation and management of AutoDock jobs for high throughput screening of ligand to receptor interactions.
Results: DockoMatic allows the user to invoke and manage AutoDock jobs on a single computer or cluster, including jobs for evaluating secondary ligand interactions. It also automates the process of collecting, summarizing, and viewing results. In addition, DockoMatic automates creation of peptide ligand .pdb files from strings of single-letter amino acid abbreviations.
Conclusions: DockoMatic significantly reduces the complexity of managing multiple AutoDock jobs by facilitating ligand and AutoDock job creation and management
Ecosystem service provision by road verges
1. Roads form a vast, rapidly growing global network that has diverse, detrimental ecological impacts. However, the habitats that border roads (âroad vergesâ) form a parallel network that might help mitigate these impacts and provide additional benefits (ecosystem services; ES).
2. We evaluate the capacity of road verges to provide ES by reviewing existing research and considering their relevant characteristics: area, connectivity, shape, and contextual ES supply and demand. We consider the present situation, and how this is likely to change based on future projections for growth in road extent, traffic densities and urban populations.
3. Road verges not only provide a wide range of ES, including biodiversity provision, regulating services (e.g. air and water filtration) and cultural services (e.g. health and aesthetic benefits by providing access to nature) but also displace other habitats and provide ecosystem disservices (e.g. plant allergens and damage to infrastructure). Globally, road verges may currently cover 270,000 km2 and store 0.015 Gt C/year, which will further increase with 70% projected growth in the global road network.
4. Road verges are well placed to mitigate traffic pollution and address demand for ES in surrounding ESâimpoverished landscapes, thereby improving human health and wellâbeing in urban areas, and improving agricultural production and sustainability in farmland. Demand for ES provided by road verges will likely increase due to projected growth in traffic densities and urban populations, though traffic pollution will be reduced by technological advances (e.g. electric vehicles). Road verges form a highly connected network, which may enhance ES provision but facilitate the dispersal of invasive species and increase vehicleâwildlife collisions.
5. Synthesis and applications. Road verges offer a significant opportunity to mitigate the negative ecological effects of roads and to address demand for ecosystem services (ES) in urban and agricultural landscapes. Their capacity to provide ES might be enhanced considerably if they were strategically designed and managed for environmental outcomes, namely by optimizing the selection, position and management of plant species and habitats. Specific opportunities include reducing mowing frequencies and planting trees in large verges. Road verge management for ES must consider safety guidelines, financial costs and ecosystem disservices, but is likely to provide longâterm financial returns if environmental benefits are considered
Tracing Galaxy Formation with Stellar Halos I: Methods
If the favored hierarchical cosmological model is correct, then the Milky Way
system should have accreted ~100-200 luminous satellite galaxies in the past
\~12 Gyr. We model this process using a hybrid semi-analytic plus N-body
approach which distinguishes explicitly between the evolution of light and dark
matter in accreted satellites. This distinction is essential to our ability to
produce a realistic stellar halo, with mass and density profile much like that
of our own Galaxy, and a surviving satellite population that matches the
observed number counts and structural parameter distributions of the satellite
galaxies of the Milky Way. Our model stellar halos have density profiles which
typically drop off with radius faster than those of the dark matter. They are
assembled from the inside out, with the majority of mass (~80%) coming from the
\~15 most massive accretion events. The satellites that contribute to the
stellar halo have median accretion times of ~9 Gyr in the past, while surviving
satellite systems have median accretion times of ~5 Gyr in the past. This
implies that stars associated with the inner halo should be quite different
chemically from stars in surviving satellites and also from stars in the outer
halo or those liberated in recent disruption events. We briefly discuss the
expected spatial structure and phase space structure for halos formed in this
manner. Searches for this type of structure offer a direct test of whether
cosmology is indeed hierarchical on small scales.Comment: 22 pages, 16 figures, submitted to Ap
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