4,292 research outputs found
Jupiter - Friend or Foe? IV: The influence of orbital eccentricity and inclination
For many years, it was assumed that Jupiter prevented the Earth from being
subject to a punishing impact regime that would greatly hinder the development
of life. Here, we present the 4th in a series of studies investigating this
hypothesis. Previously, we examined the effect of Jupiter's mass on the impact
rate experienced by Earth. Here, we extend that approach to consider the
influence of Jupiter's orbital eccentricity and inclination on the impact rate.
We first consider scenarios in which Jupiter's orbital eccentricity was
somewhat higher and somewhat lower than that in our Solar System. We find that
Jupiter's orbital eccentricity plays a moderate role in determining the impact
flux at Earth, with more eccentric orbits resulting in a higher impact rate of
asteroids than for more circular orbits. This is particularly pronounced at
high "Jupiter" masses. For short-period comets, the same effect is clearly
apparent, albeit to a lesser degree. The flux of short-period comets impacting
the Earth is slightly higher for more eccentric Jovian orbits.
We also consider scenarios in which Jupiter's orbital inclination was greater
than that in our Solar System. Increasing Jupiter's orbital inclination greatly
increased the flux of asteroidal impactors. However, at the highest tested
inclination, the disruption to the Asteroid belt was so great that the belt
would be entirely depleted after an astronomically short period of time. In
such a system, the impact flux from asteroid bodies would therefore be very
low, after an initial period of intense bombardment. By contrast, the influence
of Jovian inclination on impacts from short-period comets was very small. A
slight reduction in the impact flux was noted for the moderate and high
inclination scenarios considered in this work - the results for inclinations of
five and twenty-five degrees were essentially identical.Comment: 5 figures, plus 12 as an appendi
Simulations of the Population of Centaurs II: Individual Objects
Detailed orbit integrations of clones of five Centaurs -- namely, 1996 AR20,
2060 Chiron, 1995 SN55, 2000 FZ53 and 2002 FY36 -- for durations of 3 Myr are
presented. One of our Centaur sample starts with perihelion initially under the
control of Jupiter (1996 AR20), two start under the control of Saturn (Chiron
and 1995 SN55) and one each starts under the control of Uranus (2000 FZ53) and
Neptune (2002 FY36) respectively. A variety of interesting pathways are
illustrated with detailed examples including: capture into the Jovian Trojans,
repeated bursts of short-period comet behaviour, capture into mean-motion
resonances with the giant planets and into Kozai resonances, as well as
traversals of the entire Solar system. For each of the Centaurs, we provide
statistics on the numbers (i) ejected, (ii) showing short-period comet
behaviour and (iii) becoming Earth and Mars crossing. For example, Chiron has
over 60 % of its clones becoming short-period objects, whilst 1995 SN55 has
over 35 %. Clones of these two Centaurs typically make numerous close
approaches to Jupiter. At the other extreme, 2000 FZ53 has roughly 2 % of its
clones becoming short-period objects. In our simulations, typically 20 % of the
clones which become short-period comets subsequently evolve into
Earth-crossers.Comment: 10 pages, in press at MNRA
Recommended from our members
Colorado University: The Austere Years (The Story of Its First Quarter-Century)
This fascinating history of the University of Colorado Boulder is filled with information on the early years from the late 1880s through its first twenty five years. Illustrated with numerous black and white vintage photographs. Contents also include: Boulder City; \u22Old Main\u22; Presidents Sewall, Hale, Baker; School of Law; Applied Science; Sports (baseball, football, basketball, \u22Folsom\u22); bibliography
Decoding sequential vs non-sequential two-photon double ionization of helium using nuclear recoil
Above 54.4 eV, two-photon double ionization of helium is dominated by a
sequential absorption process, producing characteristic behavior in the single
and triple differential cross sections. We show that the signature of this
process is visible in the nuclear recoil cross section, integrated over all
energy sharings of the ejected electrons, even below the threshold for the
sequential process. Since nuclear recoil momentum imaging does not require
coincident photoelectron measurement, the predicted images present a viable
target for future experiments with new short-pulse VUV and soft X-ray sources.Comment: 4 pages, 3 figure
Quantifying Jupiter's influence on the Earth's impact flux: Implications for planetary habitability
It has long been thought that the presence of a giant planet is a
pre-requisite for the development of life on potentially habitable planets.
Without Jupiter, it was argued, the Earth would have been subject to a
punishing impact regime, which would have significantly retarded or outright
prevented the development of life on our planet.
Although this idea is widely embraced, little research has previously been
carried out to support it. Here, we present the results of several suites of
dynamical integrations used to model the influence of Jupiter's mass and orbit
on the impact rate that would be experienced by the Earth. We find that, far
from being a simple shield, Jupiter's role in determining the terrestrial
impact flux is significantly more complicated than previously thought. Far from
being a simple friend, such giant planets are perhaps more likely to imperil
the development of life on otherwise habitable planets.Comment: 16 pages, 4 figures, 1 tabl
The Populations of Comet-Like Bodies in the Solar system
A new classification scheme is introduced for comet-like bodies in the Solar
system. It covers the traditional comets as well as the Centaurs and
Edgeworth-Kuiper belt objects. At low inclinations, close encounters with
planets often result in near-constant perihelion or aphelion distances, or in
perihelion-aphelion interchanges, so the minor bodies can be labelled according
to the planets predominantly controlling them at perihelion and aphelion. For
example, a JN object has a perihelion under the control of Jupiter and aphelion
under the control of Neptune, and so on. This provides 20 dynamically distinct
categories of outer Solar system objects in the Jovian and trans-Jovian
regions. The Tisserand parameter with respect to the planet controlling
perihelion is also often roughly constant under orbital evolution. So, each
category can be further sub-divided according to the Tisserand parameter. The
dynamical evolution of comets, however, is dominated not by the planets nearest
at perihelion or aphelion, but by the more massive Jupiter. The comets are
separated into four categories -- Encke-type, short-period, intermediate and
long-period -- according to aphelion distance. The Tisserand parameter
categories now roughly correspond to the well-known Jupiter-family comets,
transition-types and Halley-types. In this way, the nomenclature for the
Centaurs and Edgeworth-Kuiper belt objects is based on, and consistent with,
that for comets.Comment: MNRAS, in press, 11 pages, 6 figures (1 available as postscript, 5 as
gif). Higher resolution figures available at
http://www-thphys.physics.ox.ac.uk/users/WynEvans/preprints.pd
Skin autofluorescence: an emerging biomarker in persons with kidney disease
Purpose of review: Skin autofluorescence (SAF) is a measure of the accumulation of advanced glycation end-products (AGEs) proposed to act as a marker of “cumulative metabolic stress”. This paper discusses mechanisms of AGE formation and reviews published literature on SAF as a biomarker and risk factor across the spectrum of kidney disease. Recent findings: SAF is elevated in adults and children on dialysis. Higher SAF is an independent risk factor for cardiovascular and all-cause mortality in persons receiving haemodialysis and for all-cause mortality in persons performing peritoneal dialysis, though the increase in discrimination when SAF was added to traditional risk factors was modest. In less advanced chronic kidney disease, higher SAF predicts all-cause mortality and progression. SAF is elevated in renal transplant recipients, but to a lesser extent than in dialysis patients. In one study higher SAF predicted graft loss and mortality. SAF has been reported to be increased in patients with acute kidney injury. Summary: A growing body of evidence attests that SAF, a marker of AGE accumulation, is a risk factor for mortality and kidney function decline in multiple types of kidney disease. Further studies are warranted to evaluate interventions to reduce SAF and the impact on clinical outcomes
New Area Recommendations For Oats
Beginning in the fall of 1957, Iowa State College oat variety recommendations will be made on the basis of four state areas. Who determined the areas? Why, none other than the oat varieties themselves
Discovery of a very X-ray luminous galaxy cluster at z=0.89 in the WARPS survey
We report the discovery of the galaxy cluster ClJ1226.9+3332 in the Wide
Angle ROSAT Pointed Survey (WARPS). At z=0.888 and L_X=1.1e45 erg/s (0.5-2.0
keV, h_0=0.5) ClJ1226.9+3332 is the most distant X-ray luminous cluster
currently known. The mere existence of this system represents a huge problem
for Omega_0=1 world models.
At the modest (off-axis) resolution of the ROSAT PSPC observation in which
the system was detected, ClJ1226.9+3332 appears relaxed; an off-axis HRI
observation confirms this impression and rules out significant contamination
from point sources. However, in moderately deep optical images (R and I band)
the cluster exhibits signs of substructure in its apparent galaxy distribution.
A first crude estimate of the velocity dispersion of the cluster galaxies based
on six redshifts yields a high value of 1650 km/s, indicative of a very massive
cluster and/or the presence of substructure along the line of sight. While a
more accurate assessment of the dynamical state of this system requires much
better data at both optical and X-ray wavelengths, the high mass of the cluster
has already been unambiguously confirmed by a very strong detection of the
Sunyaev-Zel'dovich effect in its direction (Joy et al. 2001).
Using ClJ1226.9+3332 and ClJ0152.7-1357 (z=0.835), the second-most distant
X-ray luminous cluster currently known and also a WARPS discovery, we obtain a
first estimate of the cluster X-ray luminosity function at 0.8<z<1.4 and
L_X>5e44 erg/s. Using the best currently available data, we find the comoving
space density of very distant, massive clusters to be in excellent agreement
with the value measured locally (z<0.3), and conclude that negative evolution
is not required at these luminosities out to z~1. (truncated)Comment: accepted for publication in ApJ Letters, 6 pages, 2 figures, uses
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