984 research outputs found
Jupiter - friend or foe? II: the Centaurs
It has long been assumed that the planet Jupiter acts as a giant shield,
significantly lowering the impact rate of minor bodies upon the Earth, and thus
enabling the development and evolution of life in a collisional environment
which is not overly hostile. However, in the past, little work has been carried
out to examine the validity of this idea. In the second of a series of papers,
we examine the degree to which the impact risk resulting from objects on
Centaur-like orbits is affected by the presence of a giant planet, in a
continuing attempt to fully understand the impact regime under which life on
Earth has developed. The Centaurs, which occupy orbits beyond Jupiter, have
their origins in the Edgeworth-Kuiper belt that extends beyond Neptune. The
giant planets peturb the Centaurs, sending a significanr fraction into the
inner Solar System where they become visible as short-period comets. In this
work we present results which show that the presence of a giant planet can act
to significantly change the impact rate of short-period comets on the Earth,
and that a giant planet often actually increases the impact flux greatly over
that which would be expected were it not present. (Shortened version of
abstract.)Comment: 13 pages, 1 Figur
Jupiter – friend or foe? I: the asteroids
The asteroids are a major source of potential impactors on the Earth today. It has long been assumed that the giant planet Jupiter acts as a shield, significantly lowering the impact rate on the Earth from both cometary and asteroidal bodies. Such shielding, it is claimed, enabled the development and evolution of life in a collisional environment, which is not overly hostile. The reduced frequency of impacts, and of related mass extinctions, would have allowed life the time to thrive, where it would otherwise have been suppressed. However, in the past, little work has been carried out to examine the validity of this idea. In the first of several papers, we examine the degree to which the impact risk resulting from a population representative of the asteroids is enhanced or reduced by the presence of a giant planet, in an attempt to understand fully the impact regime under which life on Earth developed. Our results show that the situation is far less clear cut that has previously been assumed, that is, the presence of a giant planet can act to enhance the impact rate of asteroids on the Earth significantly
Determining Habitability: Which exoEarths should we search for life?
Within the next few years, the first Earth-mass planets will be discovered
around other stars. Some of those worlds will certainly lie within the
classical "habitable zone" of their parent stars, and we will quickly move from
knowing of no exoEarths to knowing many. For the first time, we will be in a
position to carry out a detailed search for the first evidence of life beyond
our Solar System. However, such observations will be hugely taxing and time
consuming to perform, and it is almost certain that far more potentially
habitable worlds will be known than it is possible to study. It is therefore
important to catalogue and consider the various effects which make a promising
planet more or less suitable for the development of life. In this work, we
review the various planetary, dynamical and stellar influences that could
influence the habitability of exoEarths. The various influences must be taken
in concert when we attempt to decide where to focus our first detailed search
for life. While there is no guarantee that any given planet will be inhabited,
it is vitally important to ensure that we focus our time and effort on those
planets most likely to yield a positive result.Comment: 32 pages, 2 figures, 1 table; Accepted to appear in the International
Journal of Astrobiolog
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Environmental data management at Fernald
FERMCO supports DOE`s ongoing initiatives for the continuous improvement of site restoration through the development and application of innovative technologies. A major thrust of FERMCO`s efforts has been the enhancement of environmental data management technology for the site. The understanding of environmental data is the fundamental basis for determining the need for environmental restoration, developing and comparing remedial alternatives, and reaching a decision on how to clean up a site. Environmental data management at Fernald is being focused on two major objectives: to improve the efficiency of the data management process, and to provide a better understanding of the meaning of the data at the earliest possible time. Environmental data at Fernald is typically a soil or groundwater sample collected by one of the field geologists. These samples are then shipped to one or more laboratories for analysis. After the analyses are returned from the laboratories the data are reviewed and qualified for usability. The data are then used by environmental professionals for determining nature and extent of contamination. Additionally, hazardous waste materials whether generated during production or during cleanup, may be sampled to characterize the waste before shipment or treatment. The data management process, which uses four major software systems, is presented graphically
Jupiter - friend or foe? III: the Oort cloud comets
It has long been assumed that the planet Jupiter acts as a giant shield,
significantly lowering the impact rate of small bodies on the Earth. However,
until recently, very little work had been carried out examining the role played
by Jupiter in determining the frequency of such collisions. In this work, the
third of a series of papers, we examine the degree to which the impact rate on
Earth resulting from the Oort cloud comets is enhanced or lessened by the
presence of a giant planet in a Jupiter-like orbit, in an attempt to more fully
understand the impact regime under which life on Earth has developed. Our
results show that the presence of a giant planet in a Jupiter-like orbit
significantly alters the impact rate of Oort cloud comets on the Earth,
decreasing the rate as the mass of the giant increases. The greatest
bombardment flus is observed when no giant planet is present.Comment: 21 pages, 4 Figures. Accepted for publication in the International
Journal of Astrobiolog
Kernel density classification and boosting: an L2 sub analysis
Kernel density estimation is a commonly used approach to classification. However, most of the theoretical results for kernel methods apply to estimation per se and not necessarily to classification. In this paper we show that when estimating the difference between two densities, the optimal smoothing parameters are increasing functions of the sample size of the complementary group, and we provide a small simluation study which examines the relative performance of kernel density methods when the final goal is classification. A relative newcomer to the classification portfolio is “boosting”, and this paper proposes an algorithm for boosting kernel density classifiers. We note that boosting is closely linked to a previously proposed method of bias reduction in kernel density estimation and indicate how it will enjoy similar properties for classification. We show that boosting kernel classifiers reduces the bias whilst only slightly increasing the variance, with an overall reduction in error. Numerical examples and simulations are used to illustrate the findings, and we also suggest further areas of research
Habitable Zones of Host Stars During the Post-MS Phase
A star will become brighter and brighter with stellar evolution, and the
distance of its habitable zone will become farther and farther. Some planets
outside the habitable zone of a host star during the main sequence phase may
enter the habitable zone of the host star during other evolutionary phases. A
terrestrial planet within the habitable zone of its host star is generally
thought to be suited to life existence. Furthermore, a rocky moon around a
giant planet may be also suited to life survive, provided that the planet-moon
system is within the habitable zone of its host star. Using Eggleton's code and
the boundary flux of habitable zone, we calculate the habitable zone of our
Solar after the main sequence phase. It is found that Mars' orbit and Jupiter's
orbit will enter the habitable zone of Solar during the subgiant branch phase
and the red giant branch phase, respectively. And the orbit of Saturn will
enter the habitable zone of Solar during the He-burning phase for about 137
million years. Life is unlikely at any time on Saturn, as it is a giant gaseous
planet. However, Titan, the rocky moon of Saturn, may be suitable for
biological evolution and become another Earth during that time. For low-mass
stars, there are similar habitable zones during the He-burning phase as our
Solar, because there are similar core masses and luminosities for these stars
during that phase.Comment: 6 pages, 7 figures. Accepted by Ap & S
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