220 research outputs found
Outgassing of icy bodies in the solar system - I. The sublimation of hexagonal water ice through dust layers
Our knowledge about the physical processes determining the activity of comets
were mainly influenced by several extremely successful space missions, the
predictions of theoretical models and the results of laboratory experiments.
However, novel computer models should not be treated in isolation but should be
based on experimental results. Therefore, a new experimental setup was
constructed to investigate the temperature dependent sublimation properties of
hexagonal water ice and the gas diffusion through a dry dust layer covering the
ice surface. We show that this experimental setup is capable to reproduce known
gas production rates of pure hexagonal water ice. The reduction of the gas
production rate due to an additional dust layer on top of the ice surface was
measured and compared to the results of another experimental setup in which the
gas diffusion through dust layers at room temperature was investigated. We
found that the relative permeability of the dust layer is inversely
proportional to its thickness, which is also predicted by theoretical models.
However, the measured absolute weakening of the gas flow was smaller than
predicted by models. This lack of correspondence between model and experiment
may be caused by an ill-determination of the boundary condition in the
theoretical models, which further demonstrates the necessity of laboratory
investigations. Furthermore, the impedance of the dust layer to the ice
evaporation was found to be similar to the impedance at room temperature, which
means that the temperature profile of the dust layer is not influencing the
reduction of the gas production. Finally, we present the results of an extended
investigation of the sublimation coefficient, which is an important factor for
the description of the sublimation rate of water ice and, thus, an important
value for thermophysical modeling of icy bodies in the solar system.Comment: Submitted to Icaru
What drives the dust activity of comet 67P/Churyumov-Gerasimenko?
We use the gravitational instability formation scenario of cometesimals to
derive the aggregate size that can be released by the gas pressure from the
nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances
and different volatile ices. To derive the ejected aggregate sizes, we
developed a model based on the assumption that the entire heat absorbed by the
surface is consumed by the sublimation process of one volatile species. The
calculations were performed for the three most prominent volatile materials in
comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of
the dust aggregates able to escape from the nucleus into space widens when the
comet approaches the Sun and narrows with increasing heliocentric distance,
because the tensile strength of the aggregates decreases with increasing
aggregate size. The activity of CO ice in comparison to H_20 ice is capable to
detach aggregates smaller by approximately one order of magnitude from the
surface. As a result of the higher sublimation rate of CO ice, larger
aggregates are additionally able to escape from the gravity field of the
nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in
radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been
observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7
AU. Furthermore, the model predicts the release of decimeter-sized aggregates
(trail particles) close to the heliocentric distance at which the gas-driven
dust activity vanishes. However, the gas-driven dust activity cannot explain
the presence of particles smaller than ~1 mm in the coma because the high
tensile strength required to detach these particles from the surface cannot be
provided by evaporation of volatile ices. These smaller particles can be
produced for instance by spin-up and centrifugal mass loss of ejected larger
aggregates
Activity of comets: Gas Transport in the Near-Surface Porous Layers of a Cometary Nucleus
The gas transport through non-volatile random porous media is investigated
numerically. We extend our previous research of the transport of molecules
inside the uppermost layer of a cometary surface (Skorov and Rickmann, 1995;
Skorov et al. 2001). We assess the validity of the simplified capillary model
and its assumptions to simulate the gas flux trough the porous dust mantle as
it has been applied in cometary physics. A new microphysical computational
model for molecular transport in random porous media formed by packed spheres
is presented. The main transport characteristics such as the mean free path
distribution and the permeability are calculated for a wide range of model
parameters and compared with those obtained by more idealized models. The focus
in this comparison is on limitations inherent in the capillary model. Finally a
practical way is suggested to adjust the algebraic Clausing formula taking into
consideration the nonlinear dependence of permeability on layer porosity. The
retrieved dependence allows us to accurately calculate the permeability of
layers whose thickness and porosity vary in the range of values expected for
the near-surface regions of a cometary nucleus.Comment: 25 pages, 9 figure
LE SABIR CONTEMPORAIN: QUELQUES ÉLÉMENTS POUR TRADUIRE DE FRANÇAIS EN FRANÇAIS
This paper looks at the quickly spreading tendency in most European languages (with specific reference to French), whose symptoms include the proliferation of anglicisms, barbarisms, vulgarisms, systematic wrong usage, as well as rebus spelling and sms language, within the standard language.Oui, j’ai une patrie : la langue française.Albert Camu
Local manifestations of cometary activity
Comets are made of volatile and refractory material and naturally experience
various degrees of sublimation as they orbit around the Sun. This gas release,
accompanied by dust, represents what is traditionally described as activity.
Although the basic principles are well established, most details remain
elusive, especially regarding the mechanisms by which dust is detached from the
surface and subsequently accelerated by the gas flows surrounding the nucleus.
During its 2 years rendez-vous with comet 67P/Churyumov-Gerasimenko, ESA's
Rosetta has observed cometary activity with unprecedented details, in both the
inbound and outbound legs of the comet's orbit. This trove of data provides a
solid ground on which new models of activity can be built. In this chapter, we
review how activity manifests at close distance from the surface, establish a
nomenclature for the different types of observed features, discuss how activity
is at the same time transforming and being shaped by the topography, and
finally address several potential mechanisms.Comment: This paper is a review chapter in the upcoming book "Comets: Post 67P
Perspectives" edited by ISSI and Space Science Reviews. Accepted on 08 April
201
TRANSLATING LITERARY IRONY: ELEMENTS FOR A PRACTICAL FRAMEWORK PIERRE SKOROV
This paper looks at the versatile and elusive concept of irony in literature from a translator’s perspective. It offers elements for a basic definition and for a classiÂfication of literary irony. It aims to provide the literary translator with a practical framework that would make it an easier task to identify and understand the various types of irony present in literature
Outgassing of icy bodies in the Solar System - II. Heat transport in dry, porous surface dust layers
In this work, we present a new model for the heat conductivity of porous dust
layers in vacuum, based on an existing solution of the heat transfer equation
of single spheres in contact. This model is capable of distinguishing between
two different types of dust layers: dust layers composed of single particles
(simple model) and dust layers consisting of individual aggregates (complex
model). Additionally, we describe laboratory experiments, which were used to
measure the heat conductivity of porous dust layers, in order to test the
model. We found that the model predictions are in an excellent agreement with
the experimental results, if we include radiative heat transport in the model.
This implies that radiation plays an important role for the heat transport in
porous materials. Furthermore, the influence of this new model on the Hertz
factor are demonstrated and the implications of this new model on the modeling
of cometary activity are discussed. Finally, the limitations of this new model
are critically reviewed.Comment: Submitted to Icaru
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