47 research outputs found
An inflationary disk phase to explain extended protoplanetary dust disks
Understanding planetesimal formation is an essential first step to
understanding planet formation. The distribution of these first solid bodies
will drive the locations where planetary embryos can grow. We seek to
understand the parameter space of possible protoplanetary disk formation and
evolution models of our Solar System. A good protoplanetary disk scenario for
the Solar System must meet at least the following three criteria: 1) an
extended dust disk (at least 45 au); 2) formation of planetesimals in at least
two distinct locations; and 3) transport of high temperatures condensates
(i.e., calcium-aluminium-rich inclusion, CAIs) to the outer disk. We explore a
large parameter space to study the effect of the disk viscosity, the timescale
of infall of material into the disk, the distance within which material is
deposited into the disk, and the fragmentation threshold of dust particles.
We find that scenarios with a large initial disk viscosity (),
relatively short infall timescale ( kyr), and a small
centrifugal radius (~au; the distance within which material falls
into the disk) result in disks that satisfy the criteria for a good
protoplanetary disk of the Solar System. The large initial viscosity and short
infall timescale result in a rapid initial expansion of the disk, which we dub
the inflationary phase of the disk. Furthermore, a temperature-dependent
fragmentation threshold, which mimics that cold icy particles break more
easily, results in larger and more massive disks. This results in more "icy"
than "rocky" planetesimals. Such scenarios are also better in line with our
Solar System, which has small terrestrial planets and massive giant planet
cores. Finally, we find that scenarios with large cannot transport CAIs
to the outer disk and do not produce planetesimals at two locations within the
disk.Comment: accepted to be published in A&
Limitations in the determination of surface emission distributions on comets through modelling of observational data -- A case study based on Rosetta observations
The European Space Agency's (ESA) Rosetta mission has returned a vast data
set of measurements of the inner gas coma of comet 67P/Churyumov-Gerasimenko.
These measurements have been used by different groups to determine the
distribution of the gas sources at the nucleus surface. The solutions that have
been found differ from each other substantially and illustrate the degeneracy
of this issue. It is the aim of this work to explore the limitations that
current gas models have in linking the coma measurements to the surface. In
particular, we discuss the sensitivity of Rosetta's ROSINA/COPS, VIRTIS, and
MIRO instruments to differentiate between vastly different spatial
distributions of the gas emission from the surface. We have applied a state of
the art 3D DSMC gas dynamics code to simulate the inner gas coma of different
models that vary in the fraction of the surface that contains ice and in
different sizes of active patches. These different distributions result in jet
interactions that differ in their dynamical behaviour. We have found that
ROSINA/COPS measurements by themselves cannot detect the differences in our
models. While ROSINA/COPS measurements are important to constrain the regional
inhomogeneities of the gas emission, they can by themselves not determine the
surface emission distribution of the gas sources to a spatial accuracy of
better than a few hundred metres (400 m). Any solutions fitting the ROSINA/COPS
measurements is hence fundamentally degenerate, be it through a forward or
inverse model. Only other instruments with complementary measurements can
potentially lift this degeneracy as we show here for VIRTIS and MIRO. Finally,
as a by-product, we have explored the effect of our activity distributions on
lateral flow at the surface that may be responsible for some of the observed
aeolian features.Comment: Icarus (in press
Dust Emission and Dynamics
When viewed from Earth, most of what we observe of a comet is dust. The
influence of solar radiation pressure on the trajectories of dust particles
depends on their cross-section to mass ratio. Hence solar radiation pressure
acts like a mass spectrometer inside a cometary tail. The appearances of
cometary dust tails have long been studied to obtain information on the dust
properties, such as characteristic particle size and initial velocity when
entering the tail. Over the past two decades, several spacecraft missions to
comets have enabled us to study the dust activity of their targets at much
greater resolution than is possible with a telescope on Earth or in near-Earth
space, and added detail to the results obtained by the spacecraft visiting
comet 1P/Halley in 1986. We now know that the dynamics of dust in the inner
cometary coma is complex and includes a significant fraction of particles that
will eventually fall back to the surface. The filamented structure of the
near-surface coma is thought to result from a combination of topographic
focussing of the gas flow, inhomogeneous distribution of activity across the
surface, and projection effects. It is possible that some
larger-than-centimetre debris contains ice when lifted from the surface, which
can affect its motion. Open questions remain regarding the microphysics of the
process that leads to the detachment and lifting of dust from the surface, the
evolution of the dust while travelling away from the nucleus, and the extent to
which information on the nucleus activity can be retrieved from remote
observations of the outer coma and tail.Comment: Chapter in press for the book Comets III, edited by K. Meech and M.
Combi, University of Arizona Pres
Superparticle Method for Simulating Collisions
For problems in astrophysics, planetary science and beyond, numerical
simulations are often limited to simulating fewer particles than in the real
system. To model collisions, the simulated particles (aka superparticles) need
to be inflated to represent a collectively large collisional cross section of
real particles. Here we develop a superparticle-based method that replicates
the kinetic energy loss during real-world collisions, implement it in an
-body code and test it. The tests provide interesting insights into dynamics
of self gravitating collisional systems. They show how particle systems evolve
over several free fall timescales to form central concentrations and
equilibrated outer shells. The superparticle method can be extended to account
for the accretional growth of objects during inelastic mergers.Comment: accepted in Ap
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
Gas flow in near surface comet like porous structures: Application to 67P/Churyumov-Gerasimenko
We performed an investigation of a comet like porous surface to study how sub-surface sublimation with subsequent flow through the porous medium can lead to higher gas temperatures at the surface. A higher gas temperature of the emitted gas at the surface layer, compared to the sublimation temperature, will lead to higher gas speeds as the gas expands into the vacuum thus altering the flow properties on larger scales (kilometres away from the surface). Unlike previous models that have used modelled artificial structures, we used Earth rock samples with a porosity in the range 24 – 92 % obtained from X-ray micro computed tomography (micro-CT) scans with resolution of some μm. Micro-CT scanning technology provides 3D images of the pore samples. The direct simulation Monte Carlo (DSMC) method for the rarefied gas dynamics is directly applied on the digital rock samples in an unstructured mesh to determine the gas densities, temperatures and speeds within the porous medium and a few centimetres above the surface. The thicknesses of the rock samples were comparable to the diurnal thermal skin depth (5cm). H2O was assumed to be the outgassing species. We correlated the coma temperatures and other properties of the flow with the rock porosities. The results are discussed as an input to analysis of data from the Microwave Instrument on Rosetta Orbiter (MIRO) on the 67P/Churyumov-Gerasimenko
Water vapor deposition from the inner gas coma onto the nucleus of Comet 67P/Churyumov-Gerasimenko
Rosetta has detected water ice existing on the surface of Comet 67P/Churyumov-Gerasimenko in various types of features. One of particular interest is the frost-like layer observed at the edge of receding shadows during the whole mission, interpreted as the recondensation of a thin layer of water ice. Two possible mechanisms, (1) subsurface ice sublimation and (2) gas coma deposition, have been proposed for producing this recondensation process and diurnal cycles of water ice. Previous studies have demonstrated both mechanisms based on simplified models. More precise and modern models are yet insufficient when addressing the gas-coma-deposition mechanism. We aim to study the recondensation from the inner water gas coma of the 67P/Churyumov-Gerasimenko with more physical constraints including the OSIRIS images, nucleus shape model, and insolation conditions. We compute, for the first time, the backflux distributions from the coma with various boundary conditions. Numerical simulations of this gas-coma-deposition process show that the equivalent water ice deposition can be up to several microns in an hour of accumulation time close to the perihelion passage, which is comparable with the simulation results of the other subsurface-ice sublimation mechanism
Cometary Comae-Surface Links:The Physics of Gas and Dust from the Surface to a Spacecraft
A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading
Determining the dust environment of an unknown comet for a spacecraft flyby: The case of ESA’s Comet Interceptor mission
Context. An assessment of the dust environment of a comet is needed for data analysis and planning spacecraft missions, such as ESA’s Comet Interceptor (CI) mission. The distinctive feature of CI is that the target object will be defined shortly before (or even after) launch; as a result, the properties of the nucleus and dust environment are poorly constrained, and therefore make the assessment of the dust environment challenging.
Aims. The main goal of the work is to provide realistic estimations of a dust environment based on very general parameters of possible target objects.
Methods. Contemporary numerical models of a dusty-gas coma were used to obtain spatial distribution of dust for a given set of parameters. By varying parameters within a range of possible values, we obtained an ensemble of possible dust distributions. Then, this ensemble was statistically evaluated in order to define the most probable cases and hence reduce the dispersion. This ensemble can not only be used to estimate the likely dust abundance along a flyby trajectory of a spacecraft, for example, but also to quantify the associated uncertainty.
Results. We present a methodology of the dust environment assessment for the case when the target comet is not known beforehand (or when its parameters are known with large uncertainty). We provide an assessment of dust environment for the CI mission. We find that the lack of knowledge of any particular comet results in very large uncertainties (~3 orders of magnitude) for the dust densities within the coma. The most sensitive parameters affecting the dust densities are the dust size distribution, the dust production rate, and coma brightness, often quantified by Afρ. Further, the conversion of a coma’s brightness (Afρ) to a dust production rate is poorly constrained. The dust production rate can only be estimated down to an uncertainty of ~0.5 orders of magnitude if the dust size distribution is known in addition to the Afρ.
Conclusions. To accurately predict the dust environment of a poorly known comet, a statistical approach needs to be taken to properly reflect the uncertainties. This can be done by calculating an ensemble of comae covering all possible combinations within parameter space as shown in this work