24 research outputs found
A new method to determine the grain size of planetary regolith
Airless planetary bodies are covered by a dusty layer called regolith. The
grain size of the regolith determines the temperature and the mechanical
strength of the surface layers. Thus, knowledge of the grain size of planetary
regolith helps to prepare future landing and/or sample-return missions. In this
work, we present a method to determine the grain size of planetary regolith by
using remote measurements of the thermal inertia. We found that small bodies in
the Solar System (diameter less than ~100 km) are covered by relatively coarse
regolith grains with typical particle sizes in the millimeter to centimeter
regime, whereas large objects possess very fine regolith with grain sizes
between 10 and 100 micrometer.Comment: Accepted by Icaru
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
Laboratory Experiments to Understand Comets
In order to understand the origin and evolution of comets, one must decipher
the processes that formed and processed cometary ice and dust. Cometary
materials have diverse physical and chemical properties and are mixed in
various ways. Laboratory experiments are capable of producing simple to complex
analogues of comet-like materials, measuring their properties, and simulating
the processes by which their compositions and structures may evolve. The
results of laboratory experiments are essential for the interpretations of
comet observations and complement theoretical models. They are also necessary
for planning future missions to comets. This chapter presents an overview of
past and ongoing laboratory experiments exploring how comets were formed and
transformed, from the nucleus interior and surface, to the coma. Throughout
these sections, the pending questions are highlighted, and the perspectives and
prospects for future experiments are discussed.Comment: 36 pages, 13 figures, Chapter accepted for publication on February
24th 2023, now in press for the book Comets III, edited by K. Meech, M.
Combi, D. Bockelee-Morvan, S. Raymond and M. Zolensky, University of Arizona
Pres
Simulation and experiment of gas diffusion in a granular bed
The diffusion of gas through porous material is important to understand the
physical processes underlying cometary activity. We study the diffusion of a
rarefied gas (Knudsen regime) through a packed bed of monodisperse spheres via
experiments and numerical modelling, providing an absolute value of the
diffusion coefficient and compare it to published analytical models. The
experiments are designed to be directly comparable to numerical simulations, by
using precision steel beads, simple geometries, and a trade-off of the sample
size between small boundary effects and efficient computation. For direct
comparison, the diffusion coefficient is determined in Direct Simulation Monte
Carlo (DSMC) simulations, yielding a good match with experiments. This model is
further-on used on a microscopic scale, which cannot be studied in experiments,
to determine the mean path of gas molecules and its distribution, and compare
it against an analytical model. Scaling with sample properties (particle size,
porosity) and gas properties (molecular mass, temperature) is consistent with
analytical models. As predicted by these, results are very sensitive on sample
porosity and we find that a tortuosity depending linearly on
the porosity can well reconcile the analytical model with
experiments and simulations. Mean paths of molecules are close to those
described in the literature, but their distribution deviates from the
expectation for small path lengths. The provided diffusion coefficients and
scaling laws are directly applicable to thermophysical models of idealised
cometary material.Comment: accepted by MNRA
Sub-mm/mm optical properties of real protoplanetary matter derived from Rosetta/MIRO observations of comet 67P
Optical properties are required for the correct understanding and modelling
of protoplanetary and debris discs. By assuming that comets are the most
pristine bodies in the solar system, our goal is to derive optical constants of
real protoplanetary material. We determine the complex index of refraction of
the near-surface material of comet 67P/Churyumov-Gerasimenko by fitting the
sub-millimetre/millimetre observations of the thermal emission of the comet's
sub-surface made by the Microwave Instrument for the Rosetta Orbiter (MIRO)
with synthetic temperatures derived from a thermophysical model and
radiative-transfer models. According to the two major formation scenarios of
comets, we model the sub-surface layers to consist of pebbles as well as of
homogeneously packed dust grains. In the case of a homogeneous dusty surface
material, we find a solution for the length-absorption coefficient of for a wavelength of 1.594 mm and for a wavelength of 0.533 mm and a constant thermal
conductivity of . For the pebble scenario, we
find for the pebbles and a wavelength of 1.594 mm a complex refractive index of
for pebble
radii between 1 mm and 6 mm. Taking into account other constraints, our results
point towards a pebble makeup of the cometary sub-surface with pebble radii
between 3 mm and 6 mm. The derived real part of the refractive index is used to
constrain the composition of the pebbles and their volume filling factor. The
optical and physical properties are discussed in the context of protoplanetary
and debris disc observations.Comment: Accepted for publication in MNRA
Evidence for the formation of comet 67P/Churyumov-Gerasimenko through gravitational collapse of a bound clump of pebbles
The processes that led to the formation of the planetary bodies in the Solar System are still not fully understood. Using the results obtained with the comprehensive suite of instruments on-board ESA’s Rosetta mission, we present evidence that comet 67P/Churyumov-Gerasimenko likely formed through the gentle gravitational collapse of a bound clump of mm-sized dust aggregates (“pebbles”), intermixed with microscopic ice particles. This formation scenario leads to a cometary make-up that is simultaneously compatible with the global porosity, homogeneity, tensile strength, thermal inertia, vertical temperature profiles, sizes and porosities of emitted dust, and the steep increase in water-vapour production rate with decreasing heliocentric distance, measured by the instruments on-board the Rosetta spacecraft and the Philae lander. Our findings suggest that the pebbles observed to be abundant in protoplanetary discs around young stars provide the building material for comets and other minor bodies
Asteroid Ryugu before the Hayabusa2 encounter
Asteroid (162173) Ryugu is the target object of Hayabusa2, an asteroid exploration and sample return mission led by Japan Aerospace Exploration Agency (JAXA). Ground-based observations indicate that Ryugu is a C-type near-Earth asteroid with a diameter of less than 1 km, but the knowledge of its detailed properties is very limited prior to Hayabusa2 observation. This paper summarizes our best understanding of the physical and dynamical properties of Ryugu based on ground-based remote sensing and theoretical modeling before the Hayabusa2’s arrival at the asteroid. This information is used to construct a design reference model of the asteroid that is used for the formulation of mission operation plans in advance of asteroid arrival. Particular attention is given to the surface properties of Ryugu that are relevant to sample acquisition. This reference model helps readers to appropriately interpret the data that will be directly obtained by Hayabusa2 and promotes scientific studies not only for Ryugu itself and other small bodies but also for the solar system evolution that small bodies shed light on.Additional co-authors: Guy Libourel, Roy Lichtenheldt, Alessandro Maturilli, Scott R. Messenger, Tatsuhiro Michikami, Hideaki Miyamoto, Stefano Mottola, Thomas Müller, Akiko M. Nakamura, Larry R. Nittler, Kazunori Ogawa, Tatsuaki Okada, Ernesto Palomba, Naoya Sakatani, Stefan E. Schröder, Hiroki Senshu, Driss Takir, Michael E. Zolensky and International Regolith Science Group (IRSG) in Hayabusa2 projec