ABSTRACT
A method is presented for efficiently calculating surface temperatures for highly resolved celestial body shapes. A thorough investigation of the necessary conditions leading to reach model ...convergence shows that the speed of surface temperature convergence depends on factors such as the quality of initial boundary conditions, thermal inertia, illumination conditions, and resolution of the numerical depth grid. The optimization process to shorten the simulation time while increasing or maintaining the accuracy of model results includes the introduction of facet-specific boundary conditions such as pre-computed temperature estimates and pre-evaluated simulation times. The individual facet treatment also allows for assigning other facet-specific properties such as local thermal inertia. The approach outlined in this paper is particularly useful for very detailed digital terrain models in combination with unfavourable illumination conditions such as little-to-no sunlight at all for a period of time as experienced locally on comet 67P/Churyumov–Gerasimenko. Possible science applications include thermal analysis of highly resolved local (landing) sites experiencing seasonal, environment, and lander shadowing. In combination with an appropriate roughness model, the method is very suitable for application to disc-integrated and disc-resolved data. Further applications are seen where the complexity of the task has led to severe shape or thermophysical model simplifications such as in studying surface activity or thermal cracking.
The physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. In this article, we review some of the main physical ...properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasising the progress made during the Rosetta mission. Comets have a low density of
480
±
220
kg
m
−
3
and a low permittivity of 1.9–2.0, consistent with a high porosity of 70–80%, are weak with a very low global tensile strength
<
100
Pa, and have a low bulk thermal inertia of
0
–
60
J
K
−
1
m
−
2
s
−
1
/
2
that allowed them to preserve highly volatiles species (e.g. CO, CO
2
, CH
4
, N
2
) into their interior since their formation. As revealed by 67P/Churyumov-Gerasimenko, the above physical properties vary across the nucleus, spatially at its surface but also with depth. The broad picture is that the bulk of the nucleus consists of a weakly bonded, rather homogeneous material that preserved primordial properties under a thin shell of processed material, and possibly covered by a granular material; this cover might in places reach a thickness of several meters. The properties of the top layer (the first meter) are not representative of that of the bulk nucleus. More globally, strong nucleus heterogeneities at a scale of a few meters are ruled out on 67P’s small lobe.
ABSTRACT
Deriving thermophysical properties such as thermal inertia from thermal infrared observations provides useful insights into the structure of the surface material on planetary bodies. The ...estimation of these properties is usually done by fitting temperature variations calculated by thermophysical models to infrared observations. For multiple free model parameters, traditional methods such as least-squares fitting or Markov chain Monte Carlo methods become computationally too expensive. Consequently, the simultaneous estimation of several thermophysical parameters, together with their corresponding uncertainties and correlations, is often not computationally feasible and the analysis is usually reduced to fitting one or two parameters. Data assimilation (DA) methods have been shown to be robust while sufficiently accurate and computationally affordable even for a large number of parameters. This paper will introduce a standard sequential DA method, the ensemble square root filter, for thermophysical modelling of asteroid surfaces. This method is used to re-analyse infrared observations of the MARA instrument, which measured the diurnal temperature variation of a single boulder on the surface of near-Earth asteroid (162173) Ryugu. The thermal inertia is estimated to be 295 ± 18 $\mathrm{J\, m^{-2}\, K^{-1}\, s^{-1/2}}$, while all five free parameters of the initial analysis are varied and estimated simultaneously. Based on this thermal inertia estimate the thermal conductivity of the boulder is estimated to be between 0.07 and 0.12,$\mathrm{W\, m^{-1}\, K^{-1}}$ and the porosity to be between 0.30 and 0.52. For the first time in thermophysical parameter derivation, correlations and uncertainties of all free model parameters are incorporated in the estimation procedure that is more than 5000 times more efficient than a comparable parameter sweep.
Thermal and mechanical material properties determine comet evolution and even solar system formation because comets are considered remnant volatile-rich planetesimals. Using data from the ...Multipurpose Sensors for Surface and Sub-Surface Science (MUPUS) instrument package gathered at the Philae landing site Abydos on comet 67P/Churyumov-Gerasimenko, we found the diurnal temperature to vary between 90 and 130 K. The surface emissivity was 0.97, and the local thermal inertia was 85 ± 35 J m
−2
K
−1
s
-1/2
. The MUPUS thermal probe did not fully penetrate the near-surface layers, suggesting a local resistance of the ground to penetration of >4 megapascals, equivalent to >2 megapascal uniaxial compressive strength. A sintered near-surface microporous dust-ice layer with a porosity of 30 to 65% is consistent with the data.
The structure of the upper layer of a comet is a product of its surface activity. The Rosetta Lander Imaging System (ROLIS) on board Philae acquired close-range images of the Agilkia site during its ...descent onto comet 67P/Churyumov-Gerasimenko. These images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. At the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. The thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. The presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. Modeling suggests that abrasion driven by airfall-induced particle "splashing" is responsible for the observed formations.
•Philae’s ROLIS camera acquired 6 images of the surface of 67P after the final landing.•Illumination of the comet surface was provided by LEDs of four colors.•A local horizon is visible in one image ...corner, beyond which we can see the coma.•Seen at low phase angle, the comet surface displays a bimodal brightness distribution.•The surface seems to consist of plates with a consolidated crust, separated by cracks.
After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta’s Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colors. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images.
The formation of water is a repetitive process and depends on the physical conditions in the different stages of the solar nebula and early solar system. Our solar nebula model considers the thermal ...and chemical evolution of a collapsing globular cloud core. We simulate the collapse with a semi-analytical model which is based on a multi-zone density distribution. This model describes the formation of a central protostellar object surrounded by a disk and a thin outer envelope. It considers an adiabatic equation of state, viscous gas flow and a resistive magnetic field. Due to the low temperatures in the hydrostatic stage of the core, icy layers of water mixed with other molecules build on the dust grains. In the course of the collapse the ice sublimates and drives a complex chemical evolution located in a warm region around the proto-stellar object called hot corino. Moreover, the relatively high temperatures in this region allow the gas phase formation of water together with other molecules. The abundances of the chemical compounds are computed from rate equations solved in a Lagrangian grid. We can show that there was high water density in the early and late accretion zone of the Earth. This water was sublimated from the dust or formed by hot neutral reactions in the gas phase. Thus, according to our collapse model, there were two sources delivering the water incorporated into the Earth.
An important condition for the habitability of rocky planets is the existence of water in or on their upper lithospheric layer. We will show that the available amount of this water depends on the ...conditions in the parental cloud the planetary system has formed from. These clouds can be giant gas clusters with a complex structure associated with bright nebulae or smaller gas aggregations appearing as quiescent dark regions. It has been observed that in both cloud types young stars are formed in dense cores consisting mainly of molecular hydrogen. We assume that the physical and chemical state of these cores, which defines the initial conditions of star formation, is also representative for the initial state of the solar nebula 4.6Giga years ago. Based on this assumption, we have developed a radial symmetric model to study the physical and chemical evolution of the earliest period of the solar nebula described by a cloud core with 1.01 solar mass and a radius of about 104AU. The evolution of this core is simulated for a few Mega years, while its molecular gas being in a hydrostatic equilibrium. The related radial distributions of the gas and dust temperature can be calculated from thermal balance equations. These equations depend on the radial profile of the dust to gas density which follows from the continuity equation of the dust phase. The velocity of the dust grains is influenced by the radiation pressure of the local interstellar radiation field and the gas drag. The resulting temperature and dust profiles derived from our model depend on the grain size distribution of the dust. These profiles determine the chemical evolution of the cloud core.
It is shown that in the dust phase about 106 to 107 times more water is produced than in the gas phase. Further, the total mass of the water formed in the core varies only marginally between 0.11 and 0.12wt% for a life time of the core between 1 and 6.5Mega years, respectively. Roughly 84% of the oxygen atoms are incorporated into water molecules, if the intensity of the radiation field is about 1 Habing. The number of oxygen atoms decreases to 77% if this intensity triples. The water amount produced in the gas phase depends stronger on the interstellar radiation field and the living time of the core than the water amount formed on dust. For the 1 Habing radiation intensity the size distribution of the dust grains has nearly no influence. Finally, a number of species representing compounds mainly formed in the dust or in the gas phase was selected (H2O, CO, etc.) in order to use them for a validation of our model. Thereto, we have compared the abundances of these compounds simulated with the model to the related data from observations published in the literature. For almost all cases except N2H+ a sufficient agreement was found.
•A model for a quasi-stationary cloud core, precursor of a collapsing core, is developed.•It evaluates the core stability and the validity of the chosen equation of state (EOS).•It is combined with a chemical module to compute the produced water amount.•UV radiation influences the core stability and but only slightly the water amount.•The likely EOS of the thermally balanced core is a modified polytropic EOS.
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