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.
Aims. Using data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal ...inertia (TI) and roughness in several regions on the largest lobe of the nucleus. Methods. We have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. The model takes heat conduction, self-heating, and shadowing effects into account. To reproduce the brightness temperatures measured by the MIRO instrument, the thermal model is coupled to a radiative transfer model to derive the TI. To reproduce the spatially resolved infrared measurements of the VIRTIS instrument, the thermal model is coupled to a radiance model to derive the TI and surface roughness. These methods are applied to Rosetta data from September 2014. Results. The resulting TI values from both instruments are broadly consistent with each other. From the millimetre channel on MIRO, we determine the TI in the subsurface to be <80 JK−1 m−2 s−0.5 for the Seth, Ash, and Aten regions. The submillimetre channel implies similar results but also suggests that higher values could be possible. A low TI is consistent with other MIRO measurements and in situ data from the MUPUS instrument at the final landing site of Philae. The VIRTIS results give a best-fitting value of 80 JK−1 m−2 s−0.5 and values in the range 40–160 JK−1 m−2 s−0.5 in the same areas. These observations also allow the subpixel scale surface roughness to be estimated and compared to images from the OSIRIS camera. The VIRTIS data imply that there is significant roughness on the infrared scale below the resolution of the available shape model and that, counter-intuitively, visually smooth terrain (centimetre scale) can be rough at small (micrometre–millimetre) scales, and visually rough terrain can be smooth at small scales.
Aims. We investigate the influence of the Yarkovsky force on the long-term orbital evolution of Jupiter Trojan asteroids. Methods. Clones of the observed population with different sizes and different ...thermal properties were numerically integrated for 1 Gyr with and without the Yarkovsky effect. The escape rate of these objects from the Trojan region as well as changes in the libration amplitude, eccentricity, and inclination were used as a metric of the strength of the Yarkovsky effect on the Trojan orbits. Results. Objects with radii R ≤ 1 km are significantly influenced by the Yarkovsky force. The effect causes a depletion of these objects over timescales of a few hundred million years. As a consequence, we expect the size-frequency distribution of small Trojans to show a shallower slope than that of the currently observable population (R ≳ 1 km), with a turning point between R = 100 m and R = 1 km. The effect of the Yarkovsky acceleration on the orbits of Trojans depends on the sense of rotation in a complex way. The libration amplitude of prograde rotators decreases with time while the eccentricity increases. Retrograde rotators experience the opposite effect, which results in retrograde rotators being ejected faster from the 1:1 resonance region. Furthermore, for objects affected by the Yarkovsky force, we find indications that the effect tends to smooth out the differences in the orbital distribution between the two clouds.
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 near-Earth asteroid (162173) Ryugu, the target of Hayabusa2 space mission, was observed via both orbiter and the lander instruments. The infrared radiometer on the MASCOT lander (MARA) is the ...only instrument providing spectrally resolved mid-infrared (MIR) data, which is crucial for establishing a link between the asteroid material and meteorites found on Earth. Earlier studies revealed that the single boulder investigated by the lander belongs to the most common type found on Ryugu. Here we show the spectral variation of Ryugu's emissivity using the complete set of in-situ MIR data and compare it to those of various carbonaceous chondritic meteorites, revealing similarities to the most aqueously altered ones, as well as to asteroid (101955) Bennu. The results show that Ryugu experienced strong aqueous alteration prior to any dehydration.
Comets contain the best-preserved material from the beginning of our planetary system. Their nuclei and comae composition reveal clues about physical and chemical conditions during the early solar ...system when comets formed. ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) onboard the Rosetta spacecraft has measured the coma composition of comet 67P/Churyumov-Gerasimenko with well-sampled time resolution per rotation. Measurements were made over many comet rotation periods and a wide range of latitudes. These measurements show large fluctuations in composition in a heterogeneous coma that has diurnal and possibly seasonal variations in the major outgassing species: water, carbon monoxide, and carbon dioxide. These results indicate a complex coma-nucleus relationship where seasonal variations may be driven by temperature differences just below the comet surface.
► Monte Carlo simulation for atmospheric consequences of a (late) heavy bombardment. ► Probability distribution of impact velocities for Earth and Mars from Nice model (dynamical simulations). ► ...Scaling of models for impact erosion and impactor volatiles retention analyzed. ► Cases for Mars and the Earth. ► Atmospheric accumulation dominates erosion except for extremely volatilepoor asteroids.
Consequences of a heavy bombardment for the atmospheres of Earth and Mars are investigated with a stochastic model. The main result is the dominance of the accumulation. The atmospheric pressure is strongly increasing both for Earth and Mars in the course of an enhanced bombardment. The effect of atmospheric erosion is found to be minor, regarding escape during meteorite entry, in the expanding vapor plume, and ejection due to free-surface motion. The initial atmospheric surface pressure if comparable to the modern value turns out as a less important additive constant of the final pressure. Impactor retention and atmospheric erosion are parametrized in terms of scaling laws, compatible with recent numerical simulations. The dependence on impactor size, atmospheric and planetary parameters is analyzed among alternative models and numerical results. The stochastic model is fed with the net replenishment originating from impactor material and the loss of preexisting atmospheric gas. Major input parameters are the total cumulative impactor mass and the relative mass of atmophile molecules in comets and asteroids. Input size distributions of the impactor ensemble correspond to presently observed main belt asteroids and KBOs. Velocity distributions are taken from dynamical simulations for the Nice model. Depending on the composition of large cometary impactors, the Earth could acquire a more massive atmosphere, a few bars in terms of surface pressure, mostly as CO and CO2. For Mars accumulation of 1–4 bars of CO and CO2 requires an asteroidal ‘late veneer’ of the order of 1024g containing 2% atmophiles.
Abstract
We collect observational evidence that supports the scheme of mass transfer on the nucleus of comet 67P/Churyumov–Gerasimenko. The obliquity of the rotation axis of 67P causes strong ...seasonal variations. During perihelion the southern hemisphere is four times more active than the north. Northern territories are widely covered by granular material that indicates back fall originating from the active south. Decimetre sized chunks contain water ice and their trajectories are influenced by an antisolar force instigated by sublimation. OSIRIS observations suggest that up to 20 per cent of the particles directly return to the nucleus surface taking several hours of traveltime. The back fall covered northern areas are active if illuminated but produce mainly water vapour. The decimetre chunks from the nucleus surface are too small to contain more volatile compounds such as CO2 or CO. This causes a north–south dichotomy of the composition measurements in the coma. Active particles are trapped in the gravitational minimum of Hapi during northern winter. They are ‘shock frozen’ and only re-activated when the comet approaches the sun after its aphelion passage. The insolation of the big cavity is enhanced by self-heating, i.e. reflection and IR radiation from the walls. This, together with the pristinity of the active back fall, explains the early observed activity of the Hapi region. Sobek may be a role model for the consolidated bottom of Hapi. Mass transfer in the case of 67P strongly influences the evolution of the nucleus and the interpretation of coma measurements.
We performed a stereo-photogrammetric (SPG) analysis of more than 1500 Rosetta/OSIRIS NAC images of comet 67P/Churyumov-Gerasimenko (67P). The images with pixel scales in the range 0.2−3.0 m/pixel ...were acquired between August 2014 and February 2016. We finally derived a global high-resolution 3D description of 67P’s surface, the SPG SHAP7 shape model. It consists of about 44 million facets (1−1.5 m horizontal sampling) and a typical vertical accuracy at the decimeter scale. Although some images were taken after perihelion, the SPG SHAP7 shape model can be considered a pre-periheliondescription and replaces the previous SPG SHAP4S shape model. From the new shape model, some measures for 67P with very low 3σ uncertainties can be retrieved: 18.56 km3 ± 0.02 km3 for the volume and 537.8 kg/m3 ± 0.7 kg/m3 for the mean density assuming a mass value of 9.982 × 1012 kg.
The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal evolution of the ...colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. We identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties.
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