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تسجيل مستخدم جديدAmmonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids
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The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semivolatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dominating over refractory organic matter and more volatile species. Similar absorption features appear in the spectra of some asteroids, implying a compositional link between asteroids, comets, and the parent interstellar cloud.
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The ESA Rosetta mission has acquired unprecedented measurements of comet 67/P-Churyumov-Gerasimenko (hereafter 67P) nucleus surface, whose composition, as determined by in situ and remote sensing instruments including VIRTIS (Visible, InfraRed and Th
ermal Imaging Spectrometer) appears to be made by an assemblage of ices, minerals, and organic material. We performed a refined analysis of infrared observations of the nucleus of comet 67P carried out by the VIRTIS-M hyperspectral imager. We found that the overall shape of the 67P infrared spectrum is similar to that of other carbon-rich outer solar system objects suggesting a possible genetic link with them. More importantly, we are also able to confirm the complex spectral structure of the wide 2.8-3.6 micron absorption feature populated by fainter bands. Among these, we unambiguously identified the presence of aliphatic organics by their ubiquitous 3.38, 3.42 and 3.47 micron bands. This novel infrared detection of aliphatic species on a cometary surface has strong implications for the evolutionary history of the primordial solar system and give evidence that comets provide an evolutionary link between interstellar material and solar system bodies.
The Desert Fireball Network observed a significant outburst of fireballs belonging to the Southern Taurid Complex of meteor showers between October 27 and November 17, 2015. At the same time, the Cameras for Allsky Meteor Surveillance project detecte
d a distinct population of smaller meteors belonging to the irregular IAU shower #628, the s-Taurids. While this returning outburst was predicted and observed in previous work, the reason for this stream is not yet understood. 2015 was the first year that the stream was precisely observed, providing an opportunity to better understand its nature. We analyse the orbital elements of stream members, and establish a size frequency distribution from millimetre to metre size range. The stream is highly stratified with a large change of entry speed along Earths orbit. We confirm that the meteoroids have orbital periods near the 7:2 mean-motion resonance with Jupiter. The mass distribution of this population is dominated by larger meteoroids, unlike that for the regular Southern Taurid shower. The distribution index is consistent with a gentle collisional fragmentation of weak material. A population of metre-sized objects is identified from satellite observations at a rate consistent with a continuation of the size-frequency distribution established at centimetre size. The observed change of longitude of perihelion among the s-Taurids points to recent (a few centuries ago) activity from fragmentation involving surviving asteroid 2015TX24. This supports a model for the Taurid Complex showers that involves an ongoing fragmentation cascade of comet 2P/Encke siblings following a breakup some 20,000 years ago.
Context: Surveys in the visible and near-infrared spectral range have revealed the presence of low-albedo asteroids in cometary like orbits (ACOs). In contrast to Jupiter family comets (JFCs), ACOs are inactive, but possess similar orbital parameters
. Aims: In this work, we discuss why ACOs are inactive, whereas JFCs show gas-driven dust activity, although both belong to the same class of primitive solar system bodies. Methods: We hypothesize that ACOs and JFCs have formed under the same physical conditions, namely by the gravitational collapse of ensembles of ice and dust aggregates. We use the memory effect of dust-aggregate layers under gravitational compression to discuss under which conditions the gas-driven dust activity of these bodies is possible. Results: Owing to their smaller sizes, JFCs can sustain gas-driven dust activity much longer than the bigger ACOs, whose sub-surface regions possess an increased tensile strength, due to gravitational compression of the material. The increased tensile strength leads to the passivation against dust activity after a relatively short time of activity. Conclusions: The gravitational-collapse model of the formation of planetesimals, together with the gravitational compression of the sub-surface material simultaneously, explains the inactivity of ACOs and the gas-driven dust activity of JFCs. Their initially larger sizes means that ACOs possess a higher tensile strength of their sub-surface material, which leads to a faster termination of gas-driven dust activity. Most objects with radii larger than $2 , mathrm{km}$ have already lost their activity due to former gravitational compression of their current surface material.
We study the visible and near-infrared (NIR) spectral properties of different ACO populations and compare them to the independently determined properties of comets. We select our ACOs sample based on published dynamical criteria and present our own
observational results obtained using the 10.4m Gran Telescopio Canarias (GTC), the 4.2m William Herschel Telescope (WHT), the 3.56m Telescopio Nazionale Galileo (TNG), and the 2.5m Isaac Newton Telescope (INT), all located at the El Roque de los Muchachos Observatory (La Palma, Spain), and the 3.0m NASA Infrared Telescope Facility (IRTF), located at the Mauna Kea Observatory, in Hawaii. We include in the analysis the spectra of ACOs obtained from the literature. We derive the spectral class and the visible and NIR spectral slopes. We also study the presence of hydrated minerals by studying the 0.7 $mu$m band and the UV-drop below 0.5 $mu$m associated with phyllosilicates. We present new observations of 17 ACOs, 11 of them observed in the visible, 2 in the NIR and 4 in the visible and NIR. We also discuss the spectra of 12 ACOs obtained from the literature. All but two ACOs have a primitive-like class spectrum (X or D-type). Almost 100% of the ACOs in long-period cometary orbits (Damocloids) are D-types. Those in Jupiter family comet orbits (JFC-ACOs) are $sim$ 60% D-types and $sim$ 40% X-types. The mean spectral slope $S$ of JFC-ACOs is 9.7 $pm$ 4.6 %/1000 AA and for the Damocloids this is 12.2 $pm$ 2.0 %/1000 AA . No evidence of hydration on the surface of ACOs is found from their visible spectra. The slope and spectral class distribution of ACOs is similar to that of comets. The spectral classification, the spectral slope distribution of ACOs, and the lack of spectral features indicative of the presence of hydrated minerals on their surface, strongly suggest that ACOs are likely dormant or extinct comets.
We study the distributions of effective diameter ($D$), beaming parameter ($eta$), and visible geometric albedo ($p_V$) of asteroids in cometry orbits (ACOs) populations, derived from NASAs Wide-field Infrared Explorer (WISE) observations, and compar
e these with the same, independently determined properties of the comets. The near-Earth asteroid thermal model (NEATM) is used to compute the $D$, $p_V$ and $eta$. We obtained $D$ and $p_V$ for 49 ACOs in Jupiter family cometary orbits (JF-ACOs) and 16 ACOs in Halley-type orbits (Damocloids). We also obtained $eta$ for 45 of them. All but three JF-ACOs (95% of the sample) present a low albedo compatible with a cometary origin. The $p_V$ and $eta$ distributions of both ACO populations are very similar. For the entire sample of ACOs, the mean geometric albedo is $bar{p_V} = 0.05 pm 0.02$, ($bar{p_V} = 0.05 pm 0.01$ and $bar{p_V} =0.05 pm 0.02$ for JF-ACOs and Damocloids, respectively) compatible with a narrow albedo distribution similar to that of the Jupiter family comets (JFCs), with a $bar{p_V} sim 0.04$. The $bar{eta} =1.0 pm 0.2$. We find no correlations between $D$, $p_V$ , or $eta$. We compare the cumulative size distribution (CSD) of ACOs, Centaurs, and JFCs. Although the Centaur sample contains larger objects, the linear parts in their log-log plot of the CSDs presents a similar cumulative exponent ($beta = 1.85 pm 0.30$ and $1.76 pm 0.35$, respectively). The CSD for Damocloids presents a much shallower exponent $beta = 0.89 pm 0.17$. The CSD for JF-ACOs is shallower and shifted towards larger diameters with respect to the CSD of active JFCs, which suggests that the mantling process has a size dependency whereby large comets tend to reach an inactive stage faster than small ones. Finally, the population of JF-ACOs is comparable in number that of JFCs, although there are more tens-km JF-ACOs than JFCs.
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