Ulrich Ott

University of West Hungary, Faculty of Natural Sciences, Savaria Campus, H-9700 Szombathely, Hungary
Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

Noble gases are not rare in the Universe, but they are rare in rocks. As a consequence, it has been possible to identify in detailed analyses a variety of components whose existence is barely visible in other elements: radiogenic and cosmogenic gases produced in situ, as well as a variety of “trapped” components – both of solar (solar wind) origin and the “planetary” noble gases. The latter are most abundant in the most primitive chondritic meteorites and are distinct in elemental and isotopic abundance patterns from planetary noble gases sensu strictu, e.g., those in the atmospheres of Earth and Mars, having in common only the strong relative depletion of light relative to heavy elements when compared to the solar abundance pattern. In themselves, the “planetary” noble gases in meteorites constitute again a complex mixture of components including such hosted by pre-solar stardust grains.

The pre-solar components bear witness of the processes of nucleosynthesis in stars. In particular, krypton and xenon isotopes in pre-solar silicon carbide and graphite grains keep a record of physical conditions of the slow-neutron capture process (s-process) in asymptotic giant branch (AGB) stars. The more abundant Kr and Xe in the nanodiamonds, on the other hand, show a more enigmatic pattern, which, however, may be related to variants of the other two processes of heavy element nucleosynthesis, the rapid neutron capture process (r-process) and the p-process producing the proton-rich isotopes.
“Q-type” noble gases of probably “local” origin dominate the inventory of the heavy noble gases (Ar, Kr, Xe). They are hosted by “phase Q”, a still ill-characterized carbonaceous phase that is concentrated in the acid-insoluble residue left after digestion of the main meteorite minerals in HF and HCl acids. While negligible in planetary-gas-rich primitive meteorites, the fraction carried by “solubles” becomes more important in chondrites of higher petrologic type. While apparently isotopically similar to Q gas, the elemental abundances are somewhat less fractionated relative to the solar pattern, and they deserve further study. Similar “planetary” gases occur in high abundance in the ureilite achondrites, while small amounts of Q-type noble gases may be present in some other achondrites. A “subsolar” component, possibly a mixture of Q and solar noble gases, is found in enstatite chondrites. While no definite mechanism has been identified for the introduction of the planetary noble gases into their meteoritic host phases, there are strong indications that ion implantation has played a major role.
The planetary noble gases are concentrated in the meteorite matrix. Ca-Al-rich inclusions (CAIs) are largely planetary-gas-free, however, some trapped gases have been found in chondrules. Micrometeorites (MMs) and interplanetary dust particles (IDPs) often contain abundant solar wind He and Ne, but they are challenging objects for the analysis of the heavier noble gases that are characteristic for the planetary component. The few existing data for Xe point to a Q-like isotopic composition. Isotopically Q-Kr and Q-Xe show a mass dependent fractionation relative to solar wind, with small radiogenic/nuclear additions. They may be closer to “bulk solar” Kr and Xe than Kr and Xe in the solar wind, but for a firm conclusion it is necessary to gain a better understanding of mass fractionation during solar wind acceleration.

Reference
Ott U (in press) Planetary and pre-solar noble gases in meteorites. Chemie der Erde – Geochemistry
[doi:10.1016/j.chemer.2014.01.003]
Copyright Elsevier

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M. González, P. J. Gutiérrez and L. M. Lara

Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía s/n 18008 Granada, Spain

Aims. In this work, we simulate the global behavior of comet Hale-Bopp with our thermophysical model starting with simple, homogeneous conditions, so that dust mantling and the active area develop consistently depending on the properties of the simulated nucleus. We aim to obtain a range of compatibility between our model and the observations, that can be used as constraints on some of the characteristics of cometary nuclei.
Methods. Our thermophysical model includes crystallization (and release of trapped CO), sublimation/recondensation, heat and gas transport through the nucleus, and dragged dust release. We run a battery of simulations with different parameter sets selected according to our current knowledge of comets and compare our results with observational data. Initial calculations are performed for a comet radius R₀ = 30 km. To match the calculated integrated H₂O production to the observed rate, we renormalize to a new R, which must be within 20 and 40 km, that is a range compatible with several estimates. Further selection is performed comparing the simulated water and carbon monoxide production rate profiles with the observational profiles and checking that the observational upper/lower limits of the H₂O production are fulfilled.
Results. We have found a reasonable agreement between our model and the data for H₂O and CO production rates, without the need of distributed sources, for the following initial conditions: the nucleus is composed of water, carbon monoxide, and dust with a moderate dust proportion, tending to be icy, with a dust-to-ice ratio of between 0.5 and 1. The water ice must be initially amorphous with 15 to 20% of trapped carbon monoxide. The icy matrix has a thermal inertia between 100 and 200 J m^-2 K s^−1/2, considering the initial composition with crystalline ice at 140 K. The dust follows an exponential size distribution with particles from 0.1 μm to 1 mm and leaves the comet dragged by the expelled vapor with a dragging efficiency (dust-to-gas ratio) of 3.

Reference
González M, Gutiérrez PJ and Lara LM (2014) Thermophysical simulations of comet Hale-Bopp. Astronomy & Astrophysics 563:A98.
[doi:10.1051/0004-6361/201322702]
Reproduced with permission © ESO

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Hainaut et al.¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹ European Southern Observatory (ESO), Karl Schwarzschild Straße, 85 748 Garching bei München, Germany

The object P/2013 P5 PANSTARRS was discovered in August 2013, displaying a cometary tail, but its orbital elements indicated that it was a typical member of the inner asteroid main belt. We monitored the object from 2013 August 30 until 2013 October 05 using the CFHT 3.6 m telescope (Mauna Kea, HI), the NTT (ESO, La Silla), the CA 1.23 m telescope (Calar Alto), the Perkins 1.8m (Lowell) and the 0.6 m TRAPPIST telescope (La Silla). We measured its nuclear radius to be r ≲ 0.25−0.29 km, and its colours g′ − r′ = 0.58 ± 0.05 and r′ − i′ = 0.23 ± 0.06, typical for an S-class asteroid, as expected for an object in the inner asteroid belt and in the vicinity of the Flora collisional family. We failed to detect any rotational light curve with an amplitude <0.05 mag and a double-peaked rotation period <20 h. The evolution of the tail during the observations was as expected from a dust tail. A detailed Finson-Probstein analysis of deep images acquired with the NTT in early September and with the CFHT in late September indicated that the object was active since at least late January 2013 until the time of the latest observations in 2013 September, with at least two peaks of activity around 2013 June 14 ± 10 d and 2013 July 22 ± 3 d. The changes of activity level and the activity peaks were extremely sharp and short, shorter than the temporal resolution of our observations (~1 d). The dust distribution was similar during these two events, with dust grains covering at least the 1–1000 μm range. The total mass ejected in grains <1 mm was estimated to be 3.0 × 10⁶ kg and 2.6 × 10⁷ kg around the two activity peaks. Rotational disruption cannot be ruled out as the cause of the dust ejection. We also propose that the components of a contact binary might gently rub and produce the observed emission. Volatile sublimation might also explain what appears as cometary activity over a period of 8 months. However, while main belt comets best explained by ice sublimation are found in the outskirts of the main belt, where water ice is believed to be able to survive buried in moderately large objects for the age of the solar system deeply, the presence of volatiles in an object smaller than 300 m in radius would be very surprising in the inner asteroid belt.

Reference
Hainaut et al. (2014) Continued activity in P/2013 P5 PANSTARRS – Unexpected comet, rotational break-up, or rubbing binary asteroid?. Astronomy & Astrophysics 563:A75.
[doi:10.1051/0004-6361/201322864]
Reproduced with permission © ESO

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