Cliff collapse on Comet 67P/Churyumov-Gerasimenko -- I. Aswan

The Aswan cliff on Comet 67P/Churyumov-Gerasimenko collapsed on 2015 July 10. Thereby, relatively pristine comet material from a depth of ~12 m was exposed at the surface. Observations of the collapse site by the microwave instrument Rosetta/MIRO have been retrieved from 8 months prior to collapse, as well as from 5, 7, and 11 months post-collapse. The MIRO data are analysed with thermophysical and radiative transfer models. The pre-collapse observations are consistent with a 30 MKS thermal inertia dust mantle with a thickness of at least 3 cm. The post-collapse data are consistent with: 1) a dust/water-ice mass ratio of 0.9$\pm$0.5 and a molar $\mathrm{CO_2}$ abundance of ~30 per cent relative to water; 2) formation of a dust mantle after ~7 months, having a thickness of a few millimetres or a fraction thereof; 3) a $\mathrm{CO_2}$ ice sublimation front at 0.4 cm that withdrew to 2.0 cm and later to 20$\pm$6 cm; 4) a thermal inertia ranging 10-45 MKS; 5) a gas diffusivity that decreased from $0.1\,\mathrm{m^2\,s^{-1}}$ to $0.001\,\mathrm{m^2\,s^{-1}}$; 6) presence of a solid-state greenhouse effect parts of the time. The data and the analysis provide a first empirical glimpse of how ice-rich cometary material ages and evolves when exposed to solar heating.Comment: 22 pages, 24 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer revie

Collisional heating of icy planetesimals. I. Catastrophic collisions

Planetesimals in the primordial disc may have experienced a collisional cascade. If so, the comet nuclei later placed in the Kuiper belt, scattered disc, and Oort Cloud would primarily be fragments and collisional rubble piles from that cascade. However, the heating associated with the collisions cannot have been strong enough to remove the hypervolatiles that are trapped within more durable ices, because comet nuclei are rich in hypervolatiles. This places constraints on the diameter of the largest bodies allowed to participate in collisional cascades, and limits the primordial disc lifetime or population size. In this paper, the thermophysical code NIMBUS is used to study the thermal evolution of planetesimals before, during, and after catastrophic collisions. The loss of CO during segregation of $\mathrm{CO_2:CO}$ mixtures and during crystallisation of amorphous $\mathrm{H_2O}$ is calculated, as well as mobilisation and internal relocation of $\mathrm{CO_2}$. If an amorphous $\mathrm{H_2O}$ host existed, and was protected by a $\mathrm{CO_2:CO}$ heat sink, only diameter $D<20\,\mathrm{km}$ (inner disc) and $D<64\,\mathrm{km}$ (outer disc) bodies could have been involved in a collisional cascade. If $\mathrm{CO_2}$ was the only CO host, the critical diameters drop to $D<20$-$32\mathrm{km}$. Avoiding disruption of larger bodies requires a primordial disc lifetime of $<9\,\mathrm{Myr}$ at $15\,\mathrm{au}$ and $<50$-$70\,\mathrm{Myr}$ at $30\,\mathrm{au}$. Alternatively, if a $450\,\mathrm{Myr}$ disc lifetime is required to associate the primordial disc disruption with the Late Heavy Bombardment, the disc population size must have been 6-60 times below current estimates.Comment: 20 pages, 11 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer revie

An investigation of errors in estimates of the cometary nuclei active area fractions

The final version is available at: http://www.aanda.orgActive area fractions of cometary nuclei are often estimated by comparing the observed water production rates with theoretical rates obtained by applying the fast rotator or subsolar point approximations to spherical model nuclei. Any discrepancy between observed and theoretical production rates is interpreted as a certain degree of dust mantling (or in some cases hyper activity) of the object. We here investigate the typical errors introduced in such active area fraction estimates by the usage of oversimplified spherical model nuclei. This is done by first calculating the production rates of slowly rotating irregular model bodies with different activity patterns on their surfaces and arbitrary spin axis orientations, for which solar illumination is treated properly. Next, the production rates of the spherical model objects under averaged insolation are compared to the production rates of the complex model objects in an attempt to recover the known active area fraction of the latter bodies. We then find that the fast rotator and subsolar point approximations generally yield large over– and underestimates of the active area fraction, depending on the characteristics of the simulated complex nuclei. Acceptable relative errors (<100%) only occur at small heliocentric distances, and the subsolar point approximation yields somewhat better results than the fast rotator approximation.Peer reviewe

Evaluation of Agents Interactions in a Context-Aware System

The evaluation of Multi-Agent Systems (MAS) is a complex problem and it does not have a single form. Much effort has been spent on suggesting and implementing new architectures of MAS. Often these new architectures are not even compared to any other existing architectures in order to evaluate their relative benefits. The present work focuses on interactions, the most important characteristic of any complex software as autonomous agents according to [25], as a problematic of evaluation. So, in this paper, we describe the assignment of evaluation values to Agents interaction in a specific MAS architecture. This evaluation is based on the weight of the messages brought by an interaction.Funded by projects CICYT TIN2008-06742-C02-02/TSI,CICYT TEC2008-06732-C02-02/TEC, SINPROB, CAM MADRINET S-0505/TIC/0255, and DPS2008-07029-C02-02.Publicad

Zinc and calcium apparent absorption from an infant cereal: a stable isotope study in healthy infants

Fractional apparent absorption of Zn and Ca from a wheat-milk-based infant cereal was studied in six healthy infants (18-30 weeks old). Mineral absorption was measured by a stable-isotope technique based on faecal excretion of the isotopes. Each test meal (40 g cereal) was extrinsically labelled with 70Zn and 42Ca before intake. All faecal material passed during the 21 d following intake of the labelled test meal was collected on trace-element-free nappies. Individual stool samples were analysed for their content of ‘OZn and 42Ca by thermal ionization mass spectrometry. Apparent absorption was calculated as intake minus total faecal excretion of the isotopes over 68-92 h after administration. The fractional apparent absorption values for Zn and Ca were 33.9 (SD 164) % (range 19.2-639 %) and 53.5 (SD 12.6) %) (range 36.7-71.7 %) respectively. Re-excretion of absorbed 70Zn (> 68-92 h to 21 d after intake of the labelled meal) was 044 (SD 038) %] of administered dose while only one infant re-excreted detectable amounts of 42Ca (1.74%) of administered dose). The analysis of individual stool samples confirmed that 72 h is a sufficient time period for complete collections of non-absorbed isotopes in faecal material from infants during the weaning period and that re-excretion of initially absorbed 70Zn and 42Ca (> 68-92 h to 21 d after intake of the labelled meal) is negligibl

Continent urinary tract reconstruction - the Lund experience.

The Department of Urology in Lund, Sweden, has a long association with innovations in reconstructive urology. The authors from that department describe their experience over a long period with orthotopic bladder substitution and continent cutaneous urinary diversion. They conclude that continent urinary tract reconstruction is associated with a high incidence of early and late complications. They also found that for storage and emptying, their Lundiana pouch was superior to the Goldwasser neobladder. OBJECTIVE: To assess the early and late complications and functional results in patients undergoing continent reconstruction of the urinary tract, i.e. orthotopic bladder substitution (OBS) or continent cutaneous diversion (CCD). PATIENTS AND METHODS: The medical records of all patients undergoing OBS (Goldwasser technique) or CCD ('Lundiana' technique) for malignant or benign disease during 1987-1999 and followed to December 2001 were reviewed. There were 67 patients with neobladders, 77 with a Lundiana pouch who had undergone radical cystectomy and 22 with a Lundiana pouch operated for benign disorders. RESULTS: Early complications requiring reoperation occurred in 12% of the cystectomy group, with no difference with type of reconstruction, and in 10% with benign diseases. Four patients (3%) undergoing radical cystectomy died from early cardiovascular complications, two after surgery for intra-abdominal complications. Intestinally related complications and wound dehiscence requiring re-operation occurred in nine and six patients, respectively. The incidence of late complications requiring open surgery was 22% and 23% after cystectomy with OBS and CCD, respectively. The value in patients with benign diseases undergoing CCD was also 23%. Stone formation in the pouch was common, occurring in 12% in patients with OBS and in 10% after CCD. The pouch perforated or ruptured in four patients. The incidence of uretero-intestinal stricture using the Le Duc technique was 2.4% and renal function was well preserved. The incidence of revisional surgery of the Lundiana pouch outlet for incontinence was low and all patients but four were continent. The functional outcome in patients with OBS was less good; some needed pouch augmentation or an artificial urinary sphincter. Most patients used incontinence products and many needed clean intermittent self-catheterization. CONCLUSION: Continent urinary tract reconstruction is associated with a high incidence of early and late complications. For storage and emptying, the CCD Lundiana pouch is superior to the OBS of Goldwasser

Short-term variability of a sample of 29 trans-Neptunian objects and Centaurs

We present results of 6 years of observations, reduced and analyzed with the same tools in a systematic way. We report completely new data for 15 objects, for 5 objects we present a new analysis of previously published results plus additional data and for 9 objects we present a new analysis of data already published. Lightcurves, possible rotation periods and photometric amplitudes are reported for all of them. The photometric variability is smaller than previously thought: the mean amplitude of our sample is 0.1mag and only around 15% of our sample has a larger variability than 0.15mag. The smaller variability than previously thought seems to be a bias of previous observations. We find a very weak trend of faster spinning objects towards smaller sizes, which appears to be consistent with the fact that the smaller objects are more collisionally evolved, but could also be a specific feature of the Centaurs, the smallest objects in our sample. We also find that the smaller the objects, the larger their amplitude, which is also consistent with the idea that small objects are more collisionally evolved and thus more deformed. Average rotation rates from our work are 7.5h for the whole sample, 7.6h for the TNOs alone and 7.3h for the Centaurs. All of them appear to be somewhat faster than what one can derive from a compilation of the scientific literature and our own results. Maxwellian fits to the rotation rate distribution give mean values of 7.5h (for the whole sample) and 7.3h (for the TNOs only). Assuming hydrostatic equilibrium we can determine densities from our sample under the additional assumption that the lightcurves are dominated by shape effects, which is likely not realistic. The resulting average density is 0.92g/cm^3 which is not far from the density constraint that one can derive from the apparent spin barrier that we observe.Comment: Accepted for publication in A&

Secondary gas in debris discs released following the decay of long-lived radioactive nuclides, catastrophic or resurfacing collisions

Kuiper-like belts of planetesimals orbiting stars other than the Sun are most commonly detected from the thermal emission of small dust produced in collisions. Emission from gas, most notably CO, highlights the cometary nature of these planetesimals. Here we present models for the release of gas from comet-like bodies in these belts, both due to their thermophysical evolution, most notably the decay of long-lived radioactive nuclides and collisional evolution, including catastrophic and gentler resurfacing collisions. We show that the rate of gas release is not proportional to the rate of dust release, if non-catastrophic collisions or thermal evolution dominate the release of CO gas. In this case, care must be taken when inferring the composition of comets. Non-catastrophic collisions dominate the gas production at earlier times than catastrophic collisions, depending on the properties of the planetesimal belt. We highlight the importance of the thermal evolution of comets, including crucially the decay of long-lived radioactive nuclides, as a source of CO gas around young (<50Myr) planetary systems, if large (10-100s kms) planetesimals are present.Comment: Submitted to MNRAS, 16 page