Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere

High-resolution spectra of Pluto in the 1.66 um region, recorded with the VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to constrain the spatial and vertical distribution of methane in Pluto's atmosphere and to search for mid-term (4 year) variability. A sensitivity study to model assumptions (temperature structure, surface pressure, Pluto's radius) is performed. Results indicate that (i) no variation of the CH4 atmospheric content (column-density or mixing ratio) with Pluto rotational phase is present in excess of 20 % (ii) CH4 column densities show at most marginal variations between 2008 and 2012, with a best guess estimate of a ~20 % decrease over this time frame. As stellar occultations indicate that Pluto's surface pressure has continued to increase over this period, this implies a concomitant decrease of the methane mixing ratio (iii) the data do not show evidence for an altitude-varying methane distribution; in particular, they imply a roughly uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high concentrations of low-temperature methane near the surface can be ruled out. Our results are also best consistent with a relatively large (> 1180 km) Pluto radius. Comparison with predictions from a recently developed global climate model GCM indicates that these features are best explained if the source of methane occurs in regional-scale CH4 ice deposits, including both low latitudes and high Northern latitudes, evidence for which is present from the rotational and secular evolution of the near-IR features due to CH4 ice. Our "best guess" predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17 ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible longitudinal variations.Comment: 21 pages, 6 figure

Comparison of five dose calculation algorithms in a heterogeneous media using design of experiment

Purpose Design of experiments (DoE) provides a methodology to reveal the influence of input values on the measured output with a limited number of trials. The purpose of this study was to describe how DoE can be used to evaluate the performances of several dose calculation systems in heterogeneous media, including algorithms like Pencil Beam (PB), Anisotropic Analytical Algorithm (AAA), Acuros XB (AXB), Monte Carlo (MC) and Collapsed Cone Volume (CCV). Method This study was carried out using a CIRS Model 002LFC IMRT Thorax Phantom customized with a water-equivalent heterogeneity inside the lung. The calculated dose distributions were compared to Gafchromic® EBT3 film measurements. The beam configurations were selected using DoE to study the influence of five parameters simultaneously (energy, collimator angulation, gantry angulation, X and Y jaws) and to optimize the number of experiments. An analysis of variance was performed over the entire irradiation field and over various regions of interest (tumour, shadow of tumour and lungs). Results DoE enabled to quantify and determine the statistically significant factors, leading to an evaluation of the dose calculation systems in the lung case. The resulting scoring could be as follow (from best to worst): AXB_Dm, CCV, AXB_Dw, XVMC_Dm, XVMC_Dw, AAA and last PB. Differences between the algorithms were specially observed in the tumour and the shadow regions. Conclusion DoE is a robust statistical method to compare several dose calculation systems. The various analyses lead to the conclusion that AXB handled more accurately most of the situations investigated in heterogeneous media

32 Comparison of the dose calculation algorithms in a heterogeneous media using experimental design

International audienceIntroductionThis study compares the performances of different dose calculation algorithms in heterogeneous medium. Then, the properties of two algorithms on clinical cases are considered for the thoracic cases in conformal 3D radiotherapy (RC3D).Methods(1) A study on the CIRS phantom was performed by comparing the dose distributions measured with Gafchromic EBT3® with those calculated by the AAA [Analytical Anisotropic Algorithm v13, Varian], AXB [Acuros XB v13.7, Varian], PB [Pencil Beam v4.5.5, Brainlab], MC [Monte Carlo v4.5.5, Brainlab] and CCV [Collapsed Cone Volume v2.0.1, Mobius Medical System]. The performances of these different algorithms were tested using an experimental design of Taguchi Math Eq to demonstrate the influence of the following factors, in 18 tests: energy, collimator angle, gantry angle, X and Y field size.A comparison by subtracting the dose distributions between those measured and computed was achieved for the 18 tests. (2) The impact of the selection of the algorithm was performed on treatment plans initially calculated with AAA, on a set of 15 patients with thoracic cancer in RC3D. By keeping the same UM number, these treatment plans were compared in terms of coverage of the 95% PTV, a conformity index, doses to the PTV and organs at risk. If necessary, the planimetry was modified in AXB (weighting, filtering and normalization point) in order to validate the treatment plan clinically.Results(1) In heterogeneous medium, the average dose differences between the measured and calculated doses are, respectively, for the AXB, AAA, CCV, MC and PB: 0.01.8%, 1.22.2%, −0.12.0%, 1.21.9% and 7.04.1%. (2) For treatment plans with similar MU numbers, an average dose difference of −1.2% in the level of the 95% coverage of the PTV and a modification of conformity index from 1.10 (AAA) to 0.76 (AXB) are obtained, attesting to a lower coverage of the PTV with AXB, related to a difference in the evaluation of heterogeneities. Once these treatment plans were modified, the resulting planimetry was similar to the clinically validated one, with a mean difference of MU greater than 1.5% against treatment plans calculated with AAA.ConclusionsThe best performances are shown with the MC and the AXB algorithms. The experimental design allows reliable comparison of the algorithms with 18 tests. The PEX highlights bigger deviations for small field modelling. Regarding the dosimetry study, AXB can be used for RC3D thoracic cancer treatment. A complementary study for the same localization is ongoing for the VMAT treatment technique

Determination of MC-based predictive models for personalized and fast kV-CBCT organ dose estimation

International audiencePurpose or Objective: Monte Carlo (MC) simulations were shown to be a powerful tool to calculate accurately 3D dose distributions of kV-CBCT scans for a patient, based on planning CT images. However, this methodology is still heavy and time consuming, preventing its large use in clinical routine. This study hence explores a method to derive empirical functions relating organ doses to patient morphological parameters, in order to perform a fast and personalized estimation of doses delivered to critical organs by kV-CBCT scans used in IGRT protocols.Material and Methods: Doses to critical organs were first computed using a PENELOPE-based MC code previously validated [H. Chesneau et al., ESTRO 2016], for a set of fifty clinical cases (40 children and 10 adults) covering a broad range of anatomical localizations (head-and-neck, pelvis, thorax, abdomen) and scanning conditions for the Elekta XVI CBCT. Planning CT images were converted into voxellized patient geometries, using a dedicated tissue segmentation procedure: 5 to 7 biological tissues were assigned for soft tissues, whereas ten different bone tissues were required for accurate dosimetry in the kV energy range. Correlations between calculated mean organ doses and several morphological parameters (age, weight, height, BMI, thorax and hip circumference …) were then studied for each anatomical localization to derive appropriate empirical fitting functions.Results: As expected, results on the paediatric cohort show dose variations highly correlated with the patient morphology, varying in the range 3:1 between a 17-y old teenager and a 2-y old baby, for the same CBCT scan. Except for the head-and-neck localization, for which the mean organ doses show no significant variations with the morphology, doses to all major organs at risk can be predicted using linear or exponential functions for thorax, pelvis and abdomen scans. The use of morphological parameters directly measured on the planning CT allows to reach better correlations than global parameters such as BMI, because they represent most relevant indicators of the patient morphology at the scan time.Conclusion: This study demonstrates that it is possible to derive mathematical models predicting the doses delivered to major critical organs by kV-CBCT scans according to morphological parameters. This method allows a fast and personalized estimation of imaging doses usable in clinicalroutine

A post-New Horizons Global climate model of Pluto including the N ₂, CH ₄ and CO cycles

We have built a new 3D Global Climate Model (GCM) to simulate Pluto as observed by New Horizons in 2015. All key processes are parametrized on the basis of theoretical equations, including atmospheric dynamics and transport , turbulence, radiative transfer, molecular conduction, as well as phases changes for N 2 , CH 2 and CO. Pluto's climate and ice cycles are found to be very sensitive to model parameters and initial states. Nevertheless, a reference simulation is designed by running a fast, reduced version of the GCM with simplified atmospheric transport for 40,000 Earth years to initialize the surface ice distribution and sub-surface temperatures, from which a 28-Earth-year full GCM simulation is performed. Assuming a topographic depression in a Sputnik-planum (SP)-like crater on the anti-Charon hemisphere, a realistic Pluto is obtained, with most N 2 and CO ices accumulated in the crater, methane frost covering both hemispheres except for the equatorial regions, and a surface pressure near 1.1 Pa in 2015 with an increase between 1988 and 2015, as reported from stellar occultations. Temperature profiles are in qualitative agreement with the observations. In particular, a cold atmospheric layer is obtained in the lowest kilometers above Sputnik Planum, as observed by New Horizons's REX experiment. It is shown to result from the combined effect of the topo-graphic depression and N 2 daytime sublimation. In the reference simulation with surface N 2 ice exclusively present in Sputnik Planum, the global circulation is only forced by radiative heating gradients and remains relatively weak. Surface winds are locally induced by topography slopes and by N 2 condensation and sublimation around Sputnik Planum. However, the circulation can be more intense depending on the exact distribution of surface N 2 frost. This is illustrated in an alternative simulation with N 2 condensing in the South Polar regions and N 2 frost covering latitudes between 35 • N and 48 • N. A global condensation flow is then created, inducing strong surface winds everywhere, a prograde jet in the southern high latitudes, and an equatorial superrotation likely forced by barotropic instabilities in the southern jet. Using realistic parameters, the GCM predict atmospheric concentrations of CO and CH 4 in good agreement with the observations. N 2 and CO do not condense in the atmosphere, but CH 4 ice clouds can form during daytime at low altitude near the regions covered by N 2 ice (assuming that nucleation is efficient enough). This global climate model can be used to study many aspects of the Pluto environment. For instance, organic hazes are included in the GCM and analysed in a companion paper (Bertrand and Forget, Icarus, this issue)

Volatile transport modeling on Triton with new observational constraints

International audienceNeptune's moon Triton shares many similarities with Pluto, including volatile cycles of N2, CH4 and CO, and represents a benchmark case for the study of surface-atmosphere interactions on volatile-rich Kuiper Belt objects. The observations of Pluto by New Horizons acquired during the 2015 flyby and their analysis with volatile transport models (VTMs) shed light on how volatile sublimation-condensation cycles control the climate and shape the surface of such objects. Within the context of New Horizons observations as well as recent Earth-based observations of Triton, we adapt a Plutonian VTM to Triton, and test its ability to simulate its volatile cycles, thereby aiding our understanding of its climate. Here we present numerical VTM simulations exploring the volatile cycles of N2, CH4 and CO on Triton over long-term and seasonal timescales (cap extent, surface temperatures, surface pressure, sublimation rates) for varying model parameters (including the surface ice reservoir, albedo, thermal inertia, and the internal heat flux). We explore what scenarios and model parameters allow for a best match of the available observations. In particular, our set of observational constraints include Voyager 2 observations (surface pressure and cap extent), ground-based near-infrared (0.8-2.4 μm) disk-integrated spectra (the relative surface area of volatile vs. non-volatile ice) and the evolution of surface pressure as retrieved from stellar occultations. Our results show that Triton's poles act as cold traps for volatile ices and favor the formation of polar caps extending to lower latitudes through glacial flow or through the formation of thinner seasonal deposits. As previously evidenced by other VTMs, North-South asymmetries in surface properties can favor the development of one cap over the other. Our best-case simulations are obtained for a bedrock surface albedo of 0.6-0.7, a global reservoir of N2 ice thicker than 200 m, and a bedrock thermal inertia larger than 500 SI (or smaller but with a large internal heat flux). The large N2 ice reservoir implies a permanent N2 southern cap (several 100 m thick) extending to the equatorial regions with higher amounts of volatile ice at the south pole, which is not inconsistent with Voyager 2 images but does not fit well with observed full-disk near-infrared spectra. Our results also suggest that a small permanent polar cap exists in the northern (currently winter) hemisphere if the internal heat flux remains relatively low (e.g. radiogenic, -2). A non-permanent northern polar cap was only obtained in some of our simulations with high internal heat flux (30 mW m-2). The northern cap will possibly extend to 30°N in the next decade, thus becoming visible by Earth-based telescopes. On the basis of our model results, we also discuss the composition of several surface units seen by Voyager 2 in 1989, including the bright equatorial fringe and dark surface patches. Finally, we provide predictions for the evolution of ice distribution, surface pressure and CO and CH4 atmospheric mixing ratios in the next decades. According to our model, the surface pressure should slowly decrease but remain larger than 0.5 Pa by 2060. We also model the thermal lightcurves of Triton for different climate scenarios in 2022, which serve as predictions for future James Webb Space Telescope observations

O-2-O-2 and O-2-N-2 collision-induced absorption mechanisms unravelled

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