Context. The Herschel open time key programme TNOs are Cool: A survey of the trans-Neptunian region aims to derive physical and thermal properties for a set of similar to 140 Centaurs and ...trans-Neptunian objects (TNOs), including resonant, classical, detached and scattered disk objects. One goal of the project is to determine albedo and size distributions for specific classes and the overall population of TNOs. Aims. We present Herschel-PACS photometry of 18 Plutinos and determine sizes and albedos for these objects using thermal modeling. We analyze our results for correlations, draw conclusions on the Plutino size distribution, and compare to earlier results. Methods. Flux densities are derived from PACS mini scan-maps using specialized data reduction and photometry methods. In order to improve the quality of our results, we combine our PACS data with existing Spitzer MIPS data where possible, and refine existing absolute magnitudes for the targets. The physical characterization of our sample is done using a thermal model. Uncertainties of the physical parameters are derived using customized Monte Carlo methods. The correlation analysis is performed using a bootstrap Spearman rank analysis. Results. We find the sizes of our Plutinos to range from 150 to 730 km and geometric albedos to vary between 0.04 and 0.28. The average albedo of the sample is 0.08 +/- 0.03, which is comparable to the mean albedo of Centaurs, Jupiter family comets and other TNOs. We were able to calibrate the Plutino size scale for the first time and find the cumulative Plutino size distribution to be best fit using a cumulative power law with q = 2 at sizes ranging from 120-400 km and q = 3 at larger sizes. We revise the bulk density of 1999 TC36 and find (sic) = 0.64(-0.11)(+0.15) g cm(-3). On the basis of a modified Spearman rank analysis technique our Plutino sample appears to be biased with respect to object size but unbiased with respect to albedo. Furthermore, we find biases based on geometrical aspects and color in our sample. There is qualitative evidence that icy Plutinos have higher albedos than the average of the sample.
•We use Cassini/CIRS spectra to infer the latitudinal distribution of CH4 in Titan’s stratosphere.•Large variations of the methane mole fraction are found, ranging from 1.0% to 1.5%.•They may result ...from convective events at latitudes of preferential cloud occurrence.
Cassini/CIRS spectra in the far- and mid-infrared region are used to determine the abundance of methane in Titan’s lower stratosphere and investigate its distribution with latitude. The CIRS spectra include emission from both the CH4 ν4 band at 7.7μm and pure rotational lines longwards of 50μm, which show differential sensitivities to thermal profile and methane mole fraction. We analyze nadir and limb data taken over the first part of the Cassini mission (August 2005 to June 2010), including a selection of 12 latitudes that provides a reasonably complete and regular sampling of both hemispheres. Unexpectedly, but in a consistent manner for limb and nadir geometries, large variations of the methane mole fraction near 15mbar (∼85km) are found, with values ranging from ∼1.0% (at low latitudes and near ±50–55°) to ∼1.5% (at ±30–35° and polar latitudes). Error bars on the retrieved methane mole fraction are 0.07–0.12% at low latitudes in the Southern hemisphere and 0.14–0.21% northward of 40°N. A 1.0% methane mole fraction at low latitudes permits us to reconcile the HASI-measured temperatures below 147km altitude (2.7mbar) with inferences from CIRS. The roughly hemispherically-symmetric distribution of methane gas is reminiscent of that observed or predicted for the tropospheric methane clouds, which on a yearly-averaged basis, show preferential occurrences at tropical and polar latitudes. We speculate that convective events at these latitudes result into local stratospheric methane enrichment, which may persist year-round due to dynamical mixing times in the lower stratosphere only moderately shorter than a Titan year.
In this letter we explore the environment of Pluto and Charon in the far infrared with the main aim of identifying the signs of any possible dust ring, should it exist in the system. Our study is ...based on observations performed at 70 μm with the PACS instrument onboard the Herschel Space Observatory at nine epochs between March 14 and 19, 2012. The far-infrared images of the Pluto–Charon system are compared to those of the point spread function (PSF) reference quasar 3C 454.3. The deviation between the observed Pluto–Charon and reference PSFs are less then 1σ, indicating that clear evidence for an extended dust ring around the system was not found. Our method is capable of detecting a hypothetical ring with a total flux of ~3.3 mJy at a distance of ~153 000 km (~8.2 Pluto–Charon distances) from the system’s barycentre. We place upper limits on the total disk mass and on the column density in a reasonable disk configuration and analyse the hazard during the flyby of NASA’s New Horizons in July 2015. This realistic model configuration predicts a column density of 8.7 × 10-10 g cm-2 along the path of the probe and an impactor mass of 8.7 × 10-5 g.
We report on the first identification of hydrogen isocyanide (HNC) in Titan’s atmosphere, from observations using the HIFI instrument on the Herschel⋆ Space Observatory. An emission line from the HNC ...J = 6 → 5 rotational transition at 543.897 GHz was measured in Titan on June 14 and December 31, 2010. Radiative transfer modeling indicates that the bulk of HNC is located above 400 km, with a column density in the range (0.6−1.5) × 1013 cm-2, but the observations cannot establish its vertical profile. In particular HNC could be restricted to the upper thermosphere (~1000 km), in which case its local abundance relative to HCN could be as high as ~0.3. HNC is probably formed mostly at ionospheric levels (950–1150 km) from dissociative recombination of HCNH+ and possibly other heavier nitrile ions. Ionospheric loss of HNC occurs by protonation with XH+ ions. Additional formation (e.g. from N(4S) +3CH2) and loss routes (e.g. from isomerization to HCN) in the neutral atmosphere remain to be investigated.
We present the seasonal and geographical variations of the martian water vapor monitored from the Planetary Fourier Spectrometer Long Wavelength Channel aboard the Mars Express spacecraft. Our ...dataset covers one martian year (end of Mars Year 26, Mars Year 27), but the seasonal coverage is far from complete. The seasonal and latitudinal behavior of the water vapor is globally consistent with previous datasets, Viking Orbiter Mars Atmospheric Water Detectors (MAWD) and Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES), and with simultaneous results obtained from other Mars Express instruments, OMEGA and SPICAM. However, our absolute water columns are lower and higher by a factor of 1.5 than the values obtained by TES and SPICAM, respectively. In particular, we retrieve a Northern midsummer maximum of 60 pr-μm, lower than the 100-pr-μm observed by TES. The geographical distribution of water exhibits two local maxima at low latitudes, located over Tharsis and Arabia. Global Climate Model (GCM) simulations suggest that these local enhancements are controlled by atmospheric dynamics. During Northern spring, we observe a bulge of water vapor over the seasonal polar cap edge, consistent with the northward transport of water from the retreating seasonal cap to the permanent polar cap. In terms of vertical distribution, we find that the water volume mixing ratio over the large volcanos remains constant with the surface altitude within a factor of two. However, on the whole dataset we find that the water column, normalized to a fixed pressure, is anti-correlated with the surface pressure, indicating a vertical distribution intermediate between control by atmospheric saturation and confinement to a surface layer. This anti-correlation is not reproduced by GCM simulations of the water cycle, which do not include exchange between atmospheric and subsurface water. This situation suggests a possible role for regolith–atmosphere exchange in the martian water cycle.
The European Space Agency Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko, carries a relatively small and lightweight millimeter-submillimeter spectrometer ...instrument, the first of its kind launched into deep space. The instrument will be used to study the evolution of outgassing water and other molecules from the target comet as a function of heliocentric distance. During flybys of the asteroids (2867) Steins and (21) Lutetia in 2008 and 2010 respectively, the instrument will measure thermal emission and search for water vapor in the vicinity of these asteroids.The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients in Comet 67P/Churyumov-Gerasimenko and the asteroids (2867) Steins and (21) Lutetia. A 4096 channel CTS (Chirp Transform Spectrometer) spectrometer having 180 MHz total bandwidth and 44 kHz resolution is, in addition to the continuum channel, connected to the submillimeter receiver. The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species – CO, CH3OH, NH3 and three, oxygen-related isotopologues of water, H216O, H217O and H218O. The basic quantities measured with the MIRO instrument are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This paper provides a short discussion of the scientific objectives of the investigation, and a detailed discussion of the MIRO instrument system.
Aims. The goal is to determine the composition of Pluto’s atmosphere and to constrain the nature of surface-atmosphere interactions. Methods. We perform high-resolution spectroscopic observations in ...the 2.33–2.36 μm range, using CRIRES at the VLT. Results. We obtain (i) the first detection of gaseous methane in this spectral range, through lines of the ν3 + ν4 and ν1 + ν4 bands (ii) strong evidence (6-σ confidence) for gaseous CO in Pluto. For an isothermal atmosphere at 90 K, the CH4 and CO column densities are 0.75 and 0.07 cm-am, within factors of 2 and 3, respectively. Using a physically-based thermal structure model of Pluto’s atmosphere also satisfying constraints from stellar occultations, we infer CH4 and CO mixing ratios \hbox{$q_{\rm CH_4}= 0.6^{+0.6}_{-0.3}$}qCH4=0.6-0.3+0.6% (consistent with results from the 1.66 μm range) and \hbox{$q_{\rm CO} = 0.5^{+1}_{-0.25}\,\times\,10^{-3}$}qCO=0.5-0.25+1 × 10-3. The CO atmospheric abundance is consistent with its surface abundance. As for Triton, it is probably controlled by a thin, CO-rich, detailed balancing layer resulting from seasonal transport and/or atmospheric escape.
Context. Comet 17P/Holmes underwent a dramatic outburst in October 2007, caused by the sudden fragmentation of its nucleus and the production of a large quantity of grains scattering sunlight. Aims. ...We report on 90 GHz continuum observations carried out with the IRAM Plateau de Bure interferometer on 27.1 and 28.2 October 2007 UT, i.e., 4−5 days after the outburst. These observations probed the thermal radiation of large dust particles, and therefore provide the best constraints on the mass in the ejecta debris. Methods. The thermal emission of the debris was modelled and coupled to a time-dependent description of their expansion after the outburst. The analysis was performed in the Fourier plane. Visibilities were computed for the two observing dates and compared to the data to measure their velocity and mass. Optical data and 250-GHz continuum measurements published in the literature were used to further constrain the dust kinematics and size distribution. Results. Two distinct dust components in terms of kinematic properties are identified in the data. The large-velocity component, with typical velocities V0 of 50−100 m s-1 for 1 mm particles, displays a steep size distribution with a size index estimated to q = −3.7 (±0.1), assuming a minimum grain size of 0.1 μm. It corresponds to the fast expanding shell observed in optical images. The slowly-moving “core” component (V0 = 7−9 m s-1) detected near the nucleus has a size index |q| < 3.4 and contains a higher proportion of large particles than the shell. The dust mass in the core is in the range 0.1−1 that of the shell. Using optical constants pertaining to porous grains (50% porosity) made of astronomical silicates mixed with water ice (48% in mass), the total dust mass Mdust injected by the outburst is estimated to 4−14 × 1011 kg, corresponding to 3−9% the nucleus mass.
•The first detection of Io’s SO2 atmosphere at 4.0μm is reported, providing a new window for Io’s atmospheric studies.•Our spatially-resolved data permit us a detailed characterization of Io’s ...atmosphere.•Longitudinal, diurnal, and latitudinal variations are observed.•A mean gas temperature of ∼170K is determined.
We report on high-resolution and spatially-resolved spectra of Io in the 4.0μm region, recorded with the VLT/CRIRES instrument in 2008 and 2010, which provide the first detection of the ν1+ν3 band of SO2 in Io’s atmosphere. Data are analyzed to constrain the latitudinal, longitudinal, and diurnal distribution of Io’s SO2 atmosphere as well as its characteristic temperature. Equatorial SO2 column densities clearly show longitudinal asymmetry, but with a maximum of ∼1.5×1017cm−2 at central meridian longitude L=200–220 and a minimum of ∼3×1016cm−2 at L=285–300, the longitudinal pattern somewhat differs from earlier inferences from Ly α and thermal IR measurements. Within the accuracy of the measurements, no evolution of the atmospheric density from mid-2008 to mid-2010 can be distinguished. The decrease of the SO2 column density towards high latitudes is apparent, and the typical latitudinal extent of the atmosphere found to be ±40° at half-maximum. The data show moderate diurnal variations of the equatorial atmosphere, which is evidence for a partially sublimation-supported atmospheric component. Compared to local noon, factor of 2 lower densities are observed ∼40° before and ∼80° after noon. Best-fit gas temperatures range from 150 to 220K, with a weighted mean value of 170±20K, which should represent the column-weighted mean kinetic temperature of Io’s atmosphere. Finally, although the data include clear thermal emission due to Pillan (in outburst in July 2008) and Loki, no detectable enhancements in the SO2 atmosphere above these volcanic regions are found, with an upper limit of 4×1016cm−2 at Pillan and 1×1017cm−2 at Loki.
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