Antonella M. Barucci

TNOs are Cool: A survey of the trans-Neptunian region

Aims. Trans-Neptunian objects (TNOs) are bodies populating the Kuiper belt and they are believed to retain the most pristine and least altered material of the solar system. The Herschel open time key programme entitled “TNOs are Cool: A survey of the trans-Neptunian region” has been awarded 373 h to investigate the albedo, size distribution and thermal properties of TNOs and Centaurs. Here we focus on the brightest targets observed by both the PACS and SPIRE multiband photometers: the dwarf planet Haumea, six TNOs (Huya, Orcus, Quaoar, Salacia, 2002 UX25, and 2002 TC302), and two Centaurs (Chiron and Chariklo).Methods. Flux densities are derived from PACS and SPIRE instruments using optimised data reduction methods. The spectral energy distribution obtained with the Herschel PACS and SPIRE instruments over 6 bands (centred at 70, 100, 160, 250, 350, and 500 μm), with Spitzer-MIPS at 23.7 and 71.4 μm, and with WISE at 11.6 and 22.1 μm in the case of 10199 Chariklo, has been modelled with the NEATM thermal model in order to derive the albedo, diameter, and beaming factor. For the Centaurs Chiron and Chariklo and for the 1000 km sized Orcus and Quaoar, a thermophysical model was also run to better constrain their thermal properties.Results. We derive the size, albedo, and thermal properties, including thermal inertia and surface emissivity, for the 9 TNOs and Centaurs. Several targets show a significant decrease in their spectral emissivity longwards of ~300 μm and especially at 500 μm. Using our size estimations and the mass values available in the literature, we also derive the bulk densities for the binaries Quaoar/Weywot (2.18-0.36+0.43 g/cm3), Orcus/Vanth (1.53-0.13+0.15 g/cm3), and Salacia/Actea (1.29-0.23+0.29 g/cm3). Quaoar’s density is similar to that of the other dwarf planets Pluto and Haumea, and its value implies high contents of refractory materials mixed with ices.

ESO large program on physical studies of Transneptunian Objects and Centaurs: Visible photometry – First results

We present the first results of BVRI photometry of Transneptunian Objects (TNOs) and Centaurs obtained through the ESO Large Program on physical studies of these icy bodies in the outer solar system. In total 28 objects were observed of which 18 are new measurements. Combining our new BVRI photometry with the data summary published by Hainaut & Delsanti (2002) results in a database of 94 objects: 45 Cubewanos, 22 Plutinos, 13 scattered disk objects, 14 Centaurs. The reddening range seems to be similar among the four dynamical classes (− 5t o 55%/l00 nm) and only one outlier (1994 ES2) exists. The spectral gradient distribution of the Cubewanos peaks between 25 to 35%/l00 nm, while for the three other types the maximum seems to fall below 20%/l00 nm. A clustering of red Cubewanos with perihelia beyond ∼41 AU in low eccentricity and low inclination orbit suggests that these objects are less affected by the physical processes that potentially produce neutral colors, i.e. resurfacing by collision and by intrinsic activity. For Cubewanos and scattered disk objects, the range of reddening increases with decreasing perihelion distance and with increasing orbital excitation. A correlation of the spectral slope with inclination is present for Cubewanos and scattered disk objects, and is non-existent for the other dynamical types. It is unclear whether these trends (or their absence) are discriminative for the correctness of the resurfacing scenarios. If intrinsic activity is responsible for resurfacing, the start of the effect inside ∼41 AU from the Sun may be indicative for the driving agent, while in the collision scenario the survival of the red Cubewano cluster in the central region of the Kuiper-Belt argues for the existence of a population of bodies the surface of which is heavily radiation processed without impact resurfacing.

TNOs are Cool: A survey of the trans-Neptunian region III. Thermophysical properties of 90482 Orcus and 136472 Makemake

Context. The goal of the Herschel open time programme “TNOs are Cooll” is to derive the physical and thermal properties for a large sample of Centaurs, and trans-Neptunian objects (TNOs), including resonant, classical, detached and scattered disk objects. Aims. Based on observations of two targets we tried (i) to optimise the SPIRE observing technique for faint (close to the background confusion noise), slowly moving targets; (ii) to test different thermal model techniques; (iii) to determine radiometric diameter and albedo values; (iv) to compare with Spitzer results whenever possible. Methods. We obtained SPIRE photometry on two targets and PACS photometry on one of the targets. Results. We present results for the two targets, (90482) Orcus and (136472) Makemake, observed with SPIRE and for one of those targets, Makemake, observed with PACS. We adopt pv = 0.27 and D = 850 km as our best estimate of the albedo and diameter of Orcus using single terrain models. With two-terrain models for Makemake, the bright terrain is fitted by, 0.78 \textless pv \textless 0.90, and the dark terrain 0.02 \textless pv \textless 0.12, giving 1360 \textless D \textless 1480 km. Conclusions. A single terrain model was derived for Orcus through the SPIRE photometry combined with MIPS data. The Makemake data from MIPS, PACS and SPIRE combined are not compatible with a single terrain model, but can be modelled with a two-terrain fit. These science demonstration observations have shown that the scanning technique, which allows us to judge the influence of background structures, has proved to be a good basis for this key programme.

First observations of H2O and CO2 vapor in comet 67P/Churyumov-Gerasimenko made by VIRTIS onboard Rosetta

Context. Outgassing from cometary nuclei involves complex surface and subsurface processes that need to be understood to investigate the composition of cometary ices from coma observations. Aims. We investigate the production of water, carbon dioxide, and carbon monoxide from the nucleus of comet 67P/Churyumov-Gerasimenko (67P). These species have different volatility and are key species of cometary ices. Methods. Using the high spectral-resolution channel of the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H), we observed the ν3 vibrational bands of H2O and CO2 at 2.67 and 4.27 μm, respectively, from 24 November 2014 to 24 January 2015, when comet 67P was between 2.91 and 2.47 AU from the Sun. Observations were undertaken in limb-viewing geometry at distances from the surface of 0 to 1.5 km and with various line-of-sight (LOS) orientations in the body-fixed frame. A geometry tool was used to characterize the position of the LOS with respect to geomorphologic regions and the illumination properties of these regions. Results. The water production of 67P did not increase much from 2.9 to 2.5 AU. High water column densities are observed for LOS above the neck regions, suggesting they are the most productive in water vapor. While water production is weak in regions with low solar illumination, CO2 is outgassing from both illuminated and non-illuminated regions, which indicates that CO2 sublimates at a depth that is below the diurnal skin depth. The CO2/H2O column density ratio varies from 2 to 60%. For regions that are in sunlight, mean values between 2 and 7% are measured. The lower bound value is likely representative of the CO2/H2O production rate ratio from the neck regions. For carbon monoxide, we derive column density ratios CO/H2O < 1.9% and CO/CO2< 80%. An illumination-driven model, with a uniformly active surface releasing water at a mean rate of 8 × 1025 s-1, provides an overall agreement to VIRTIS-H data, although some mismatches show local surface inhomogeneities in water production. Rotational temperatures of 90–100 K are derived from H2O and CO2 averaged spectra.

Physical properties of OSIRIS-REx target asteroid (101955) 1999 RQ36 - Derived from Herschel, VLT/ VISIR, and Spitzer observations

In September 2011, the Herschel Space Observatory performe d an observation campaign with the PACS photometer observin g the asteroid (101955) 1999 RQ 36 in the far infrared. The Herschel observations were analyse d, together with ESO VLT-VISIR and Spitzer-IRS data, by means of a thermophysical model in orde r to derive the physical properties of 1999 RQ 36. We find the asteroid has an e ffective diameter in the range 480 to 511 m, a slightly elongate d shape with a semi-major axis ratio of a /b=1.04, a geometric albedo of 0.045 +0.015 −0.012, and a retrograde rotation with a spin vector between -70 and -90 ◦ ecliptic latitude. The thermal emission at wavelengths below 12 μm -originating in the hot sub-solar regionshows that there may be large variations in roughness on the surface along the equatorial zone of 1999 RQ 36, but further measurements are required for final proof. We de t rmine that the asteroid has a disk-averaged thermal inertia of Γ=650 Jm−2s−0.5K−1 with a 3-σ confidence range of 350 to 950 Jm −2s−0.5K−1, equivalent to what is observed for 25143 Itokawa and suggestive that 1999 RQ 36 has a similar surface texture and may also be a rubble-pile in nature. The low albedo indicates that 1999 RQ 36 very likely contains primitive volatile-rich material, co nsistent with its spectral type, and that it is an ideal target for the OSIRIS-REx sample return mi ssion.

Spitzer observations of spacecraft target 162173 (1999 JU3)

Context. Near-Earth asteroid 162173 (1999 JU3) is the primary target of the Japanese Aerospace Exploration Agency (JAXA) Hayabusa-2 sample return mission, and is also on the list of potential targets for the European Space Agency (ESA) Marco Polo sample return mission. Earth-based studies of this object are fundamental to these missions. Aims. Our aim is to provide new constraints on the surface properties of this asteroid. Methods. We present a mid-infrared spectrum (5–38 μm) obtained with NASA’s Spitzer Space Telescope in May 2008 and results from the application of thermal models. Results. These observations place new constraints on the surface properties of this asteroid. To fit our spectrum we used the near-Earth asteroid thermal model (NEATM) and the more complex thermophysical model (TPM). However, the position of the spin-pole, which is uncertain, is a crucial input parameter for constraining the thermal inertia with the TPM; hence, we consider two pole orientations. First is the extreme case of an equatorial retrograde geometry from which we derive a rigorous lower limit to the thermal inertia of 150 Jm −2 s −0.5 K −1 . Second, when we adopt the pole orientation of Abe et al. (2008a, 37 th COSPAR Scientific Assembly) our best-fit thermal model yields a value for the thermal inertia of 700 ± 200 Jm −2 s −0.5 K −1 and even higher values are allowed by the uncertainty in the spectral shape due to the absolute flux calibration. Our best estimates of the diameter (0.90 ± 0.14 km) and geometric albedo (0.07 ± 0.01) of asteroid 162173 are consistent with values based on previous mid-infrared observations. Conclusions. We establish a rigorous lower limit to the thermal inertia, which is unlikely but possible, and would be consistent with a fine regolith similar to wthat is found for asteroid 433 Eros. However, the thermal inertia is expected to be higher, possibly similar to or greater than that on asteroid 25143 Itokawa. An Accurately determining the spin-pole of asteroid 162173 will narrow the range of possible values for its thermal inertia.

Variegation of comet 67P/Churyumov-Gerasimenko in regions showing activity

We carried out an investigation of the surface variegation of comet 67P/Churyumov-Gerasimenko, the detection of regions showing activity, the determination of active and inactive surface regions of the comet with spectral methods, and the detection of fallback material. Methods. We analyzed multispectral data generated with Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) narrow angle camera (NAC) observations via spectral techniques, reflectance ratios, and spectral slopes in order to study active regions. We applied clustering analysis to the results of the reflectance ratios, and introduced the new technique of activity thresholds to detect areas potentially enriched in volatiles. Results. Local color inhomogeneities are detected over the investigated surface regions. Active regions, such as Hapi, the active pits of Seth and Ma’at, the clustered and isolated bright features in Imhotep, the alcoves in Seth and Ma’at, and the large alcove in Anuket, have bluer spectra than the overall surface. The spectra generated with OSIRIS NAC observations are dominated by cometary emissions of around 700 nm to 750 nm as a result of the coma between the comet’s surface and the camera. One of the two isolated bright features in the Imhotep region displays an absorption band of around 700 nm, which probably indicates the existence of hydrated silicates. An absorption band with a center between 800–900 nm is tentatively observed in some regions of the nucleus surface. This absorption band can be explained by the crystal field absorption of Fe2+, which is a common spectral feature seen in silicates.

Near-Infrared Color Properties of Kuiper Belt Objects and Centaurs: Final Results from the ESO Large Program

We present near-IR JHK broadband photometry for 17 Kuiper Belt objects (KBOs) and Centaurs. The observations were performed within the ESO Large Program on the Physical Properties of Kuiper Belt Objects and Centaurs from 2001 January to 2002 August. We used the ISAAC instrument at the ESO 8 m Very Large Telescope. We compiled visible-near-IR colors for a total of 51 published objects and performed a statistical analysis. Color-color correlations show that the same coloring process is probably acting on Centaur and KBO surfaces in the visible-near-IR range. Centaurs with H - K smaller than the Sun (0.06) systematically display the reddest B - V colors (at the 2.5 σ level). These Centaur surfaces are suspected of harboring material that has spectral signatures around 1.7-2.2 μm (water ice is a possibility; it was reported for the three objects that have published spectroscopy). We report no statistically significant evidence for a bimodal structure of the VJHK Centaur colors (Kolmogorov-Smirnov and dip tests on up to 17 objects). The Centaur H - K colors show some robust evidence (significance level >99.99%) for a continuous structure. We also report a statistically significant bimodal structure of the Centaur B - R distribution, which is compatible with the results published by Peixinho et al. in 2003 with different data. Classical KBOs show no trends at the 3 σ level. The V - J color is marginally correlated with perihelion distance q (which is consistent with results reported by Doressoundiram et al. in 2005 on B - R colors). Resonant and scattered disk objects are underrepresented (seven and nine objects, respectively) and show no statistically significant trend. Some of the marginal trends are mentioned as worthy of subsequent monitoring.

Opposition Effect of Kuiper Belt Objects: Preliminary Estimations

Information on the surface structure of the Kuiper Belt objects can be obtained from studies of their opposition brightening. Although KBOs are observed at a very limited phase angle range they represent a unique opportunity to study the backscattering phenomenon down almost to zero phase angle. Preliminary estimations of the opposition effect amplitude and width based on composite phase curves of four KBOs and two Centaurs showed the existence of a very narrow opposition surge of about 0.1–0.2 mag at phase angles less than 0.1–0.2 deg. It may indicate a high porosity of the KBOs regoliths. Further observations are needed to confirm this phenomenon.

Thermal properties of (4) Vesta derived from Herschel measurements

Aims. We report in this paper the results provided by new infrared observations of the asteroid (4) Vesta obtained between April 2010 and June 2010 with the Herschel space observatory (ESA) in the wavelength range 70–500 μm when the asteroid was seen from the Northern hemisphere. Over this period, the thermal flux of Vesta was observed at different rotational phases and a large fraction of the surface was visible. Methods. A thermophysical model was used to derive thermal properties such as infrared emissivity, thermal inertia and surface roughness. Results. We found an average thermal inertia Γ= 20 +20 −10 Jm −2 K −1 s −1/2 . The surface roughness on the Northern hemisphere, while poorly constrained, appears to be low on Vesta with a mean slope angle ¯ θ ∼ 23 ◦ . Surface temperatures range from 40 K to 248 K. We cannot confirm any longitudinal variation of the thermal flux as it remains within the error bars. More observations are needed to confirm if the eastern hemisphere could have either a slightly higher thermal inertia suggesting a rockier surface or a smooth surface with a higher beaming factor. We emphasize an important decrease of the infrared emissivity of Vesta with the wavelength, ranging from 0.9 at 70 μm to 0.70 at 500 μm.