Thermophysical Characterization of Potential Spacecraft Target (101955) 1999 RQ36

We report on thermal emission measurements of 1999 RQ36 from Spitzer. The derived size is in agreement with radar measurements, and we find a moderately high thermal inertia and homogeneous surface properties

Near-Infrared Rotational Variability in Comet-Asteroid Transition Object 944 Hidalgo

Dynamical arguments indicate that 944 Hidalgo is most likely an extinct or dormant comet. Hidalgo\u27s Tisserand invariant (T = 2.07) suggests strongly that this object came either from the Kuiper belt or the Oort cloud (e.g., Weissman et al. 2002). We obtained low-resolution near-infrared spectra in the 0.8-2.4 micron region on UT Oct 22, 23, Nov 19 and Dec 11, 2004, using the SpeX instrument on NASA\u27s Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii (Oct. and Nov.) and the NICS instrument on the Telescopio Nazionale Galileo (TNG) on La Palma, Spain (Dec.). Our reflectance spectra show a range of slopes. To characterize these slope differences, we normalized each spectrum to 1.0 reflectance at 1.25 microns and measured the reflectance at 2.2 microns. These values are listed in the table for the seven spectra obtained on the two dates when we have temporal coverage, Oct 22 and 23. The uncertainty in each reflectance value is ± 3%. Hidalgo\u27s rotational light curve has a period of 10.06 hours and amplitudes ranging from 0.31 to 0.6 magnitudes in the visible (Harris and Warner 2006, Minor Planet Center). We define the time of our first observation on Oct. 22 as zero rotational phase and give the other six phases in the table. The table shows a systematic temporal variation of the spectral slope consistent with the rotational period. Based on an unpublished visible light curve obtained 10 days earlier (C. Hergenrother personal communication) we determine that one of the small ends of Hidalgo corresponds to our reddest” spectrum (phase 0.36) while one of the broad sides has the flattest spectrum (phase 0.77)

Rotationally Resolved Spitzer Spectra of Comet-Asteroid Transition Object 944 Hidalgo

Last year (Campins et al. 2006), we reported near-infrared rotational variability in ground-based spectra of comet-asteroid transition object 944 Hidalgo. Since then, we carried out a rotationally resolved study of Hidalgo at mid-infrared wavelengths using the Infrared Spectrograph (IRS) on NASA\u27s Spitzer Space Telescope. We obtained 7 to 38 micron spectra of Hidalgo at 10 different rotational phases. These observations were carried out on July 24, 2006, when Hidalgo was at heliocentric and Spitzer-centric distances of 4.83 AU and 4.84 AU. In an initial analysis, we normalized the spectra with a thermal model fit to the continuum (which varied as the cross section of this non-spherical object changed with rotational phase). No detectable rotational variability in the emissivity was found across the wavelength range. All the spectra show clear emissions from silicates. These emissions are qualitatively similar to those seen in the spectra of Trojan asteroids (Emery et al. 2006) and in the spectrum of comet Hale-Bopp (Crovisier et al. 1997). Given the lack of emissivity variability, we averaged all our spectra and compared them with the other Spitzer spectrum of Hidalgo, which was obtained as part of the guaranteed time observations (GTO) on February 10, 2005 when Hidalgo was at heliocentric and Spitzer-centric distances of 1.96 AU and 1.71 AU. Although the 2005 spectrum has better signal-to-noise than the combined 2006 spectra, the two are identical within the uncertainties, save for changes in the thermal continuum. It is not clear why there is spectral variability in the near-infrared and not the longer wavelengths. One possible explanation is that the mineralogy across Hidalgo\u27s surface is similar but some areas have been affected differently by space weathering, i.e., one or more collisions may have exposed fresh material on some of Hidalgo\u27s surface

Thermal Infrared Observations and Thermophysical Characterization of OSIRIS-REx Target Asteroid (101955) Bennu

Near-Earth Asteroids (NEAs) have garnered ever increasing attention over the past few years due to the insights they offer into Solar System formation and evolution, the potential hazard they pose, and their accessibility for both robotic and human spaceflight missions. Among the NEAs, carbonaceous asteroids hold particular interest because they may contain clues to how the Earth got its supplies of water and organic materials, and because none has yet been studied in detail by spacecraft. (101955) Bennu is special among NEAs in that it will not only be visited by a spacecraft, but the OSIRIS-REx mission will also return a sample of Bennu’s regolith to Earth for detailed laboratory study. This paper presents analysis of thermal infrared photometry and spectroscopy that test the hypotheses that Bennu is carbonaceous and that its surface is covered in fine-grained (sub-cm) regolith. The Spitzer Space Telescope observed Bennu in 2007, using the Infrared Spectrograph (IRS) to obtain spectra over the wavelength range 5.2–38 μm and images at 16 and 22 μm at 10 different longitudes, as well as the Infrared Array Camera (IRAC) to image Bennu at 3.6, 4.5, 5.8, and 8.0 μm, also at 10 different longitudes. Thermophysical analysis, assuming a spherical body with the known rotation period and spin-pole orientation, returns an effective diameter of 484 ± 10 m, in agreement with the effective diameter calculated from the radar shape model at the orientation of the Spitzer observations (492 ± 20 m, Nolan, M.C., Magri, C., Howell, E.S., Benner, L.A.M., Giorgini, J.D., Hergenrother, C.W., Hudson, R.S., Lauretta, D.S., Margo, J.-L., Ostro, S.J., Scheeres, D.J. [2013]. Icarus 226, 629–640) and a visible geometric albedo of 0.046 ± 0.005 (using Hv = 20.51, Hergenrother, C.W. et al. [2013]. Icarus 226, 663–670). Including the radar shape model in the thermal analysis, and taking surface roughness into account, yields a disk-averaged thermal inertia of 310 ± 70 J m−2 K−1 s−1/2, which is significantly lower than several other NEAs of comparable size. There may be a small variation of thermal inertia with rotational phase (±60 J m−2 K−1 s−1/2). The spectral analysis is inconclusive in terms of surface mineralogy; the emissivity spectra have a relatively low signal-to-noise ratio and no spectral features are detected. The thermal inertia indicates average regolith grain size on the scale of several millimeters to about a centimeter. This moderate grain size is also consistent with low spectral contrast in the 7.5–20 μm spectral range. If real, the rotational variation in thermal inertia would be consistent with a change in average grain size of only about a millimeter. The thermophysical properties of Bennu’s surface appear to be fairly homogeneous longitudinally. A search for a dust coma failed to detect any extended emission, putting an upper limit of about 106 g of dust within 4750 km of Bennu. Three common methodologies for thermal modeling are compared, and some issues to be aware of when interpreting the results of such models are discussed. We predict that the OSIRIS-REx spacecraft will find a low albedo surface with abundant sub-cm sized grains, fairly evenly distributed in longitude

Infrared Spectra of Comet-Asteroid Transition Object 944 Hidalgo

Asteroid 944 Hidalgo is suspected of being an extinct comet. Understanding the origin of this enigmatic object is relevant to several areas of planetary astronomy, and the study of its surface composition may be diagnostic of its origin. Silicates have been detected in active comets, and on Jupiter Trojans. Our team investigated Hidalgo in the 8-30 micron range to determine the mineral composition and presence of surface silicates. We chose this wavelength region because it is most diagnostic for the detection of silicates. We applied to use NASA\u27s Spitzer Space Telescope as Hidalgo is too faint at these wavelengths for ground- based telescopes. Once the data were collected, the continuum was modeled and subtracted from the raw spectra. The result is a plot of emissivity versus wavelength that shows clear emission features from 8-13 microns, and around 20 microns; both of which have been identified with silicates. Our spectrum is compared with those of Jupiter Trojans, which are believed to be related to comets, and comet Hale-Bopp. With the project complete, we have demonstrated the presence of silicate emissions in Hidalgo and strong similarity with spectra of Jupiter Trojans and of active comets. These results argue in favor of Hidalgo having formed further from the Sun than main belt asteroids. We conclude that our findings are consistent, but not definitive, with Hidalgo being of cometary origin. Understanding the composition of this body and others like it is important for determining the origin of Earth\u27s water

Thermal and Physical Characterization of the OSIRIS-REx Target Asteroid (101955) 1999 RQ36

The OSIRIS-REx mission, the third in NASA’s New Frontier line, will launch in 2016, visit the near-Earth asteroid (101955) 1999 RQ36, and return samples of its regolith to Earth in 2023. Ground-based observations have already revealed a great deal about 1999 RQ36, including the spectral type (B-type), size, and rotation period. To further characterize the composition, surface grain size, and thermophysical properties, we observed 1999 RQ36 with the Spitzer Space Telescope during the time period 3-9 May 2007. Thermal spectra from 5.2 to 38 μm were measured with the Infrared Spectrograph (IRS) of opposite hemispheres of the body. Photometry at 3.6, 4.5, 5.8, and 8.0 μm was obtained with the Infrared Array Camera and at 16 and 22 μm with the IRS peak-up imaging mode. With the imaging modes, we targeted 10 equally distributed longitudes in order to search for rotational heterogeneities. The thermal inertia derived from the model fit is 600 +/- 150 J m-2s-1/2K-1. This moderately high thermal inertia suggests a regolith with grains less than 2 cm in diameter. Thermal inertia an important parameter for estimating the strength of the Yarkovsky effect, and has been used with measurements of the semi-major axis drift rate to estimate the bulk density of 1999 RQ36 (Chesley et al. 2012). The inferred size of RQ36 is in excellent agreement with radar observations, and the geometric albedo is very low (pv 0.03). There is no evidence for spectral features larger than the noise (S/N 40) in the final spectrum. The imaging data show no evidence for dust around the asteroid. Additional observations with Spitzer are planned for September 2012. We will present the current results and new observations along with an analysis of the thermal lightcurve in the context of the shape model derived from radar data

Debris Field of the July 19, 2009, Impact in Jupiter and Its Long-term Evolution

A multi-platform suite of imaging and spectroscopic observations of Jupiter\u27s atmosphere tracked the evolution of the debris field of an unknown impactor on 2009 July 19. The initial debris field is similar to those of intermediate Shoemaker-Levy 9 fragments, suggesting a body hundreds of meters in size, if icy, entering from the west and slightly north. The field is detectable in the visible as dark material and in the near-IR by high-altitude particulate reflectivity; it was quickly redistributed by different zonal flows across its latitudinal range. At first, the particulate field was highly correlated with areas of enhanced temperatures and enhanced ammonia and ethane emission, but this was no longer true by mid-August. As of Sept. 2, the debris field was undetectable in the thermal, detectable in the visible with good seeing, and still prominent near 2 microns wavelength. Visibly, the impact scar consists of two dark regions along the same latitude, ostensibly different from the central bright region associated with the near-IR debris pattern. Both morphologies show eastern and western extensions propagating away from the original impact site, which appear to be influenced by flows around vortices previously undetected in Jupiter atmosphere. These observations define the flow field just north of Jupiter\u27s southern polar vortex at higher altitudes than tracked in Jupiter\u27s main cloud deck

Long-Term Evolution of the Aerosol Debris Cloud Produced by the 2009 Impact on Jupiter

We present a study of the long-term evolution of the cloud of aerosols produced in the atmosphere of Jupiter by the impact of an object on 19 July 2009 (Sánchez-Lavega, A. et al. [2010]. Astrophys. J. 715, L155–L159). The work is based on images obtained during 5 months from the impact to 31 December 2009 taken in visible continuum wavelengths and from 20 July 2009 to 28 May 2010 taken in near-infrared deep hydrogen–methane absorption bands at 2.1–2.3 μm. The impact cloud expanded zonally from ∼5000 km (July 19) to 225,000 km (29 October, about 180° in longitude), remaining meridionally localized within a latitude band from 53.5°S to 61.5°S planetographic latitude. During the first two months after its formation the site showed heterogeneous structure with 500–1000 km sized embedded spots. Later the reflectivity of the debris field became more homogeneous due to clump mergers. The cloud was mainly dispersed in longitude by the dominant zonal winds and their meridional shear, during the initial stages, localized motions may have been induced by thermal perturbation caused by the impact’s energy deposition. The tracking of individual spots within the impact cloud shows that the westward jet at 56.5°S latitude increases its eastward velocity with altitude above the tropopause by 5–10 m s−1. The corresponding vertical wind shear is low, about 1 m s−1 per scale height in agreement with previous thermal wind estimations. We found evidence for discrete localized meridional motions with speeds of 1–2 m s−1. Two numerical models are used to simulate the observed cloud dispersion. One is a pure advection of the aerosols by the winds and their shears. The other uses the EPIC code, a nonlinear calculation of the evolution of the potential vorticity field generated by a heat pulse that simulates the impact. Both models reproduce the observed global structure of the cloud and the dominant zonal dispersion of the aerosols, but not the details of the cloud morphology. The reflectivity of the impact cloud decreased exponentially with a characteristic timescale of 15 days; we can explain this behavior with a radiative transfer model of the cloud optical depth coupled to an advection model of the cloud dispersion by the wind shears. The expected sedimentation time in the stratosphere (altitude levels 5–100 mbar) for the small aerosol particles forming the cloud is 45–200 days, thus aerosols were removed vertically over the long term following their zonal dispersion. No evidence of the cloud was detected 10 months after the impact