J. Licandro

Water ice and organics on the surface of the asteroid 24 Themis

It has been suggested that Earth’s current supply of water was delivered by asteroids, some time after the collision that produced the Moon (which would have vaporized any of the pre-existing water). So far, no measurements of water ice on asteroids have been made, but its presence has been inferred from the comet-like activity of several small asteroids, including two members of the Themis dynamical family. Here we report infrared spectra of the asteroid 24 Themis which show that ice and organic compounds are not only present on its surface but also prevalent. Infrared spectral differences between it and other asteroids make 24 Themis unique so far, and our identification of ice and organics agrees with independent results that rule out other compounds as possible sources of the observed spectral structure. The widespread presence of surface ice on 24 Themis is somewhat unexpected because of the relatively short lifetime of exposed ice at this distance (∼3.2 au) from the Sun. Nevertheless, there are several plausible sources, such as a subsurface reservoir that brings water to the surface through ‘impact gardening’ and/or sublimation.

Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail

We report on the discovery and analysis of a striking neutral sodium gas tail associated with comet C/1995 O1 Hale-Bopp. Sodium D-line emission has been observed at heliocentric distance r<1.4 AU in some long-period comets and the presence of neutral sodium in the tailward direction of a few bright comets has been noted, but the extent, and in particular the source, has never been clear. Here we describe the first observations and analysis of a neutral sodium gas tail in comet Hale-Bopp, entirely different from the previously known ion and dust tails. We show that the observed characteristics of this third type of tail are consistent with itbeing produced by radiation pressure due to resonance fluorescence of sodium atoms and that the lifetime for photoionization is consistent with recent theoretical calculation.We report on the discovery and analysis of a striking neutral sodium gas tail associated with comet C/1995 O1 Hale-Bopp. Sodium D line emission has been observed at heliocentric distance r≤1.4 AU in some long-period comets, and the presence of neutral sodium in the tailward direction of a few bright comets has been noted, but the extent, and in particular the source, has never been clear. Here we describe the first observations and analysis of a neutral sodium gas tail in comet Hale-Bopp, which is entirely different from the previously known ion and dust tails. We show that the observed characteristics of this third type of tail are consistent with it being produced by radiation pressure due to resonance fluorescence of sodium atoms and that the lifetime for photoionization is consistent with recent theoretical calculations.

(65) Cybele: detection of small silicate grains, water-ice, and organics

Context. (65) Cybele is the most representative member of a population of primitive asteroids in the outer edge of the main belt, the Cybele asteroids. Recent dynamical models suggest that a significant fraction of them originated in the primordial transneptunian disk, so the study of the physical properties of these asteroids is potentially a useful test of these models. Aims. Our aim is to obtain information on the surface composition of this asteroid. In particular we want to obtain information on the composition and properties of the regolith and the possible presence of ices and organic materials. Methods. We present 2–4 μm and 5–14 μm spectroscopy of (65) Cybele obtained with the NASA IRTF telescope and Spitzer Space Telescope respectively. We compare the results with spectra of Trojan asteroids and asteroid (24) Themis. We analyze the 2–4 μm spectrum using scattering models and we apply thermal models to the 5–14 μm data. Results. The 2–4 μm spectrum of (65) Cybele presents an absorption band centered at ∼3.1 μm and more weaker bands in the 3.2–3.6 μm region, very similar to those observed in (24) Themis. No hydrated silicates are detected. From the spectrum in the 5–14 μ mr egion an effective diameter D = 290 ± 5 km, a beaming paramete η = 0.967 ± 0.014, and a geometric visible albedo pV = 0.05 ± 0.01 are derived using the NEATM thermal model. The emisivity spectrum in the 5–14 μm range exhibits an emission plateau at about 9 to 12 μm with an spectral contrast of ∼5%. This emission is similar to that of Trojan asteroids and active comets and may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. The lower amplitude of the silicate emission in Cybele’s spectrum with respect to that of Trojan asteroids could be attributed to larger dust particles and/or a slightly denser structure. Conclusions. The surface of (65) Cybele is covered by a fine anhydrous silicate grains mantle, with a small amount of water ice and complex organic solids. This is similar to comet surface where non-equilibrium phases coexist. The presence of water-ice and anhydrous silicates is indicative that hydration did not happened or is incomplete, suggesting that the temperatures were always sufficiently low.

The Origin of Asteroid 101955 (1999 RQ36)

Near-Earth asteroid (NEA) 101955 (1999 RQ36; henceforth RQ36) is especially accessible to spacecraft and is the primary target of NASAs OSIRIS-REx sample return mission; it is also a potentially hazardous asteroid. We combine dynamical and spectral information to identify the most likely main-belt origin of RQ36 and we conclude that it is the Polana family, located at a semimajor axis of about 2.42 AU. We also conclude that the Polana family may be the most important inner-belt source of low-albedo NEAs. These conclusions are based on the following results. (1) Dynamical evidence strongly favors an inner-belt, low-inclination (2.15 AU < a < 2.5 AU and i < 10°) origin, suggesting the ν6 resonance as the preferred (95% probability) delivery route. (2) This region is dominated by the Nysa and Polana families. (3) The Polana family is characterized by low albedos and B-class spectra or colors, the same albedo and spectral class as RQ36. (4) The Sloan Digital Sky Survey colors show that the Polana family is the branch of the Nysa-Polana complex that extends toward the ν6 resonance; furthermore, the Polana family has delivered objects of the size of RQ36 and larger into the ν6 resonance. (5) A quantitative comparison of visible and near-infrared spectra does not yield a unique match for RQ36; however, it is consistent with a compositional link between RQ36 and the Polana family.

Spectroscopy of B-type asteroids: Subgroups and meteorite analogs

[1] B-type asteroids have a negative slope from -0.5 to ∼1.1 μm and beyond. What causes this? Visible to near-infrared reflectance spectra (0.4-2.5 μm) are assembled for 22 B-type asteroids. The spectra fall naturally into three groups: (1) those with negative (blue) spectral shapes like 2 Pallas (7 objects), (2) those with concave curve shapes like 24 Themis (11 objects), and (3) everything else (4 objects). The asteroid spectra are compared to mineral and meteorite spectra from the Reflectance Experiment Laboratory library of 15,000 samples, in a least squares search for particulate analogs, constrained by spectral brightness. The Pallas group objects show a trend of analogs from the CV, CO, and CK meteorite groups. Only three of the seven Pallas-like objects are determined to be dynamically related (2, 1508, and 6411). The Themis group objects show a trend of analogs from the CI, CM, CR, CI-Unusual, and CM-Unusual meteorites (as expected from the work of Hiroi et al. (1996)). Seven of the 11 Themis-like objects are dynamically related (24, 62, 222, 316, 379, 383, and 431). Allowing for reasonable uncertainties in the spectral matches, we find no need to invoke mineralogies that do not exist in the meteorite collection to explain B-type spectra or their negative slopes. Our Themis group results are as expected and are consistent with previous work, but our Pallas group results are new and, in some cases, in conflict with previous work.

The slow rotation of 253 Mathilde

Abstract CCD photometry of the NEAR mission fly-by target 253 Mathilde is presented. Measurements taken during 52 nights of observations, from February to June 1995, allow a rotation period of 17.406±0.010 days and a lightcurve amplitude of 0.45±0.02 mag to be determined. A B-V color index of 0.67±0.02 and a V-R of 0.35±0.02 are measured, which are compatible with C-type membership. The determination of the phase relation results in H = 10.28±0.03 and G = 0.12±0.06. Indications that the lightcurve is not strictly singly-periodic are found. A power-spectrum analysis detects a secondary frequency f 2 = 0.0322±0.0010 d −1 , which is interpreted as evidence for a complex rotation state.

Thermal properties, sizes, and size distribution of Jupiter-family cometary nuclei

We present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 using imaging capabilities of the Spitzer Space Telescope. The comets were typically 4–5 AU from the Sun when observed and most showed only a point-source with little or no extended emission from dust. For those comets showing dust, we used image processing to photometrically extract the nuclei. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We have six main conclusions: (a) The average beaming parameter of the JFC population is 1.03 ± 0.11, consistent with unity; coupled with the large distance of the nuclei from the Sun, this indicates that most nuclei have Tempel 1-like thermal inertia. Only two of the 57 nuclei had outlying values (in a statistical sense) of infrared beaming. (b) The known JFC population is not complete even at 3 km radius, and even for comets that approach to ∼2 AU from the Sun and so ought to be more discoverable. Several recently-discovered comets in our survey have small perihelia and large (above ∼2 km) radii. (c) With our radii, we derive an independent estimate of the JFC nuclear cumulative size distribution (CSD), and we find that it has a power-law slope of around −1.9, with the exact value depending on the bounds in radius. (d) This power-law is close to that derived by others from visible-wavelength observations that assume a fixed geometric albedo, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3–6 km. (f) Our CSD is consistent with the idea that the intrinsic size distribution of the JFC population is not a simple power-law and lacks many sub-kilometer objects.

Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation

Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies. Pluto possesses an atmosphere whereas Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos. Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto, and is currently at a distance to the Sun intermediate between the two. Although Makemake’s size (1,420 ± 60 km) and albedo are roughly known, there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere. Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo pV = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4–12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm−3 is inferred from the data.

Origin of the near-Earth asteroid Phaethon and the Geminids meteor shower

Aims. In this paper we establish a compositional and dynamical connection between two B-type objects: main belt asteroid (2) Pallas and near-Earth asteroid (3200) Phaethon. The final purpose is to help understand the origin of this very interesting object. Methods. We first compare visible and near-infrared spectra of asteroids Phaethon and Pallas. We then compare the reflectance spectra of Phaethon with all the available visible spectra of B-type asteroids belonging to the Pallas family. One last spectral comparison is then performed to search for any correspondence between Phaethon and any B-type asteroid in the main belt. Numerical simulations are also carried out to explore the dynamical connection between the orbital neighborhoods of Pallas and Phaethon. Results. Main differences between Phaethon and Pallas lie in the visible wavelength part of their reflectance spectra. We have also found that the nine asteroids belonging to the Pallas family have visible spectra that are different from that of Pallas and strikingly similar to that of Phaethon. Spectral comparison excludes any other B-type asteroid in the main belt as a possible parent body of Phaethon. Numerical simulations establish a dynamical pathway that connects Phaethon with Pallas and its family members. Conclusions. The spectral similarities between Phaethon and Pallas family members, together with their established dynamical connection, supports Pallas as the most likely parent body of Phaethon and therefore, the associated Geminids meteor stream. We suggest that differences in asteroid sizes are the most likely explanation for the differences in the visible reflectance spectra between Phaethon and Pallas.

The surface of (136108) Haumea (2003 EL61), the largest carbon-depleted object in the trans-Neptunian belt

Context. Previously known as 2003 EL61, (136108) Haumea, is the largest member of a group of trans-Neptunian objects (TNOs) with similar orbits and “unique” spectral characteristics in the form of a neutral slope in the visible and the deepest water ice absorption bands observed in the trans-Neptunian belt (TNb). Studying the surface of 2003 EL61 provides useful constraints of the origin of this particular group of TNOs and about the outer Solar System’s history. Aims. We attempt to study the composition of the surface of 2003 EL61. Methods. We present visible and near-infrared spectra of 2003 EL61 obtained with the 4.2 m WHT and the 3.6 m TNG telescopes at the “Roque de los Muchachos” Observatory (Canary Islands, Spain). Near-infrared spectra were obtained at different rotational phases covering almost one complete rotational period. Spectra are fitted using scattering models based on Hapke theory and constraints on the surface composition are derived. Results. The observations confirm previous results that the 2003 EL61 spectrum is neutral in color and exhibits deep water-ice absorption bands. They also provide new facts about the surface of this object: the lack of significant variations in the spectral slope (in the near-infrared) and the depth of the water-ice absorption bands at different rotational phases clearly evident in the data, suggest that the surface of 2003 EL61 is homogeneous. The scattering models indicate that a 1:1 intimate mixture of crystalline and amorphous water ice is the most probable surface composition of this big TNO, and constrain the presence of other minor constituents to a maximum traction of 8%. Conclusions. The derived composition suggests that: a) cryovolcanism is unlikely to be the resurfacing process that retains the surface of this TNO, and the other members of this population, covered mainly by water ice; b) the surface is older than 10 8 yr which constrains the timescale of any catastrophic event. such as the collision suggested to be the origin of this population, to at least 10 8 yr; c) the surface of 2003 EL61 is depleted of carbon-bearing species. According to the orbital parameters of the population, this implies that is a possible source of carbon-depleted, Jupiter-Family comets.