Humberto Campins

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.

The identification of crystalline olivine in cometary silicates

An intermediate-resolution spectrum of the 8-13 micron region in comet Halley is obtained which shows a prominent silicate emission feature with structure not observed before in other comets or in interstellar silicates. The presence of a strong 11.3 micron peak reported by Bregman and coworkers is confirmed, and evidence is found for additional structure in the band. By comparison with spectra of interplanetary dust particles and laboratory silicates, it is concluded that small crystalline olivine particles are a major component of the silicates in this comet; other silicates (e.g., amorphous or hydrated) must also be present. The identification of crystalline olivine in this part of the spectrum is supported by the observation of four peaks in 20-50 micron airborne spectra of this comet which have also been attributed to olivine. 32 refs.

Absolute calibration of photometry at 1 through 5 microns

The solar-analog method is used to determine the absolute calibration of photometry in the J, H, K, L, and M bands to an accuracy of 3 percent, (5 percent at M). This calibration agrees well with the direct calibration obtained by Blackwell et al. (1983); this agreement gives confidence that the calibration is known to an accuracy of 2 percent to 3 percent. The agreement also implies that the behavior of solar type stars is well understood in the infrared and that such stars make reliable standards for calibration of nonstandard photometric bands and for planetary reflectance studies. Both methods indicate that Vega is slightly brighter than indicated by atmospheric models.

The nucleus of Comet P/Tempel 2

Simultaneous optical photometry and IR radiometry of Comet P/Tempel 2 are presented. Periodic variations of brightness are present and in phase at all wavelengths. Because the optical and thermal rotational light curves are in phase, it is concluded that the variations are caused by the changing apparent cross section of an elongated nucleus rotating with a period near 8.9 h. The variation of flux with aperture makes it possible to separate the contributions of the nucleus and the coma. The contribution by the coma is about 25 percent at maximum light in the optical and undetectable at the level of 10 percent at all times in the thermal IR. 34 refs.

(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.

Airborne and groundbased spectrophotometry of comet P/Halley from 5–13 micrometers

Spectrophotometry from 5-10 micrometers (delta lambda/lambda approximately 0.02) of comet Halley was obtained from the Kuiper Airborne Observatory on 1985 December 12.1 and 1986 April 8.6 and 10.5, UT. 8-13 micrometers data were obtained on 17.2 December 1985 from the Nickel Telescope at Lick Observatory. The spectra show a strong broad emission band at 10 micrometers and a weak feature at 6.8 micrometers. We do not confirm the strong 7.5 micrometers emission feature observed by the Vega 1 spacecraft. The 10 micrometers band, identified with silicate materials, has substructure indicative of crystalline material. The band can be fitted by combining spectra data from a sample of interplanetary dust particles. The primary component of the silicate emission is due to olivine. The 6.8 micrometers emission feature can be due either to carbonates or the C-H deformation mode in organic molecules. The lack of other emission bands is used to place limits on the types of organic molecules responsible for the emission observed by others at 3.4 micrometers. Color temperatures significantly higher than the equilibrium blackbody temperature indicate that small particles are abundant in the coma. Significant spatial and temporal variations in the spectrum have been observed and show trends similar to those observed by the spacecraft and from the ground. Temporal variability of the silicate emission relative to the 5-8 micrometers continuum suggests that there are at least two physically separated components of the dust.

The bare nucleus of comet Neujmin 1

Simultaneous visible and infrared observations of comet P/Neujmin 1 1984c are presented which show that the comet has a large (mean radius 10 km), dark (geometric albedo 2-3 percent) nucleus with a surface which is mostly inert material but which still shows a low level of gaseous activity. This is the first physical evidence that cometary nuclei can leave behind an inert body after the coma activity ceases. No asteroid or asteroid class has been found to match the reflectance and albedo of this comet except possibly some D asteroids. 57 references.

An investigation of the nucleus and coma of Comet P/Arend-Rigaux

Narrow-band photometry of Comet P/Arend-Rigaux in the visible and near-ultraviolet are presented for six nights during the 1884/1985 apparition, including three nights of simultaneous observations in the thermal infrared made with the NASA IRTF. The results indicate a period of rotation of 13.47 hr, dimensions for the nucleus of 13 x 8 x 8 km (under the assumption of prolate spheroid shape), and a steady decrease in the comets gas production from a month before perihelion to more that two months after perihelion. Results for the nucleus and the coma suggest that the nucleus is totally covered with a nonvolatile mantle. 51 references.

Infrared mapping of M82 : a starburst in an edge-on barred galaxy

New 1-30-micron images of the starburst galaxy M82 are presented. The appearance of M82 at 2 microns indicates that a bar about 1 kpc long is located at its center. This bar may dramatically influence the gas dynamics and may have caused the formation of the central gas and dust toroid associated with the luminous episode of star formation. The mid-IR maps define the structure of the toroid out to 30 microns and show that the extent of the emitting region is at least 1.3 kpc. The absorption optical depth determined from 1-2 micron colors corresponds to A(v) less than about 8 and is distributed in a ridge spanning the starburst region and closely resembling the distribution of H I optical depth. Finally, the contribution that very small grains may make to the 10-micron emission is considered, and it is concluded, by comparison to Galactic H II regions, that their mid-IR emission is significant.

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.