Masateru Ishiguro

Regolith migration and sorting on asteroid Itokawa.

High-resolution images of the surface of asteroid Itokawa from the Hayabusa mission reveal it to be covered with unconsolidated millimeter-sized and larger gravels. Locations and morphologic characteristics of this gravel indicate that Itokawa has experienced considerable vibrations, which have triggered global-scale granular processes in its dry, vacuum, microgravity environment. These processes likely include granular convection, landslide-like granular migrations, and particle sorting, resulting in the segregation of the fine gravels into areas of potential lows. Granular processes become major resurfacing processes because of Itokawas small size, implying that they can occur on other small asteroids should those have regolith.

Detailed images of asteroid 25143 Itokawa from Hayabusa.

Rendezvous of the Japanese spacecraft Hayabusa with the near-Earth asteroid 25143 Itokawa took place during the interval September through November 2005. The onboard camera imaged the solid surface of this tiny asteroid (535 meters by 294 meters by 209 meters) with a spatial resolution of 70 centimeters per pixel, revealing diverse surface morphologies. Unlike previously explored asteroids, the surface of Itokawa reveals both rough and smooth terrains. Craters generally show unclear morphologies. Numerous boulders on Itokawas surface suggest a rubble-pile structure.

Evidence of hydrated and/or hydroxylated minerals on the surface of asteroid 4 Vesta

[1]xa0Asteroid 4 Vesta is thought to be a differentiated object with intact internal structure due to its achondritic surface. In this paper, we report K-L band spectroscopic observations of Vesta. We have detected the presence of a 3-μm absorption feature at about the 1% level on the surface of Vesta at longitudes between 155° and 195°. This result indicates that OH and/or H2O-bearing minerals are present in this region of Vesta; the sources are plausibly fragments of carbonaceous chondrite impactors or solar wind implantation. The detection of 3-μm absorption features from Vesta, regarded as the smallest terrestrial planet, may provide clues to the origin of volatile materials on terrestrial planets.

First Detection of an Optical Dust Trail along the Orbit of 22P/Kopff

We present the first evidence of a cometary dust trail in optical wavelengths along the orbit of 22P/Kopff, observed when the parent comet was at a heliocentric distance of 3.01 AU. We find that the surface brightness and the width of the trail become, respectively, fainter and wider as the distance from the comet nucleus increases, except for a region with delta mean anomaly ΔMA ≤ 002. This suggests that the majority of the centimeter-sized dust particles were ejected before the comets previous perihelion passage and that they spread due to their initial velocity with respect to the comet. By comparing this trail with the IRAS data at wavelengths of 12 and 25 μm, we infer that the trail is composed of very low albedo particles (~0.01).

Discovery of the Dust Trail of the Stardust Comet Sample Return Mission Target: 81P/Wild 2

We have succeeded in detecting a dust trail along the orbit of the short-period comet 81P/Wild 2, the target of the Stardust mission, which will fly by 81P/Wild 2 in 2004 January. This is the first discovery in optical wavelengths for which an IR counterpart (such as from IRAS) has not been reported. The detected trail extends from the nucleus to a point 23 (0.14 AU) back along the orbit. Its width is 9 (1.4 × 104 km) near the nucleus. Based on a comparison between models and the observed properties of the dust trail, it is likely that the trail is composed of dust particles with a diameter of ~1 mm (corresponding to 0.5 mg in mass). During the flyby phase of the Stardust spacecraft, it is likely that the spacecraft will experience impacts of such large dust particles along the comets orbit as well as from smaller grains in the cometary coma. With an impact velocity of trail particles of 6.1 km s-1, the impact fluence of 0.5 mg trail particles onto the Stardust spacecraft will be ~0.80 day-1 inside the dust trail. However, the Whipple bumper shields should prevent physical disruption of the spacecraft by impacting particles of up to 1 cm in size. Thus, unless the 1 mm trail dust impacts cause severe attitude control problems for the spacecraft, they may not be critically hazardous to the mission but instead may provide a unique opportunity for the first in situ flux measurement of a comet dust trail, using the Whipple bumper shield dust flux monitors.

First Detection of Visible Zodiacal Dust Bands from Ground-based Observations

We present the first ground-based observational evidence of the zodiacal dust bands originally discovered by the Infrared Astronomical Satellite (IRAS) and confirmed by the Cosmic Background Explorer (COBE). Our photometric observations have been performed on Mauna Kea, Hawaii, between 1997 October 29 and November 2, using a 24 mm wide-angle lens attached to a cooled CCD camera, and a blue filter centered at 440 nm. Photometric data of the morning zodiacal light have revealed the presence of zodiacal dust bands at ecliptic latitudes β=0°, 3°, and ±10°, as well as additional faint structures at approximately β=±5°, between solar elongations =75° and 90°. The brightness of dust bands is approximately 2%-3% of the background zodiacal light. Moreover, our observation of the Gegenschein has discovered dust bands at β=+2°, -4°, and -9° at 165°≤≤185°. Using the separation of the inner dust band pair observed in two different regions of , we estimate the parallactic distance of this band pair to be about 1.6 AU from the Sun.

Saturated Hydrocarbons in Comet 153P/Ikeya-Zhang: Ethane, Methane, and Monodeuterio-Methane

Emissions of the methane ν3 band and the ethane ν7 band were detected in the high-dispersion infrared spectra of comet 153P/Ikeya-Zhang taken by the Subaru Telescope with the Infrared Camera and Spectrograph (IRCS) on 2002 May 28 and 29 UT. The rotational temperature is estimated to be about 40-50 K from R0 and R1 lines of the methane ν3 band. The production rates of methane and ethane were estimated by applying the rotational temperature of 50 K. Resultant production rates of methane and ethane are (2.13 ± 0.40) × 1026 and (1.75 ± 0.48) × 1026 molecules s-1, respectively. The abundance ratio between ethane and methane is 0.82 ± 0.27, which is comparable with other comets within error bars. Although the emissions of monodeuterio-methane (CH3D) were searched for carefully, there is no evidence of them. The 2 σ (95% confidence) upper limit of the CH3D/CH4 ratio is 0.3 in comet 153P/Ikeya-Zhang. Comparing the relative abundance of methane and ethane in the interstellar ices, we discussed the formation conditions of cometary nuclei.

THE DISCOVERY OF A FAINT GLOW OF SCATTERED SUNLIGHT FROM THE DUST TRAIL OF THE LEONID PARENT COMET 55P/TEMPEL-TUTTLE

A meteoric cloud is the faint glow of sunlight scattered by small meteoroids in the dust trail along the orbit of a comet as seen by an earthbound observer. While these clouds were previously only known from anecdotes of past meteor storms, we now report the detection of a meteoric cloud by modern techxad niques in the direction of the dust trail of comet 55P/Tempel-Tuttle, the parent of the Leonid meteor stream. Our photometric observations, performed on Mauna Kea, Hawaii, reveal the cloud as a local enhancement in sky brightness during the Leonid shower in 1998. The radius of the trail, deduced from the spatial extent of the cloud, is approximately 0.01 AU and is consistent with the spatial extent mapped out by historic accounts of meteor storms. The brightness of the cloud is approximately D2%E3% of the background zodiacal light and cannot be explained by simple model calculations based on the zenith hourly rate and population index of the meteor stream in 1998. If the typical size of cloud particles is 10 km and the albedo is 0.1, the brightness translates into a number density of 1.2 ] 10~10 m~3. The meteoroid cloud would be the product of the whole dust trail and not only the part that was crossed in 1998. Subject headings : comets : individual (55P/Tempel-Tuttle) E dust, extinction E interplanetary medium E meteors, meteoroids

Search for Nonmethane Hydrocarbons on Pluto

We have carried out infrared high-resolution spectroscopy of the Pluto-Charon system in the L band with the adaptive optics system on the Subaru telescope. The spectrum is dominated by the strong and broad absorption features of methane but includes some additional features. Comparing the spectrum with model calculations, we suggest that absorption features around 3.1, 3.2, and 3.35 μm could be an indication of nonmethane hydrocarbons on Plutos uppermost surface. Implications of the estimated mass ratio between hydrocarbons for the formation and evolution of Pluto are discussed.

The first detection of water absorption on a D type asteroid

[1]xa0The D type asteroids are among the darkest objects known in our solar system. Here, we present infrared spectra of one of the main-belt D type asteroids, 773 Irmintraud. In contrast to previous observations of D type asteroids, we found a gap of reflectance around 3 μm in wavelength. The 3 μm gap is one of the spectral signatures of OH or H2O as water ice or in hydrous minerals, which had formed in the processes of aqueous alteration in the early solar system. We suggest that D type asteroids, which are all the while considered as unaltered primitive material, could be aqueously altered. Our data do support the presence of water on a compositionally primitive D type asteroid.