S-type Asteroids Research Articles

Most-Common Meteorites Find a Home Among the Asteroids

Meteoriticists have long been puzzled by the fact that the most common meteorites, so-called ordinary chondrites, don't appear to have come from the most common asteroids, the S-types. Last week, however, a group of researchers attending the spring meeting of the American Geophysical Union here announced that the 31-kilometer-long S-type asteroid Eros now being orbited by the NEAR Shoemaker spacecraft is made of the same stuff as ordinary chondrites. That conclusion comes from NEAR Shoemaker's first-ever analysis of the elemental composition of an asteroid.

Density and Porosity of Stone Meteorites: Implications for the Density, Porosity, Cratering, and Collisional Disruption of Asteroids

The grain density, bulk density, and porosity were measured for 19 ordinary chondrite and 5 carbonaceous chondrite meteorites, and the grain density was measured for an additional 30 ordinary chondrite and 7 carbonaceous chondrite meteorites. The unweathered ordinary chondrites generally have porosities ranging from 5 to 25%, while the carbonaceous chondrites have porosities in the range 10 to 30%. These high porosities indicate that the asteroidal parent bodies from which these chondritic meteorites are derived are porous, or contain porous regions. The grain densities of all 49 ordinary chondrites are greater than 3.1 g/cc, the grain densities of all 9 anhydrous carbonaceous chondrites are greater than 3.0 g/cc, and the grain densities of the hydrated carbonaceous chondrites range from 2.5 to 3.0 g/cc. Comparison of these grain densities with spacecraft determinations of the densitiy of the S-type asteroid Ida, 2.6 g/cc, and the anhydrous C-type asteroid Mathilde, 1.3 g/cc, suggests that if these asteroids are composed of material similar in mineralogy to that of the stone meteorites, then Ida and Mathilde must exhibit significant porosity. Cratering and collisional disruption are sensitive to the porosity of the target. The parent bodies of the ordinary and carbonaceous chondrite meteorites appear to contain regions of significant porosity, which may affect their compression and compaction during impact events.

Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation on olivine and pyroxene samples

For the purpose of simulating the surface alteration process called “space weathering”, experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6–8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.

The Lightcurve and Geometric Albedo of 433 Eros during the 1998 Apparition

We obtained R-band photometry of 433 Eros at the Table Mountain 0.6-m telescope during its 1998 apparition. Our five nights of photometry yields a rotational period P sid=5.27048±0.00064 h and visual geometric albedo for 433 Eros of 0.27 +0.03 −0.06, high, but within the range of S-type asteroids. Utilizing lightcurve data from the 1974/1975 apparition we solved for rotational pole position (RA=00:46.9, DEC=+19:30.5; J2000) and axial ratios of 1.0:0.388:0.375, which are both significantly different from previously derived values.

Small Asteroids Point to a Source for Meteorites

PLANETARY SCIENCESeen in the telescope, the subtle reddish cast of S-type asteroids--the most common type--looks nothing like the gray of ordinary chondrites, the most common type of meteorite. At last month's Asteroids, Comets, and Meteors meeting in Ithaca, New York, however, a pair of astronomers reported that the smaller the S asteroid, the more its color resembles that of chondrites. Because smaller asteroids are also thought to be the youngest, the relation could mean that some kind of weathering process reddens the surface of larger, older asteroids.

A CCD Search for Lagrangian Asteroids of the Earth–Sun System

The L4 and L5 points of the Earth–Sun system were imaged using the University of Hawaii 2.24-m telescope on 1994 May 5–7 and July 6–8 UT. We used a thinned Tektronix 2048 × 2048 CCD, subtending a field 7.5 arcmin on a side. Our objective in this search was to locate sub-kilometer-sized asteroids orbiting near the libration point or, failing this, to set a rough upper limit on the number density of objects contained in such a population. Previous searches of these areas have used various less sensitive photographic techniques, but no asteroids have ever been discovered at the L4 or L5 points of the Earth–Sun system. The limiting sensitivity of this search wasR∼ 22.8, corresponding to C-type asteroids ∼350 m in diameter or S-type asteroids ∼175 m in diameter. No objects were discovered in the approximately 0.35 square degrees covered, leading to our crude estimate that the upper limit for the number density of objects at or above our detection threshold is approximately 3 objects per square degree. In this paper, we discuss the various considerations governing our search strategy. We also discuss the relevence of this search technique in looking for near-Earth asteroids in general. Finally, in this paper we point out areas of future theoretical work that can yield important information on Earth's libration regions, and we discuss the outlook for using large mosaic arrays in future asteroid searches of this nature.

Albedo measurements on meteorite particles

We have measured single-particle albedos for several chondrite and achondrite meteorites and for carbonaceous material. We have introduced a new, relative method for measuring the albedo, including a new method for evaluating the particle cross-section. We determined albedos of 0.50±0.25 for ordinary chondrite, 0.15±0.02 for C2 carbonaceous chondrite, 0.64±0.13 for achondritic shergottite, 0.33±0.14 for enstatite chondrite meteorites, and 0.21±0.03 for carbon particles. Making use of the measured single-particle albedos, we have predicted geometric albedos for atmosphereless solar system bodies covered with regoliths of such particles. The geometric albedos are consistent with the values derived for C- and S-type asteroids in earlier works. However, for enstatite chondrite, the geometric albedo is much lower than that accepted for E-type asteroids. The predicted geometric albedos are 0.04–0.07 for C2 chondritic material and 0.13–0.24 for chondritic material. These ranges describe the usually accepted values for C- and S-type asteroids. Our results put important constraints on the radiative transfer models for the regoliths of asteroids and comets.

Size and Orbit Dependent Trends in the Reflectance Colors of Earth-Approaching Asteroids

New observations show that reflectance colors of Earth-approaching asteroids depend on their sizes and orbits. Most bodies larger than ∼2 km have reddish colors similar to common main-belt asteroids of spectral type S. Smaller bodies, and those that may have recently migrated from the main belt, have relatively neutral spectra in the 0.5- to 0.8-μm wavelength range. These trends may result from the action of “space-weathering,” recently proposed to explain similar spectral variations on the surfaces of S-type asteroids and the disparity between the colors of ordinary chondrite meteorites and their probable main-belt parents.

Surface Properties of (6) Hebe: A Possible Parent Body of Ordinary Chondrites

We report for the first time rotationally resolved spectroscopic observations, as well as new photometric and polarimetric measurements, of the large S-type asteroid (6) Hebe, always at near-equatorial aspects. We have found evidence for only minor variations in Hebe's visible reflectance spectrum over a rotational cycle, comparable to our measurement accuracy (a few percent) and consistent with an undifferentiated silicate assemblage on the surface. We have also confirmed previous results of the existence of small polarization changes, but they are not correlated with the (complex and asymmetric) photometric lightcurve of the asteroid. A plausible interpretation of these data is that Hebe's surface is composed of undifferentiated materials, but presents texture, albedo, and spectral changes possibly related to the occurrence of energetic cratering events. Given the comparatively large size of Hebe, its proximity to resonance-related chaotic zones of the orbital element space (Farinellaet al. 1993,Celest. Mech.56,287–305, andIcarus101,174–187), and its assignment by Gaffeyet al. (1993,Icarus106,573–602) to the S(IV) taxonomic subclass, our results support the idea that this asteroid may be the source of a significant fraction of the ordinary chondrite meteorites.

Polarimetric Observations of 4179 Toutatis in 1992/1993

Our polarimetric measurements of asteroid 4179 Toutatis in 1992 and 1993 have been reexamined, taking into account careful error analyses of the raw data. The resulting values of the polarization degreePand its position angle at wavelengths of 0.42, 0.46, 0.53, 0.65, 0.70, and 0.76 μm are reported for a range of phase angle, α, between 13°.6 and 93°.7. A weak decrease inPwith wavelength has been found at α ≥ 50°. In addition, a small increase inPwith increasing phase angle has been confirmed, i.e., a polarimetric slope of 0.09–0.15% deg−1. Furthermore, we found a time variation of polarization and an occurrence of higher polarization in the dark phase of the light curve compiled by J. R. Spenceret al.(1995,Icarus117,71–89). Our polarimetric results plus other available data on 4179 Toutatis have confirmed that 4179 Toutatis is an S-type asteroid with an irregular shape and inhomogeneous surface composed in part of light-transmitting materials such as a mixture of olivine and pyroxene.

Partial melting and melt migration in the acapulcoite-lodranite parent body

We review petrologic evidence that the acapulcoites and lodranites formed by < 1 vol% to probably 20 vol% whole rock partial melting of a chondritic precursor material. At low degrees of partial melting, only Fe,NiFeS cotectic melting occurred. Migration distances for partial melts were short, resulting in the formation of acapulcoites with essentially chondritic troilite and plagioclase contents, but achondritic textures. At high degrees of partial melting, both Fe,NiFeS and basaltic (plagioclase-pyroxene) partial melts formed, and the melts may have migrated out of the source rock. The partial melt residues, which are more or less depleted in Fe,NiFeS and plagioclase, are the lodranites. Melt migration was complex: most acapulcoites, which experienced relatively low degrees of partial melting, lost little if any of the partial melt. One acapulcoite, LEW 86220, represents a unique case in which Fe,NiFeS and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region, where they were trapped. In cases of the relatively high degrees of partial melting experienced by lodranites, melts may have been partly, selectively, or totally removed from the rocks, and Fe,NiFeS and/or basaltic partial melts may have been removed to different degrees and may, in fact, have been trapped on occasion in greater than chondritic proportions. We model vein and dike formation and melt migration by calculating the excess pressures and vein and dike sizes for varying degrees of partial melting. Our calculations are broadly consistent with observations, indicating that melt migration is inefficient at low degrees of partial melting and extremely efficient at high degrees of partial melting. Although the size of the acapulcoite-lodranite parent body and the volatile contents of the chondritic precursor rocks are poorly constrained, the lack of basaltic rocks in the world's meteorite collections complementary to the lodranites suggests that basaltic partial melts may have been accelerated off the body by explosive volcanism of the type envisioned by Wilson and Keil (1991) and ejected into space. The diversity of rocks from the acapulcoite-lodranite parent body may provide a basis for better understanding the diverse range of spectral subtypes recognized among the S-type asteroids.

Shape and Albedo Variations of Asteroid 15 Eunomia

Near-infrared reflected and thermal emitted lightcurves were measured concurrently for Asteroid 15 Eunomia and were used to calculate relative projected area and relative albedo variations over its surface. The technique for calculating relative variations in projected area and albedo from simultaneously measured reflected and emitted lightcurves can be used to detect changes in the shape and surface brightness of asteroids for which such data exist.Rotational spectral reflectance data obtained in 1981 were used to correlate variations in shape and relative surface albedo with changes in surface composition. Using these data combined with this relative projected area/relative albedo technique, it is confirmed that 15 Eunomia is an elongate asteroid with a generally ovoid (“egg”) shape. Its surface composition varies from metalliferous with major olivine and lesser pyroxene in subequal metal/silicate ratios (much like stony–iron meteorites) for the more “pointed” end to a higher concentration of high-Fe pyroxene and a higher general concentration of silicate (a basaltic composition) for the “blunt” end (Gaffey and Ostro, 1987,Lunar Planet Sci. XVIII,310–311).Asteroid 15 Eunomia may be a cross section through a completely or partially differentiated S-type asteroid parent body. The basaltic blunt end of 15 Eunomia may show a sample of the surface of the parent body with the metalliferous end showing a portion of the core/mantle boundary. If true, this would indicate a mantle thickness on the parent body of approximately 300 km.

Near Infrared Spectrometer for the Near Earth Asteroid Rendezvous Mission

The Near-Infrared Spectrometer (NIS) instrument on the Near-Earth Asteroid Rendezvous (NEAR) spacecraft is designed to map spectral properties of the mission target, the S-type asteroid 433 Eros, at near-infrared wavelengths diagnostic of the composition of minerals forming S asteroids. NIS is a grating spectrometer, in which light is directed by a dichroic beam-splitter onto a 32-element Ge detector (center wavelengths, 816–1486 nm) and a 32-element InGaAs detector (center wavelengths, 1371–2708 nm). Each detector reports a 32-channel spectrum at 12-bit quantization. The field-of-view is selectable using slits with dimensions calibrated at 0.37° × 0.76° (narrow slit) and 0.74° × 0.76° (wide slit). A shutter can be closed for dark current measurements. For the Ge detector, there is an option to command a l0x boost in gain. A scan mirror rotates the field-of-view over a 140° range, and a diffuse gold radiance calibration target is viewable at the sunward edge of the field of regard. Spectra are measured once per second, and up to 16 can be summed onboard. Hyperspectral image cubes are built up by a combination of down-track spacecraft motion and cross-track scanning of the mirror. Instrument software allows execution of data acquisition macros, which include selection of the slit width, number of spectra to sum, gain, mirror scanning, and an option to interleave dark spectra with the shutter closed among asteroid observations. The instrument was extensively characterized by on-ground calibration, and a comprehensive program of in-flight calibration was begun shortly after launch. NIS observations of Eros will largely be coordinated with multicolor imaging from the Multispectral Imager (MSI). NIS will begin observing Eros during approach to the asteroid, and the instrument will map Eros at successively higher spatial resolutions as NEAR’s orbit around Eros is lowered incrementally to 25 km altitude. Ultimate products of the investigation will include composition maps of the entire illuminated surface of Eros at spatial resolutions as high as as ~300 m.

IRTS Observation of the Near-Infrared Spectrum of the Zodiacal Light

We present the near-infrared observation of the zodiacal light with the IRTS (Infrared Telescope in Space), a small cryogenically cooled orbital infrared telescope. The observed spectra of the sky brightness at wavelengths from 1.4 μm to 4.0 μm are characterized by a smooth stellar-like spectrum at wavelengths shorter than 3.2 μm, and enhanced emission at the long wavelengths, which is attributed to thermal emission from interplanetary dust (IPD). The measured sky brightness has a clear dependence on the ecliptic latitude, implying that the zodiacal light is a dominant emission component of the near-infrared sky. The spectrum of the zodiacal light, itself, was obtained by subtracting other emission components. Although it is fairly close to the solar spectrum at wavelengths between 1.8 and 3.2 μm, it clearly shows a decrement in the scattering efficiency at the short-wavelength end. The zodiacal light at 1.4–3.2 μm is about twice as bright as that extrapolated from optical measurements. These results indicate that large dust is responsible for the near-infrared zodiacal light; they also suggest that the IPD that has a spectral reflectance similar to the S-type asteroids could be responsible for the near-infrared zodiacal light.

Optical Effects of Regolith Processes on S-Asteroids as Simulated by Laser Shots on Ordinary Chondrite and Other Mafic Materials

Laboratory results from a simulation of possible optical effects of impact melting and repeated crystallization on asteroidal surfaces are presented. Quick melting and crystallization of surface materials were simulated by impulse laser treatment of powdered materials in vacuum. The mafic materials used in this study were: ordinary chondrite (OC) ElenovkaL5, carbonaceous chondrite Allende CV3, terrestrial olivine, clinopyroxene, and olivine–clinopyroxene mixture 1:1. The reflectance spectra of powdered samples before and after laser treatment were recorded in the range 0.3–25μm. The laboratory laser alteration produced a reduction in overall reflectance and spectral contrast, a noticeable increase in spectral slope, the shifts in the wavelength positions of absorption band centers, and a decrease in band area ratio. The continuum slope of the ordinary chondrite Elenovka after laser treatment is comparable to that of S-type asteroids. Laser impulse alteration raises the spectrally derived olivine/orthopyroxene ratio of Elenovka from 2.8 to 5.4. This value is far outside the OC range, but falls within the olivine/orthopyroxene range for S-asteroids. The results suggest that quick melting and subsequent crystallization can enhance the spectral similarity between S-asteroids and ordinary chondrites.

Ida and Dactyl: Spectral Reflectance and Color Variations

Galileo SSI color data between 0.4 and 1.0 μm demonstrate that both Ida and Dactyl are S-type asteroids with similar, but distinct spectra. Small but definite color variations are also observed on Ida itself and involve both the blue part of the spectrum and the depth of the 1-μm pyroxene–olivine band. Ida's surface can be classified into two color terrains: Terrain A has a shallower 1-μm absorption and a steeper visible red slope than does Terrain B. Qualitatively, the color–albedo systematics of these two terrains follow those noted for color units on Gaspra and the variations in 1-μm band depth with weathering described by Gaffeyet al.(Gaffey, M. J., J. F. Bell, R. H. Brown, T. H. Burbine, J. Piatek, K. L. Reed, and D. A. Chaky 1993.Icarus106, 573–602). Terrain A, with its slightly lower albedo, its shallower 1-μm band, and its slightly steeper visible red slope relative to Terrain B could be interpreted as the “more processed,” “more mature,” or the “more weathered” of the two terrains. Consistent with this interpretation is that Terrain A appears to be the ubiquitous background on most of Ida, while Terrain B is correlated with some small craters as well as with possible ejecta from the 10-km Azzurra impact structure. Because of these trends, it is less likely that differences between Terrains A and B are caused by an original compositional inhomogeneity within the body of Ida, although they do fall within the range known to occur within the Koronis family.The spectrum of Dactyl is similar to, but definitely different from, that of Terrain B on Ida. It does not conform to the pattern that obtains between the colors and albedos of Terrains A and B: the satellite's 1-μm band is deeper than that of Terrain B, but its albedo is lower, rather than higher. By itself, the deeper band depth could be interpreted, following Gaffeyet al., to mean that Dactyl is a less weathered version of Terrain B on Ida, but such an interpretation is at odds with Dactyl's redder spectral slope. Thus, the explanation for the color difference between Dactyl and Ida is likely to be different from that which accounts for the differences between the two terrains on Ida. Given that Dactyl and Ida have very similar photometric properties (Helfenstein, P., J. Veverka, P. C. Thomas, D. P. Simonelli, K. Klassen, T. V. Johnson, F. Fanale, J. Granahan, A. S. McEwen, M. J. S. Belton, and C. R. Chapman 1996Icarus120, 48–65), thus ruling out any dramatic texture differences between the two surfaces, the most likely explanation is that the satellite has a slightly different composition (more pyroxene?) than Ida. The spectral difference is within the range reported by Binzelet al.(Binzel, R. P., S. Xu, and S. J. Bus 1993.Icarus106, 608–611.) for members of the Koronis family, and could be caused by compositional inhomogeneities of the Koronis parent body rather than by post-breakup regolith processes.

UBVRI-Polarimetry of Asteroid 4179 Toutatis

UBVRI polarimetric observations of NEA 4179 Toutatis were carried out at the Crimean Astrophysical Observatory for six nights in December 1992-January 1993. Phase curves of polarization over the phase angle range 15.8-51.4° were obtained in each photometric band. Spectral dependences of positive polarization, polarimetric slope h, and inversion angle α inv in UBVRI wavelength range were obtained for the first time. The degree of the linear positive polarization decreases with the increase of wavelength supporting the inversion effect of the polarization spectral dependence of S-type asteroids. The obtained observation data show a fact unknown for asteroids before—the presence of Toutatis polarization unconnected with the scattering plane. For the first time the polarimetric estimate of geometric albedo p v = 0.13 was obtained. Along with other data this value points out that Toutatis is an S-type asteroid having a tentative size of about 3 km.

Bulk density of asteroid 243 Ida from the orbit of its satellite Dactyl

DURING its reconnaissance of the asteroid 243 Ida, the Galileo spacecraft returned images of a second object, 1993(243)1 Dactyl1— the first confirmed satellite of an asteroid. Sufficient data were obtained on the motion of Dactyl to determine its orbit as a function of Ida's mass. Here we apply statistical and dynamical arguments to constrain the range of possible orbits, and hence the mass of Ida. Combined with the volume of Ida2, this yields a bulk density of 2.6 ± 0.5 g cm−3. Allowing for the uncertainty in the porosity of Ida, this density range is consistent with a bulk chon-dritic composition, and argues against some (but not all) classes of meteoritic igneous rock types that have been suggested as compositionally representative of S-type asteroids like Ida.

Interplanetary dust particles collected from the stratosphere: physical, chemical, and mineralogical properties and implications for their sources

The suggestion that significant quantities of interplanetary dust are produced by both main-belt asteroids and comets is based on the Infrared Astronomical Satellite detection of dust trails or bands associated with these objects. Gravitational focusing strongly biases all near-Earth collections of interplanetary dust in favor of particles with the lowest geocentric velocities, that is the dust from main-belt asteroids spiraling into the Sun under the influence of Poynting-Robertson radiation drag. The major dust bands in the main-belt appear to be associated with the catastrophic disruptions which produced the Eos, Themis and Koronis families of asteroids. If dust particles are produced in the catastrophic collision process, then Poynting-Robertson radiation drag is such an efficient transport mechanism from the main-belt to 1 AU that near-Earth collections of interplanetary dust should include, and perhaps be dominated by, this material. The physical, chemical and mineralogical properties of this asteroidal dust can provide constraints on the properties of the asteroidal parent bodies. Interplanetary dust particles from 5 to 100 μm in diameter have been recovered from the stratosphere of the Earth by NASA sampling aircraft since the mid1970s. The densities of a large fraction of these interplanetary dust particles are significantly lower than the densities of their constituent silicate mineral phases, indicating significant porosities. Direct examination of ultra-microtome thin-sections of interplanetary dust particles also shows significant porosities. The majority of the particles are chemically and mineralogically similar to, but not identical to, the carbonaceous chondrite meteorites. Most stony interplanetary dust particles have carbon contents exceeding those of Allende, a carbonaceous chondrite meteorite having a low albedo. The population of interplanetary dust does not appear to exhibit the full range of compositional diversity inferred from reflection spectroscopy of the main-belt asteroids. In particular, higher albedo particles corresponding to S-type asteroids are underrepresented or absent from the stratospheric collections, and primitive carbonaceous particles seem to be overrepresented in the stratospheric collections compared to the fraction of mainbelt asteroids classified as primitive. This suggests that much of the interplanetary dust may be generated by a stochastic process, probably preferentially sampling a few most recent collisional events.

Polarimetry of asteroids

Polarimetric data for 4179 Toutatis were obtained during 23 nights from 10 December 1992 to 29 January 1993 at wavelengths of 0.41, 0.46, 0.54 and 0.70 μm. It is found that: (1) no peak of linear polarization P occurs, at least in our available data, between a phase angle α = 0.3° and 101.3°; (2) the inversion angle, where P = 0, is about 20°; and (3) the polarimetric slope near the inversion angle is about 0.1% deg −1. A comparison of our results with published polarimetric data for asteroids has confirmed that 4179 Toutatis is an S-type asteroid. Model calculations of the phase angle dependence of polarization suggest that Toutatis is covered by a coarse regolith on its inhomogeneous rough surfaces.