Composition Of Asteroids Research Articles

Spheroidal and ellipsoidal harmonic expansions of the gravitational potential of small Solar System bodies. Case study: Comet 67P/Churyumov‐Gerasimenko

AbstractGravitational features are a fundamental source of information to learn more about the interior structure and composition of planets, moons, asteroids, and comets. Gravitational field modeling typically approximates the target body with a sphere, leading to a representation in spherical harmonics. However, small celestial bodies are often irregular in shape and hence poorly approximated by a sphere. A much better suited geometrical fit is achieved by a triaxial ellipsoid. This is also mirrored in the fact that the associated harmonic expansion (ellipsoidal harmonics) shows a significantly better convergence behavior as opposed to spherical harmonics. Unfortunately, complex mathematics and numerical problems (arithmetic overflow) so far severely limited the applicability of ellipsoidal harmonics. In this paper, we present a method that allows expanding ellipsoidal harmonics to a considerably higher degree compared to existing techniques. We apply this novel approach to model the gravitational field of comet 67P, the final target of the Rosetta mission. The comparison of results based on the ellipsoidal parameterization with those based on the spheroidal and spherical approximations reveals that the latter is clearly inferior; the spheroidal solution, on the other hand, is virtually just as accurate as the ellipsoidal one. Finally, in order to generalize our findings, we assess the gravitational field modeling performance for some 400 small bodies in the Solar System. From this investigation we generally conclude that the spheroidal representation is an attractive alternative to the complex ellipsoidal parameterization, on the one hand, and the inadequate spherical representation, on the other hand.

Spectral calibration for deriving surface mineralogy of Asteroid (25143) Itokawa from Hayabusa Near-Infrared Spectrometer (NIRS) data

We present spectral calibration equations for determining mafic silicate composition of near-Earth asteroid (25143) Itokawa from visible/near-infrared (VNIR) spectra (0.85–2.1μm) measured using the Near Infrared Spectrometer (NIRS) on board the Japanese Hayabusa spacecraft. Itokawa was the target of the Hayabusa sample return mission and has a surface composition similar to LL chondrites. Existing laboratory spectral calibrations (e.g., Dunn, T.L. et al. [2010]. Icarus 208, 789–797) use a spectral wavelength range that is wider (0.75–2.5μm) than that of the NIRS instrument (0.85–2.1μm), making them unfit for interpreting the Hayabusa spectral data currently archived in the Planetary Data System (PDS). We used laboratory near-infrared (NIR) reflectance spectra of ordinary chondrites (H, L and LL) from the study of Dunn et al. (Dunn, T.L. et al. [2010]. Icarus 208, 789–797), which we resampled to the NIRS wavelength range. Using spectral parameters extracted from these resampled spectra we established a relationship between band parameters and mafic silicate compositions (olivine and low-Ca pyroxene). We found a correlation >90% between mafic silicate compositions (fayalite and ferrosilite mol.%) estimated by our spectral method, and electron microprobe measured values from Dunn et al. (Dunn, T.L. et al. [2010]. Icarus 208, 789–797). The standard deviation between the measured and estimated values is 1.5 and 1.1mol.% for fayalite and ferrosilite, respectively. To test the validity of the new equations we blind tested them using nine laboratory spectra of L and LL chondrites with well-known compositions. We found that the absolute difference between the measured and computed values is in the range 0.1–1.6mol.%. Our study demonstrates that this new calibration is robust and can be applied to Hayabusa NIRS data despite its limited spectral range (0.85–2.1μm). We applied the equations to a subset of uncalibrated (no photometric corrections) NIRS spectra and we obtained fayalite and ferrosilite values that are consistent with Itokawa having a LL chondrite surface composition. We intend to develop a photometric model to calibrate the NIRS spectra to standard viewing geometry and apply the new equations to create a global mineralogical map of Itokawa.

The small binary asteroid (939) Isberga

In understanding the composition and internal structure of asteroids, their density is perhaps the most diagnostic quantity. We aim here at characterizing the surface composition, mutual orbit, size, mass, and density of the small main-belt binary asteroid (939) Isberga. For that, we conduct a suite of multi-technique observations, including optical lightcurves over many epochs, near-infrared spectroscopy, and interferometry in the thermal infrared. We develop a simple geometric model of binary systems to analyze the interferometric data in combination with the results of the lightcurve modeling. From spectroscopy, we classify Ibserga as a Sq-type asteroid, consistent with the albedo of 0.14-0.06+0.09 (all uncertainties are reported as 3-σ range) we determine (average albedo of S-types is 0.197±0.153, see Pravec et al. (Pravec et al. [2012]. Icarus 221, 365–387). Lightcurve analysis reveals that the mutual orbit has a period of 26.6304±0.0001h, is close to circular (eccentricity lower than 0.1), and has pole coordinates within 7° of (225°, +86°) in Ecliptic J2000, implying a low obliquity of 1.5-1.5+6.0deg. The combined analysis of lightcurves and interferometric data allows us to determine the dimension of the system and we find volume-equivalent diameters of 12.4-1.2+2.5km and 3.6-0.3+0.7km for Isberga and its satellite, circling each other on a 33km wide orbit. Their density is assumed equal and found to be 2.91-2.01+1.72gcm-3, lower than that of the associated ordinary chondrite meteorites, suggesting the presence of some macroporosity, but typical of S-types of the same size range (Carry [2012]. Planet. Space Sci. 73, 98–118). The present study is the first direct measurement of the size of a small main-belt binary. Although the interferometric observations of Isberga are at the edge of MIDI capabilities, the method described here is applicable to others suites of instruments (e.g., LBT, ALMA).

The composition of M-type asteroids II: Synthesis of spectroscopic and radar observations

This work updates and expands on results of our long-term radar-driven observational campaign of main-belt asteroids (MBAs) focused on Bus–DeMeo Xc- and Xk-type objects (Tholen X and M class asteroids) using the Arecibo radar and NASA Infrared Telescope Facilities (Ockert-Bell, M.E., Clark, B.E., Shepard, M.K., Rivkin, A.S., Binzel, R.P., Thomas, C.A., DeMeo, F.E., Bus, S.J., Shah, S. [2008]. Icarus 195, 206–219; Ockert-Bell, M.E., Clark, B.E., Shepard, M.K., Issacs, R.A., Cloutis, E.A., Fornasier, S., Bus, S.J. [2010]. Icarus 210, 674–692; Shepard, M.K. et al. [2008a]. Icarus 193, 20–38; Shepard, M.K. et al. [2008b]. Icarus 195, 184–205; Shepard, M.K. et al. [2010]. Icarus 215, 547–551). Eighteen of our targets were near-simultaneously observed with radar and those observations are described in Shepard et al. (Shepard, M.K. et al. [2010]. Icarus 215, 547–551). We combine our near-infrared data with available visible wavelength data for a more complete compositional analysis of our targets. Compositional evidence is derived from our target asteroid spectra using two different methods, a χ2 search for spectral matches in the RELAB database and parametric comparisons with meteorites. We present four new methods of parametric comparison, including discriminant analysis. Discriminant analysis identifies meteorite type with 85% accuracy. This paper synthesizes the results of these two analog search algorithms and reconciles those results with analogs suggested from radar data (Shepard, M.K. et al. [2010]. Icarus 215, 547–551). We have observed 29 asteroids, 18 in conjunction with radar observations. For eighteen out of twenty-nine objects observed (62%) our compositional predictions are consistent over two or more methods applied. We find that for our Xc and Xk targets the best fit is an iron meteorite for 34% of the samples. Enstatite chondrites were best fits for 6 of our targets (21%). Stony-iron meteorites were best fits for 2 of our targets (7%). A discriminant analysis suggests that asteroids with no absorption band can be compared to iron meteorites and asteroids with both a 0.9 and 1.9μm absorption band can be compared to stony-iron meteorites.

Overview of the composition of asteroid 4 Vesta: Constraints from the Dawn spacecraft mission and HEDs

Overview of the composition of asteroid 4 Vesta: Constraints from the Dawn spacecraft mission and HEDs

Surface composition and taxonomic classification of a group of near-Earth and Mars-crossing asteroids

In the past, constraining the surface composition of near-Earth asteroids (NEAs) has been difficult due to the lack of high quality near-IR spectral data (0.7–2.5μm) that contain mineralogically diagnostic absorption bands. Here we present visible (∼0.43–0.95μm) and near-infrared (∼0.7–2.5μm) spectra of nine NEAs and five Mars-crossing asteroids (MCs). The studied NEAs are: 4055 Magellan, 19764 (2000 NF5), 89830 (2002 CE), 138404 (2000 HA24), 143381 (2003 BC21), 159609 (2002 AQ3), 164121 (2003 YT1), 241662 (2000 KO44) and 2007 ML13. The studied MCs are: 1656 Suomi, 2577 Litva, 5407 (1992 AX), 22449 Ottijeff and 47035 (1998 WS). The observations were conducted with the NTT at La Silla, Chile, the 2.2m telescope at Calar Alto, Spain, and the IRTF on Mauna Kea, Hawai’i. The taxonomic classification (Bus system) of asteroids showed that all observed MC asteroids belong to the S-complex, including the S, Sr and Sl classes. Seven of the NEAs belong to the S-complex, including the S, Sa, Sk and Sl classes, and two NEAs were classified as V-types. The classification of the NEA 164121 (2003 YT1) as a V-type was made on the basis of its near-infrared spectrum since no visible spectrum is available for this asteroid. A mineralogical analysis was performed on six of the asteroids (those for which near-IR spectra were obtained or previously available). We found that three asteroids (241662 (2000 KO44), 19764 (2000 NF5), 138404 (2000 HA24)) have mafic silicate compositions consistent with ordinary chondrites, while three others (4055 Magellan, 164121 (2003 YT1), 5407 (1992 AX)) are pyroxene-dominated basaltic achondrite assemblages. In the case of 5407 (1992 AX) we found that its basaltic surface composition contrasts its taxonomic classification as a S-type.

Rotational spectra of (162173) 1999 JU3, the target of the Hayabusa2 mission

Context. The Hayabusa2 mission, which will be launched by JAXA in 2014, will return samples from the C-type near-Earth asteroid (162173) 1999 JU3. To better plan the mission, it is important to obtain as many physical characteristics of the asteroid as possible from ground-based observations. Moreover, these can then be calibrated with the in-situ and laboratory studies and be used to better understand other similar objects. Regarding the surface composition of the target asteroid, previous spectroscopic studies in the visible provided conflicting results for the possible presence of a deep absorption band, which is usually related to aqueous alteration processes.Aims. Our goal is to better understand the surface composition of asteroid (162173) 1999 JU3, and how it relates to the spectral differences observed by diverse authors at different epochs and telescopes. Moreover, to support the JAXA mission planning, we aim to constrain the level of aqueous alteration and thermal activity undergone by the object. Methods. The adopted methodology was to observe different regions of the surface of the asteroid that rotates around its axis. Spectroscopic observations that cover about 70% of its surface were therefore obtained at the SOAR telescope in Chile on 2012 July 9−10. Results. Our results indicate that the surface of asteroid (162173) 1999 JU3 presents featureless spectra with very little variation.Conclusions. No sign of an absorption feature that could be related to aqueous alteration processes is detectable in the observed spectra.

Workshop on Formation of the First Solids in the Solar System—Honoring Klaus Keil for his Distinguished Career in Meteoritics and Cosmochemistry

Workshop on Formation of the First Solids in the Solar System—Honoring Klaus Keil for his Distinguished Career in Meteoritics and Cosmochemistry

Mineralogies and source regions of near-Earth asteroids

Near-Earth Asteroids (NEAs) offer insight into a size range of objects that are not easily observed in the main asteroid belt. Previous studies on the diversity of the NEA population have relied primarily on modeling and statistical analysis to determine asteroid compositions. Olivine and pyroxene, the dominant minerals in most asteroids, have characteristic absorption features in the visible and near-infrared (VISNIR) wavelengths that can be used to determine their compositions and abundances. However, formulas previously used for deriving compositions do not work very well for ordinary chondrite assemblages. Because two-thirds of NEAs have ordinary chondrite-like spectral parameters, it is essential to determine accurate mineralogies. Here we determine the band area ratios and Band I centers of 72 NEAs with visible and near-infrared spectra and use new calibrations to derive the mineralogies 47 of these NEAs with ordinary chondrite-like spectral parameters. Our results indicate that the majority of NEAs have LL-chondrite mineralogies. This is consistent with results from previous studies but continues to be in conflict with the population of recovered ordinary chondrites, of which H chondrites are the most abundant. To look for potential correlations between asteroid size, composition, and source region, we use a dynamical model to determine the most probable source region of each NEA. Model results indicate that NEAs with LL chondrite mineralogies appear to be preferentially derived from the ν6 secular resonance. This supports the hypothesis that the Flora family, which lies near the ν6 resonance, is the source of the LL chondrites. With the exception of basaltic achondrites, NEAs with non-chondrite spectral parameters are slightly less likely to be derived from the ν6 resonance than NEAs with chondrite-like mineralogies. The population of NEAs with H, L, and LL chondrite mineralogies does not appear to be influenced by size, which would suggest that ordinary chondrites are not preferentially sourced from meter-sized objects due to Yarkovsky effect.

Composition of near-Earth Asteroid (4179) Toutatis

Surface composition of near-Earth Asteroid (4179) Toutatis is consistent with an undifferentiated L-chondrite composition. This is inconsistent with early observations that suggested high pyroxene iron content and a differentiated body.

Effects of NEO composition on deflection methodologies

During the early stages of NEO mitigation campaign planning, it is indispensable to characterise the hazardous object. However, exhaustive characterisation in close proximity to the object may not always be feasible when the lead time is in the order of a few years to a decade, hence causing uncertainty in its physical and compositional properties. This study aims to evaluate the reliability and robustness of different deflection methodologies subject to uncertainty in the asteroid composition. The Evidence Theory is used to quantify epistemic uncertainty in the asteroid properties. The kinetic impact, nuclear intercept, and the solar sublimation [mitigation approaches] are applied to a set of virtual Earth-threatening asteroids with different Keplerian elements, sizes (30–150m), and mission lead times (5–18 years) with the goal of a minimum 2½ Earth-radii deviation distance. A typical S-type rubble pile configuration is used as the baseline asteroid composition for the study.

Composition of near-Earth Asteroid 2008 EV5: Potential target for robotic and human exploration

We observed Potentially Hazardous Asteroid (PHA) 2008 EV5 in the visible (0.30–0.92μm) and near-IR (0.75–2.5μm) wavelengths to determine its surface composition. This asteroid is especially interesting because it is a potential target for two sample return mission proposals (Marco Polo-R and Hayabusa-2) and human exploration due to its low delta-v for rendezvous. The spectrum of 2008 EV5 is essentially featureless with exception of a weak 0.48-μm spin-forbidden Fe3+ absorption band. The spectrum also has an overall blue slope. The albedo of 2008 EV5 remains uncertain with a lower limit at 0.05 and a higher end at 0.20 based on thermal modeling. The Busch et al. (Busch et al. [2011]. Icarus 212, 649–660) albedo estimate of 0.12±0.04 is consistent with our thermal modeling results. The albedo and composition of 2008 EV5 are also consistent with a C-type taxonomic classification (Somers, J.M., Hicks, M.D., Lawrence, K.J. [2008]. Bull. Am. Astron. Soc. 40, 440). The best spectral match is with CI carbonaceous chondrites similar to Orgueil, which also have a weak 0.48-μm feature and an overall blue slope. This 0.48-μm feature is also seen in the spectrum of magnetite. The albedo of CI chondrites is at the lower limit of our estimated range for the albedo of 2008 EV5.

Constraining albedo, diameter and composition of near-Earth asteroids via near-infrared spectroscopy

We present a method to constrain the albedo and diameters of near-Earth asteroids (NEAs) based on thermal flux in their near-infrared spectra (0.7–2.5μm) using the Standard Thermal Model. Near-infrared spectra obtained with the SpeX instrument on NASA Infrared Telescope Facility are used to estimate the albedo and diameters of 12 NEAs (1992 JE, 1992 UY4, 1999 JD6, 2004 XP14, 2005 YY93, 2007 DS84, 2005 AD13, 2005 WJ56, 1999 JM8, 2005 RC34, 2003 YE45, and 2008 QS11). Albedo estimates were compared with average albedo for various taxonomic classes outlined by Thomas et al. (Thomas, C.A. et al. [2011]. Astron. J. 142(3)) and are consistent with their results. Spectral band parameters, like band centers, are derived and compared to spectra of laboratory mineral mixtures and meteorites to constrain their composition and possible meteorite analogs. Based on our study we estimate the albedos and diameters of these NEAs and compare them with those obtained by other techniques such as ground-based mid-infrared, Spitzer thermal infrared and Arecibo radar. Our results are broadly consistent with the results from other direct methods like radar. Determining the compositions of low albedo asteroids is a challenge due to the lack of deep silicate absorption features. However, based on weak absorption features and albedo, we suggest possible meteorite analogs for these NEAs, which include black chondrites, CM2 carbonaceous chondrites and enstatite achondrites. We did not find any specific trends in albedo and composition among the asteroids we observed.

The Surface Composition and Temperature of Asteroid 21 Lutetia As Observed by Rosetta/VIRTIS

The Visible, InfraRed, and Thermal Imaging Spectrometer (VIRTIS) on Rosetta obtained hyperspectral images, spectral reflectance maps, and temperature maps of the asteroid 21 Lutetia. No absorption features, of either silicates or hydrated minerals, have been detected across the observed area in the spectral range from 0.4 to 3.5 micrometers. The surface temperature reaches a maximum value of 245 kelvin and correlates well with topographic features. The thermal inertia is in the range from 20 to 30 joules meter(-2) kelvin(-1) second(-0.5), comparable to a lunarlike powdery regolith. Spectral signatures of surface alteration, resulting from space weathering, seem to be missing. Lutetia is likely a remnant of the primordial planetesimal population, unaltered by differentiation processes and composed of chondritic materials of enstatitic or carbonaceous origin, dominated by iron-poor minerals that have not suffered aqueous alteration.

Ptolemy operations and results during the Lutetia flyby

The September 5th 2008 Rosetta flyby of asteroid 2867 Šteins raised tantalising questions concerning the possibility of outgassing and extant exospheres surrounding small bodies within the asteroid belt. With this in mind and recognising the size and composition of asteroid 21 Lutetia it was decided to attempt a detection of such possible outgassing using a number of Rosetta instruments, including Ptolemy, a miniature gas chromatograph isotope ratio mass spectrometer aboard the lander Philae. Ptolemy collected 5 sets of mass spectra at hourly intervals during the Lutetia encounter; 2 background measurements either side of close approach and an exosphere detection measurement as the spacecraft passed over the subsolar point. An increase in pressure was detected at the subsolar point but the cause (exosphere or spacecraft) was unresolved with the realisation that any such detection is hindered by outgassing from the spacecraft and instrument itself. Constraints are placed on the properties of Lutetia exosphere, structure and production rates. The experience gained is used to provide suggestions for future missions or spacecraft events aimed at the detection and characterisation of the exospheres of Solar System bodies.

The properties of asteroid (2867) Steins fromSpitzerSpace Telescope observations and OSIRIS shape reconstruction

We report on the thermal properties and composition of asteroid (2867) Steins derived from an analysis of new Spitzer Space Telescope (SST) observations performed in March 2008, in addition to previously published SST observations performed in November 2005. We consider the three-dimensional shape model and photometric properties derived from OSIRIS images obtained during the flyby of the Rosetta spacecraft in September 2008, which we combine with a thermal model to properly interpret the observed SST thermal light curve and spectral energy distributions. We obtain a thermal inertia in the range 100\pm50 JK-1m-2s-1/2 and a beaming factor (roughness) in the range 0.7-1.0. We confirm that the infrared emissivity of Steins is consistent with an enstatite composition. The November 2005 SST thermal light curve is most reliably interpreted by assuming inhomogeneities in the thermal properties of the surface, with two different regions of slightly different roughness, as observed on other small bodies, such as the nucleus of comet 9P/Tempel 1. Our results emphasize that the shape model is important to an accurate determination of the thermal inertia and roughness. Finally, we present temperature maps of Steins, as seen by Rosetta during its flyby, and discuss the interpretation of the observations performed by the VIRTIS and MIRO instruments.

The composition of M-type asteroids: Synthesis of spectroscopic and radar observations

We have conducted a radar-driven observational campaign of 22 main-belt asteroids (MBAs) focused on Bus–DeMeo Xc- and Xk-type objects (Tholen X and M class asteroids) using the Arecibo radar and NASA Infrared Telescope Facilities (IRTF). Sixteen of our targets were near-simultaneously observed with radar and those observations are described in a companion paper (Shepard, M.K., and 19 colleagues [2010]. Icarus, in press). We find that most of the highest metal-content asteroids, as suggested by radar, tend to exhibit silicate absorption features at both 0.9 and 1.9 μm, and the lowest metal-content asteroids tend to exhibit either no bands or only the 0.9 μm band. Eleven of the asteroids were observed at several rotational longitudes in the near-infrared and significant variations in continuum slope were found for nine in the spectral regions 1.1–1.45 μm and 1.6–2.3 μm. We utilized visible wavelength data (Bus, S.J., Binzel, R.P. [2002b]. Icarus 158, 146–177; Fornasier, S., Clark, B.E., Dotto, E., Migliorini, A., Ockert-Bell, M., Barucci, M.A. [2010]. Icarus 210, 655–673.) for a more complete compositional analysis of our targets. Compositional evidence is derived from our target asteroid spectra using two different methods: (1) a χ 2 search for spectral matches in the RELAB database, and (2) parametric comparisons with meteorites. This paper synthesizes the results of the RELAB search and the parametric comparisons with compositional suggestions based on radar observations. We find that for six of the seven asteroids with the highest iron abundances, our spectral results are consistent with the radar evidence (16 Psyche, 216 Kleopatra, 347 Pariana, 758 Mancunia, 779 Nina, and 785 Zwetana). Three of the seven asteroids with the lowest metal abundances, our spectral results are consistent with the radar evidence (21 Lutetia, 135 Hertha, 497 Iva). The remaining seven asteroids (22 Kalliope, 97 Klotho, 110 Lydia, 129 Antigone, 224 Oceana, 678 Fredegundis, and 771 Libera) have ambiguous compositional interpretations when comparing the spectral analogs to the radar analogs. The number of objects with ambiguous results from this multi-wavelength survey using visible, near-infrared, and radar wavelengths indicates that perhaps a third diagnostic wavelength region (such as the mid-infrared around 2–4 μm, the mid-infrared around 10–15 μm, and/or the ultraviolet around 0.2–0.4 μm) should be explored to resolve the discrepancies.

Volatile element chemistry during metamorphism of ordinary chondritic material and some of its implications for the composition of asteroids

We used chemical equilibrium calculations to model thermal metamorphism of ordinary chondritic material as a function of temperature, pressure, and trace element abundance and use our results to discuss volatile mobilization during thermal metamorphism of ordinary chondrite parent bodies. We compiled trace element abundances in H-, L-, and LL-chondrites for the elements Ag, As, Au, Bi, Br, Cd, Cs, Cu, Ga, Ge, I, In, Pb, Rb, Sb, Se, Sn, Te, Tl, and Zn, and identified abundance trends as a function of petrographic type within each class. We calculated volatility sequences for the trace elements in ordinary chondritic material, which differ significantly from the solar nebula volatility sequence. Our results are consistent with open-system thermal metamorphism. Abundance patterns of Ag and Zn remain difficult to explain.

ASTRONOMY Found meteorites could aid asteroid studies

Graced by astronomical serendipity, scientists now have in their hands the fragments of an asteroid they’d tracked, from its journey through space to its collision with Earth last October The meteorites sprayed over the Nubian Desert in northern Sudan ( Nature 2009, 458 , 485). Asteroids contain the best record of the origin of the solar system, but scientists have not been able to link them to the meteorites they’ve collected so far. The location, size, and composition of asteroids are needed to study their roles in solar system formation. “We have 40,000 samples of meteorites, but we don’t know where a single one of them came from,” Michael E. Zolensky, a cosmic mineralogist at NASA’s Goddard Space Flight Center, in Greenbelt, Md., said at a press conference held on March 24. “What’s different this time is that we know exactly which asteroid these meteorites came from,” he said. Astronomers followed the asteroid’s trajectory as it broke ...

Sky and ground observations determine an asteroid's composition

Sky and ground observations determine an asteroid's composition Sky and ground observations determine an asteroid's composition