The Porosity of Dark Meteorites and the Structure of Low-Albedo Asteroids

Britt, D. T.

doi:10.1006/icar.2000.6374

Cited by 63 publications

(32 citation statements)

References 28 publications

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“…Solid densities derived for C-class targets are consistent with carbonaceous chondritic composition, as was previously determined for MBAs by Magri et al (1999). The Eros-calibrated estimates imply similarity to the hydrated CI and CM meteorite subtypes, whose grain densities range from 2.2 to 2.9 g cm-3 (Britt and Consolmagno, 2000); the uncalibrated estimates tend to be higher, suggesting instead a link with the anhydrous CO and CV subtypes (3.1 to 3.9 g cm-3).…”

Section: Methodsmentioning

confidence: 84%

“…We choose &, ]i d values as a function of taxonomic class based on the grain densities of meteoritic analogs, keeping in mind the weathering-based bias toward low measured grain densities ("Eros-Calibrated Method"). Our adopted solid densities are 3.75 g cm-3 for S, Q, and QSV objects and 3.3 g cm-3 for V. We choose 3.5 g cm-3 for C and BFGP, although some of these carbonaceous asteroids are likely to be hydrated (especially the G-class objects) and hence may have lower solid densities (Britt and Consolmagno, 2000). The M taxon is even more difficult to handle, given the large compositional difference between hydrated and unhydrated members (see "Comparison of Two Density-Estimation Methods and Compositional Inferences").…”

Section: Estimating Near-surface Porosities Introductionmentioning

confidence: 99%

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Radar constraints on asteroid regolith properties using 433 Eros as ground truth

Magri

Consolmagno

Ostrch

et al. 2001

Meteorit & Planetary Scien

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Abstract-Radar data enable us to estimate an asteroid's near-surface bulk density, thus providing a joint constraint on near-surface porosity and solid density. We investigate two different approaches to simplifying this joint constraint: estimating solid densities by assuming uniform porosities for all asteroids; and estimating porosities by assuming uniform mineralogy within each taxonomic class. Methods used to estimate asteroids' near-surface solid densities from radar data have not previously been calibrated via independent estimates. Recent spacecraft results on the chondritic nature of 433 Eros now permit such a check, and also support porosity estimation for S-class objects.We use radar albedos and polarization ratios estimated for 36 main-belt asteroids and nine nearEarth asteroids to estimate near-surface solid densities using two methods, one of which is similar to the uncalibrated algorithms used in previous studies, the other of which treats Eros as a calibrator. We also derive porosities for the same sample by assigning solid densities for each taxonomic class in advance. Density-estimation results obtained for Eros itself are consistent with the uncalibrated method being valid in the mean; those derived for the full sample imply that uncalibrated solid densities are, at most, a few tens of percent too large on average. However, some derived densities are extremely low, whereas most porosity estimates are physically plausible. We discuss the relative merits of these two approaches.

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Section: Methodsmentioning

confidence: 84%

Section: Estimating Near-surface Porosities Introductionmentioning

confidence: 99%

Radar constraints on asteroid regolith properties using 433 Eros as ground truth

Magri

Consolmagno

Ostrch

et al. 2001

Meteorit & Planetary Scien

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“…The high porosity of CI's (up to 35%) complicates the measurement of their densities, and different techniques yield different results. For the bulk and grain densities, Britt & Consolmagno (2000) give 2120 and 2270 kg m −3 , in that order, and Consolmagno et al (2008) give 1600 and 2460 kg m −3 . Specifically for the Orgueil CI1 chondrite, published values of the grain density are 2430 kg m −3 (Consolmagno & Britt 1998) and 2760 kg m −3 (Bland et al 2004), where the latter value being close to the density of 2870 kg m −3 for our mixture (Sect.…”

Section: Meteoritesmentioning

confidence: 92%

Constraints on the subsurface structure and density of the nucleus of Comet 67P/Churyumov-Gerasimenko from Arecibo radar observations

Kamoun

Lamy

Tóth

et al. 2014

A&A

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Context. Little is known about the internal structure of cometary nuclei. In addition to understanding their accretion in the early solar nebula and their subsequent evolution in the solar system, we find this question to be of acute and timely interest in the case of 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) due to be visited by the Rosetta spacecraft in the second half of 2014. In particular, the successful landing of the Philae surface module depends critically upon the bulk density of the nucleus and the structure of its surface layer. Aims. In addition to fostering our general knowledge of these properties, it is important to exploit all possible information to assist in preparing the delivery of Philae. Methods. We performed an in-depth analysis of the observations done with the radar system of the Arecibo Observatory in November 1982 when comet 67P/C-G had a close encounter with Earth at a geocentric distance of 0.4 AU taking our present knowledge of the properties of its nucleus (size, rotational state) into account. Results. In the absence of a detectable radar echo, we determined a maximum radar cross section of 0.7 km 2 , leading to a maximum radar albedo of 0.05. This low albedo probably results from a combination of a low radar reflectivity material and a lightly packed upper layer of the nucleus with substantial roughness (rms slope of ≈55• ), consistent with its low thermal inertia. Based on radar observations of other cometary nuclei and asteroids, it is unlikely that the albedo can be lower than 0.04 so that we were able to constrain the dielectric permittivity of the subsurface layer to a narrow range of 1.9 to 2.1. Laboratory measurements and our modeling of mixtures of ice and dust have led to a porosity in the range of approximately 55 to 65% and a density in the range of ≈600 to ≈1000 kg m −3 for the top ≈2.5 m layer of the nucleus. This would be the bulk density range for a homogeneous nucleus and would place the success of the landing at risk, but an inhomogeneous nucleus with an overall density below this range remains a possibility.

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“…While dark type asteroids are often assumed to have a high fraction of porosity (>40%) based on measured bulk densities and comparison with carbonaceous chondrites (their meteorite anologues; Barucci et al (2008)), bright type asteroids are expected to contain a smaller fraction of porosity (in particular microporosity), and this is particularly the case of E-type asteroids, even if no bulk density has yet been measured for any of them. However these bodies are generally supposed to be evolved, and their meteorite analogues to be enstatite achondrites (Britt & Consolmagno 2000), which do not consist of porous material. Therefore, while searching for internal structures that would permit the formation of a large crater, we tried to remain as close as possible to the common belief that E-type bodies do not contain a large fraction of microporosity.…”

Section: Models Of Steins' Internal Structurementioning

confidence: 99%

A large crater as a probe of the internal structure of the E-type asteroid Steins

Jutzi

Michel²,

Benz

2010

A&A

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Context. The detection of a large crater (2 km in diameter) on Steins, a diamond-shape asteroid with a diameter of only about 4.6 km, was a large surprise given the size of the asteroid itself. Steins belongs to the rare class of E-type asteroids, which had not been observed by any spacecraft before the Rosetta mission of the European Space Agency (ESA) in 2008. Aims. We demonstrate that this large crater places constraints on both the internal structure of Steins and its age based on crater counting. Methods. Numerical simulations of impacts were performed to reproduce the large crater, assuming four different initial internal structures of the asteroid: monolithic with or without microporosity, and rubble pile with or without microporosity. Results. To reproduce this crater, Steins must be either a rubble pile, which also contains microporosity, or a monolithic body with or without microporosity. The lack of porosity in meteorite analogues favors a structure without microporosity, which, according to our results, must have been monolothic. Moreover, the asteroid would be transformed into a rubble pile structure as a result of the crater formation, allowing it to be reshaped in its current shape by the YORP spin-up thermal effect. Conclusions. A combination of modeling and observations of surface features can thus serve as a probe of the internal structure of a small body and constrains its age estimate. Since the surface was totally refreshed when the large crater was formed, crater counting on Steins indicates the time that has passed since this impact event occurred.

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The Porosity of Dark Meteorites and the Structure of Low-Albedo Asteroids

Cited by 63 publications

References 28 publications

Radar constraints on asteroid regolith properties using 433 Eros as ground truth

Radar constraints on asteroid regolith properties using 433 Eros as ground truth

Constraints on the subsurface structure and density of the nucleus of Comet 67P/Churyumov-Gerasimenko from Arecibo radar observations

A large crater as a probe of the internal structure of the E-type asteroid Steins

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