Lunar and Planetary Science XXXII (2001) 1611.pdf ASTEROID BULK DENSITY: IMPLICATIONS FOR THE STRUCTURE OF ASTEROIDS. D. T. Britt1 and G. J. Consolmagno SJ2, 1Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996, [email protected]. 2Vatican Observatory, V-00120, Vatican City State, [email protected]. Introduction: The numbers of asteroid bulk density teorite analogue microporosity from the bulk porosity of measurements have been rapidly increasing thanks to an asteroid provides a rough estimate of the asteroid’s spacecraft missions [1,2], observations of asteroid satel- large-scale macroporosity. An asteroids macroporosity lites [3,4], observations of asteroid mutual gravitational are the fractures, cracks, and voids that are large enough events [5,6,7], and radar observations. These data, along to effect its coherent strength. It is the estimate of with data on the grain densities of analogue meteorites macroporosity that can provide insight into the internal can provide significant insight on asteroid porosity and structure of an asteroid. Note that this estimate requires internal structure. Shown in Figure 1 are bulk density two assumptions: First that we know the asteroid’s me- measurements for 11 asteroids, two bulk density aver- teorite analog and, second, that the meteorites delivered ages for C and S type asteroids, and bulk density meas- to Earth are a representative sample of that material. urements for the small, asteroid-like Martian moons Both assumptions are open to debate for almost any of Phobos and Deimos. Also included for comparison are these asteroids so the estimates presented here should be the average grain densities of CI, CM, and CV carbona- viewed with healthy skepticism. ceous chondrite meteorites (shown as red labeled verti- Shown in Figure 2 are asteroidal estimated macropo- cal lines) and range of ordinary chondrite meteorite rosities for the measured asteroids. These data appear to grain densities (shown as a gray box). Note that the divide into three rough groups. The first group include definition of bulk density is the mass of an object di- the large asteroids 1 Ceres, 2 Pallas, and 4 Vesta. Their vided by its volume including the volume of its pore bulk densities are very close to the bulk densities of their spaces. Grain density is the mass of an object divided analogue meteorites, indicating essentially zero macro- by the volume occupied by the mineral grains, excluding porosity. These asteroids are probably strong, coherent the volume of its pore spaces. The difference between objects that have not been disrupted through solar sys- bulk and grain density provides insight into the porosity tem history. It is interesting that all the three asteroids of an object. with diameters > 500 km all fall in the zero macroporos- Discussion: Figure 1 shows the large range of as- ity group. teroid bulk densities that, in most cases, tend to be sub- The second group includes the S asteroids 433 Eros stantially below their most likely meteorite analogues, and 243 Ida as well as 762 Pulcova and 121 Hermione. indicating significant porosity. The bulk porosity of These asteroids have between 15 and 25% macroporos- these asteroids can be estimated by using the grain den- ity indicating that they have been extensively fractured. sity data of the analogue meteorites to constrain the Spacecraft images of both 433 Eros and 243 Ida show amount of pore space that would be required for a object numerous morphological indications of pervasive frac- of that composition and measured bulk density. For turing [8,4]. However, this fracturing was probably not example 433 Eros has a measured bulk density of 2.67 ± extensive enough to disrupt the object and asteroids with 0.03 g/cm3 and probably has an L-chondrite composition less than approximately 25% macroporosity probably which implies a grain density of 3.75 g/cm3 [8]. To have some measure of coherent strength. In terrestrial make the L-chondrite grain density consistent with the geology, well-sorted sedimentary rocks can have up to asteroidal bulk density would require a bulk porosity of 30% porosity and still be coherent. However, over 30% 28.8% [9]. Similarly, 253 Mathilde has a measured bulk porosity usually indicates loose rubble or soils (volcanic density of 1.3 ± 0.2 g/cm3 and a possible CM composi- scoria can have up to 90% porosity, but this is an tion that implies a grain density of 2.71 g/cm3 [2]. This unlikely analogue for most asteroids). gives a bulk porosity of 52%. The third group are those with greater than 30% Both these bulk porosity numbers seem high and are macroporosity. These objects are probably pervasively a bit misleading for assessing the structure of the aster- fractured and may have been disrupted and reassembled oid. We can take the analysis a step farther by recogniz- by mutual gravity, some of which have more empty space ing that most meteorites have some level of micron-scale than solid material. Asteroid 16 Psyche is likely the most microporosity (ordinary chondrites for instance average porous object observed so far. Its reflectance spectra about 10% microporosity, CM carbonaceous chondrites and radar albedo strongly indicate a metallic surface average about 12% microporosity) that does not seri- composition. Assuming a iron meteorite grain density 3 ously effect the meteorite’s cohesive strength. The im- of 7.4 g/cm , this would require a bulk porosity of 75%! plication is that microporosity would also not effect the Iron meteorites have essentially zero microporosity, so parent asteroid’s strength. Subtracting the average me- the bulk porosity would in this case likely equals the macroporosity suggesting a pervasively disrupted object Lunar and Planetary Science XXXII (2001) 1611.pdf ASTEROID BULK DENSITY D. T. Britt and G. J. Consolmagno SJ that has been loosely reassembled and held together by 314.10-127. [7] Hilton J. L. (1999) AJ, 177, 1077- mutual gravitation. 1086. [8] Veverka J. et al. (2000) Science, 289, 2088– References: [1] Yeomans D. K. et al. (2000) Sci- 2097. [9] Britt D. T. et al. (2001) This Volume. ence, 289, 2085–2088. [2] Veverka J. et al., (1997) Sci- ence, 278, 2109-2114. [3] Merline W. J. (1999) Nature, 401, 565-568. [4] Belton, M.J.S. et al., (1995) Nature 373, 785-788. [5] Viateau, B (2000) Ast. & Astrophy. 354, 725-731. [6] Standish E. M. et al. (1995) JPL IOM Figure 1: Asteroid and Meteorite Density 1E+22 y 1 Ceres (G) s 2 Pallas (B) 4 Vesta (V) 1E+20 16 Psyche (M) 45 Eugenia (C) 762 Pulcova (F) CM Grain Densit 121 Hermione (C) 1E+18 90 Antiope (C) Average C Average S Chondrite Grain Densitie y 253 Mathilde (C) n 243 Ida (S) CV Grain Density 1E+16 Mass in Kg (log scale) Phobos CI Grai 433 Eros (S) Ordinar Deimos 1E+14 0.511.522.533.54 Density (g/cm3) Figure 2: Estimated Macroporosity 1E+22 1 Ceres (G) 4 Vesta (V) 2 Pallas (B) 1E+20 16 Psyche (M) 762 Pulcova (F) 45 Eugenia (C) 121 Hermione (C) 90 Antiope (C) 1E+18 e Average S Average C 253 Mathilde (C) 243 Ida (S) 1E+16 Mass in Kg (log scale) 433 Eros (S) Phobos Rubble Piles? Coherent Objects? Deimos Transition Zon 1E+14 0% 10% 20% 30% 40% 50% 60% 70% 80% Macroporosity .