& Planetary Science 47, Nr 3, 319–329 (2012) doi: 10.1111/j.1945-5100.2012.01331.x

Stony thermal properties and their relationship with meteorite chemical and physical states

C. P. OPEIL SJ1, G. J. CONSOLMAGNO SJ2*, D. J. SAFARIK3, and D. T. BRITT4

1Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467–3804, USA 2Specola Vaticana, V-00120, Vatican City, Vatican City State 3Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 4Department of Physics, University of Central Florida, Orlando, Florida 32816–2385, USA *Corresponding author. E-mail: [email protected] (Received 20 July 2011; revision accepted 26 December 2011)

Abstract–In our ongoing survey of meteorite physical properties, we have to date measured the thermal conductivity for seventeen stony at temperatures ranging from 5 K to 300 K. Here, we report new results for nine ordinary , one enstatite , and the basaltic Frankfort () and Los Angeles (shergottite). We find that thermal conductivity is significantly lower than would be expected from averaging the laboratory conductivities of their constituent minerals, with a dependence on temperature different from the expected conductivity of pure minerals. In addition, we find a linear relationship between the inverse of the porosity of the samples measured and their thermal conductivity, regardless of meteorite composition or type. We conclude that thermal conductivity is controlled by the presence of shock-induced microcracks within the meteorites, which provide a barrier to the transmission of thermal energy via phonons. In contrast to conductivity, our first measurement of heat capacity as a function of temperature (on Los Angeles) suggests that heat capacity is primarily a function of oxide composition and is not strongly affected by the physical state of the sample.

INTRODUCTION vibrational energy called phonons. The transmission of phonons depends on the material within the sample, and Thermal properties are essential parameters in is expected to be a strong function of temperature modeling a number of important processes in the solar especially at temperatures below 300 K. At very low system. For example, the Yarkovsky and YORP effects temperatures, below 50 K, theory for crystalline materials on orbital and spin perturbations depend on the (e.g, insulators) where phonons dominate predicts that inverse thermal inertia, which varies with the square root thermal conductivity should be controlled by the phonon of thermal conductivity and heat capacity. The thermal activation energy; as temperature increases, the production evolution of and the sublimation rates of of phonons increases, resulting in values of the thermal , icy satellites, and other icy bodies thought to conductivity increasing as T3. Above 100 K, as the number have a significant meteorite-like rocky component will of phonons increases, they become more likely to scatter depend on the thermal diffusivity, which is linearly off each other, resulting in a reduction in conductivity dependent on thermal conductivity divided by heat that varies as 1 ⁄ T. Thus, one expects conductivity to capacity. In addition to its utility to modelers, however, increase sharply from temperatures near absolute zero the thermal properties of meteorites provide an important to a maximum around 50 K, above which the conductivity way of characterizing the meteorite’s composition and its drops off by a factor that closely approaches 1 ⁄ T at physical state (Consolmagno et al. 2008; Opeil et al. temperatures above 200 K. This is in fact the pattern 2010). seen in the thermal conductivity we have measured for Thermal conductivity, a sample’s ability to transport two enstatite chondrites (out of the 17 samples in this heat, is usually modeled as the passage of packets of study).

319 The , 2012. 320 C. P. Opeil et al.

Amorphous materials and metals, on the other hand, 2.5 W mK)1 and that of enstatite and is 4.5 to show a different behavior; for instance, we found a 5WmK)1 (comparable to our value). continually rising conductivity over this temperature Our first results were consistent with earlier work on range in the (Opeil et al. ordinary chondrites by Matsui and Osako (1979) and 2010). Yomogida and Matsui (1983), who had derived values of Phonons will also be scattered by metal grains, grain thermal conductivity from measurements of thermal boundaries, and isolated pore spaces. Thus, one should diffusivity. (We note as well ongoing diffusivity expect the thermal conductivity to also be a function of measurements by Szurgot and collaborators; c.f. Szurgot the metal content and porosity of the sample. But [2011]. Their results, published to date in abstract form, perhaps even more importantly, phonons may encounter promise to extend significantly the number of data points larger barriers to thermal transport such as sheet cracks for meteorite thermal diffusivity at room temperature.) and porosity caused by shock (cf. Friedrich et al. 2008); However, those authors had found that thermal the presence of such cracks should result in a significant conductivities ranged from sample to sample over nearly drop in thermal conductivity. an order of magnitude even within a given meteorite Measuring heat capacity allows us access to the class. Furthermore, as our original results only measured intrinsic characteristic thermodynamic property of a one sample of each meteorite type, it was not possible to particular substance. Specific heat capacity, C, is the draw conclusions about the effects on conductivity of fundamental measurable physical quantity that properties other than composition. Thus, our next step characterizes the amount of heat energy (dQ) required to has been to measure more ordinary chondrites, including change the temperature (dT) of a unit mass of substance multiple samples of the same meteorite, to look for such (m) by a given amount; expressed in simple mathematical variations within a class that can be related to other ) ) terms, C = m 1 dQ ⁄ dT with SI units of [J (kg K 1)]. physical properties of the meteorite. This heat is contained within the minerals via the various In addition, with the interest in the thermal vibrational modes of the components present, and thus evolution of basaltic parent bodies, including Mars and one expects that the heat capacity of a meteorite should the HED (concomitant with the arrival of be primarily a function of its composition, with little the Dawn spacecraft at asteroid 4 Vesta), the dependence on its physical state. As with phonons, these measurement of basaltic meteorite conductivities has vibrational states are suppressed at lower temperatures. become of recent interest. Thus, we have included a For this reason, it is essential for a complete understanding shergottite, thought to be from Mars, and one of the of meteorite formation and asteroid evolution that this putative Vesta meteorites in the suite measured here. All quantity be measured over a large temperature range, most meteorite samples were provided from the Vatican importantly at low temperatures that reflect the actual meteorite collection. temperature environment of the formation and evolution of this material in the solar system. MEASUREMENT TECHNIQUE

PREVIOUS RESULTS We have measured seven new chondrites and two basaltic meteorites for this study. They include the EL6 In an earlier paper (Opeil et al. 2010), we reported meteorite Pillistfer (our previous enstatite meteorite, the measurement of the thermal conductivity at low Abee, is an EH); the H5 chondrites Barbotan, temperatures (5 K to 300 K) of five stony meteorites: Collescipilli, and Pułtusk and the H6 La Cienega (we had Chronstad (H5), Lumpkin (L6), Abee (EH4), Northwest previously measured Cronstad, H5); and the L6 Africa 5515 (CK4 find), and Cold Bokkeveld (CM2). chondrites Bath Furnace and Holbrook (we had (An iron meteorite, Campo de Cielo, was also discussed previously measured Lumpkin, L6). The two basaltic in that paper.) The conductivity of all the stony samples meteorites included in our study are the shergottite Los except the Abee gave values that were Angeles and the howardite Frankfort. lower by as much as an order of magnitude compared All the ordinary chondrites except La Cienega are with conductivity values reported in the literature for falls, but most date from the 19th century (the oldest, pure minerals (Clauser and Huenges 1995). In addition, Barbotan, fell in 1790; the most recent, Holbrook, in we found that all except Abee had conductivities that 1912) and show varying degrees of terrestrial weathering, were nearly constant with temperature above 100 K. The as can be seen in Fig. 1. La Cienega was found in 2007 L and CK sample conductivities at 200 K were both and shows little evidence of weathering. close to 1.5 W mK)1, that of the H was 1.9 W mK)1, Among the ordinary chondrites, two different and that of the CM sample was 0.5 W mK)1;by samples of the L chondrites Bath Furnace and Holbrook contrast, the literature value at 300 K for serpentine is were measured. In addition, one of the Bath Furnace Meteorite thermal properties 321

previously reported. In both cases, the characteristic peak conductivity is seen near 50 K, followed by the expected 1 ⁄ T drop off. The conductivity found at room temperature is comparable to that of enstatite measured at room temperature. Note that above 100 K, there is little difference in the conductivity seen for this sample and that of Abee. Although both meteorites conduct heat the way that one would expect for samples of pure enstatite, this correspondence is only coincidental; both meteorites are in fact quite different physically and compositionally from pure minerals. According to the literature (c.f. Grady 2000), Abee is a , with a shock state of the various components ranging from S2 to S5, and it is 32.5% by mass iron. The reported metal content for Pillistfer is only slightly lower, 27.8%, and it has a Fig. 1. Sample of Bath Furnace mounted for measurement. reported shock state of S2. Of the samples from the Vatican collection that we measured, Abee is notable for its jet-black color, evidence of shock blackening, while samples was rotated by 90 and remeasured to determine Pillistfer has a light gray color, probably the least shocked whether there was a significant anisotropy in the of the enstatite chondrites in the Vatican collection. In conductivity of this meteorite. spite of its reported lower metal content, in fact, our As with our previous work (Opeil et al. 2010), all sample of Pillistfer has a higher magnetic susceptibility, measurements were made using a Quantum Design 5.43 (log units), compared with 5.3 measured for our Physical Property Measurement System, Thermal sample of Abee (Rochette et al. 2008). Our sample of Transport Option (QD-PPMS–TTO) that allows thermal Abee has a porosity of 2.19 ± 1.5% (Macke et al. 2010). conductivity measurements [W ⁄ m-K] in a temperature No porosity data exist for the particular sample of range of 2–400 K; the system utilizes a basic cryogenic ⁄ Pillistfer measured (it being too small for reliable volume field system to establish precise control of temperature measurements) but Macke et al. (2010) report porosity (Dilley et al. 2002). A QD option P640 High-Vacuum measurements for four other samples of Pillistfer, which system adsorption pump in the cryogenic dewar range from zero to 5.2% with an average of 2.4%. Both thermally isolates the sample. The accuracy of the porosities are significantly lower than typical porosities measurements has been confirmed on a 7740 Pyrex for ordinary or carbonaceous chondrites. standard, and temperature calibrations were performed using the Quantum Design Ni-alloy standard. Ordinary Chondrites All samples were cut into 0.5–1 cm prisms, to which gold-coated, oxygen-free, high-conductivity copper Matsui and Osako (1979) and Yomogida and Matsui (OFHC-Cu) disks were attached with silver (Ag) epoxy. (1983) reported the conductivities of 21 ordinary Thermal conductivity is determined by applying a heat chondrites by measuring thermal diffusivity at six pulse ‘‘Q’’ from a heater attached to one end of the sample temperatures from 100 to 350 K, assuming a heat to create a user-specified temperature difference between capacity, and calculating from that the thermal two calibrated Cernox thermometers located at the sample conductivity. Their values ranged from less than ends. Heat flows out of the sample into a cold-foot located 0.5 W mK)1 up to nearly 4 W mK)1; L chondrites were on the sample puck. The sample is held in a cryostatic generally lower in conductivity than H chondrites, but chamber whose temperature is automatically stepped from there was a notable overlap among the two groups. We ) 300 K to below 5 K at a rate of 0.5 K min 1 in a vacuum have plotted their results as shaded regions in Figs. 2 and 3. ) (pressure is held to <1.33 · 10 4 Pa). As seen in Fig. 2, the results from Opeil et al. (2010) fell in the lower range of their values. However, we find (see RESULTS Fig. 3) that our new measurements fill the entire range of meteorite conductivities previously reported by the Enstatite Chondrite Matsui group. There is a wide range, and great overlap, of thermal conductivities among classes. Our results for the EL6 chondrite Pillistfer are Furthermore, we find a significant range in consistent with those of the EH chondrite Abee conductivities when measuring different samples of 322 C. P. Opeil et al.

meteorites Frankfort, a howardite (likely to be typical of material on the surface of asteroid 4 Vesta) and Los Angeles, a shergottite (believed to have originated on Mars). From 300 K to 100 K, the conductivity of Frankfort decreases gradually from 1.6 to 1.2 W mK)1; that of Los Angeles drops from 0.9 to 0.5 W mK)1. At lower temperatures, the thermal conductivity of both meteorites continues to drop, but more rapidly, to values below 0.1 W mK)1 at 10 K. We find that for both meteorites, the conductivity is significantly lower than would be expected from averaging the laboratory conductivities of their constituent minerals. These results are similar to results from measurements of ordinary chondrites. The monotonic decrease in conductivity with temperature over this range is different from the expected conductivity of pure Fig. 2. Previous results for thermal conductivities of meteorites minerals, which tend to vary as 1 ⁄ T. This indicates that, from Opeil et al. (2010), compared with terrestrial minerals near 300 K indicated with diamonds (data from Clauser and in common with the ordinary chondrites, the Huenges 1995) and those published by Matsui and Osako conductivity we measure is controlled by the physical (1979) and Yomogida and Matsui (1983), indicated by the structure of the meteorites, presumably the presence of shaded regions labeled as Y&M. shock-induced microcracks that provide a barrier to the transmission of thermal energy via phonons. Thus, these measurements should accurately describe the nominally the same material, or indeed even the same conductivity of any material in the sampled regolith of sample, as is illustrated in Fig. 4. For example, looking their parent bodies. The texture and mineralogy of at conductivities at 200 K (in the temperature range Frankfort are characteristic of a breccia that mixes where conductivity is generally constant with surface and subsurface materials in a shock-lithified temperature), we find that one sample of Holbrook (L6) matrix, which is typical of regolith material. The cooling has a conductivity of only 0.44 W mK)1, while the rates calculated for Los Angeles are significantly slower conductivity of a second sample of the same meteorite is than other shergottites (Rubin et al. 2000), suggesting nearly three times that value, at 1.2 W mK)1. Likewise, that it was formed at depth. However, results from one piece of Bath Furnace (L6) has a conductivity at surface samples or samples transported by impact from 200 K of 2.3 W mK)1, while a second piece of the same the interior of a body (and thus presumably strongly meteorite showed conductivities of 2.7 and 3.2 W mK)1 shocked in the process) may significantly underestimate when measured in two orthogonal directions. the actual conductivity of material deeper in the parent Note that both the Holbrook and Bath Furnace bodies, depending on how characteristically the shock meteorites are the same chemical and petrographic type, history of these meteorites reflects material still in place L6, yet one piece of Bath Furnace measured in one inside these bodies. direction is more than seven times as conductive as one The thermal conductivities of meteorites measured of the Holbrook pieces. These differences are far greater here (except for the enstatite chondrite Pillistfer) can be than any measurement error (which are generally on the fit well by a fourth-order quadratic of the form order of 0.01 W mK)1), and must represent differences K = A + BT + CT2 + DT3 + ET4. The fits are in structure in the conductivity within the samples, shown in Fig. 5 and the values of the coefficients A including significant anisotropy, as seen in the second through E for each sample are listed in Table 1. Bath Furnace sample. It is clear from these results that the major determinant in the thermal conductivity of CONDUCTIVITY, METAL CONTENT, stony meteorites is their physical state, rather than their AND POROSITY chemical composition. One of the major constituents of ordinary chondrites Basaltic Achondrites is metallic iron, which is a much better conductor of heat than other minerals. And while the orientation of the We also measured the thermal conductivity at pore spaces obviously would control how they affect temperatures ranging from 300 K to 5 K for the basaltic conductivity, the likelihood of finding any such cracks Meteorite thermal properties 323

Fig. 3. Thermal conductivity of meteorites as a function of temperature. New results (this paper) are indicated with solid symbols; results previously reported in Opeil et al. (2010) are open symbols. Different classes of meteorites are indicated by the different colors, and the individual samples are identified, to the right, in the order of their conductivity at 300 K, from highest to lowest conductivities. should be related to the average porosity of the sample. where P is the fractional porosity of the sample. This Thus, it is instructive to compare our conductivity values relationship is also seen when the data from Yomogida with those of the average porosity measured for the hand and Matsui (1983) are included, as is shown in Fig. 6b; samples, and with magnetic susceptibility, which is here, however, two of their data points of very low directly related to the average metal content of an porosity lower the confidence level of the result, which is ordinary chondrite. k = 3.8 + 6.9 ⁄ P, R = 0.81. We have measured porosities in hand samples of The fact that one sample, Bath Furnace, gave two twelve of the meteorites in our data set, and the Matsui significantly different results when measured in two group likewise measured porosities for many of their different directions immediately tells us that one cannot samples (see Table 2). In Fig. 6a, we see that there is a expect a simple, perfect relationship between thermal strong linear relationship between thermal conductivity conductivity and any intensive property such as at 200 K (where the values for most of our samples are mineralogy or average porosity, as the conductivity will not strong functions of temperature) and the inverse of vary with the orientation of the pore cracks, at least for the porosity. Even recognizing that this simple sample sizes comparable to the extent of the cracks. relationship does not take into account the anisotropic However, given the multiple impact history that effects of shock-induced cracks (note, for example, the asteroids have experienced, one should expect that on three values of Bath Furnace seen with the value of scales much larger than the extent of individual cracks 1 ⁄ porosity = 23), there is a strong correlation (at the (and much larger than the millimeter-sized samples R = 0.95 level) between inverse porosity and measured here), the orientation of cracks will be random, conductivity in our data set; a line fit through our data and thus an average relationship between porosity and gives a relationship of conductivity k = 3.6 + 6.8 ⁄ P conductivity, as indicated here, should be a reasonable 324 C. P. Opeil et al.

Likewise, our least-conductive meteorite is the water and volatile rich CM sample, Cold Bokkeveld; but this sample is also the most porous one for which a thermal conductivity has been measured. Thus, while it is possible that the volatile and water content may have an effect on its conductivity, as in the case of metal content, we cannot disentangle the effect of composition from that of porosity. More data are needed to explore the relationship further.

SHERGOTTITE HEAT CAPACITY

Given that thermal diffusivity and thermal inertia both depend equally on conductivity and heat capacity, it is clear that measuring heat capacity for meteorites is critical. One essential problem in our understanding of heat capacity is that, like thermal conductivity, it is expected to vary strongly (by a factor of two or more) Fig. 4. Two samples each of the L chondrites Bath Furnace over the temperature range expected for the interiors (squares and triangles) and Holbrook (circles) were measured. of asteroids and small bodies. However, to date, heat In addition, one Bath Furnace sample (marked above with triangles as Bath Furnace 2 and Bath Furnace 3) was rotated capacity has been even less well studied than thermal 90 and measured again. Significantly different results were conductivity. The few published values for heat capacity found, suggesting that thermal conductivity is controlled by the have only been measured at or above room temperature. anisotropic physical state of the material, most likely crack Matsui and Osako (1979) directly measured the heat sheets, and not by composition. capacity of five Yamato meteorites (four ordinary chondrites and a howardite) at 300 K, 350 K, and 400 K, while Yomogida and Matsui (1983) used laboratory description of the typical conductivity to be found within data of the constituent minerals of ordinary chondrites an asteroid. to calculate their heat capacities; their calculated Szurgot (2011) reported the thermal conductivity at values, which they preferred, were 50% higher than room temperature for a number of stony iron and iron their directly measured results. meteorites as well as stones, and showed that conductivity In the past year, we measured the heat capacity of one over this range can be linearly related to density; clearly, meteorite, the shergottite Los Angeles, at the Los Alamos the amount of iron present in a stony iron meteorite National Laboratory using the Quantum Design Physical directly affects its thermal properties. Does this trend Properties Measurement System (QD-PPMS) similar to continue within the various classes of stony meteorites? that used for our thermal conductivity measurements, but The magnetic susceptibility of stony meteorites varies equipped with the additional instrumentation (P650 with the amount of metallic iron present, and so one package) that allows the measurement of heat capacity. might expect that samples with higher susceptibilities As with the thermal conductivity measurements, this would have higher thermal conductivity. We have system utilizes a basic cryogenic ⁄ field system to establish magnetic susceptibility data for twelve of the meteorites precise control of temperature, while specialized measured in our sample, and Fig. 7 shows that there is components perform the heat capacity measurement. A certainly a hint of a relationship, with the highest QD option P640 High-Vacuum system adsorption pump conductivity meteorites having the highest susceptibility. in the cryogenic dewar thermally isolates the sample. The However, below a certain metal content, the conductivity sample was mounted on a compact calorimeter puck with appears to stay constant even as susceptibility drops thermal grease and inserted into the PPMS sample further. Furthermore, the two meteorites with the highest chamber. The specific heat capacity option P650 includes metal content, which contribute the most to the visible hardware and software that allowed fully automated high- trend, are also the enstatite chondrites with the lowest sensitivity heat capacity measurements to be taken porosity. Thus, it is not clear that metal content itself is between 1.9 K and 400 K. The option uses a hybrid what leads to the trend seen in Fig. 7, or if it is merely an adiabatic relaxation method that combines the best artifact of the most metal-rich meteorites also being the measurement accuracy with robust error analysis. least porous, and the metal-poor meteorites the most Unfortunately, using this system is quite time consuming; porous. a single run, measuring the set-up (platform and thermal Meteorite thermal properties 325

Fig. 5. An expanded view of Fig. 3, showing the range of conductivities at the lower end of the thermal conductivity scale. Red data points are H chondrites, blue points are L chondrites, golden points are basaltic achondrites. Newly measured meteorites are indicated with solid symbols and named to the right of their respective datapoints. Lines through the data represent fourth order quadratic fits to the new data (see Table 1). Note that the basaltic achondrite conductivities are very similar to those for ordinary chondrites, in spite of their having significantly different compositions––in particular, little metallic iron.

Table 1. Quadratic fits to the new thermal conductivity data reported here (except for the E chondrite Pillistfer) can be fit by a quadratic equation of the form: K=A+ BT + CT2 + DT3 + ET4. Meteorite Type AB· 102 C · 104 D · 106 E · 109 r2 La Cienega )0.218 4.975 )3.904 1.257 )1.438 0.997 Barbotan H chondrite )0.367 7.126 )5.433 1.751 )1.976 0.988 Collescipoli H chondrite )0.070 1.634 )1.179 0.378 )0.436 0.997 Pułtusk H chondrite )0.113 2.283 )1.522 0.466 )0.531 1.000 Bath Furnace 1 )0.139 5.333 )4.063 1.290 )1.475 0.996 Bath Furnace 2 L chondrite )0.158 6.383 )5.012 1.672 )2.029 0.997 Bath Furnace 3 L chondrite )0.128 5.846 )3.744 1.026 )1.033 0.999 Holbrook 1 L chondrite )0.024 0.888 )0.623 0.191 )0.214 0.999 Holbrook 2 L chondrite )0.102 2.485 )1.882 0.625 )0.753 0.999 Frankfurt Howardite )0.084 3.053 )2.482 0.844 )0.974 0.991 Los Angeles Shergottite 0.051 0.774 )0.345 0.087 )0.086 0.999 grease without the sample, followed by the actual (Krupka et al. 1985). Los Angeles is a basalt of roughly measurement with the sample) requires up to 48 h of equal volume percent abundances of plagioclase chamber time, significantly limiting the number of (An41Or4 to An58Or1), shocked to , and samples that can be measured. high-ferroan calcium (Fe45Wo13 to Fs45Wo37 In contrast to the thermal conductivity, which we to Fs72Wo24), with smaller amounts of Fe-rich olivine have found is controlled by the physical state of the and other trace minerals (Rubin et al. 2000). Note that meteorite, one expects that the heat capacity of a Los Angeles itself is only a quarter (by volume) meteorite should closely follow its composition. Our wollastonite. Nonetheless, there is a remarkably close results support this hypothesis. fit between our sample of Los Angeles and that for In Fig. 8, we compare our Los Angeles results with pure wollastonite, the calcium-rich pyroxene, in data for a variety of pure from the literature contrast to the other pyroxenes. We conclude that 326 C. P. Opeil et al.

Table 2. Density, porosity, and magnetic susceptibility compared with conductivity at 200 K. Meteorite Kind Bulk density Grain density Porosity Magnetic suscept. Cond. @ 200 K Notes New this paper Pillistfer EL 6 3.61 3.7 2.4% 5.55 5.51 1 Pułtusk H 5 3.44 3.72 7.5% 5.27 1.25 1 Barbotan H 5 3.49 3.75 6.9% 5.21 3.05 1 Collescipoli H 5 5.37 0.82 2 La Cienega H 6 1.90 Holbrook 1 L 6 3.18 3.55 10.4% 4.58 0.45 1 Holbrook 2 L 6 3.18 3.55 10.4% 4.58 1.15 1 Bath Furnace 3 L 6 3.50 3.66 4.3% 5.05 3.15 1 Bath Furnace 2 L 6 3.50 3.66 4.3% 5.05 2.72 1 Bath Furnace 1 L 6 3.50 3.66 4.3% 5.05 2.26 1 Frankfort Howardite 2.90 3.32 12.7% 3.43 1.31 1 Los Angeles Shergottite 2.83 3.08 8.1% 3.52 0.77 1 Opeil et al. 2010 Abee EH 6 3.52 3.62 3.0% 5.47 5.33 1 Cronstad H 5 1.88 Lumpkin L 6 1.47 NWA 5515 CK 4 2.70 1.48 2 Cold Bokkeveld CM 2 2.36 2.78 15.0% 3.68 0.50 1 Yomogida and Matsui 1983 ALH 77288 H 6 3.69 2.0% 3.53 3 ALH 77294 H 5 3.35 12.9% 0.75 3 Gilgoin Station H 5 3.61 5.0% 3.60 3 Gladstone H 5 3.56 5.0% 2.16 3 Monroe H 4 3.58 5.9% 2.35 3 Wellman H 5 3.58 6.1% 3.85 3 Y-74156 H 4 3.45 9.2% 1.54 3 Y-74647 H 4.5 3.49 9.1% 1.15 3 ALH 769 L 6 2.89 19.4% 0.55 3 ALH 77231 L 6 3.07 14.3% 1.20 3 ALH 78103 L 6 3.23 13.4% 0.73 3 ALH 78251 L 6 3.22 13.2% 0.85 3 Arapahoe L 5 3.52 2.5% 2.31 3 Bruderheim L 6 3.31 8.0% 1.03 3 Farmington L 5 3.40 5.5% 2.14 3 Kunashak L 6 3.41 5.2% 1.86 3 Leedey,A L 6 3.25 10.4% 0.40 3 Leedey,B L 6 3.24 10.6% 0.47 3 MET 78003 L 6 3.33 7.8% 1.54 3 New Concord L 6 3.27 9.2% 0.78 3 Y-74191 L 3 3.23 10.3% 1.24 3 Y-75097 L 4 3.28 10.3% 0.97 3 Notes: NWA = Northwest Africa; ALH = Allan Hills; Y = Yamato; MET = Meteorite Hills. 1. Data from Macke (2010); average of all samples measured. 2. Data from Macke (2010); sample from Vatican collection. 3. Data from Yomogida and Matsui (1983). this close match is primarily a function of the FeO above 450 C. Furthermore, the low thermal and CaO content of these samples, rather than conductivity reported here (measured on the same the physical arrangement of the oxides within the sample as that used for the heat capacity measurements) minerals. suggests that this sample has significant cracking that Note that Los Angeles is a significantly shocked inhibits the transport of heat, similar to what is seen for meteorite, as is evidenced by the presence of maskelynite. ordinary chondrites. Macke et al. (2011) report a Min et al. (2004) estimate that it may have experienced a porosity of 8.1 ± 1.0% for Los Angeles (measured on a shock of higher than 45 GPa and a shock temperature different sample than the one used here), again similar to Meteorite thermal properties 327

(a) (b)

Fig. 6. Conductivity varies linearly with the inverse of porosity. a) Only data collected by the authors (circles). b) The data from Yomogida and Matsui (1983) (diamonds) added to our data; although the scatter is now larger, the fit is essentially unchanged.

Fig. 7. There is a weak correlation in our data between conductivity and magnetic susceptibility, a measure of the iron content of the sample, at least for samples above a threshold metal abundance. However, the most metal-rich samples, the enstatite chondrites, are also the least porous, and so it may be Fig. 8. The heat capacity of the shergottite meteorite Los porosity rather than metal content that we see here. Angeles (black dots) compared with literature values for the heat capacities of various pyroxene minerals (Krupka et al. 1985). Note the close correlation between the meteorite and the Ca-rich pyroxene wollastonite. ordinary chondrites. Nonetheless, this physical processing, which clearly controls the thermal conductivity of the sample, does not seem to have significantly affected its CONCLUSIONS heat capacity. This very intriguing result for one basaltic meteorite, In the absence of data for various meteorite types, that the heat capacity is well modeled by the elemental most modelers have assumed that the thermal properties abundance of the sample and is not significantly affected of asteroidal or meteoritic materials could be estimated by its shock state, needs confirmation for other meteorite from the laboratory measurements of minerals typically types, especially ordinary chondrites. This remains for found in those meteorites. Our results here, confirming future work. earlier work, indicate that this may not be too far off for 328 C. P. Opeil et al. heat capacity (though further data are needed to confirm while inside an asteroid. Meteorites now in our labs this result) so long as one recognizes its strong must have experienced strong shocks both when they dependence on temperature, which has been neglected in were ejected from their parent asteroid and when they many models. But this assumption may be seriously in impacted the Earth. Thus, our hand samples may be error for thermal conductivity. The thermal conductivity biased toward materials that are more shocked than of meteorites is significantly lower, by a factor of three to material within an asteroid itself. Given that the effect ten, than that of the pure minerals from which they are of shock on originally unshocked material is to made. compress and thus reduce meteorite porosity, this The one macroscopic quantity of a meteorite that suggests that material within an asteroid may be even appears to be a good predictor of its thermal properties more porous, and thus lower in thermal conductivity, is its porosity. We have found a linear relationship than the samples we report here. However, we note between meteorite thermal conductivity and the inverse (Consolmagno et al. 2008) that there is only a very of the porosity. Even here, however, we have seen an weak dependence of porosity on shock state (above anisotropy in thermal conductivity of 25% within a shock state 1). Except for essentially unshocked (state single sample, and a similar variation in conductivity 1) meteorites, once a meteorite has been compressed from sample to sample of the same meteorite. and cracked by shock, repeated or stronger shocks do Presumably, however, the parent body experienced not significantly alter its porosity. Given the likelihood numerous shock events resulting in a random orientation that material in asteroids has been shocked many times, of internal cracks throughout the fabric of its material, it seems unlikely that the final larger shocks which and so the anisotropies seen in our millimeter-scaled delivered material from the asteroids to our labs would samples will average out over sample sizes significantly have significantly altered the material’s porosity or larger than the extent of the cracks causing this thermal properties. Nonetheless, this is an assumption anisotropy. that one must keep in mind when using these data to For temperatures of interest to planetary scientists, model asteroid interiors. the variation of thermal conductivity with temperature With the data presented here, we have roughly appears to be small. It appears that once phonon doubled the number of meteorites for which thermal activation energies have been reached at temperatures conductivities have been directly measured. However, above 50–100 K, the transport of these phonons is many meteorite types remain unmeasured, and many controlled primarily by the presence and orientation of more ordinary chondrites of varying physical and cracks, which inhibit the flow of heat more or less chemical nature need to be measured to confirm and uniformly at all temperatures. Above 100 K, for most refine the trends suggested by the data in hand. And the cases, the thermal conductivity of a meteorite can be measurement of heat capacities as a function of assumed to be nearly constant with temperature. temperature for many other meteorite types is also a high However, both literature data on minerals of interest and priority for future work. in our one measurement so far of a meteorite, we find a very strong (roughly linear) relationship between Acknowledgments––CO acknowledges support from the temperature and heat capacity from very low temperatures Trustees of Boston College and DTB was supported by up to at least 300 K. NASA Grants NX09AD91G and NNG06GG62G from Care must, of course, be taken in applying these the Planetary Geology and Geophysics Program. We results directly to modeling the thermal properties of thank Jason Lanshley at the Los Alamos National asteroids. 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