3D porosity structure of the first material in the solar system

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Porosity is an important material property that greatly affects a wide range of physical processes on asteroids. It significantly influences crater mechanics; not only does it contribute to the attenuation of impact shock waves, but it also determines the amount and distribution of residual heat generated1,2,3. Porosity affects the permeability and movement of gases and fluids through an object, controlling the extent and type (e.g. open or closed system) of aqueous weathering, e.g., 4. It also influences the thermal conductivity of a material and therefore its thermal inertia.5, which has implications for the movement of heat and energy inside an asteroid. Porosity has also been shown to have a significant effect on the outcome of kinetic impacts, as it could be used to deflect near-Earth asteroids (NEAs)6. Therefore, the determination of porosity, as well as its distribution and structure, has important implications for the physical, hydrological and dynamical evolution of an asteroid.

Recently, two sample return missions to carbonaceous asteroids (162,173) Ryugu and (101,955) Bennu, respectively by JAXA’s Hayabusa2 and Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) missions ) from NASA, revealed that the asteroids are significantly more microporous than expected7.8and preliminary analysis of the Ryugu samples confirm that they are more highly microporous (46%) than their analogue CI meteorites9. Microporosity in this context is defined as porosity on the scale of the analogous meteorite, and is therefore composed of inter and intragrain porosity and small fractures (hundreds of microns in width or less). The origin of this surprisingly high porosity is not known. It could represent porosity created from a secondary process such as meteoroid bombardment or cracking due to diurnal heat stress8. However, it is speculated that it is more likely the original primary porosity of the carbonaceous chondrite material that accreted to the asteroid.7.8.

Carbonaceous chondrite (CC) porosity has previously been measured in the laboratory using both mass (He pycnometry) and direct imaging (scanning or transmission electron imaging (SEM/TEM)) methods; X-ray tomography ( XCT)). 10,11,12,13. These studies have shown that the CC types which are the closest analogues of the carbonaceous asteroids Bennu and Ryugu, the unclustered CMs, CIs and C2s, have a high porosity (~23-40%) which is mainly composed of pores of submicron to micron size.10,11,12,14,15,16,17. While aggregate porosity measurements are likely to be accurate, they lack detail on the type (intragranular, intergranular, fracture, etc.), morphology, or location of the porosity. Direct pore imaging provides this detail, but 2D imaging methods such as SEM or TEM require destructive preparation (sectioning) of the sample. It also examines only a limited area (for TEM, on the order of ~100 µm2), which may not be representative of the sample, and does not provide 3D context, which we will show can be critical in interpreting the origin and evolution of porosity. Additionally, knowledge of the 3D porosity distribution in a sample is important for studies of carbonaceous chondrites that use freeze-thaw weathering to concentrate components of interest such as chondrules, refractory inclusions, presolar grains or clasts, for example, 18.

XCT is able to examine porosity in larger, more representative samples while preserving 3D spatial context. XCT is a non-destructive imaging technique that produces a series of two-dimensional (2D) images (slices) where the gray scales of each image represent the X-ray attenuation, which depends to first order on the density and number atomic (Z) material19. A few studies have used XCT to examine porosity in chondrites, but have been limited by the scale of observation due to the only attempt to identify discrete pores.13,20,21. Typically, a discrete 3D feature (such as a pore) can only be accurately measured when it is at least a few (~3) voxels in diameter19.22. Therefore, when imaging small (6-12 mm3) chondrite chips, Friedrich and Rivers 20 found that they could not measure all porosity in highly porous samples (>15%) due to the large number of pores below scanning resolution (2.6 µm/voxel). Conversely, Dionnet et al. 13 used very high imaging resolution (0.13 µm/voxel) for CM Paris but also found much lower than expected porosity (4.6%) compared to previous estimates (30%15), probably due to the non-representativeness of their tiny sample (40 µm in diameter).

However, XCT imaging with a heavy noble gas such as Kr or Xe, which strongly attenuates X-rays, has allowed inspection of very fine-scale porosity in terrestrial samples.23,24,25. By XCT imaging a porous sample twice, once in air or vacuum and once infiltrated by gas, then subtracting the former from the latter, we obtain a 3D map of where the gas has infiltrated, and therefore connected porosity. In such maps, each voxel value corresponds to a partial porosity, or the fraction of the voxel that contains porous space, thus revealing the location of all interconnected porosities, at all scales. The noble gas technique has two other advantages over direct XCT pore imaging. First, it can differentiate between a low attenuation material, such as an organic material, and a nanoporous region with porosity below the spatial resolution, which is otherwise indistinguishable from a low attenuation material. Second, it provides information about pore connectivity, since isolated pores will not change in X-ray attenuation with the introduction of gas. In this work, we first apply this XCT noble gas imaging method to an extraterrestrial sample, CM Murchison (a meteorite analogue for carbonaceous asteroids), to demonstrate and refine the technique for application to current and future sample return missions to these microporous and complex targets.

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