3D porosity structure of the earliest solar system material

Porosity is an important material property that greatly affects a wide range of physical processes on asteroids. It significantly influences cratering mechanics; not only does it contribute to the attenuation of impact shock waves but it also determines the amount and distribution of waste heat generated1,2,3. Porosity affects permeability and the movement of gases and fluids through an object, controlling the extent and type (eg, open- vs. closed- system) of aqueous alteration eg, 4. It also influences a material’s thermal conductivity and therefore its thermal inertia5, which has implications for the movement of heat and energy within an asteroid. Porosity also has also been shown to have a significant effect on the outcome of kinetic impacts, as might be used to deflect Near Earth Asteroids (NEAs)6. Therefore, determination of the porosity, as well as its distribution and structure, has significant implications for the physical, hydrological, and dynamic evolution of an asteroid.

Recently, two sample return missions to carbonaceous asteroids (162,173) Ryugu and (101,955) Bennu, by JAXA’s Hayabusa2 and NASA’s Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer (OSIRIS-REx) missions, respectively, have revealed that the asteroids are significantly more microporous than expected7.8and the preliminary analysis of samples from Ryugu confirm that they are more highly microporous (46%) than their analog CI meteorites9. Microporosity in this context is defined as porosity on the scale of the analog meteorite, and thus is composed of inter- and intragrain porosity and small fractures (hundreds of microns in width or less). The origin of this unexpectedly high porosity is not known. It could represent porosity created from a secondary process such as meteoroid bombardment or cracking due to diurnal thermal stress8. However, it is hypothesized that it is more likely the original, primary porosity of the carbonaceous chondrite material that accreted to the asteroid7.8.

The porosity of carbonaceous chondrites (CCs) have been previously measured in the lab using both bulk (He pycnometry) and direct imaging (scanning or transmission electron imaging (SEM / TEM); X-ray computed tomography (XCT)) methods eg, 10,11,12,13. These studies have shown that the CC types that are the closest analogs to carbonaceous asteroids Bennu and Ryugu, the CMs, CIs, and ungrouped C2s, have a high porosity (~ 23–40%) that is primarily composed of submicron to micron sized pores10,11,12,14,15,16,17. While bulk porosity measurements are likely to be accurate, they lack any detail on the type (intragranular, intergranular, fracture, etc.), morphology, or location of the porosity. Direct imaging of the pores provides this detail, but 2D imaging methods such as SEM or TEM requires 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 the interpretation of the origin and evolution of the porosity. Further, knowledge of the 3D porosity distribution within a sample is important for studies of carbonaceous chondrites that employ freeze – thaw disaggregation to concentrate components of interest such as chondrules, refractory inclusions, presolar grains, or clasts eg, 18.

XCT is able to examine porosity within larger, more representative samples while preserving 3D spatial context. XCT is a nondestructive imaging technique that produces a series of two-dimensional (2D) images (slices) where the gray scales in each image represent X-ray attenuation, which is to first order dependent on the density and atomic number (Z) of the material19. A few studies have used XCT to examine porosity within chondrites but have been limited by the scale of observation due to only attempting to identify discrete pores13,20,21. Typically, a discrete 3D feature (such as a pore) can be accurately measured only when it has a diameter of at least a few (~ 3) voxels19,22. Consequentially, when imaging small (6–12 mm3) chondrite chips, Friedrich and Rivers 20 found that they could not measure all the porosity within highly porous samples (> 15%) due to the large number of pores below the scanning resolution (2.6 /m / voxel). Conversely, Dionnet, et al. 13 used a very high imaging resolution (0.13 /m / voxel) for CM Paris but also found a porosity much lower than expected (4.6%) compared to previous estimates (30%15), most likely due to the unrepresentativeness of their tiny sample (40 µm in diameter).

However, XCT imaging with a heavy noble gas such as Kr or Xe, which is highly attenuating to X-rays, has allowed inspection of extremely fine-scale porosity in terrestrial samples23,24,25. By XCT imaging a porous sample twice, once in air or under vacuum and once infiltrated by the gas, and then subtracting the former from the latter, one obtains a 3D map of where the gas has infiltrated, and thus the connected porosity. In such maps, each voxel value corresponds to partial porosity, or the fraction of the voxel that contains pore space, thus revealing the location of all interconnected porosity, at all scales. The noble gas technique has two other advantages over direct XCT pore imaging. First, it can differentiate between low attenuation material, such as organics, and a nano-porous region with porosity below the spatial resolution, which is otherwise indistinguishable from a low attenuation material. Second, it provides information on the connectivity of the pores, as isolated pores will not change in X-ray attenuation with the introduction of the gas. In this work, we apply this XCT noble gas imaging method for the first time to an extraterrestrial sample, CM Murchison (an analog meteorite for carbonaceous asteroids), to demonstrate and refine the technique for application to current and future sample return missions to these highly microporous and complex targets.

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