Parent body of (162173) Ryugu: High porosity, early accretion, small size
Authors: Neumann Wladimir , Grott Matthias , Trieloff Mario , Jaumann Ralf , Biele Jens , Hamm Maximilian , Kührt Ekkehard , Sakatani Naoya 
Published: July 2021

Introduction: Observations of C-type NEAs and properties of carbonaceous chondrites indicate a high porosity of C-type asteroids. Hayabusa2 demonstrated that Ryugu is a low-density rubble-pile[1] dominated by large surface boulders[2]. Boulder thermal properties imply a microporosity of &#981;boulder&#8776;28-55%[3,4], substantially higher than for meteorites and indicative of distinct evolution for Ryugu&#8217;s &#160;parent body from that of water-rich CI/CM chondrites, despite spectral similarities. High microporosity values are consistent with a bulk density of 1190&#177;20 kg m-3[5], a bulk porosity of &#8776;50%[1], and a macroporosity of &#8776;16%[6].We calculated the temperature and porosity evolution for primordial planetesimals and constrained the range of parameters resulting in microporosities compatible with Ryugu&#8217;s and likely burial depths for the surface boulders. By varying the accretion time t0 and radius R and by comparing the porosity distribution with &#981;boulder, hydration, and partial dehydration, we constrained a field within the (R,t0)-diagram appropriate for bodies that are likely to have produced such material.Methods: Within the sequence of the parent body evolution, a series of suggested disruption and accretion of intermediate bodies events, and the accretion of Ryugu[5], the boulder microporosity results mainly from parent body processes, e.g., 26Al-induced internal heating, hydration, compaction, and dehydration prior to the first disruption.We used global thermal evolution and compaction models[7] that predict the microporosity established due to creep processes driven by the joint action of temperature and pressure. No notable creep can be expected in late formed and small objects such as Ryugu or in intermediate objects between the first and last disruption events. Thus, we calculated the interior microporosity of primordial planetesimals with different sizes and accretion times in order to reproduce &#981;boulder.Fig. 1: Final porosity profiles for R=1, 5, 10, and 100 km (left to right, top to bottom). Profiles for different accretion times are shown and compared with the &#981;boulder.Results: The bulk porosity &#981;bulk calculated shows that large (i.e., slowly cooling and with a higher pressure) and early accreted (i.e., 26Al-rich) planetesimals compact more efficiently, retaining only a thin porous blanket. By contrast, low-pressure small and late accreted bodies remain highly porous. While &#981;bulk provides no information about the porosity distribution, it shows that intermediate conditions with a trade-off between R and t0 should result in high volume fractions of material matching &#981;boulder. Of importance for the accretion of Ryugu from the rubble of a larger object is the location of such material within the parent body. The final porosity distribution is shown in Fig. 1 and compared with &#981;boulder. Profiles for R=1, 5, 10, and 100 km and several accretion times shown represent different types of structures with up to three layers defined by the comparison &#981;>&#981;boulder, &#981;&#8776;&#981;boulder, and &#981;