In Nature Astronomy there are two different papers looking at the same weird feature of the asteroids Bennu and Ryugu. These asteroids are essentially rubble piles, and while their compositions differ, they are both primarily composed of dark rubble that includes minerals that include water in their structure or formed in the presence of water. Sprinkled atop both these asteroids are compositionally different bright boulders that are from something that was hot and dry.

In the Bennu-related paper, with first author Daniella DellaGiustina, they discuss using OSIRIS-REx’s Visible and Infrared Spectrometer to study these boulders’ composition. They discovered that the boulders appear consistent with the mineral pyroxene, which is common on Vesta and vestoids – small asteroids chipped off Vesta during past impacts. According to co-author Hannah Kaplan: Our leading hypothesis is that Bennu inherited this material from its parent asteroid after a vestoid (a fragment from Vesta) struck the parent. Then, when the parent asteroid was catastrophically disrupted, a portion of its debris accumulated under its own gravity into Bennu, including some of the pyroxene from Vesta.

Let me put this another way. Once upon a time, a reasonably large asteroid was minding its own business as it orbited the solar system and it was struck by a chunk of Vesta that stuck. At some later date, an even larger object hit this asteroid, breaking it into the myriad pieces we see today. This means Bennu is a mix of some parent object, some vestoid that hit that parent object, and additionally at least one more object responsible for its shattered nature.

The solar system is a rough place to be. 

The team looking at Ryugu was using JAXA’s Hayabusa2, which has a different suite of instruments that allow different kinds of explorations to be done. Using Hayabusa2’s spectrometer, researcher Eri Tatsumi is able to say: Ryugu is considered a C-type, or carbonaceous, asteroid, meaning it’s primarily composed of rock that contains a lot of carbon and water. As expected, most of the surface boulders are also C-type; however, there are a large number of S-type, or siliceous, rocks as well. These are silicate-rich, lack water-rich minerals and are more often found in the inner, rather than outer, solar system.

While this isn’t as definitive as saying “bits of Vesta are on Ryugu”, it is still the same story of impact after impact adding new minerals while destroying previous structures. 

More Information

NASA Goddard article 

“Exogenic basalt on asteroid (101955) Bennu,” D. N. DellaGiustina et al., 2020 Sep. 21, Nature Astronomy

The University of Tokyo press release 

“Collisional history of Ryugu’s parent body from bright surface boulders,” E. Tatsumi et al., 2020 Sep. 22, Nature Astronomy