Bennu Descended from an Ocean World

by | February 28, 2024, 12:00 PM | Solar Systems

A view of the outside of the OSIRIS-REx sample collector. Sample material from asteroid Bennu can be seen on the middle right. Scientists have found evidence of both carbon and water in initial analysis of this material. The bulk of the sample is located inside. Credit: NASA/Erika Blumenfeld & Joseph Aebersold

Language is messy, and changing definitions of how words are used can sometimes make science messy, too.

Late one night last week, I was binge-scrolling science on social media when I stumbled across reports that initial analysis of Bennu samples indicated that it was made of debris from an ocean world.

As a child of the 80s, I think of Earth as a water world – a pale blue dot covered in oceans and home to whales that one day Mr. Spock will bond with and transport to the future… or something like that. 

And I wasn’t the only one thinking like that – another human, whose name is withheld to protect their identity, posted about how wild it would be if OSIRIS-REx had flown all that way only to discover a bit of Earth that got splashed into space with some dinosaurs. 

And I went to sleep hoping that would be exactly what happened.

This is how I learned that today, “ocean world” refers to small moons and objects like Pluto that have ice shells over liquid oceans that are kept warm by very violent physics. Earth is a terrestrial world, and compared to tiny moons like Enceladus, we just don’t have all that much water as a function of volume. 

The “ocean worlds” of the Solar System. Credit: NASA/JPL

In a citation that brings me joy, the Guinness World Records notes, “Models based on the gravitational field of the moon suggest that Enceladus’ ocean may have a volume of roughly 15,000,000 km³.” With a 500-km diameter, this makes it 23% ocean by volume. Newly acknowledged ocean world Ganymede is 46% water! They still aren’t mostly water, but they have got water.

Meanwhile, Earth, the Guinness record holder for total water, has 1,361,620,510 km³ of water, which is just 0.125% of our world. We are – by volume – not a water world. We are just a rock covered in water. 

Back to Bennu. Bennu, much to my sadness, is not a pile of rubble that resulted from something bad happening to Earth or an Earth-like world.

Instead, Bennu is a rubble pile made from the destruction of a world like Europa, Enceladus, or maybe even giant Ganymede. At some point in our solar system’s past, one of these ocean-filled worlds got the structure knocked out of it, and the remaining gravel pile became part of what is now Bennu.

And while this story is less human-centric than if that had been a gravel pile of Earth out there, it is actually scientifically super important. We can dig around on our world and the Moon and study our origins pretty well. We can’t – at least with our current technology and science funding – go out and dig around on any of these ocean worlds. Bennu is thus giving us a chance to study the stuff of planets we won’t be able to visit for a decade, and even then, won’t be able to dig chunks out of. This is new. This is exciting.

I still wish it was a piece of Earth that was ejected in the demise of the dinos, but I just like dinosaurs.