The Space Heist of Bennu

By Allison Kubo
Artistic representation of OSIRIS-REx over the asteroid Bennu. NASA/GODDARD/University of Arizona.

In 2016, the OSIRIS-REx probe left Earth but unlike most other probes on their journey out to space, OSIRIS-REx does intend to return home. If all goes well, OSIRIS-REx will return to its home in 2023 carrying a precious sample.

As of 2018, OSIRIS-REx has been orbiting the asteroid Bennu. Bennu is about the size of the Empire Statebuilding but it is older than the Earth itself. Bennu is a relic from the solar system’s formation. On October 20, 2020 after two years of careful observation, OSIRIS-REx reached out it’s arm and touched the asteroid at Nightingale Crater with the goal of getting at least 60 g of sample from the surface and then returning it to Earth. To understand the importance of Bennu and why we would go all the way there for approximately one chocolate bar’s worth of icy dirt, we have to go back to the beginning.

Before our sun, there were likely multiple stars which supernovae to spread the mass out into a large protoplanetary disk. This disk was made almost exclusively of hydrogen and helium with small amounts of the heavier element such as carbon, oxygen, and iron. After dust collided with dust, and asteroids collided with asteroid and protoplanet collided with protoplanet the planets were formed. But the composition of the Earth which has high amounts of oxygen, iron, nickel, and silicone does not resemble at all what we think that the protoplanetary disk started as. We have many “heavy elements” that are only recreated in supernova events. That is because Earth has undergone differentiation. Earth has had the volatile elements such as hydrogen and helium leave or join the atmosphere while the heavier elements remained and the heaviest elements mostly magnesium, iron, and nickel, sunk into our solid metal core. This presents a large problem here on earth if we want to study the early solarsystem, we don’t have any samples of it. Differentiation in the Earth has completely changed its composition from that of the early solar system. If we want to investigate the origins of Earth and especially of life on Earth, we have to go elsewhere. We have to go to Bennu.

Bennu is a type of asteroid called carbonaceous asteroid that is thought to have been the origins of our planet and more importantly the origins of all the water and carbohydrates on the planet. As the name suggests, carbonaceous materials have high amounts of water, carbons, and even potentially the building blocks of life, amino acids. A small amount of carbonaceous chondrites that have fallen to Earth as meteorites which gave scientists an early look at what an undifferentiated solar system might look like. The presence of water indicates that these asteroids did not experience large amounts of heat which would burn off the water and other volatile compounds. Little heat in terms of the solarsystem building means few collisions and no differentiation which is why Bennu is often called “primitive”. Bennu represents the composition of parts of the early solar system.

It is proposed that Bennu and the carbonaceous chondrites are the source of Earth’s water. After the Earth formed 4.5 billion years ago, it had little water. Most of the water had evaporated in the violent impacts that formed it. Only half a billion years later did the Earth, according to geologic evidence, form it’s oceans. Looking at the isotopic ratios of hydrogen and nitrogen, we can start to find the source of the life sheltering water. The closest match to Earth’s oceans is carbonaceous chondrites like Bennu.

Bennu represents a step forward into understanding the origins of life and water on the Earth.

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