Asteroid Bennu Has an Identity Crisis, and Jupiter Might Be to Blame

Asteroid sample return missions have brought well preserved samples of the early solar system (that hadn’t been roasted in a fireball) back to Earth for the first time. But those samples, like the OSIRIS-REx sample from asteroid (101955) Bennu, sometimes provide more questions than answers. A new paper, published in Science Advances from researchers at ETH Zurich and Lawrence Livermore National Laboratory, highlights one of those questions - why Bennu appears to have been born in both the inner and outer solar system - and a possible answer to it.

Samples from Bennu are one of the most sought after resources in the world of geology. So when a half a gram of the material made its way to ETH Zurich, Maria Schönbächler and her lab began analyzing it immediately. Specifically, the Professor of Isotope Geochemistry and her students were looking for exactly what her title implied - nucleosynthetic isotopes.

Isotopes acted like markers in the early solar system. When the Sun formed 4.6 billion years ago, the dust surrounding it didn’t stir evenly. Different parts of the new star’s orbital plane contained different blends of isotopes - elements like iron, titanium, and chromium had slightly different numbers of neutrons depending on where they were found in this protoplanetary disk.

Fraser interviews Dr. Jamie Molaro about the OSIRIS-REx mission to Bennu

Studies of other asteroids and meteorites (which are just asteroids that have fallen to Earth) have noted a distinct bifurcation into two separate families. Non-Carbonaceous (NC) asteroids are made from material in the inner solar system, where terrestrial planets like Earth formed. Carbonaceous (CC) asteroids, on the other hand, are made in the cold, volatile-rich outer solar system.

Before we ever got a sample back from it, scientists had categorized Bennu as a CI chondrite - a sub-type of the more common CC asteroids, and one that we’ve only ever had 10 actual meteorite found so far, making up less than 0.0127% of the ones we’ve collected, though this is likely because most of them burn up in the atmosphere.

So studying a pristine sample from such an asteroid was even more of a rare opportunity, and the researchers expected the isotopes they found to match up with what would be expected for CC asteroids throughout the solar system. For Bennu’s titanium and chromium isotopes, that proved to be correct. But for iron, Bennu appeared to be more like the NC asteroids that formed nearer the Sun.

Fraser talks about the DNA precursors we found in the OSIRIS-REx samples

Digging a little deeper, the researchers used radioactive manganese-53 decay to date the asteroid to about 2 million years after the first solid rocks started appearing in our solar system. In the outer solar system, it would have taken far longer for an asteroid to assemble out of the more sparsely available parts. Also, Bennu seemed to lack the “heavy” hydrogen and nitrogen that we see in comets we know were formed in this region.

As an alternative, the researchers put the blame squarely on one particular culprit - Jupiter. At the point in the solar system’s history where Bennu had formed, Jupiter had already reached twenty three times the Earth’s mass. It had carved its path out of the disk’s gas and dust, and left some residual pressure ridges both inside and outside of its orbit. This allowed tiny dust grains to cross Jupiter’s orbit inwards towards the Sun without much difficulty. But it trapped larger, millimeter-sized molten droplets called chondrules in the pressure gaps outside its orbit, stopping their inward migration to the gravitational center of the solar system.

Those pressure ridges also happened to very nearly coincide with the “ice line” - the point around the Sun where the temperature dropped enough to freeze water. When Bennu was formed, this line was just inside the orbit of Jupiter. As the dust hit this water-ice line with its pressure ridges, they began to collapse in on themselves, creating what could be thought of as a dust traffic jam. Eventually, these traffic jams formed the seeds of asteroids like Bennu.

Dust continued to stream in from the outer solar system, creating what looked like cosmic sedimentary layers on Bennu itself, and blending the inner disk ingredients with the dust grains from the outer disk. Which therefore explains why Bennu looks both like it was formed in the inner and outer solar system - it appears to have been formed near the water/ice line.

Since then it’s migrated further inward, eventually becoming a Near Earth Asteroid (NEA), and making itself a prime target for a sample return mission. That process took millions of collisional shoves and gravitational nudges over billions of years, but its fundamental chemistry remained the same. It just took a plucky robotic explorer, half a gram of dust, and some very fancy instrumentation to finally uncover how this particular form of asteroid came to be.

Learn More:

ETH Zurich - Mystery surrounding the formation of asteroid Bennu solved

M. Schönbächler et al - Nucleosynthetic constraints on the origin of Bennu and CI-like asteroids

UT - Asteroid Bennu Is Like A Time Capsule From The Early Solar System

UT - OSIRIS-REx Returned Carbon and Water from Asteroid Bennu