Universe Today
Asteroids don’t get the love they deserve. They don’t get “cool points” because they’re not a planet or a potential life-harboring moon. They’re “just a bunch of rocks”. But asteroids are so much more, as they are time capsules of the early solar system that have survived billions of years untouched by weathering or plate tectonics. One of the most intriguing asteroids that has been explored is asteroid Bennu, and specifically how its physical characteristics greater differed from Earth-based observations in 2007 after NASA OSIRIS-REx spacecraft visited Bennu in 2018.
Now, an international team of almost 50 researchers have potentially disclosed the reasons behind the large discrepancy in asteroid Bennu’s surface appearance and composition. Specifically, while ground-based observations and data indicated Bennu’s surface was comprised of small rock fragments, the OSIRIS-REx spacecraft revealed a surface full of large boulders and varying physical characteristics. Their findings have been recently published in Nature Communications.
Using a series of laboratory techniques that were used on asteroid Ryugu samples, the researchers tested the physical properties and thermal inertia of the Bennu samples to ascertain their connection to the boulders observed on Bennu. Thermal inertia is the ability of an object to retain heat for specific periods of time, and Bennu was found to have low thermal inertia, resulting in the initial hypothesis that its surface is covered in small rock fragments.
Additionally, the researchers hypothesized that the boulders contained a lot of empty space, also called porosity, potentially resulting in their low thermal inertia, as hollow objects don’t retain heat as well as full objects. For context, a solid baseball has higher thermal inertia than a hollow baseball, meaning the solid baseball can keep its heat for longer periods of time than the hollow baseball. After careful analysis, the researchers found that Bennu’s low thermal inertia results from cracks within the rocks on its surface, which is a similar finding for Ryugu. They also note these findings could challenge Earth-based observations going forward, and that direct spacecraft analysis is a more efficient method.
“The analysis of returned samples from Bennu and Ryugu permits a ground-truthing of findings from remote-sensing data,” the study notes. “In particular, the physical mechanism responsible for the low thermal inertia of the dark boulders on both bodies has remained an unresolved question. Given that both asteroids’ surfaces are dominated by boulders, rather than unconsolidated regolith, and that the boulders and samples of both asteroids share many similarities in appearance and composition, it is plausible that their similarly low thermal inertia has a common explanation.”
Bennu is hypothesized to have formed from a parent body that was struck long ago, resulting in chunks breaking off into space. Eventually, these chunks coalesced because of gravity, forming asteroid Bennu. Because of its low rotation time of 4.3 hours, Bennu’s surface experiences constant temperature swings, known as heat stress or thermal fatigue. As a result, the rocks on the surface are constantly expanding from the heat and contracting from the cold, creating cracks over time. Additionally, the researchers propose that micrometeoroid impacts could have also caused cracks.
Asteroids like Bennu are often thought to inhabit the Main Asteroid belt between Mars and Jupiter. While this is the case for more than 90 percent of asteroids in the solar system, Bennu inhabits a small population of asteroids known as near-Earth asteroid (NEA). More specifically, Bennu is designated as a Potentially Hazardous Asteroid (PHA) since its orbital path crosses Earth’s orbit once every six years. Despite this, scientists estimate a 1-in-2,700 chance of striking Earth sometime in the late 22nd century, approximately the year 2182, or 21 years after the United Federation of Planets is slated to be founded.
What new insight about asteroid Bennu will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!
