Asteroids don’t get the love they deserve. They do not find “cool stuff” because it is not a planet or a moon that can save life. It is “just a bunch of rocks”. But asteroids are abundant, as they are time capsules of the early solar system that have survived billions of years untouched by weather or plate tectonics. One of the most interesting asteroids ever explored is asteroid Bennu, and especially how its physical characteristics differ from those seen by Earth in 2007 after NASA’s OSIRIS-REx spacecraft visited Bennu in 2018.
Now, an international team of about 50 researchers may reveal the reasons that made a big difference in the appearance and shape of the asteroid Bennu. In particular, although ground-based observations and data indicate that the surface of Bennu was composed of small pieces of rock, the OSIRIS-REx spacecraft revealed a surface full of large rocks and different physical features. Their findings were recently published in Nature Communications.
Using a series of laboratory techniques that were used on the Ryugu asteroid samples, the researchers analyzed the physical properties and thermal inertia of the Bennu samples to confirm their relationship to the rocks seen on Bennu. Thermal inertia is the ability of an object to retain heat for certain periods of time, and Bennu was found to have a low thermal inertia, which led to the first hypothesis that its surface is covered with small pieces of rock.
In addition, the researchers hypothesized that the rocks have an empty space, which is also called porosity, which may cause a low temperature, since hollow materials do not retain heat as well as solid materials. For context, a solid baseball has a higher thermal inertia than a hollow baseball, meaning that a solid baseball can retain its heat longer than a hollow baseball. After careful analysis, the researchers found that Bennu’s low temperature is caused by cracks in the rocks on its surface, which is similar to Ryugu. They also note that these findings may challenge Earth’s observations further, and that direct observation of the atmosphere is a more efficient method.
“Analysis of samples recovered from Bennu and Ryugu allows for ground truthing of findings from remote sensing data,” the study said. “In particular, the physical mechanism responsible for the decrease in temperature of dark rocks on both bodies remains an unsolved question.” Due to the fact that the surface of both asteroids is dominated by large rocks, rather than unconsolidated regolith, and that the rocks and samples of the two asteroids have many similarities in appearance and structure, it is clear that the same description has the same explanation.
It is thought that Bennu was born from the body of a parent that was struck long ago, causing pieces to break up in space. Eventually, these particles collided due to gravity, forming the asteroid Bennu. Because of its short rotation period of 4.3 hours, Bennu’s surface experiences frequent temperature fluctuations, known as thermal stress or thermal exhaustion. As a result, the stones on the surface of the water constantly expand from the heat and are affected by the cold, causing cracks over time. In addition, researchers suggest that micrometeoroid impacts also caused the wings.
Asteroids like Bennu are generally thought to reside in the Main Asteroid belt between Mars and Jupiter. Although more than 90 percent of the asteroids in the solar system are like that, Bennu resides in a small group of asteroids known as near-Earth asteroids (NEA). In particular, Bennu is designated as a Potentially Hazardous Asteroid (PHA) as its orbit crosses Earth’s orbit once every six years. However, scientists estimate a 1 in 2,700 chance of hitting Earth sometime in the late 22nd century, around the year 2182.
What new insight into asteroid Bennu will researchers make in the coming years and decades? Only time will tell, and that’s why we’re science!
As always, keep doing science and keep looking up!
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