Why is the asteroid Bennu’s surface rocky and not smooth?

Although Bennu is less than half a kilometer in width, it could cause severe damage to Earth if it did impact. Astronomers take the task of understanding the dangers posed by such impacts seriously. Credit: NASA/Goddard/University of Arizona
Scientists expected that the tiny asteroid Bennu would look like a sandy beach when the OSIRIS REx spacecraft arrived. It turned out to be a construction site.

Scientists think they now know why.

Bennu, an asteroid measuring half a kilometer in diameter, is shaped like two Apollo Command Modules that are connected base-tobase. It is what's known as a rubble pile asteroid. It's basically a bag of rocks held together with gravity.

We used to believe that asteroids were monoliths. They were just large solid rocks. But that was before we discovered many small ones from spacecraft. Although they are not common, rubble piles are quite common. It is believed that an asteroid collides and the impact breaks them apart. If the impact speed is low enough, the pieces will reassemble themselves due to gravity. The result is a pile filled with rubble.

The surface is subject to micrometeorite impact, solar wind and thermal stress. As the surface rotates in and out of sunlight (continuous expansion and contraction of rocks) the surface will erode, generating small grains known as regolith. OSIRIS-REx, a spacecraft that was sent to Bennu in order to study its surface, arrived at Bennu with a lot of pebbles and sand. It looked more like sand on a beach.

Regolith, however, was rare and the surface looked more like a construction site following a demolition. There was just rubble everywhere. Why was there no regolith?

Scientists may have solved this mystery using a clever method, according to a new paper.

Bennu's surface is not perfectly smooth. This OSIRIS-REx image, taken from 5km up, shows boulders in all sizes. The largest (the one with the lightest color left of center), is more than 7 meters in width. Credit: NASA/Goddard/University of Arizona Photo: NASA/Goddard/University of Arizona

Bennu's rocks are porous and look like pumice. These tiny holes, which aren't filled with air, are caused by the lack of any air on the asteroid. These rocks take time to heat up in sunlight and slow to cool down at night. The thermal inertia is the rate at which something heats or cools. Porous rocks have high thermal inertia.

Regolith is made up of unconsolidated particles and heats up faster when the Sun sets than rock.

OSIRIS-REx's cameras can detect thermal emission and map the heat and cooling of different Bennu parts. Scientists discovered that the most regolith was seen in areas where the rock is porous. This is why?

The property of compressibility. A small rock that hits Bennu will break down a chunk of solid material, causing some regolith. If it hits porous material, however, most of the energy will be used to compress that rock and not shatter it. If any, there is less regolith. Thermal stress is the same. Porous rocks are less subject to day/night cycle stress than solid rocks, so they don’t have as many finer particles such as regolith.

The TAGSAM came into contact with Bennu's surface before (left) and afterwards (right). The 20 cm rock (arrowed), that was exposed to the sun after contact, has been melted. Credit: NASA/Goddard/University of Arizona

It's pretty cool. Scientists predict that solid stony asteroids would have more regolith in the future than crumbly ones such as Bennu (and Ryugu), which were visited recently by the Japanese spacecraft Hayabusa2). These asteroids are high carbon and thus called carbonaceous.

Carbonaceous chondrites are a subset of carbonaceous asteroids. They are rich in breccias. Breccias are rocks with small, or even fine-grained pieces mixed in with larger pieces. Compaction of carbonaceous asteroids such as Bennu may allow them to cement this stuff over time, which could lead them to become carbonaceous chondrites.

There's more to this than just the science. The best thing to do is push an asteroid off its collision path with Earth by, for instance, inserting a spaceprobe in it. A small change in velocity can help the asteroid move on a path that is not near Earth.

On the surface of the asteroid Bennu, a long rock (center), which may be the remains of an even larger boulder that was shattered, lies. It measures almost 19 meters in length, which is about the same length as a tennis court. Credit: NASA/Goddard/University of Arizona

However, if the asteroid's surface is porous, this method won't work. The spacecraft's momentum is reduced by compacting rocks rather than moving the entire asteroid. This reduces the effectiveness of the hit. We might instead have used the heat of the explosion to vaporize the rocks at the surface. The suddenly expanding gas will propel the asteroid along a new path. Before we can save the world, we have to determine if an asteroid's surface is porous or hard. This study could help us to make such an effort.

OSIRIS-REx is an acronym for "Origins Spectral Interpretation Resource Identification Security-Regolith Explorer". The "regolith-explorer" component has been found to be very important to the "security". Bennu is a Potentially Hazardous Asteroid. It's larger than 140m wide and lies within 7.5million kilometers of Earth. In fact, it's so close that there's an 1 in 2700 chance that it will impact Earth in September 2182. It's a long shot and it could happen in the future, but it's better to be safe than sorry. It is a good idea to study it and other similar ones.

While I am a huge fan of asteroid research, I also support their not hitting us. This is a win-win situation.