Monday, September 25, 2017

Uncovering the Nature of the Ancient Asteroid Belt

Artist's impression of the asteroid belt. Image credit: NASA/JPL-Caltech

An international team of astronomers has recently discovered a gigantic void in the main asteroid belt - a relatively unpopulated region, where the few asteroids occupying it could tell a story about the origins and evolution of our solar system. This area, populated by only a handful of asteroids, could provide important insights into the nature of the ancient asteroid belt, improving our understanding on how planets form.

The main asteroid belt, located between Mars and Jupiter, contains numerous irregularly shaped planetesimals. Approximately one-third of the objects in this belt are members of an asteroid family, sharing similar orbital parameters and spectral features what indicates a common origin in collisions of larger bodies. Therefore, identifying and studying asteroid families could provide new hints about the history of main belt asteroids.

Now, a group of researchers has used a new search technique, which allowed them to find asteroids that apparently formed over 4 billion years ago - before the gas giants in the outer solar system moved into their current orbits. The scientists detailed their findings in a paper entitled “Identification of a primordial asteroid family constrains the original planetesimal population”. The study was published August 3, 2017, in an online edition of Science.

“By finding old families we can start to get a picture of which asteroids are not members of any known family and may be primordial, or original members of the asteroid belt. This is a big step because that is likely the starting point for planet formation,” Dr. Kevin Walsh of the Southwest Research Institute (SwRI), one of the co-authors of the study, told Astrowatch.net.

When the scientists run models building planets they have to make assumptions on the sizes and locations of the building blocks - the first big rocky bodies, or the first asteroids. They get big differences if they start with 1-kilometer or with 300-kilometer asteroids. 

“Here, we find a handful of asteroids that cannot be members of this very old family, or any other family, and they are all 35-kilometer and larger. This points us to better starting points of planet formation, while also telling us important facts about how the first planetesimals formed,” Walsh said.

He explained that asteroid families can be linked a few ways. For younger families they are typically found a groups of asteroids with tightly clumped orbits (often visible by eye or other linking techniques). Over time they drift apart due to this thermal effect (the so-called Yarkovsky effect) - in a way that depends on their size, so that smaller asteroids drift faster than larger asteroids. This size-dependent shift creates an identifiable shape when we compare all the asteroids in terms of their orbits (semimajor axis, or distance from the sun) and their size.

“It makes a V-shape as some drift inwards and some drift outwards. Both techniques often rely on how the asteroids look, their albedo (how reflective they are - light or dark) or their spectra in order to match similar types of asteroids that presumably have similar origins,” Walsh said.

Finding the new asteroids in the newly found void relied on spotting the characteristic V-shape in the distribution of asteroids that is created over billions of years of them drifting away from where they were formed.

The scientists do not have many precise dates on very old families, and some of the oldest ones known are in very crowded parts of the asteroid belt, where it is difficult to precisely calculate their age. However, it is estimated that some of the older ones are up to 3 billion years old.

According to Walsh, the asteroids discovered by his team are dark what told them, first of all, that they can all be related. If there were a wide mix of albedos it would not suggest a common heritage from a singular family. Furthermore, dark asteroids are typically of primitive, or carbonaceous types. 

“These are minimally heated or processed materials that can be full of water-rich minerals, making up to 10 percent of their mass. They are thought to be good candidates for delivering water to the Earth when it was forming and to have been born in the outer part of the solar system (beyond Jupiter),” Walsh revealed.

He added that the large size of these newly found old asteroids is interesting for a few reasons. The researchers are biased to find the large families that are old as small families would be far less recognizable after such long timescales, so in some ways they could only discover a large family that is this old. It is interesting because it shows that collisions like this happened back then. How large and how stirred up the asteroid belt was 4 billion years ago is not really known, nor are the mechanisms that did the stirring.

“So, knowing about this family, its age and size, gives us one data point for what it would have looked like. To first order, it matches with what we expect, that the asteroid belt was likely more than ten times more massive 4 billion years ago, but already as stirred up as we find it today,” Walsh told Astrowatch.net.

Walsh concluded that the study in many ways was a demonstration that it is possible to find families of this very old age, and that in doing so it will help scientists learn a lot about the nature of the ancient asteroid belt. He assumes that this true nature could look a lot like what his team found recently - large asteroids with a shallow size distribution (where "shallow" means that there are a similar number of large and small bodies, rather than a similar mass of large and small, which would be a steep distribution). 

“I think that we will also find that big and early collisions like the one that made this ancient family might have been common, telling of a larger and collisional active first few millions of years in this part of the solar system,” Walsh noted.

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