A privileged pedigree for meteorites

Where do meteorites come from? This question has been occupying the scientific community ever since it was realised that these “rocks from the sky” are, in fact, pieces of other worlds arriving on our planet from deep space. New research done by scientists at the Armagh Observatory and Planetarium in the UK, the University of Florida at Gainsville, the Florida Space Institute and the University of Pennsylvania in the US and published this week in Nature Astronomy now suggests that meteorites, as well as the larger objects that pose an ever-present hazard to civilization if they hit our planet, come from a few large, ancient asteroids between Mars and Jupiter that were destroyed by collisions.

Snapshot of known solar system objects within the orbit of Jupiter. The white dots correspond to asteroids in the Main Belt. Source: Wikipedia Commons

 

Some tens of thousands of meteorites now lie in laboratories and private or museum collections across the globe. The vast majority originate from the so-called Main Asteroid Belt (see above), a region between the orbits of Mars and Jupiter occupied by millions of rocky objects. The smallest of those barely survive entry into the Earth’s atmosphere to land on the surface as solid fragments, such as the Bovedy meteorite that fell near Portadown, Northern Ireland in 1969. Larger asteroids, such as the object that exploded above the city of Chelyabinsk in 2013, carry enough energy to do significant damage at ground level. The systematic study of these objects over decades has revealed a remarkable degree of diversity: no two meteorites are quite the same, even among groups that share a common characteristic such as a high abundance of metal or carbon-rich material. Scientists have been hard-pressed to come up with a credible scenario to reproduce this diversity from accretion of solids within the solar nebula.

In the new study, the team led by Professor Stanley Dermott of the University of Florida at Gainesville took a fresh approach to the problem by looking at the distribution of asteroid orbits and their sizes. Studying the asteroid belt is complicated by the fact that the orbits continuously change because of the gravitational influence of Jupiter and Saturn. But by taking time averages of the orbital properties, a significant amount of structure becomes apparent: some areas show concentrations of asteroids, elsewhere the Belt is relatively barren.

Asteroids, such as (21) Lutetia pictured here, show numerous craters from past collisions with other objects over millions of years.
Credit: © ESA 2010 MPS/OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

This is the result of asteroids colliding with one another over the 4 billion year history of the solar system (right). Collisions release myriads of fragments from the “parent” asteroids, which remain in much the same orbit. In fact, these asteroid “families” account for just under 50% of the known asteroids. The remaining non-family or “background” asteroids are more evenly spread across the Belt and it is not immediately clear where they come from: are they objects that formed independently as the solar system was forming or, are they themselves pieces of larger asteroids? The latter would imply a mechanism that changes the orbit, otherwise those asteroids would show concentrations just like the family asteroids.

By comparing the mean orbits of asteroids in families with the means for the background asteroids, Dermott et al found that families and background are quite similar and that this is true for asteroids of all sizes down to a few km across. The exception is asteroids in the most extreme orbits; these objects may also have derived from the families, but their orbits been so radically modified that any memory of their origin has been effectively wiped out. But even if one excludes these latter asteroids, that still leaves 85% of the asteroids studied by Dermott et al that are probably derived from the families.

The finding has important implications for scientists who study our solar system’s history. As Prof Dermott explains: “Rather than sampling numerous objects that formed independently in orbit around the sun, our findings suggest that the diversity observed in the meteorite record is more likely to represent a comparatively few large parent asteroids.”

Still, much work remains to be done. One question left unanswered is the nature of the mechanism that transports the asteroids out of the families. Dermott et al suggest that a subtle effect called chaotic diffusion may be the culprit. According to Dr Apostolos Christou, an astronomer at the Armagh Observatory and Planetarium and co-author of the paper, the combined gravity of Mars, Jupiter and Saturn acting over hundreds of millions of years would cause orbits to become more elongated and more tilted to the plane of the solar system so that finally the asteroid no longer lies within the family.

This idea also finds resonance with a theory, proposed a decade ago by a team led by Dr Alessandro Morbidelli and colleagues at the Observatoire de la Cote d’Azur in France and the Southern Research Institute in the US, whereby all the objects that originally populated the asteroid belt were quite big, hundreds or thousands of km across. According to that paper, all the smaller asteroids we now see are fragments of the larger objects being whittled down to smaller and smaller sizes by mutual collisions. Most of these objects have been completely destroyed but we are lucky enough that some, like the asteroid (4) Vesta visited by the Dawn spacecraft a few years ago (Fig), have survived to give testament to the violent early history of our solar system.

Reference to the paper: “The common origin of family and non-family asteroids”, by Dermott, Stanley F., Christou, Apostolos A., Li, Dan, Kehoe, Thomas. J. J., Robinson, J. Malcolm, Nature Astronomy, Vol. 7 (2018), DOI: 10.1038/s41550-018-0482-4

Link to the article

Science contact: Dr Apostolos Christou (email: apostolos.christou@armagh.ac.uk, Phone: +44(0)28 3744 2320, Alt phone: +44(0)28 37522928, Mobile: +44(0)7908 169835)

Media Contact: Heather Alexander (email: heather.alexander@armagh.ac.uk, Phone: +44(0)28 37512965), Apostolos Christou