The Impact History Of Asteroids Revealed In Cosmic Dust
The asteroid belt is a collection of approximately 300,000 objects greater than 1km in diameter. These solar system “small bodies” sit between Mars and Jupiter at approximately 2.2-3.2 astronomical units from the Sun.
The asteroid belt is composed of a diverse array of materials including protoplanets – that is early-formed aggregations of solar system condensates, which acted as the building blocks for later planets; larger and partially differentiated planetesimals – essentially small planets, the vast majority of which have since been smashed apart and captured icy comets, collected from the outer solar system.
As planetary scientists, we are interested in learning more about the geology of the asteroids that make up this belt. Ultimately, we hope to discover how the 8 planets we have today relate to this complex mix of planetary debris.
One way we can study the asteroid belt is by analyzing extraterrestrial materials that arrive on Earth. Most meteorites are fragments of asteroids, liberated from their parent body by collisions. In these collisions, an abundance of cosmic dust is also produced. In space, this dust forms loose “clouds” that slowly spiral into the inner solar system over millions of years. Eventually, dust grains are either caught by the gravity of terrestrial planets or consumed by the Sun.
Today, the Earth receives approximately 60,000 tonnes of cosmic dust per year, falling everywhere over the Earth, continuously. Individual dust grains are typically less than 2mm in diameter and termed micrometeorites. Scientists like myself recover micrometeorites from deep-sea sediments, Antarctic and Greenland Ice and even from rooftops in urban areas. Potential micrometeorites are set in resin and analyzed under a scanning electron microscope.
Investigating Impact-induced Shock Deformation In Micrometeorites
In a recent study titled “Shock fabrics in fine-grained micrometeorites”, we investigated whether cosmic dust grains preserve evidence past impact events that occurred on their parent asteroids. This is important to know because it helps us to understand how the asteroid belt has evolved over geological time; allowing us to answer questions like “how common are asteroid collisions?”, “How big were these collisions?” and “do impacts provide the heat energy needed to drive geological processes?”
Previous studies on a group of meteorites known as the CM chondrites have consistently identified subtle evidence of impacts preserved as shock textures (e.g. fractures, melt veins, aligned minerals and flatted chondrules) and shocked minerals (formed only at high pressures). However, although abundant evidence of impacts can be identified in the CM chondrites, the peak pressures exerted appear to be relatively low (less than 5GPa), indicating that impacts were relatively small events, which imparted only modest amounts of energy and therefore, the pervasive shock textures found in CM chondrites most probably formed by successive impact events over long periods of time.
As a micrometeorite researcher, I wanted to know whether fine-grained micrometeorites, which are geologically similar to CM chondrites, showed the same shock histories? To answer this question, we had to develop a new technique suitable for studying the textures or “petrofabrics” within tiny micrometeorites. This is because several conventional analysis techniques within our field are often not suitable for micrometeorites that are unfortunately too small. As I have increasingly found during my PhD, studying such small samples often presents new problems that require novel solutions.
Ultimately, we developed a 2D analysis technique, where the exposed micrometeorite cross-sections were analyzed by image processing. The voids within a micrometeorite are extracted and their longest axis determined. The orientation of this long-axis is then measured with respect to an arbitrary “north” and the data plotted as a rose diagram. Rose diagrams are essentially circular histograms and commonly used in sedimentary geology. For example, we may use a rose diagram to determine whether a set of dinosaur bones, deposited on a bedding plane are aligned and thus whether the bones were deposited in a current, such as a flowing river. Similarly, we employed rose diagrams to quantitatively analyze whether each micrometeorite contained a subtle petrofabric.
The findings of our study revealed that most micrometeorites (up to 80%) contain a weak petrofabric, that was most likely formed by successive collisions hitting the parent asteroid at similar angles. Therefore, before the asteroid broke apart and released a cloud of micrometeorites, it had already sustained a series of impacts that had compacted and reprocessed the object’s interior.
By choosing to analyze shock textures in micrometeorites, we have provided a repeat study that reinforces the findings of previous researchers on CM chondrites whilst also identifying the first shock textures in fine-grained micrometeorites. As I continue my career I hope to further explore how impact events have shaped the geological history of our solar system.