Ordinary Chondrites, Cosmic Collisions and the History of Life
The reason many people are attracted to astrology is probably because it makes them feel connected to the cosmos. If the stars have an influence on even your individual life details, it gives your life cosmic significance. What if there is a way to feel connected to the cosmos without a horoscope, however? There is evidence that cosmic events have changed the course of the history of life on Earth in the geologic past. These events range from planetary impacts to Earth’s movement through galaxy itself. This also leads to the question of whether it could work the other way. Could life also influence the cosmos one day?
The most famous example of a cosmic event influencing life on Earth is the asteroid that hit Earth around 66 million years ago. The asteroid collided with Earth and formed Chicxulub Crater in what is now Yucatan Peninsula. In the popular version of the story, the asteroid also wiped out the non-avian dinosaurs, triggering the fifth mass extinction.
Based the impact crater at Chicxulub, the event is supported by a clay layer rich in iridium, a metal rare on Earth’s surface but common in asteroids, dating to about 66 million years ago. This iridium-rich layer now defines the boundary between the Mesozoic (i.e., age of the dinosaurs or reptiles) and Cenozoic (i.e., age of mammals) eras
Although this is appealing for its simplicity, the actual story is more complex. At the time that the asteroid hit, enormous volcanic eruptions had been ongoing for 700,000 years. These eruptions formed Deccan Traps, an enormous layer basalt in modern-day India.
The resulting environmental disturbances from the Deccan Traps eruptions had already made life on Earth miserable for the dinosaurs. The asteroid strike was just the proverbial straw that broke the hadrosaur’s back (I had to adapt the cliche for our purposes). Interestingly enough, the recent discovery of the Nadir crater in West Africa indicates there was another asteroid collision around the same time. About 66 million years ago was a bad time to be on Earth, apparently.
There are also lesser known impact events that may have influenced the evolution of life. One such event is significant to me since it is relevant to research I did as a graduate student a the University of North Dakota. While a graduate research assistant, our team, led by my advisor, used the NASA Infrared Telescope Facility (IRTF) to observe asteroids in the Massalia asteroid family.
An asteroid family is a group of related asteroids which are fragments of a larger protoplanet that was destroyed in a catastrophic collision. The Massalia asteroids are one of the proposed sources for the L-chondrite meteorites.
Most meteorites that fall to Earth are chunks of iron, remnants of the metal cores of large protoplanets that got destroyed in catastrophic collisions. L-chondrites are classified as ordinary chondrite meteorites and they are the most common non-iron meteorites to fall to Earth. All L-chondrite meteorites show mineralogical evidence of having been through a catastrophic collision about 500 million years ago.
Our goal was determine the composition of Massalia family asteroids based on spectroscopy, or what wavelengths of light are absorbed or reflected by minerals on the asteroid’s surface. This was to test the hypothesis that one or more of the Massalia asteroids are the source of the L-chondrite meteorites.
How does this connect to life on Earth? Interestingly, there is a high concentration of fossil L-chondrite meteorites in limestone layers that formed about 470-480 million years ago. This was during a period in Earth’s geologic history known as the Ordovician (485-440 million years ago or Ma).
The Ordovician Period experienced an explosion in biodiversity known as the Great Ordovician Biodiversification Event (GOBE), beginning around 480 million years ago and lasting 40 million years. During this time, there was a great increase in the diversity of marine life where the Cambrian Fauna, such as trilobites, were replaced by the Paleozoic Fauna, including star fish, corals, and cephalopods. This change in biodiversity also set the stage for life evolving into the forms that we know today.
As Steven Gould would tell you, often periods of environmental stress will drive the evolution of new species because of the need to adapt to harsh conditions. Could it be that the environmental pressures created by minor asteroid impacts contributed to the GOBE? Could we in part owe the current state of life on Earth and L-chondrite meteorites both to a cosmic event that happened 480 million years ago? I am not saying that the break up of the L-chondrite parent body caused the GOBE, but it is certainly plausible that impacts by fragments of the L-chondrite parent body influenced conditions on Earth at that time.
While asteroid impacts and comet strikes affect life on Earth, there is also evidence that the frequency asteroid and comet strikes may be affected by galactic forces. A recent paper found evidence from a correlation between hydrogen density in the galactic disk and oxygen isotopes in zircon crystals that the evolution of Earth’s crust could be influenced by Earth’s solar system passing through regions of the galactic disk that are especially dense in matter during the sun’s orbit around the center of the galaxy.
Why in the universe would this be related? When the sun passes through dense parts of the galactic disk, gravitational perturbations from said matter can cause the orbits of bodies near the edge of the solar system, mostly comet-like objects in what is called the Oort Cloud, to be altered so they collide with Earth. An increase in the frequency of impacts on Earth by incoming comets would affect the evolution of the crust because they would add thermal energy to the crust and thus what rocks form. In this way, galactic processes may be written into Earth’s geology.
Earth’s geology is not isolated from the wider galaxy, but appears to be directly influenced by galactic events and processes. Could the same be said of life? The evolution of life has clearly been influenced by previous impact events, so it is only logical to conclude the life on Earth is also shaped to some degree by galactic processes.
The effects of the gravity of passing star clusters on the evolution of life on Earth is likely to be minimal, except over very large timescales. On the other hand, it is possible that life my one day have a more significant influence on the galaxy. We already know that life is a driving force in Earth’s geology. If technological life on Earth does spread beyond this planet, it may that one day life will also be a driving force in the solar system and eventually the galaxy. This may never happen, but as we learn more about how our planet is connected to the cosmos, we should get used to thinking of our planetary system and the universe as being interconnected.
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