Shooting stars, or meteors, can be seen on any clear, dark night. At various times throughout the year, showers of meteors can be seen, appearing to originate form certain areas of the sky. These showers are the result of the Earth moving through trails of dust and debris. The bits of this debris are moving very fast. When they enter the atmosphere, friction with the air causes them to heat up dramatically and glow brightly as they disintegrate. The material causing the meteors in a typical shower are usually not big enough to have any remnant survive and reach the ground.
The meteors that are big enough to survive entry are very bright, often colorful, and sometimes fragment into multiple pieces. These fireballs, as they are called, also last much longer. If a piece survives to impact the ground, it is called a meteorite. Meteorites are very old, much older than any rock that originates from terrestrial processes. Meteorites are representative of the types of materials from which the Earth formed in a process called accretion.
Among the most exciting advances in modern astronomy has the been the detection of numerous exoplanets orbiting other stars in our interstellar neighborhood. While Science Fiction has long included alien planets in plot lines, these discoveries have revolutionized our understanding of how planetary systems form and evolve. Naturally, we want to know what it is like on these distant worlds. Could they be habitable? Could life reside there? How do we know what to look for?
It turns out there is another alien planet much closer to home and just as interesting to study. It lies right beneath us, the record of the early Earth. For much of the long history of our planet, Earth would have more closely resembled an alien world, completely inhospitable to humans. We know this from the geologic record.
About 2.3 billion years ago, one of the most important events in the history of life on Earth happened, the Great Oxygenation Event (GOE). From the Earth's formation 4.5 billion years ago until the GOE, there was very little oxygen in the ocean, and practically none in the atmosphere. Think about it. For half of Earth's history, there was no oxygen in the atmosphere. We could not have lived on this planet without a space suit.
It is only within the last few hundred million years that Earth began to resemble anything that we might call home.
There are clues to our planets past elsewhere in Solar System as well. The ancient, heavily cratered surface of the Moon is one example that tells of a time when the inner Solar System was bombarded by maelstrom of comets and asteroids, many of which must have hit the early Earth as well.
On Mars, water erosion has left recognizable scars, channels, and canyons. Mars was once a nicer place, probably habitable. Yet unlike the Moon, where we see remnants of a common past, Mars may very well foretell a common future.
While the primary focus of the Oldest Show on Earth is the history, tradition, and continued practice of astronomy, our quest should not ignore the only planet known to support life (at least as of this writing). Earth does not exist apart from the rest of the Solar System, it is a member of it and it has a captivating story to tell.
The dark broken layer in the bottom half of this specimen is widely considered to be the worlds oldest fossil remains. In life, this thin layer was a microbial mat lying on the sea floor. This specimen is from the Dresser formation, also in Western Australia and is dated at 3.49 billion years old.
Two Perseid meteors captured in a 5 minute exposure.
The Moon is a pristine relic from the early Solar System. The relatively fresh, well defined craters in this image are about 1 billion years old. The older, less well defined craters are approximately 1.5-3.0 billion years old. The dark, smooth looking areas are called maria, or seas. These are magma filled basins created from the enormous impacts of the Late Heavy Bombardment and date from 3.5-4.0 billion years ago.
As the early Earth formed, the energy released from all the impacting comets, asteroids, and meteoroids kept the surface very hot, probably molten. This time on Earth is called the Hadean Eon. There are no known terrestrial rocks that date from this time, lasting from about 4.5 billion years ago to 4.0 billion years ago.
During the Hadean, it is thought that another planet (commonly referred to as Theia) collided with the Earth. Theia was smaller than Earth and in the aftermath of the collision, most its material remained as part of our planet, but a large amount from both bodies was thrown out into space, forming a vast cloud of debris. This debris then coalesced, forming the Moon.
Toward the end of the Hadean, it is thought that there was a large increase in the number of asteroids and comets entering the inner Solar System. The remnants of enormous impacts from this event are still present on the Moon, attesting to the violence of what is called the Late Heavy Bombardment. Earth, being the larger of the two bodies, would have seen even more impacts at this time, though all traces have been lost to geologic processes.
Kimberella quadrata was once thought to be a primitive box jellyfish. More recent research suggests it may be a type of primitive mollusk. Kimberella has been found in both Australia, in the Ediacaran Hills, as well as along the White Sea Coast, in Russia. This animal grazed on microbial mats, lying on the sea floor over 550 million years ago, one of the first animals to have exhibited signs of mobility.
For most of the history of Earth, the micro-organisms that created stromatolites were the dominant life form on the planet and the most common, cyanobacteria, would alter the planet in ways that are difficult to fully appreciate.
Cyanobacteria are credited with the emergence of the process of photosynthesis, resulting in the release of Oxygen as waste. Over time, huge amounts of oxygen were released into the ocean, bonding with Iron in the water. The product settled to the bottom, forming large deposits between 2.0-2.3 billion years ago. These Iron deposits have been and continue to be the principle source of Iron ore. Such deposits could only be made once, thanks to the work of cyanobacteria.
Once the Oceans were saturated with Oxygen, it started to change the atmosphere too, slowly displacing the Methane and Carbon Dioxide. However, if we were able to go back and visit this time period, now called the Proterozoic Eon, we would still need to wear a space suit. Earth was not yet a home for us.
Five-hundred forty-one million years ago, something big happened. The fossil record bears evidence of a very rapid diversification of life. The first animals with shells and exoskeletons appeared, perhaps for protection, or perhaps to help channel poorly oxygenated water over delicate and inefficient gills. The compound eye appeared and the animals of the Cambrian were the first that we know could see.
During the Cambrian, the level of Oxygen in the atmosphere was still low compared to modern times. There were no plants or animals on land. There were no fish in the sea, or vertebrates for that matter. Trilobites and other arthropods were abundant. There were no reefs of coral yet. The Earth was spinning faster, making the days noticeably shorter, and the Moon was a bit closer. Life on Earth had made some tremendous steps, but this was still not the familiar planet we know today.
The Oxygen introduced into the atmosphere had enormous consequences for life on Earth. First, it was a toxic waste product to the anaerobic micro-organisms who created it, causing a global mass extinction event. It also created the opposite of a greenhouse effect, cooling the planet dramatically. The resultant Huronian Glaciation lasted until around 2.0 billion years ago. This event is the oldest known glaciation, as well as one of the coldest and longest.
The Huronian was not the only major glaciation during the Proterozoic Eon. The Cryogenian, which occurred toward the end of the eon between 720 and 635 million years ago. This event was so severe that it has lead to the Snowball Earth Hypothesis, the idea that the surface of the ocean had frozen all the way to the equator. Throughout the Proterozoic, it is thought that multicellular life was evolving but left few fossils. Shortly after the Cryogenian, coinciding with a global thaw, large soft-bodied creatures appeared in relative abundance. These life forms are collectively called Ediacaran fauna and do not bear any obvious resemblance to surviving lineages.
Most of the enigmatic Ediacaran fauna appear to have been stationary, held to the sea floor by holdfasts. There are exceptions however and some have even left scratches and tracks, made by primitive mouths and feeding parts and preserved in the same sediment that fossilized the creatures that made them. They were filter feeders and grazers.
Cooksonia was revolutionary. It was a plant that grew on land. What's more, it appears to have grown up, not just out. Cooksonia's strategy would be followed by later plants up to the present day. It all started during the Silurian Period, from about 443-419 million years ago.
The geological specimens shown on this page reside in the private collection of Chad Quandt. In addition to astronomy, Chad enjoys studying the past history of the Earth. Rocks tell us what our planet was like a very long time ago, just as the light from distant stars tell us what they were like a very long time ago.
Nestled between Mare Imbrium (Sea of Rain) and Mare Serenitatis (Sea of Serenity), the Lunar Appenines are a prominent mountain range easily visible in small telescopes. Unlike mountains on Earth, most Lunar mountains are actually the uplifted rim of enormous and ancient impact basins.
Just when and where the origin of life occurred is a tough thing to determine. There are very few places on Earth where rocks of this great antiquity can be found. The Strelley Pool formation in the Pilbara Craton of Western Australia is just one of those places. There, widespread formations of Stromatolites are exposed at the surface. Dating at nearly 3.5 billion years old, these rocks are among the oldest intact crustal rocks on Earth and they also contain tantalizing evidence of fossil life.
Stromatolites are accretionary structures that can be formed by both abiotic and biological processes. A biological stromatolite usually consists of distinct laminar bands, but also can have other structures like domes. Micro-organisms, usually cyanobacteria, live on the surface of the stromatolite in a film. Grains caught in the film help to build up the stromatolite layer by layer. If microfossils are found within a given stromatolite specimen, it is a clear indication of biological origin. For the oldest stromatolites, ones whose rocks have undergone significant heat and stress, such microfossils are both rare and difficult to identify with certainty.
Stromatolites can also form without the presence of life as well. These abiotic stromatolites might form in a briny pool, around a volcanic vent, or some other location with interesting chemistry, extremes of heat, and cycles of evaporation. In the absence of microfossils, telling which ones were caused by life is not yet a settled science. However, there are other clues such as morphological variation in the structure of the stromatolite that have been interpreted by some researchers as an indicator of biological processes. What is certain is that stromatolites were abundant in these early seas.
Slice of Seymchan Pallasite meteorite. The Pallasites are also called stoney-iron meteorites, containing both Iron and occasionally yellow/green crystals of Olivine. Pallasites are thought to have originated in the core-mantle boundary region of a planet or large asteroid shattered by a violent collision in the early Solar System. Olivine is a common mineral, present in both terrestrial and Lunar rocks. Gem quality crystals are called Peridot.
Meteorites give us a good idea what the basic constituents of the early Solar System were and it follows that this is the same stuff that we are made of as well. Meteorites are also very old, but all are relatively recent arrivals on Earth. The ones from which we formed have been melted and recycled from over 4 billion years of geologic activity.
As the Earth was forming, the molten material slowly differentiated. Heavier elements such as Iron tended to sink to the center. Lighter elements tended to rise to the upper layers. After hundreds of millions of years, the surface had cooled enough to solidify and it wasn't long after this that the oceans began to form.
In addition to metals, meteorites also contain a small percentage of water. It is from meteorites, asteroids, and comets that Earth received the water that now reside in our oceans, lakes, and streams, all brought down from space a little bit at a time. It was by chance that Earth resided at just the right distance form the Sun for this water to remain liquid on the surface (most of the time at least), providing a unique environment for interesting chemistry and the origin of life.
Though not a stromatolite, this beautiful banded rock called Tiger Iron was the indirect product of biological activity. The bands of varying darkness represent distinct periods when the local concentration of Oxygen varied due to periodic blooms of cyanobacteria, whose revolutionary photosynthesis would discharge large amounts of Oxygen into the water. Iron in the water would oxidize and deposit a new layer, creating a Banded Iron Formation (BIF).
Stromatolite layers from the Strelley Pool formation in Western Austrailia are among the oldest terrestrial rocks found anywhere, dating from approximately 3.4 billion years ago.
Another type of meteorite is the Iron meteorite. This example was recovered from the witnessed fall in Sikhote-Alin (1947). It displays the classic features of a typical Iron meteorite with a surface covered in regmaglypts, shallow thumb-shaped impressions created from ablation during entry in the atmosphere. Iron meteorites are primarily composed of both Iron and Nickel.
Detailed view of cut side of the Chondrite above showing elemental Iron and other inclusions.
This Chondrite (stoney meteorite) was recovered from the North African desert and is representative of most of the meteors that have bombarded the Earth and Moon since their formation, 4.5 billion years ago. The top surface shows obvious marks from is violent entry into Earth's atmosphere. The cut side reveals an interior composition of undifferentiated materials form the early Solar System. This rock is older than any terrestrial rock ever found.
At this point, over 90% of Earth's history has already passed. There were still no forests, no meadows, no animals living on the land, just a vast barren landscape. It wasn't until the Silurian Period that plants first started to colonize the land, beginning some 443 million years ago. The Silurian is still a long way from what we would call comfortable.
The history of the Earth, as preserved in rocks, shows a planet that has hosted life for billions of years. Tracing our ancestry back to these early animals and micro-organisms is a bit like tracing our heritage to some alien planet. The Earth has long been habitable, just not to us.
Olenellus getzi, a large and primitive species of lower Cambrian trilobites. Arthropods experienced an incredible expansion of diversity throughout the early Paleozoic Era and trilobites are among the best studied. Trilobites roamed the seas for about 300 million years, the last of them going extinct during the Permian-Triassic extinction 251 million years ago.
Last Updated 22 Dec 2020.
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