Cosmic timeline 19
A Blue Planet
Earth (or the Earth) is the third planet from the Sun, the fifth-largest and the densest of the eight planets in the Solar System. It is also the largest of the Solar System’s four terrestrial planets. It is sometimes referred to as the World, the Blue Planet, or by its Latin name, Terra.
Our Blue Planet is home to millions of species including humans, Earth is the only place in the universe where life is known to exist. Since the planet formed 4.54 billion years ago, Earth’s biosphere has constantly changed the atmosphere and other factors essential to life, enabling the proliferation of life forms as well as the formation of the ozone layer which, together with Earth’s magnetic field, blocks harmful solar radiation, permitting life on land to survive. This combination of conditions, as well as the geological history and orbit of our Blue Planet, have allowed life to keep evolving for billions of years.
Water and the early formation of our Blue Planet
It is waters of our oceans that create the “blue” of our Blue Planet. In the first era of our planet’s formation, the so-called Hadean era (named after Hades, the underworld of ancient classical mythology) it is hard to imagine the presence of water and oceans.
An artist’s impression of a magma ocean on the early Earth suggests how extreme the conditions on the Earth would have been. So, it is surprising to learn that a sizeable quantity of water would have been in the material which formed the Earth. There are some theorists who think that the lack of denser noble gases (helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn) ) in the modern atmosphere suggests that something disastrous happened to the early atmosphere of our planet. Others disagree, but the idea is that part of the ancient planet was disrupted by the impact which created the Moon. A lot of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy carbon dioxide atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230 °C because of the atmospheric pressure of the heavy carbon dioxide atmosphere.
Dating the crust of the Earth
Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks , usually based on a comparison of a naturally occurring radioactive isotope and its decay products, using known decay rates and is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself. Radiometric dating methods are used in geochronology to establish the geological time scale. The mineral Zircon is a remarkable mineral, if only for the fact it is found almost everywhere in the Earth’s crust, and it has another property that makes it very useful in radiometric dating. Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques. Because zircons can survive all kinds of geologic processes, they hold a key to unlock the history of the geological processes that have shaped our planet.
The Jack Hills are a range of hills in Mid West Western Australia. They are best known as the source of the oldest material of terrestrial origin found to date: zircons that formed nearly 4.5 billion years ago. These zircons have enabled ground-breaking research into the conditions on earth in the Hadean eon. The oxygen isotopic compositions of some of these zircons have been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth, but this idea is still the subject of debate and disagreement among scientists.