TF006-Origins of Earth’s Salty Oceans

Origins of Earth‘s Salty Oceans

Scientists have long been interested in discovering the origin of Earth‘s water and establishing why Earth’s oceans are so salty There has been speculation that earliest Earth was so hot that no liquid water existed, and all light elements (such as hydrogen and oxygen) were rapidly stripped away from Earth by the solar wind (a stream of charged particles emitted by the Sun). If this were true, then the elements needed to form water on Earth would not have been freely available. As a consequence, it was proposed that collisions with icy comets or similar gas–and water–rich materials brought water to Earth after the planet had sufficiently cooled to retain it. This concept was supported by comparisons of the gas compositions of meteorites with those of rocks from beneath Earth’s surface, notably using krypton and xenon, nonmetallic gases that do. not react with other materials. There certainly is enough ice in space to have supplied our water (and atmosphere)in this manner.

In July 2015, the space probe Philae. which landed on comet Churi, discovered not only ice and dust, but also 16 types of organic compounds, present not in a loose distribution but in discrete clumps. Suddenly, the idea gained lots of traction that comets brought not only water, but also the ingredients for life, even in ready-made clumps. Intriguingly, in October 2015 it was reported. that-as this comet slowly thaws-molecular oxygen(02) escapes in a constant and high proportion(1% to 10%) relative to water which suggests that the comet also contains a surprising amount of primordial (ancient) oxygen, which was incorporated during the comet‘s formation.

Other work favors an alternative explanation. This work found that the hydrogen isotope ratio(the proportion of different forms of hydrogen) of ice in comets may be different from that of water on Earth. It instead emphasizes that the chemical composition of water on Earth resembles that of the small percentage of water contained within rocky meteorites, and thus in asteroids, which are essentially very large meteorites. Thus, a theory was developed that the asteroids planetesimals. and protoplanets that clumped together to form Earth had carried enough water in their rock minerals to explain our oceans. It would have escaped from the planet‘s interior as steam, which in turn would have condensed into water at the surface and in the early atmosphere. Calculations indicate that this mechanism can also provide plenty of water to explain Earth‘s observed water content.

We have a more complete understanding of the origin of salt in our oceans. It represents an accumulation of dissolved minerals over tens of millions to hundreds of millions of years. These minerals were broken up and dissolved during chemical weathering We are all familiar with this process from limestone buildings that become pitted or smoothed by the action of water, wind, and weather; this is where the term weathering comes from. The key process at work is one of chemical reactions between the rock and the water, with an important role for gases that are dissolved in water, such as carbon dioxide or sulfur dioxide, since these make the water more corrosive. The chemical weathering reactions break up rock minerals into charged atoms or molecules, called ions, which are removed in solution by river water and groundwater. This is exactly what happens when you dissolve table salt in water: the mineral salt breaks down into sodium and chloride ions that are held in a solution.

The early atmosphere contained high levels of carbon dioxide, or CO2. This gas is easily dissolved in water, forming a mildly acid solution. In the Co2-rich early atmosphere, this resulted in a corrosive acid rain that was highly effective at chemically weathering rocks, and fresh volcanic rocks are especially easily weathered.The intense weathering released dissolved minerals in the form of ions into river water and groundwater. From early times onward, river and groundwater flow has transported the dissolved minerals to their final collection point, the ocean basins. Given the extremely slow input and removal of salts, it becomes clear that the oceans‘ vast store of salt has accumulated because the oceans have for ages been the end station for salt transport. Meanwhile water itself continually evaporates from the oceans-concentrating- its salts-and– the evaporated fresh water continues the weathering cycle. 

1、Scientists have long been interested in discovering the origin of Earth’s water and establishing why Earth’s oceans are so salty There has been speculation that earliest Earth was so hot that no liquid water existed, and all light elements (such as hydrogen and oxygen) were rapidly stripped away from Earth by the solar wind (a stream of charged particles emitted by the Sun). If this were true, then the elements needed to form water on Earth would not have been freely available. As a consequence, it was proposed that collisions with icy comets or similar gas-and water-rich materials brought water to Earth after the planet had sufficiently cooled to retain it. This concept was supported by comparisons of the gas compositions of meteorites with those of rocks from beneath Earth’s surface, notably using krypton and xenon, nonmetallic gases that do. not react with other materials. There certainly is enough ice in space to have supplied our water (and atmosphere)in this manner.