16-Nov-2018: How the Antarctic Circumpolar Current helps keep Antarctica frozen?

The Antarctic Circumpolar Current, or ACC, is the strongest ocean current on our planet. It extends from the sea surface to the bottom of the ocean, and encircles Antarctica.

It is vital for Earth’s health because it keeps Antarctica cool and frozen. It is also changing as the world’s climate warms. Scientists are studying the current to find out how it might affect the future of Antarctica’s ice sheets, and the world’s sea levels.

The ACC carries an estimated 165 million to 182 million cubic metres of water every second (a unit also called a “Sverdrup”) from west to east, more than 100 times the flow of all the rivers on Earth. It provides the main connection between the Indian, Pacific and Atlantic Oceans.

The tightest geographical constriction through which the current flows is Drake Passage, where only 800 km separates South America from Antarctica. While elsewhere the ACC appears to have a broad domain, it must also navigate steep undersea mountains that constrain its path and steer it north and south across the Southern Ocean.

What is the Antarctic Circumpolar Current?

A satellite view over Antarctica reveals a frozen continent surrounded by icy waters. Moving northward, away from Antarctica, the water temperatures rise slowly at first and then rapidly across a sharp gradient. It is the ACC that maintains this boundary.

The ACC is created by the combined effects of strong westerly winds across the Southern Ocean, and the big change in surface temperatures between the Equator and the poles.

Ocean density increases as water gets colder and as it gets more salty. The warm, salty surface waters of the subtropics are much lighter than the cold, fresher waters close to Antarctica. We can imagine that the depth of constant density levels slopes up towards Antarctica.

The westerly winds make this slope steeper, and the ACC rides eastward along it, faster where the slope is steeper, and weaker where it’s flatter.

Fronts and bottom water: In the ACC there are sharp changes in water density known as fronts. The Subantarctic Front to the north and Polar Front further south are the two main fronts of the ACC (the black lines in the images). Both are known to split into two or three branches in some parts of the Southern Ocean, and merge together in other parts.

Scientists can figure out the density and speed of the current by measuring the ocean’s height, using altimeters. For instance, denser waters sit lower and lighter waters stand taller, and differences between the height of the sea surface give the speed of the current.

The path of the ACC is a meandering one, because of the steering effect of the sea floor, and also because of instabilities in the current.

The ACC also plays a part in the meridional (or global) overturning circulation, which brings deep waters formed in the North Atlantic southward into the Southern Ocean. Once there it becomes known as Circumpolar Deep Water, and is carried around Antarctica by the ACC. It slowly rises toward the surface south of the Polar Front.

Once it surfaces, some of the water flows northward again and sinks north of the Subarctic Front. The remaining part flows toward Antarctica where it is transformed into the densest water in the ocean, sinking to the sea floor and flowing northward in the abyss as Antarctic Bottom Water. These pathways are the main way that the oceans absorb heat and carbon dioxide and sequester it in the deep ocean.

Changing current: The ACC is not immune to climate change. The Southern Ocean has warmed and freshened in the upper 2,000 m. Rapid warming and freshening has also been found in the Antarctic Bottom Water, the deepest layer of the ocean.

Waters south of the Polar Front are becoming fresher due to increased rainfall there, and waters to the north of the Polar Front are becoming saltier due to increased evaporation. These changes are caused by human activity, primarily through adding greenhouse gases to the atmosphere, and depletion of the ozone layer. The ozone hole is now recovering but greenhouse gases continue to rise globally.

Winds have strengthened by about 40% over the Southern Ocean over the past 40 years. Surprisingly, this has not translated into an increase in the strength of the ACC. Instead there has been an increase in eddies that move heat towards the pole, particularly in hotspots such as Drake Passage, Kerguelen Plateau, and between Tasmania and New Zealand.

We have observed much change already. The question now is how this increased transfer of heat across the ACC will impact the stability of the Antarctic ice sheet, and consequently the rate of global sea-level rise.

7-Nov-2018: Scientists theorize new origin story for Earth's water

Earth's water may have originated from both asteroid material and gas left over from the formation of the Sun. The new finding could give scientists important insights about the development of other planets and their potential to support life.

In a new study in the Journal of Geophysical Research: Planets, a journal of the American Geophysical Union, researchers propose a new theory to address the long-standing mystery of where Earth's water came from and how it got here.

The new study challenges widely-accepted ideas about hydrogen in Earth's water by suggesting the element partially came from clouds of dust and gas remaining after the Sun's formation, called the solar nebula.

To identify sources of water on Earth, scientists have searched for sources of hydrogen rather than oxygen, because the latter component of water is much more abundant in the solar system.

Many scientists have historically supported a theory that all of Earth's water came from asteroids because of similarities between ocean water and water found on asteroids. The ratio of deuterium, a heavier hydrogen isotope, to normal hydrogen serves as a unique chemical signature of water sources. In the case of Earth's oceans, the deuterium-to-hydrogen ratio is close to what is found in asteroids.

More recent research suggests hydrogen in Earth's oceans does not represent hydrogen throughout the entire planet. Samples of hydrogen from deep inside the Earth, close to the boundary between the core and mantle, have notably less deuterium, indicating this hydrogen may not have come from asteroids. Noble gases helium and neon, with isotopic signatures inherited from the solar nebula, have also been found in the Earth's mantle.

In the new study, researchers developed a new theoretical model of Earth's formation to explain these differences between hydrogen in Earth's oceans and at the core-mantle boundary as well as the presence of noble gases deep inside the planet.

Modeling Earth's beginning: According to their new model, several billion years ago, large waterlogged asteroids began developing into planets while the solar nebula still swirled around the Sun. These asteroids, known as planetary embryos, collided and grew rapidly. Eventually, a collision introduced enough energy to melt the surface of the largest embryo into an ocean of magma. This largest embryo would eventually become Earth.

Gases from the solar nebula, including hydrogen and noble gases, were drawn in by the large, magma-covered embryo to form an early atmosphere. Nebular hydrogen, which contains less deuterium and is lighter than asteroid hydrogen, dissolved into the molten iron of the magma ocean.

Through a process called isotopic fractionation, hydrogen was pulled towards the young Earth's center. Hydrogen, which is attracted to iron, was delivered to the core by the metal, while much of the heavier isotope, deuterium, remained in the magma which eventually cooled and became the mantle, according to the study's authors. Impacts from smaller embryos and other objects then continued to add water and overall mass until Earth reached its final size.

This new model would leave Earth with noble gases deep inside its mantle and a lower deuterium-to-hydrogen ratio in its core than in its mantle and oceans. The authors used the model to estimate how much hydrogen came from each source. They concluded most was asteroidal in origin, but some of Earth's water did come from the solar nebula.

For every 100 molecules of Earth's water, there are one or two coming from solar nebula.

The study also offers scientists new perspectives about the development of other planets and their potential to support life. Earth-like planets in other solar systems may not all have access to asteroids loaded with water. The new study suggests these exoplanets could have obtained water through their system's own solar nebula. This model suggests that the inevitable formation of water would likely occur on any sufficiently large rocky exoplanets in extrasolar systems.