Earth May Have Lost Some Primoridial Atmosphere to Meteors

Earth's Hadean Eon is a bit of a mystery to us, because geologic evidence from that time is scarce. Researchers at the Australian National University have used tiny zircon grains to get a better picture of early Earth. Credit: NASA

During the Hadean Eon, some 4.5 billion years ago, the world was a much different place than it is today. As the name Hades would suggest (Greek for “underworld”), it was a hellish period for Earth, marked by intense volcanism and intense meteoric impacts. It was also during this time that outgassing and volcanic activity produced the primordial atmosphere composed of carbon dioxide, hydrogen and water vapor.

Little of this primordial atmosphere remains, and geothermal evidence suggests that the Earth’s atmosphere may have been completely obliterated at least twice since its formation more than 4 billion years ago. Until recently, scientists were uncertain as to what could have caused this loss.

But a new study from MIT, Hebrew Univeristy, and Caltech indicates that the intense bombardment of meteorites in this period may have been responsible.

This meteoric bombardment would have taken place at around the same time that the Moon was formed. The intense bombardment of space rocks would have kicked up clouds of gas with enough force to permanent eject the atmosphere into space. Such impacts may have also blasted other planets, and even peeled away the atmospheres of Venus and Mars.

In fact, the researchers found that small planetesimals may be much more effective than large impactors –  such as Theia, whose collision with Earth is believed to have formed the Moon – in driving atmospheric loss. Based on their calculations, it would take a giant impact to disperse most of the atmosphere; but taken together, many small impacts would have the same effect.

Artist's concept of a collision between proto-Earth and Theia, believed to happened 4.5 billion years ago. Credit: NASA
Artist’s concept of a collision between proto-Earth and Theia, believed to happened 4.5 billion years ago. Credit: NASA

Hilke Schlichting, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says understanding the drivers of Earth’s ancient atmosphere may help scientists to identify the early planetary conditions that encouraged life to form.

“[This finding] sets a very different initial condition for what the early Earth’s atmosphere was most likely like,” Schlichting says. “It gives us a new starting point for trying to understand what was the composition of the atmosphere, and what were the conditions for developing life.”

What’s more, the group examined how much atmosphere was retained and lost following impacts with giant, Mars-sized and larger bodies and with smaller impactors measuring 25 kilometers or less.

What they found was that a collision with an impactor as massive as Mars would have the necessary effect of generating a massive a shockwave through the Earth’s interior and potentially ejecting a significant fraction of the planet’s atmosphere.

However, the researchers determined that such an impact was not likely to have occurred, since it would have turned Earth’s interior into a homogenous slurry. Given the appearance of diverse elements observed within the Earth’s interior, such an event does not appear to have happened in the past.

A series of smaller impactors, by contrast, would generate an explosion of sorts, releasing a plume of debris and gas. The largest of these impactors would be forceful enough to eject all gas from the atmosphere immediately above the impact zone. Only a fraction of this atmosphere would be lost following smaller impacts, but the team estimates that tens of thousands of small impactors could have pulled it off.

An artistic conception of the early Earth, showing a surface pummeled by large impact, resulting in extrusion of deep seated magma onto the surface. At the same time, distal portion of the surface could have retained liquid water. Credit: Simone Marchi
Artist’s concept of the early Earth, showing a surface pummeled by large impacts. Credit: Simone Marchi

Such a scenario did likely occur 4.5 billion years ago during the Hadean Eon. This period was one of galactic chaos, as hundreds of thousands of space rocks whirled around the solar system and many are believed to have collided with Earth.

“For sure, we did have all these smaller impactors back then,” Schlichting says. “One small impact cannot get rid of most of the atmosphere, but collectively, they’re much more efficient than giant impacts, and could easily eject all the Earth’s atmosphere.”

However, Schlichting and her team realized that the sum effect of small impacts may be too efficient at driving atmospheric loss. Other scientists have measured the atmospheric composition of Earth compared with Venus and Mars; and compared to Venus, Earth’s noble gases have been depleted 100-fold. If these planets had been exposed to the same blitz of small impactors in their early history, then Venus would have no atmosphere today.

She and her colleagues went back over the small-impactor scenario to try and account for this difference in planetary atmospheres. Based on further calculations, the team identified an interesting effect: Once half a planet’s atmosphere has been lost, it becomes much easier for small impactors to eject the rest of the gas.

The researchers calculated that Venus’ atmosphere would only have to start out slightly more massive than Earth’s in order for small impactors to erode the first half of the Earth’s atmosphere, while keeping Venus’ intact. From that point, Schlichting describes the phenomenon as a “runaway process — once you manage to get rid of the first half, the second half is even easier.”

This gave rise to another important question: What eventually replaced Earth’s atmosphere? Upon further calculations, Schlichting and her team found the same impactors that ejected gas also may have introduced new gases, or volatiles.

“When an impact happens, it melts the planetesimal, and its volatiles can go into the atmosphere,” Schlichting says. “They not only can deplete, but replenish part of the atmosphere.”

The "impact farm:, an area on Venus marked by impact craters and volcanic activity. Credit: NASA/JPL
The “impact farm:, an area on Venus marked by impact craters and volcanic activity. Credit: NASA/JPL

The group calculated the amount of volatiles that may be released by a rock of a given composition and mass, and found that a significant portion of the atmosphere may have been replenished by the impact of tens of thousands of space rocks.

“Our numbers are realistic, given what we know about the volatile content of the different rocks we have,” Schlichting notes.

Jay Melosh, a professor of earth, atmospheric, and planetary sciences at Purdue University, says Schlichting’s conclusion is a surprising one, as most scientists have assumed the Earth’s atmosphere was obliterated by a single, giant impact. Other theories, he says, invoke a strong flux of ultraviolet radiation from the sun, as well as an “unusually active solar wind.”

“How the Earth lost its primordial atmosphere has been a longstanding problem, and this paper goes a long way toward solving this enigma,” says Melosh, who did not contribute to the research. “Life got started on Earth about this time, and so answering the question about how the atmosphere was lost tells us about what might have kicked off the origin of life.”

Going forward, Schlichting hopes to examine more closely the conditions underlying Earth’s early formation, including the interplay between the release of volatiles from small impactors and from Earth’s ancient magma ocean.

“We want to connect these geophysical processes to determine what was the most likely composition of the atmosphere at time zero, when the Earth just formed, and hopefully identify conditions for the evolution of life,” Schlichting says.

Schlichting and her colleagues have published their results in the February edition of the journal Icarus.

Further Reading: MIT News

Why Is The Sunset Red?

Sunset

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Why is the sunset red? Awesome question. The most basic answer is that light is refracted by particles in the atmosphere and the red end of the spectrum is what is visible. To better understand that you have to have a basic understanding of how light behaves in the air, the atmosphere’s composition, the color of light, wavelengths, and Rayleigh scattering and here is all of the information that you need to understand those things.

The Earth’s atmosphere is one of the main factors in determining what color a sunset is. The atmosphere is made up mostly of gases with a few other molecules thrown in. Since it completely surrounds the Earth it affects what you see in every direction. The most common gasses in our atmosphere are nitrogen(78%) and oxygen(21%). The remaining single percent is made up of trace gasses, like argon, and water vapor and many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans. There may be more water in the air after a rainstorm, or near the ocean. Volcanoes can put large amounts of dust particles high into the atmosphere. Pollution can add different gases or dust and soot.

Next, you have to look at light waves and the color of light. Light is an energy that travels in waves. Light is a wave of vibrating electric and magnetic fields and is a part of the electromagnetic spectrum. Electromagnetic waves travel through space at the speed of light(299,792 km/sec). The energy of the radiation depends on its wavelength and frequency. A wavelength is the distance between the tops of the waves. The frequency is the number of waves that pass by each second. The longer the wavelength of the light, the lower the frequency, and the less energy it contains. Visible light is the part of the electromagnetic spectrum that our eyes can see. Light from a light bulb or the Sun may look white, but it is actually a combination of many colors. Light can be split into its different colors with a prism. A rainbow is a natural prism effect. The colors of the spectrum blend into one another. The colors have different wavelengths, frequencies, and energies. Violet has the shortest wavelength meaning that it has the highest frequency and energy. Red has the longest wavelength and lowest frequency and energy.

In order to put it all together, we have to look at the action of light in the air of our planet. Light moves in a straight line until it is interfered with(gas molecule, dust, or anything else). What happens to that light depends on the wavelength of the light and size of the particle. Dust particles and water droplets are much larger than the wavelength of visible light, so it bounces off in different directions. The reflected light appears white because it still contains all of the same colors, but gas molecules are smaller than the wavelength of visible light. When light bumps into them it acts differently. After light hits a gas molecule some of it may get absorbed. Later, the molecule radiates the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed, but the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering.

Long story short,, the answer to ‘why is the sunset red?’ is: At sunset, light must travel farther through the atmosphere before it gets to you, so more of it is reflected and scattered and the sun appears dimmer. The color of the sun itself appears to change, first to orange and then to red because even more of the short wavelength blues and greens are now scattered and only the longer wavelengths(reds, oranges) are left to be seen.

We have written many articles about the sunset for Universe Today. Here’s an article about sunrise and sunset, and here are some sunset pictures.

If you’d like more info on the Sun, check out NASA’s Solar System Exploration Guide on the Sun, and here’s a link to the SOHO mission homepage, which has the latest images from the Sun.

We’ve also recorded an episode of Astronomy Cast all about the Sun. Listen here, Episode 30: The Sun, Spots and All.

Reference:
NASA Space Place

What Is The Atmosphere?

[/caption]What is the atmosphere? It is only the thing that keeps you from being burned to death every day, helps to bring the rain that our plants need to survive, no to mention it holds the oxygen that you need to breath. Essentially, the atmosphere is is a collection of gases that makes the Earth habitable.

The atmosphere consists of 78% nitrogen, 21% oxygen, 1% water vapor, and a minute amount of other trace gases like argon, and carbon monoxide. All of these gases combine to absorb ultraviolet radiation from the Sun and warm the planet’s surface through heat retention. The mass of the atmosphere is around 5×1018kg. 75% of the atmospheric mass is within 11 km of the surface. While the atmosphere becomes thinner the higher you go, there is no clear line demarcating the atmosphere from space; however, the Karman line , at 100 km, is often regarded as the boundary between atmosphere and outer space. The effects of reentry can be felt at 120 km.

Over the vast history of Earth there have been three different atmospheres or one that has evolved in three major stages. The first atmosphere came into being as a result of a major rainfall over the entire planet that caused the build up of a major ocean. The second atmosphere began to develop around 2.7 billion years ago. The presence oxygen began to appear apparently from being released by photosynthesizing algae. The third atmosphere came into play when the planet began to stretch its legs, so to speak. Plate tectonics began constantly rearranging the continents about 3.5 billion years ago and helped to shape long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate stores. Free oxygen did not exist until about 1.7 billion years ago and this can be seen with the development of the red beds and the end of the banded iron formations. This signifies a shift from a reducing atmosphere to an oxidizing atmosphere. Oxygen showed major ups and downs until reaching a steady state of more than 15%.

The Earth’s atmosphere performs a couple of cool optical tricks. The blue color of the sky is due to Rayleigh scattering which means as light moves through the atmosphere, most of the longer wavelengths pass straight through. Very little of the red, orange and yellow light is affected by the air; however, much of the shorter wavelength light(blue) is absorbed by the gas molecules. The absorbed blue light is then radiated in every direction. So, no matter where you look, you see the scattered blue light. The atmosphere is also responsible for the aurora borealis. Auroras are caused by the bombardment of solar electrons on oxygen and nitrogen atoms in the atmosphere. The electrons literally excite the oxygen and nitrogen atoms high in the atmosphere to create the beautiful light show we know as an aurora.

The atmosphere is divided into 5 major zones. The troposphere begins at the surface and extends to between 7 km at the poles and 17 km at the equator, with some variation due to weather. The stratosphere extends to about 51 km. The mesosphere extends to about 85 km. Most meteors burn up in this zone of the atmosphere. The thermosphere extends up to between 320 and 380 km. This is where the International Space Station orbits. The temperature here can rise to 1,500 °C. The exosphere is the last bastion of the atmosphere. Here the particles are so far apart that they can travel hundreds of km without colliding with one another. The exosphere is mainly composed of hydrogen and helium.

Check out the NASA page about the Earth’s atmosphere. Here on Universe Today we have a great article about an alternative idea about the atmosphere’s origin. Astronomy Cast offers a good episode about atmospheres around the Universe.