Astronomy

How thick was Earth's primordial atmosphere?

How thick was Earth's primordial atmosphere?


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Earth's first atmosphere was made of hydrogen and helium accumulated before fusion started in the Sun. As soon as this happened it was stripped away by the Sun. How many atm of hydrogen and helium had Earth accumulated at the peak pressure of the first atmosphere?


We have no way of knowing how much hydrogen and helium was in the Earth's primordial atmosphere. What is more important is what the atmosphere consisted of when the primeval soup from which life arose was formed, just over 4 billion years ago. We have a fairly good idea what this atmosphere consisted of: mainly carbon dioxide and nitrogen, rather like Mars at the present time, but with substantial amounts of ammonia and methane. Also very small amounts of hydrogen and helium, most of which had by then been blown away by solar radiation. There was hardly any oxygen. As for pressure, all we can say for sure is that on Earth it was a good deal higher than it is today. Conditions at that time were similar, though not identical, on Venus, Earth and Mars, perhaps on Titan as well, so it is possible that all three terrestrial planets developed a version of the Earth's primeval soup, which was essential for the creation of life. Whether life emerged on any planet other than Earth, we don't yet know. Because all three planets had oceans, water vapour would also have been an important atmospheric constituent.


Lecture 29: The Earth's Atmosphere

Infrared "opacity" comes from absorption bands of H2O, CO2, CH4 and others molecules. Infrared Absorption of the Atmosphere from 1-28 microns (calculated)

Zoom into the near-Infrared (1-6micron) showing specific molecular bands

  • Photons are absorbed by the ground, heating it up
  • The warm ground radiates infrared photons (Wein's Law)
  • Most of the infrared photons emitted by the warm ground get absorbed by the atmosphere on their way out, heating the atmosphere.

The implications are very interesting: Without the Greenhouse Effect, there would be no liquid water on the Earth, only ice. Since life as we understand it requires liquid water, if there was no Greenhouse Effect, the Earth would be inhospitable to life.

The term "greenhouse effect" is something of a misnomer. Greenhouses (glass buildings mean to stay warm through winter for growing plants) work primarily by inhibiting convection (boiling motions of air warmed by solar-heating of the ground inside the greenhouse), rather than by radiative trapping of sunlight behind glass. Radiative trapping does contribute to how a garden greenhouse works, but only to a fraction of the total heating experienced inside a greenhouse.


How thick was Earth's primordial atmosphere? - Astronomy

Saturn's largest satellite (of 60 orbiting Saturn) is the mysterious world called Titan. It has slightly greater diameter (5150 km), density, and mass than Callisto. At 1.22 million kilometers from Saturn, it takes 15.9 days to orbit Saturn. With a density of 1.881X water, Titan is probably half rock, half ice. Careful observations of how the Cassini spacecraft moved in Titan's gravity field have shown that Titan's interior is only partially differentiated (like Callisto). Below the frozen surface may be an internal ocean of liquid water (or water-ammonia mixture) sandwiched between two thick ice layers surrounding a rock-ice mixture core.

What is special about Titan is that it has a thick atmosphere with a surface air pressure about 1.5 times thicker than the Earth's. Even though Titan's mass is even smaller than Mars', it is so cold (just 95 Kelvin) that it has been able to hold on to its primordial atmosphere. The atmosphere is made of cold molecular nitrogen (95%) and methane (about 5%). Other organic molecules have been detected in its atmosphere. They are formed from solar ultraviolet light and high energy particles accelerated by Saturn's magnetic field interacting with the atmospheric nitrogen and methane. The molecules of nitrogen and methane are split apart (photodissociation) and the atoms recombine to make a thick haze layer of mostly ethane that blocks our view of the surface in visible light. When the droplets of the organic molecules get large enough, they rain down to the surface as very dark deposits of liquid methane and ethane. Methane bubbling up from below the surface is thought to replenish the methane lost in its atmosphere from photodissociation. The picture of Titan, Triton and the Moon at the end of this sub-section shows hazy Titan as viewed in visible wavelengths from the Voyager spacecraft. Unfortunately, Voyager's cameras were precisely tuned to the wrong wavelengths so it could not peer through the haze layer. Therefore, all it saw was an orange fuzz ball.

Titan's brew of organic compounds is probably like the early Earth's chemistry. Its very cold temperatures may then have preserved a record of what the early Earth was like before life formed. This possibility and the possibility of lakes or oceans of methane and ethane hidden under a haze of organic compounds made Titan the special subject of a Saturn orbiter mission to follow-up the Voyager fly-by mission. The Cassini spacecraft orbited Saturn for 13 years, flying by its numerous moons including over 100 targeted flybys of Titan. Using infrared wavelengths and radar, Cassini was able to peer through the hazy atmosphere. The picture below is a mosaic of 16 images taken at infrared wavelengths coming from the surface and that pass through the atmosphere easily to Cassini's camera.

Cassini managed to sample particles from the uppermost levels of Titan's atmosphere (many hundreds of kilometers above the surface) and found that there were traces of oxygen in Titan's upper atmosphere, probably from the photodissociation of water escaping Enceladus (see below) into hydrogen and oxygen. The presence of trace amounts of oxygen enables a greater variety of chemical compounds to be made with the energy of sunlight than just nitrogen and methane alone. Scientists simulating the conditions of Titan's upper atmosphere by mixing together simple compounds together under very low densities and bathing them with various wavelength bands of light have been able to create complex organic compounds, and an experiment reported in October 2010 (see also the LPL Spotlight article or the UA news release) produced all five of the nucleotide bases of life (adenine, cytosine, uracil, thymine, and guanine) and two amino acids (glycine and alanine) when a mixture of molecular nitrogen, methane, and carbon monoxide were subjected to microwaves. Early Earth with only trace amounts of oxygen in its atmosphere might have produced the first nucleotides and amino acids in the same way.

The montage below includes a radar map of the lakes near the north pole of Titan. They are filled with liquid ethane and methane and are fed from sub-surface seepage and rainfall. Looking a lot like lakes on the Earth, you can see bays, islands, and tributary networks. The large lake at the top of the radar map is larger than Lake Superior on Earth. Kraken Mare, of which a small portion of is visible in the lower left part of the map, is as big as the Caspian Sea on the Earth. There are also lakes near the south pole of Titan. [Data used to create the montage: 939 nm image, 5 micron glint image, and radar image.]


Titan, Triton, and our Moon to the same scale.

Enceladus

Enceladus is the fourth largest moon of Saturn at 504 km in diameter. It is shown in front of the much larger Titan in the image at left from Cassini. Enceladus orbits 238,000 kilometers from Saturn in 1.37 days. Despite its small size, Enceladus is a moon of large interest because it has the highest albedo of any major moon (1.0) and it is geologically active. Tidal heating supplies only a small amount (about 1/5th) of the internal heat for this moon. Simulations show that if Enceladus has a slight wobble in its rotation of between 0.75 and 2 degrees, the wobbling could generate about five times more heat than tidal heating as well as produce it at the observed locations of greatest heat in the fissures in its southern hemisphere. Geological activity is helped by Enceladus being mostly ice---its density is 1.61X water. Recall that ices can deform and melt at lower temperatures than silicate and metal rocks.

Enceladus has geysers spurting water (vapor and ice) from its south pole that point to a large ocean of liquid water below its icy, mirror-like surface. The geysers can be seen when one is on the other side of Enceladus looking back toward the Sun. The small particles scatter the sunlight forward toward the viewer. Geyser material is able to escape Enceladus and become part of the E-ring of Saturn. Enceladus' activity appears to be localized to the southern hemisphere. Its northern hemisphere has many more craters. Sampling of the geyser material has found salts (sodium chloride and potassium chloride) and carbonates mixed in with the water. That means the liquid water layer is in contact with the rocky core instead of being sandwiched between ice layers. If there is an ocean below the icy surface, should Enceladus be another place to look for life besides Europa?

In this image above taken in November 2009, more than 30 individual jets shoot water vapor and ice up hundreds of kilometers from the south pole region.

The south pole region of Enceladus is a stark contrast from regions further north in the image on the right. In this enhanced color view, the blue "tiger stripes" stand out. The "tiger stripes" are fissures that spray icy particles, water vapor and organic compounds.

Triton

Triton has many black streaks on its surface that may be from volcanic venting of nitrogen heated to a gaseous state despite the very low temperatures by high internal pressures. The nitrogen fountains are about 8 kilometers high and then move off parallel to the surface by winds in the upper part of its thin atmosphere. Another unusual thing about Triton is its highly inclined orbit (with respect to Neptune's equator). Its circular orbit is retrograde (backward) which means the orbit is decaying---Triton is spiralling into Neptune. Triton's strange orbit and the very elliptical orbit of Neptune's other major moon, Nereid, leads to the proposal that Triton was captured by Neptune when Triton passed too close to it. If it was not captured, Triton was certainly affected by something passing close to the Neptune system.


The Miller-Urey Experiment

In 1953, American scientists Stanley Miller and Harold Urey tested the theory. They combined the atmospheric gases in the amounts that early Earth's atmosphere was thought to contain. They then simulated an ocean in a closed apparatus.

With constant lightning shocks simulated using electric sparks, they were able to create organic compounds, including amino acids. In fact, almost 15 percent of the carbon in the modeled atmosphere turned into various organic building blocks in only a week. This groundbreaking experiment seemed to prove that life on Earth could have spontaneously formed from nonorganic ingredients.


Earth's Early Atmosphere: An Update

Scientists from NAI ’s New York Center for Astrobiology at Rensselaer Polytechnic Institute have used the oldest minerals on Earth to reconstruct the atmospheric conditions present on Earth very soon after its birth. The findings, which appear in the current issue of Nature, are the first direct evidence of what the ancient atmosphere of the planet was like soon after its formation and directly challenge years of research on the type of atmosphere out of which life arose on the planet.

The scientists show that the atmosphere of Earth just 500 million years after its creation was not a methane-filled wasteland as previously proposed, but instead was much closer to the conditions of our current atmosphere. The findings, in a paper titled “The oxidation state of Hadean magmas and implications for early Earth’s atmosphere,” have implications for our understanding of how and when life began on this planet and could begin elsewhere in the universe.

For decades, scientists believed that the atmosphere of early Earth was highly reduced, meaning that oxygen was greatly limited. Such oxygen-poor conditions would have resulted in an atmosphere filled with noxious methane, carbon monoxide, hydrogen sulfide, and ammonia. To date, there remain widely held theories and studies of how life on Earth may have been built out of this deadly atmosphere cocktail.

Now, scientists at Rensselaer are turning these atmospheric assumptions on their heads with findings that prove the conditions on early Earth were simply not conducive to the formation of this type of atmosphere, but rather to an atmosphere dominated by the more oxygen-rich compounds found within our current atmosphere — including water, carbon dioxide, and sulfur dioxide.

“We can now say with some certainty that many scientists studying the origins of life on Earth simply picked the wrong atmosphere,” said Bruce Watson, Institute Professor of Science at Rensselaer.

The findings rest on the widely held theory that Earth’s atmosphere was formed by gases released from volcanic activity on its surface. Today, as during the earliest days of the Earth, magma flowing from deep in the Earth contains dissolved gases. When that magma nears the surface, those gases are released into the surrounding air.

“Most scientists would argue that this outgassing from magma was the main input to the atmosphere,” Watson said. “To understand the nature of the atmosphere ‘in the beginning,’ we needed to determine what gas species were in the magmas supplying the atmosphere.”

As magma approaches the Earth’s surface, it either erupts or stalls in the crust, where it interacts with surrounding rocks, cools, and crystallizes into solid rock. These frozen magmas and the elements they contain can be literal milestones in the history of Earth.

One important milestone is zircon. Unlike other materials that are destroyed over time by erosion and subduction, certain zircons are nearly as old as the Earth itself. As such, zircons can literally tell the entire history of the planet — if you know the right questions to ask.

The scientists sought to determine the oxidation levels of the magmas that formed these ancient zircons to quantify, for the first time ever, how oxidized were the gases being released early in Earth’s history. Understanding the level of oxidation could spell the difference between nasty swamp gas and the mixture of water vapor and carbon dioxide we are currently so accustomed to, according to study lead author Dustin Trail, a postdoctoral researcher in the Center for Astrobiology.

“By determining the oxidation state of the magmas that created zircon, we could then determine the types of gases that would eventually make their way into the atmosphere,” said Trail.

To do this Trail, Watson, and their colleague, postdoctoral researcher Nicholas Tailby, recreated the formation of zircons in the laboratory at different oxidation levels. They literally created lava in the lab. This procedure led to the creation of an oxidation gauge that could then be compared with the natural zircons.

During this process they looked for concentrations of a rare Earth metal called cerium in the zircons. Cerium is an important oxidation gauge because it can be found in two oxidation states, with one more oxidized than the other. The higher the concentrations of the more oxidized type cerium in zircon, the more oxidized the atmosphere likely was after their formation.

The calibrations reveal an atmosphere with an oxidation state closer to present-day conditions. The findings provide an important starting point for future research on the origins of life on Earth.

“Our planet is the stage on which all of life has played out,” Watson said. “We can’t even begin to talk about life on Earth until we know what that stage is. And oxygen conditions were vitally important because of how they affect the types of organic molecules that can be formed.”

Despite being the atmosphere that life currently breathes, lives, and thrives on, our current oxidized atmosphere is not currently understood to be a great starting point for life. Methane and its oxygen-poor counterparts have much more biologic potential to jump from inorganic compounds to life-supporting amino acids and DNA . As such, Watson thinks the discovery of his group may reinvigorate theories that perhaps those building blocks for life were not created on Earth, but delivered from elsewhere in the galaxy.

The results do not, however, run contrary to existing theories on life’s journey from anaerobic to aerobic organisms. The results quantify the nature of gas molecules containing carbon, hydrogen, and sulfur in the earliest atmosphere, but they shed no light on the much later rise of free oxygen in the air. There was still a significant amount of time for oxygen to build up in the atmosphere through biologic mechanisms, according to Trail.

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Earth's Primordial Atmosphere

Well, one way to tell is by measuring an element which doesn't change. That is, measure an element which is to too heavy to escape from Earth so it doesn't go anywhere, and doesn't react with anything so it doesn't change to a different form and hide from sight. One such element is 20 Ne (Neon). It hasn't escaped and it doens't change into something else. What is here is what has always been here. Scientists have measured 3 things the amount of 20 Ne in the atmosphere today, the amount of nitrogen in the atmosphere of Earth as well as in the Sun. If Earth still had its original atmosphere, then the balance of the amount of nitrogen to the amount of neon in the Earth's atmosphere would be the same as that of the Sun, namely "cosmic" amounts. What they learned from these measurements is that the amount of nitrogen compared to the amount of Neon in the Earth is 10,000 more that it should be! This means that all the extra nitrogen in the Earth's atmosphere is new and must have come from somewhere. Moreover, with small planets like the Earth, Mars and Venus the atmosphere will eventually drift away unless it is replenished somehow. Scientists call the new atmosphere a secondary atmosphere, and they think all the new gases came from volcanoes. This suggests that, with Earth at least, the atmosphere is continually replenished by volcanic activity.


Earth’s atmosphere in context

Our understanding of how planetary atmospheres evolve has aided scientists now studying other planets. For instance, information gathered by the Curiosity rover suggests that Mars once had a much thicker atmosphere, but lost most of it nearly 4 billion years ago (Mahaffy et al., 2013). Scientists at NASA deduced this by studying the abundance and isotopes of argon in the Martian atmosphere, just as Aston and Brown studied neon on Earth.

Farther afield, astronomers have begun to investigate Titan, a moon orbiting Saturn that has a thick, nitrogen-rich atmosphere. Scientists think that it might have a similar composition to the secondary atmosphere Earth would have had before the evolution of life. This illustrates how studying planetary atmospheres continues to require analyzing our own planet as well as other bodies in the solar system, and how the knowledge we gain improves our understanding of both.

Summary

This module looks at how the Earth's atmosphere has changed since the planet came into existence. Starting with clues provided by neon gas, the module traces how scientists have pieced together the story of Earth’s atmosphere. Techniques are described for determining the concentration of elements found on Earth as well as those on planets and stars that are too far away to allow scientists to collect samples.

Key Concepts

Earth’s early atmosphere had a different composition than the modern atmosphere.

Earth’s modern atmosphere evolved over billions of years due to many different geologic processes.

Our knowledge about Earth’s primordial atmosphere comes from studying the atmospheres of other planets and the composition of stars, as well as clues from the rock record.


Structure of the Atmosphere

The structure of the atmosphere is illustrated in [link]. Most of the atmosphere is concentrated near the surface of Earth, within about the bottom 10 kilometers where clouds form and airplanes fly. Within this region—called the troposphere—warm air, heated by the surface, rises and is replaced by descending currents of cooler air this is an example of convection. This circulation generates clouds and wind. Within the troposphere, temperature decreases rapidly with increasing elevation to values near 50 °C below freezing at its upper boundary, where the stratosphere begins. Most of the stratosphere, which extends to about 50 kilometers above the surface, is cold and free of clouds.

Structure of Earth’s Atmosphere. Height increases up the left side of the diagram, and the names of the different atmospheric layers are shown at the right. In the upper ionosphere, ultraviolet radiation from the Sun can strip electrons from their atoms, leaving the atmosphere ionized. The curving red line shows the temperature (see the scale on the x-axis).

Near the top of the stratosphere is a layer of ozone (O3), a heavy form of oxygen with three atoms per molecule instead of the usual two. Because ozone is a good absorber of ultraviolet light, it protects the surface from some of the Sun’s dangerous ultraviolet radiation, making it possible for life to exist on Earth. The breakup of ozone adds heat to the stratosphere, reversing the decreasing temperature trend in the troposphere. Because ozone is essential to our survival, we reacted with justifiable concern to evidence that became clear in the 1980s that atmospheric ozone was being destroyed by human activities. By international agreement, the production of industrial chemicals that cause ozone depletion, called chlorofluorocarbons, or CFCs, has been phased out. As a result, ozone loss has stopped and the “ozone hole” over the Antarctic is shrinking gradually. This is an example of how concerted international action can help maintain the habitability of Earth.

At heights above 100 kilometers, the atmosphere is so thin that orbiting satellites can pass through it with very little friction. Many of the atoms are ionized by the loss of an electron, and this region is often called the ionosphere. At these elevations, individual atoms can occasionally escape completely from the gravitational field of Earth. There is a continuous, slow leaking of atmosphere—especially of lightweight atoms, which move faster than heavy ones. Earth’s atmosphere cannot, for example, hold on for long to hydrogen or helium, which escape into space. Earth is not the only planet to experience atmosphere leakage. Atmospheric leakage also created Mars ’ thin atmosphere. Venus ’ dry atmosphere evolved because its proximity to the Sun vaporized and dissociated any water, with the component gases lost to space.


Who asked for Venus?

Of all the rocky worlds in the solar system, Venus takes the cake — and then has some more. Its atmosphere is chokingly, toxically thick. At the surface, the air is almost a hundred times denser than it is on Earth, which makes "sea level" on Venus the pressure equivalent of an ear-crunching 3,000 feet (900 meters) under the water.

The miles upon relentless miles of air above Venus trap heat, a runaway greenhouse effect in full bloom. It makes the surface so hot that it's warmer there than on Mercury, despite the latter planet sitting closer to the sun.

Want to melt some lead? Just leave a lead bar sitting around on the surface of Venus, and wait. The atmosphere will do the rest of the work.

To make this hellish nightmare of a planet even worse (because why not), Venus' atmosphere hosts a substantial amount of sulfuric acid. The acid forms clouds in the upper atmosphere, and below that a pea-soup foggy haze, before condensing to produce acid rain.

But that acid rain doesn't even make it to the surface: the temperatures and pressures quickly evaporate it before it touches the ground.

Is Venus nasty? You bet. Is it special? Well, it's certainly one-of-a-kind.


Super-Earth 55 Cancri e Could Have Thick, Earth-Like Atmosphere

A hot, rocky exoplanet called 55 Cancri e likely has an atmosphere thicker than Earth’s, with ingredients that could be similar to those of Earth’s atmosphere, according to a study by researchers from NASA’s Jet Propulsion Laboratory (JPL), Caltech, and the University of California, Berkeley.

This artist’s impression shows the super-Earth 55 Cancri e orbiting the Sun-like star 55 Cancri A. Image credit: NASA / JPL-Caltech.

55 Cancri e is one of five planets orbiting the Sun-like star 55 Cancri A that is located 40 light-years away yet visible to the naked eye in the constellation of Cancer.

Discovered in 2004, this planet has a radius twice Earth’s, and a mass 8 times greater, making it a so-called super-Earth.

55 Cancri e orbits its host star at a distance of 0.015 AU — about 25 times closer than Mercury is to our Sun — every 18 hours.

The planet is also tidally locked, meaning that it doesn’t rotate like Earth does — instead there is a permanent ‘day’ side and a ‘night’ side.

Based on a 2016 study using data from NASA’s Spitzer Space Telescope, astronomers speculated that lava would flow freely in lakes on the starlit side and become hardened on the face of perpetual darkness.

The lava on the dayside would reflect radiation from the parent star, contributing to the overall observed temperature of the planet.

Now, a deeper analysis of the same Spitzer data finds 55 Cancri e likely has an atmosphere whose ingredients could be similar to those of Earth’s atmosphere, but thicker.

Lava lakes directly exposed to space without an atmosphere would create local hot spots of high temperatures, so they are not the best explanation for the Spitzer observations.

“If there is lava on this planet, it would need to cover the entire surface. But the lava would be hidden from our view by the thick atmosphere,” said JPL/Caltech astronomer Dr. Renyu Hu.

Using an improved model of how energy would flow throughout the planet and radiate back into space, Dr. Hu and Dr. Isabel Angelo of JPL and the University of California, Berkeley, find that the night side of the planet is not as cool as previously thought.

The ‘cold’ side of 55 Cancri e is still quite toasty by Earthly standards, with an average of 2,400 to 2,600 degrees Fahrenheit (1,300 to 1,400 degrees Celsius), and the hot side averages 4,200 degrees Fahrenheit (2,300 degrees Celsius).

The difference between the hot and cold sides would need to be more extreme if there were no atmosphere.

“Scientists have been debating whether this planet has an atmosphere like Earth and Venus, or just a rocky core and no atmosphere, like Mercury. The case for an atmosphere is now stronger than ever,” Dr. Hu said.

“The atmosphere of this mysterious planet could contain nitrogen, water and even oxygen — molecules found in our atmosphere, too — but with much higher temperatures throughout,” the authors said.

“The density of the planet is also similar to Earth, suggesting that it, too, is rocky.”

“The intense heat from the host star would be far too great to support life, however, and could not maintain liquid water.”

Isabel Angelo & Renyu Hu. 2017. A Case for an Atmosphere on Super-Earth 55 Cancri e. AJ, in press arXiv: 1710.03342