Our atmosphere is about 78% nitrogen and 21% oxygen, with traces of other things like water and carbon dioxide. It’s an odd mix compared to the atmospheres of other planets. Jupiter and Saturn are dominated by hydrogen and helium, The thick atmosphere of Venus is about 96% carbon dioxide, and only 3% nitrogen, which is about the same ratio as the thin atmosphere of Mars. So why is our atmosphere so dominated by nitrogen? 

It wasn’t always this way. Like most planets Earth’s earliest atmosphere was dominated by hydrogen and helium. These two elements are by far the most abundant in the Universe. About 92% of the atoms created by the big bang were hydrogen, and most of the rest were helium. All the other elements on the periodic table are formed through astrophysical processes such as nuclear fusion in the heart of a star. To this day they make up only a small trace of cosmic elements. When planets initially form, their composition is mostly hydrogen and helium. Some of the hydrogen bonds with other elements, but most of it remains free hydrogen. Both hydrogen and helium are light elements, so they will tend to evaporate into space over time. A large planet such as Jupiter has enough gravity to hold on to most of its hydrogen and helium, which is why these elements dominate the atmospheres of gas giants. But the gravity of Earth isn’t strong enough, so Earth’s early atmosphere of helium and free hydrogen evaporated into space.

The relative abundances of elements on Earth. Credit: Gordon B. Haxel, Sara Boore, and Susan Mayfield from USGS

Of the remaining elements, carbon, nitrogen, and oxygen are the most abundant. This is due to the fact that the main fusion reaction in large stars is the CNO cycle, which produces these elements as a by-product. These react easily with other elements, and produce gasses like water (H₂O), carbon dioxide (CO₂), and ammonia (NH₃). Young Earth was much more geologically active than it is today, and volcanic activity released large quantities of these gases, and over time they came to dominate Earth’s atmosphere.

So why are the atmospheres of Venus and Mars dominated by CO₂, while Earth’s is not? It all comes down to water. Earth’s vulcanism driven atmosphere was likely dominated by carbon dioxide like Venus and Mars, but Earth also has vast oceans of liquid water. Carbon dioxide dissolves easily in water, so our oceans absorbed much of the atmospheric CO₂, leaving an atmosphere dominated by ammonia. It turns out that ammonia is unstable in Earth’s atmosphere. When struck by ultraviolet light from the Sun, it breaks apart into nitrogen and hydrogen. The liberated hydrogen evaporated into space, leaving nitrogen behind. Venus’ atmosphere likely followed a similar process, but without vast oceans to pull CO₂ out of its atmosphere. While Venus’ atmosphere is mostly carbon dioxide, it is much thicker than Earth’s, and contains four times the nitrogen.

Even with it’s vast oceans, Earth’s atmosphere would likely have been dominated by carbon dioxide were it not for the appearance of life. Early cyanobacteria used sunlight and the carbon dioxide dissolved in Earth’s oceans to produce energy, and released oxygen as a by-product. Early oxygen bonded with iron to form a layer of rust, but eventually began to build up in Earth’s atmosphere. As carbon dioxide was broken down by cyanobacteria, more CO₂ could be dissolved into the ocean. This gave rise to our modern atmosphere dominated by nitrogen and oxygen.

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One of the great strengths of astronomy is it’s ability to observe the past. Since light takes time to travel across the universe, more distant objects are seen at an earlier time than closer objects. This means we can do things such as study the evolution of galaxies, and observe the universe in its youth. It also means we can test whether physical constants change over time. But geology also gives us the ability to observe the past, and sometimes it tells us things about astronomy.

The geology of Oklo. Credit: Mossman et al., 2008

Take, for example, the natural reactor at Oklo in Gabon, Africa. It was discovered in a uranium mine in 1972, when it was noticed that some samples from the mine had a noticeably lower concentration of uranium 235. What’s more, the region also had elevated concentrations of fission byproducts such as ruthenium 99 and neodymium 142. This is the type of thing you’d expect to see in a nuclear reactor, but it was found in a region of rock about 1.7 billion years old. It is the first and only known location of a naturally occurring fission reactor.

What happened was that groundwater seeped into the uranium deposits, which acted to dampen the speed of neutrons emitted by the U235 (similar to the way we now use heavy water in modern reactors). The slower neutrons were then able to strike other U235 elements at a speed capable of inducing fission, thus creating a steady chain reaction.

The thing about fission reactions is that their rates and byproducts are critically dependent upon various constants of nature, so the Oklo reactor can be used as a way to test whether these constants have changed over time. In particular, there have been several papers looking at one particular constant known as the fine structure constant. What we’ve found is that at the time of the Oklo reactor, roughly 2 billion years ago, this constant appeared to have the same value it has today. Specifically, it could have changed by no more than one part in 10 million. It agrees with other astronomical studies that also show physical constants haven’t changed over billions of years, but it’s important because it reaffirms the fact that what we observe in the universe is consistent with what we measure here.

Astrophysics works, and we know that thanks in part to a bit of geology.

Paper: Thibault Damour & Freeman Dyson. The Oklo bound on the time variation of the fine-structure constant revisited. Nuclear Physics B, 480, 25, pp 37–54. (1996)

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There’s been news of the discovery of what could be the largest known meteor impact. The results have been published in Tectonophysics, but the results aren’t without controversy.

The layered structure of quartz crystals could be due to a meteor impact. Credit: Glikson, et al.

Currently the largest confirmed impact basin is Vredefort crater in South Africa, with a diameter of about 300 km. Slightly smaller is Sudbury basin, at about 250 km across. This new impact in Australia, if confirmed, would be about 400 km across. The evidence comes from quartz crystals found in core samples taken from a region known as Warburton Basin. These crystals have a layered fracture structure that could be caused by the impact of a large meteor. Based upon the samples, there would seem to be two impact regions each about 200 km across. It could have been caused by a meteor that split in two before impacting the Earth about 350 – 400 million years ago.

While its an interesting idea, quartz fractures such as these could also have been caused by other seismic events such as earthquakes, so quartz fractures alone is not particularly compelling evidence for a meteor impact. There is some evidence of overall basin geology that could be caused by an impact, but it is very different from other impact regions. In particular, the work argues that the impact features are now about 3 km below the surface, which makes it particularly difficult to study. It also isn’t clear that the two regions would be due to the same impact, or from two separate impacts in a similar era. The latter might seem unlikely, but we can’t rule it out.

It’s certainly possible that a large impact occurred there. We know that large impacts have occurred in Earth’s history, and the size of this new impact is perfectly plausible. But it’s important to keep in mind that finding a possible impact isn’t the same as confirming an impact basin. There’s still plenty of work to be done before Warburton Basin can be added to the list of large impact events.

Paper: A.Y. Glikson, et al. Geophysical anomalies and quartz deformation of the Warburton West structure, central Australia. Tectonophysics, Volume 643, 7, Pages 55–72 (2015)

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When we look at the Moon, we see a surface pocked with craters, scattered between seas of basalt from ancient lava flows. Since the Moon is not geologically active, it’s easy to imagine that the formation of lunar seas was triggered by large impacts. That’s actually been the dominant theory for some time. Now new research indicates that for at least one of the great seas, Oceanus Procellarum, that isn’t the case.

The results have been recently published in Nature, and shows that the great sea seems to be the result of geological activity. The team looked at data from the Gravity Recovery and Interior Laboratory (GRAIL), which is a pair of satellites that mapped the gravity of the Moon in great detail. When they analyzed the data, the team found rift zones bordering Oceanus Procellarum. These rift zones (seen on the right of the image above) are fairly straight with sharp angles, which is not the type of thing you see with impact zones.

The Pacific Ring of Fire is a similar structure on Earth

We have observed rift zones on several planets, as well as on Saturn’s moon Enceladus, but finding them on the Moon is rather surprising. The Moon is not massive enough to drive plate tectonic activity on its own, and it isn’t driven by strong tidal effects like some moons of Jupiter and Saturn. So it isn’t clear how such rift zones could have formed on the lunar surface. One idea proposed by the authors is that the Moon’s crust is rather thin, and the under layers of that region were heated by radioactive decay. The Procellarum region is known to have higher concentrations of radioactive elements such as uranium and thorium, and this could have driven rift formation in the past.

Regardless of the cause, it seems clear that the Moon was not simply a Moon battered by ancient impacts. It also had a few geological tricks of its own, and the famous Man in the Moon feature of Oceanus Procellarum is the result of one of them.

Paper: Andrews-Hanna, J. C. et al. Structure and evolution of the lunar Procellarum region as revealed by GRAIL gravity data. Nature 514, 68–71 (2014)

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About 1.85 billion years ago, in what would come to be known as Sudbury Canada, a 10 kilometer wide asteroid struck with such energy that it created an impact crater 250 kilometers wide. Today the chief industry of Sudbury is mining, all because of that ancient impact. In fact much of the mining industry is possible because of asteroid impacts in Earth’s early history.

When the Earth originally formed 4.5 billion years ago, it was molten enough that heaver elements such as iron, nickel, gold and then like settled toward the core, while lighter elements such as silicon and carbon settled near the surface. This is why the Earth has a nickel-iron core with a silicate crust. Of course a lot of the elements we desire are heavy metals, from iron and gold to rare earth elements like Neodymium. Unfortunately most of the Earth’s native heavy metals sank to the core in its early history. Most of them are completely inaccessible by modern mining.

But fortunately Earth was also bombarded by meteors in its early history, and this made mining practical in two ways. The first is that asteroids and meteors themselves contain vast quantities of heavy metals. By some estimates, a single mile-wide asteroid could contain twenty trillion dollars worth of precious metals. Since most of the asteroid bombardments occurred after Earth’s crust formed, these metals were deposited near Earth’s surface, making them easier to obtain. There is some evidence that most of accessible heavy metals are extraterrestrial in origin due to this process.

The second is due to large impacts such as Sudbury. With large impacts, part of the Earth’s crust are melted. As a result, deposited material then settles in layers as it re-cools. Heavier elements settle at the bottom of the crater, while lighter ones settle near the top. As a result, heavy metals are concentrated at the bottom layer of the crater, producing rich veins of ore. Impacts can also create other useful byproducts, such as impact diamonds and pockets of oil. Chicxulub crater (caused by the famous dinosaur extinction asteroid) near the Gulf of Mexico is a region with plentiful oil deposits, for example.

It’s worth noting that Earth’s geological history is complex, and there are other processes besides meteor impacts that can bring useful elements to Earth’s crust. But it is clear that one of those processes involves large impacts. If these impacts hadn’t occurred, it would be much for difficult for modern mining to sustain our technological way of life.

Paper: J. M. Brenan and W. F. McDonough. Core formation and metal–silicate fractionation of osmium and iridium from gold. Nature Geoscience 2, 798 – 801 (2009)

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The Sun orbits the center of the Milky way at a speed of about 230 km/s, taking about 250 million years to go around the galaxy once. It is a period of times sometimes referred to as a galactic year. But the Sun does not move in a simple circle or ellipse as the planets move around the Sun. This is due to the fact that the mass of the galaxy is not concentrated at a single point, but is instead spread across a plane with spiral arms and such. As a result, while the Sun orbits the galaxy it also moves up and down across the galactic plane. While the Sun is above the plane, the mass of the galaxy works to move it downward, and when below the plane the mass pulls it upward.  As a result the Sun oscillates through the galaxy, crossing the galactic plane once every 30 million years or so.

There has been a great deal of speculation that this oscillatory motion could have implications for life on Earth, such as triggering cometary bombardments and causing mass extinctions. There is little evidence to support this idea, since mass extinctions don’t strongly follow a 30 million year cycle, and studies of impacts on the Moon show no correlation either. But now a new study in Scientific Reports shows what seems to be a relation between galactic motion and Earth’s temperature.

The paper looked at temperature measurements of the Phanerozoic, which is the geologic period covering the last 540 million years. It covers everything from the Cambrian up to the present, which is most of the period in which complex life has been on Earth. Specifically, they looked at what is known as delta-O-18 measurements, which is a measurement of the oxygen 18 isotope relative to oxygen 16 within calcium carbonate deposits. These deposits were made by shelled organisms. Since the evaporation of water prefers O16 over O18 due to its smaller mass, delta-O-18 provides an indicator for geologic temperatures.

Oxygen isotope levels versus solar galactic position (z). Credit: Nir J. Shaviv, et al.

The team looked at 24,000 delta-O-18 measurements covering the Phanerozoic, and looked for a correlation between O18 levels and the position of the Sun relative to the galactic plane. What they found was a correlation with a confidence of 99.9%. So it seems fairly clear that our galactic position has had an effect on geologic temperatures. What isn’t clear is what could cause such a variation. The authors suggest that the motion may result in a variation of gamma rays striking the upper atmosphere, which could lead to changes in atmospheric temperature. At this point that it still pretty speculative.

Just to be clear, this paper looked at variations over long geologic scales. The motion of the Sun through the galaxy and any resulting temperature variation has no effect on the current warming trend we observe due to rising CO2 levels. Global warming, as it is often called, is not a galactic effect.

Paper: Nir J. Shaviv, et al. Is the Solar System’s Galactic Motion Imprinted in the Phanerozoic Climate? Scientific Reports 4, Article number: 6150 (2014)

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Hematite is a fairly common iron oxide mineral. The particular sample seen above comes from the iron range of northern Minnesota. It is about 2.7 billion years old, and is a type of hematite known as gray hematite. It doesn’t look very gray in the picture, since it has been exposed to air and water since its formation. Typically when you see hematite, it is carved and polished so that it looks like gray metallic stone, but in its raw form its surface is usually reddish in color, which is why it is also known as “blood ore.”

Hematite bear. Credit: Wikipedia

In northern Minnesota there are pockets of grey hematite that are so pure that it was used as a “sweetener” for iron processing.  Since it consists of about 70% iron and 30% oxygen, it could be used to raise the temperature of forged iron without adding contaminating materials.  Hematite was crucial to the American steel industry until the late 1960s when more advanced iron processing allowed the use of much cheaper taconite, which only contains about 30% iron.

Gray hematite concentrations on Mars. Credit: JPL

As interesting as this might be, you’re probably wondering what this all has to do with astronomy.  It turns out there is another planet where hematite is relatively common. In fact it helps give the planet its reddish color. What’s interesting is that gray hematite also exists on the surface of Mars. Gray hematite typically forms in the presence of liquid water.  So the presence of gray hematite is an indicator of liquid water in Mars’ past.

There is also another interesting aspect of hematite. In the oldest Minnesota hematite mine there are bacteria that thrive on iron. They don’t need sunlight, and they use oxygen from the ore itself. It’s rather similar to conditions that likely exist beneath the surface of Mars.

Of course just because such an organism could survive on Mars doesn’t mean that life exists or did exist on Mars. But it does show that the extreme conditions of Mars don’t rule out the possibility of Martian life.

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Of the tens of thousands of meteorites that have been discovered on Earth, about a hundred came from Mars, such as the one pictured here. We know they originated from Mars because their composition is different from other meteorites, and they correlate with the atmospheric and geological composition of Mars as determined by the Martian landers. They are currently the only martian material to which we have direct access.

These meteorites were blasted from Mars when comets or asteroids collided with Mars, sending fragments out into space. Eventually their orbits intersected with Earth. The martian meteorites don’t originate from a single event, but rather from various events throughout martian history. As a result these meteorites provide clues to the geological evolution of Mars. The downside is that most of these meteorites originate from the more recent period of Mars, from about 1.5 billion to 800 million years ago, so they don’t provide any clues about the earliest period of martian history.

For that we have to look at data from the martian landers. Specifically, the Spirit rover, which examined Gusev crater. The surface of Gusev crater dates to about 3.8 billion years ago, so Spirit’s analysis of Gusev rocks allows us to compare the early period of Mars with more recent periods. The results published last year in Nature have led to the conclusion that Mars had an oxygen rich atmosphere in its youth.

The team analyzed the concentration of nickel in both the meteorites and the Gusev rocks. What they found was the Gusev rocks were significantly higher in nickel than the meteorites. The reason why this is important is that in an oxygen free environment nickel tends to react with sulfur, leading to lower concentrations in the meteorites. In an environment with free oxygen, the sulphur reacts with the oxygen, making it less available to nickel. As a result rocks that form in an oxygen rich environment have higher concentrations of nickel. Therefore the Gusev rocks formed in an oxygen rich environment.

This finding is significant because it means that Mars had a wet environment with an oxygen rich atmosphere four billion years ago. Over time the water and oxygen reacted with surface rocks of Mars, leading to the rusty surface it has today. This is quite different from Earth’s geological history, where free oxygen only appeared in our atmosphere about 2.4 billion years ago.

Of course news of a wet young Mars with an oxygen atmosphere has lead many articles to write about evidence of life on Mars. On Earth the presence of free oxygen in our atmosphere was caused by the presence of life. Specifically, cyanobacteria produced (and still do produce) free oxygen as a waste product. This early oxygen first reacted with surface rocks and atmospheric gases until they became saturated. Only then did free oxygen appear in our atmosphere.

But on Earth this had serious consequences. The presence of oxygen is toxic to many anaerobic life forms, so there may have been a mass extinction of such organisms. The free oxygen also reacted with greenhouse gasses in the atmosphere, leading to a snowball Earth period that nearly wiped out life on our planet. Only later did oxygen-using organisms evolve. Since oxygen is highly reactive, it’s not clear that life could begin in an oxygen-rich environment. Such an early oxygen atmosphere may in fact be evidence against life on Mars.

It may be that the presence of oxygen may have arrested the development of life in the first place.

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