One thing 2017 has going for it is a total solar eclipse. Such eclipses are relatively common, but they often occur in hard to reach areas where not many people live. But the eclipse this Fall will wander across the central US, making it highly accessible. Such solar eclipses are only possible thanks to the favorable orbital geometries of the Sun, Moon and Earth, but its those same geometries that mean such total solar eclipses will eventually come to an end. 

Total eclipses are only possible because the Moon has about the same apparent size as the Sun. The diameter of the Sun is about 400 times larger than the diameter of the Moon, while Moon is about 389 times closer to the Earth than the Sun. So it is possible for the Moon to completely cover the Sun when they line up the right way. It doesn’t happen every month because the Moon’s orbit is tilted a few degrees from the orbital plane of Earth, so sometimes the Moon passes a bit above or below the Sun, and casts no shadow on the Earth.

An annular eclipse showing the “ring of fire.” Credit: Kevin Baird

But even when things line up perfectly, there isn’t always a total eclipse. The Moon’s orbit around the Earth isn’t perfectly circular. Likewise, the Earth’s orbit around the Sun isn’t perfectly circular either. So the apparent sizes of the Sun and Moon vary slightly, and this means sometimes the Moon can appear slightly smaller than the Sun during a solar eclipse. When this happens, it is an annular (ring) eclipse, since a thin outer ring of the Sun can still be seen.

In our present era, both total and annular eclipses can occur. But because of the tidal forces between the Earth and Moon, the Moon is gradually moving farther away from Earth. In the distant past, the Moon was closer, so annular eclipses weren’t possible. As the Moon continues to recede from Earth, total eclipses will only occur when the Moon is at a particularly close point in its orbit (perigee), while the Sun is near its most distant (aphelion). Over millions of years, annular eclipses will become the norm, and total eclipses will become increasingly rare.

So when will the last total eclipse occur? We can’t pin down an exact date, but we can get a basic estimate. The Moon currently moves away from the Earth at a rate of 3.8 centimeters per year. In the past that rate was slower, at about 2.2 centimeters a year. If we use about 3 centimeters per year as an average, then we can simply estimate how long it will take for the Moon’s apparent size at perigee to be the same size as the Sun’s apparent size at aphelion. It comes out to be about a billion years. Of course on that time scale other factors come into play. The Sun is gradually getting hotter, and expanding slowly as a result, which would shorten the time until the last total eclipse. But the Earth is also slowly moving away from the Sun as our star radiates energy and mass. Subtle gravitational effects between the Earth and other planets can also shift Earth’s orbit slightly, as well as the Moons. All of this can come into play. But if we only want a rough estimate, we can safely say that in about a billion years the days of total eclipses will come to an end.

One more reason to see one while you can.

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In the movie Gravity the driving force of the plot is a catastrophic cascade of space debris. An exploding satellite sends high speed debris into the path of other satellites, and the resulting collisions create more space debris until everything from a space shuttle to the International Space Station faces an eminent threat of destruction. Not unexpectedly, the movie portrayal of such a situation is not particularly accurate, but the risk of a debris cascade is very real. 

The projected level of debris in the future for various orbits. Credit: NASA

It’s known as the Kessler syndrome, after Donald Kessler, who first imagined the scenario in the 1970s. The problem comes down to the fact that small objects in Earth orbit can stay in orbit for a very long time. If an astronaut drops a bolt, it can stay in orbit for decades or centuries. Because the relative speed of two objects in orbit can be quite large, it doesn’t take a big object to pose a real threat to your spacecraft. On the highway a small pebble can chip your car windshield. In space it can be done by a chip of paint traveling at thousands of kilometers per hour. In the history of the space shuttle missions, there were more than 1,600 debris strikes. Because of such strikes, more than 90 space shuttle windows had to be replaced over the lifetime of shuttle missions.

While that might sound alarming, it’s actually quite manageable. Upgrades and maintenance were quite common on the shuttle missions, and we tend to err on the side of caution when it comes to replacing parts. Modern spacecraft also have ways to mitigate the risk of small impacts, such as Whipple shields made of thin layers of material spaced apart so that objects disintegrate when hitting the shield rather than the spacecraft itself. We also have a tracking system that currently tracks more than 300,000 objects bigger than 1 cm, so we can make sure that most spacecraft avoid these objects.

Map of known objects in Earth orbit (sizes exaggerated) Via Reddit.

But the risk of big collisions isn’t negligible. In 2009 the Iridium 33 and Kosmos-2251 satellites collided at high speed, destroying both spacecraft and creating more dangerous debris. It wouldn’t take many collisions like this for the debris numbers to rise dramatically, and more debris means a greater risk of collisions. In Gravity the cascade happens very quickly, triggered by a single event. The reality is not quite so grave. Instead of happening overnight, Kessler syndrome would occur gradually, raising collision risks to the point where certain orbits become logistically impractical. It could occur so gradually that we might not notice it early on, and there are some that argue it’s already underway.

The good news is that we’re aware of the threat. And, as the old saying goes, knowing is half the battle. Already we take steps to limit the amount of debris created. New spacecraft include end of life plans to remove them from orbit, either by sending them into Earths atmosphere to burn up, or sending them to a “graveyard orbit” that poses little risk to other spacecraft. There are also plans on the drawing board to clear orbits of debris, particularly in low-Earth orbit where the risk is greatest. The cascade effect is a real risk, but it’s also one we can likely manage with a bit of ingenuity.

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The dominant model for the origin of our Moon is the impact model. In this model, about 4.5 billion years ago proto-Earth was hit by a Mars-sized body known as Theia. It’s generally been thought that the collision was somewhat off-center, causing the remnants of Theia and some outer layer material from Earth to form the Moon.  But new evidence suggests the collision was more head-on. 

If the Earth-Moon system was caused by an off-center collision with Theia, then we would expect to see close similarities in the chemical compositions the Earth and Moon, but still some differences. Earlier research found some differences in the amount of isotopes, such as the ratio of oxygen-17 to oxygen-16 differing by about 12 parts per million between the Earth and Moon. But new work analyzing oxygen isotopes in lunar rocks and volcanic rocks on Earth found their oxygen isotopes to be indistinguishable. Since oxygen is common in both rock samples, the fact that they are indistinguishable suggests that the material forming the Earth and Moon were mixed together before they formed. This could be achieved by a more head-on collision between proto-Earth and Theia.

If that’s the case, then much of Theia became a part of Earth’s core, and our planet is actually the product of two worlds.  It’s an interesting twist on the impact origin of the Moon.

Paper: Edward D. Young, et al. Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact. Science Vol. 351, Issue 6272, pp. 493-496 (2016)

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We generally think of the Earth as having a constant mass. On a basic level that’s true, but the Earth’s mass does change very slightly. So is it’s mass increasing or decreasing?

Earth gains mass through dust and meteorites that are captured by its gravity. If you watched the recent meteor shower you know this can occur on a regular basis. In fact from satellite observations of meteor trails it’s estimated that about 100 – 300 metric tons (tonnes) of material strikes Earth every day. That adds up to about 30,000 to 100,000 tonnes per year. That might seem like a lot, but over a million years that would only amount to less than a billionth of a percent of Earth’s total mass.

Earth loses mass through a couple of processes. One is the fact that material in Earth’s crust undergoes radioactive decay, and therefore energy and some subatomic particles can escape our world. Another is the loss of hydrogen and helium from our atmosphere. The first process only amounts to about 15 tonnes per year, but the loss from our atmosphere amounts to about 95,000 tonnes per year.

So it’s most likely that Earth is losing a bit of mass each year, but if the rate of meteors is on the higher end of estimates, then it could be gaining a bit of mass.

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The origin of Earth’s water is a bit of a mystery. While water is common in our solar system, it’s much more common in the outer solar system, such as Jupiter’s moon Europa or Saturn’s moon Enceladus. Most of the worlds of the inner solar system are fairly dry. So how did Earth come to have large oceans on its surface?

There are two main ideas on the origin of Earth’s water. One is that Earth’s water was locked up in the original rocks and dust that formed our planet. As the material collapsed under its own gravitational weight, water was released and eventually formed the oceans we have now. The other idea is that any water in the original material escaped early on, and the current water of Earth came to our planet through the bombardment by asteroids and comets. Evidence that Venus and Mars were also wet in their early history points toward a formation origin of water, but there has generally been more evidence to support the bombardment model.

Three isotopes of atomic hydrogen. Credit: Dirk Hünniger, CC BY-SA 3.0.

This evidence comes through what’s known as the deuterium/hydrogen (D/H) ratio of Earth’s water. Deuterium is an isotope of hydrogen that has a nucleus of a proton and neutron, rather than the single proton of regular hydrogen. Chemically it reacts in the same way as hydrogen, but since it is heavier than regular hydrogen there are slight differences. For example, when a deuterium atom is part of a water molecule, the extra mass means it doesn’t evaporate as readily as regular water. Deuterium water is more likely to form in space than in the gravitational field of a planet, so the D/H ratio of water tells us about the origin of that water.

The D/H ratio for Earth’s oceans is about 150 parts per million, which is similar to that of chondrite asteroids. This would seem to support the bombardment model. But a new paper argues that such a conclusion is too simplistic. Our oceans cycle between the surface and interior of Earth, which could affect the D/H ratio. In this paper the team looked at rocks from Earth’s mantle, and they found that the water contained within these rocks has a much lower D/H ratio than that of our oceans. This suggests that mantle water formed locally rather than through astroid bombardment.

There’s still a number of unanswered questions. This latest work doesn’t disprove the bombardment model, and it’s possible that our water came from a number of sources. Further study on both fronts is needed to resolve this mystery.

Paper: Lydia J. Hallis, et al. Evidence for primordial water in Earth’s deep mantle. Science, Vol. 350 no. 6262 pp. 795-797 (2015) DOI: 10.1126/science.aac4834

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The Earth has a spherical shape (technically an oblate spheriod) but it’s gravitational field is not as spherical. There are regions of the Earth where it’s surface gravity is stronger and others where it is weaker. If we map the gravitational field of the Earth, and represent gravitational strength by elevation, then we find a distorted shape shown above, sometimes called the Potsdam Gravity Potato.

In terms of physical shape, the dark blue line shows deviation from a sphere. Credit: Wikipedia

The term arose because Podsdam, Germany is where the gravitational data was analyzed, and it looks kind of like a potato. What’s interesting about the map is that it doesn’t have the shape you would expect. Intuitively you might think that a region like the Alps with its high mountains would have a strong surface gravity, while ocean regions would have a weaker gravity. After all, if you have more stuff under you, there should be more gravity. But what we find is that there are ocean regions with strong gravity, and mountain regions with weaker gravity. The distribution of mass in Earth’s interior must be causing much of this variation.

Just how Earth’s interior causes these fluctuations is not yet fully understood. But maps such as this should provide some useful clues.

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You may have heard about a “Star Trek-like” shield that scientists have found surrounding the Earth. That’s because the University of Colorado Boulder shamefully stated as much in a press release, and websites all over the world would rather copy and paste than actually do science journalism. The press release was promoting a new paper in Nature which looks at properties of the Earth’s radiation belts. 

Basically, the authors looked at satellite data on the distribution of electrons within the radiation belts. What they found was that very high-energy electrons in the belt didn’t penetrate as close to the Earth as we had thought. There is apparently some mechanism that prevents the inward drift of these electrons. Very clearly the authors state this mechanism “does not arise because of a physical boundary within the Earth’s intrinsic magnetic field.” Apparently from that Boulder’s news service concludes that it’s a Star Trek shield.

In fact, there a likely cause for this electron barrier effect. Closer to the Earth is the magnetosphere, which contains cool diffuse plasma. This region is also sometimes known as the plasmasphere. When high-energy electrons begin drifting toward the Earth, they collide with charges in the plasmasphere, which prevents them from drifting further inward, or at least eventually cooling and becoming part of the plasmasphere itself.

To confirm the details of this interaction will require more data. But it is clear that this isn’t some kind of Star Trek shield effect. Which is unfortunate, because it would be useful to use such a shield to prevent high energy hype from reaching the popular press.

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For an inner planet, Earth is bountiful with water. The origin of that water has been a matter of some debate. One idea is that a combination of Earth’s strong magnetic field and distance from the Sun allowed Earth to retain much of the water emitted from rocks as the planet cooled. Another is that water came to Earth through cometary or asteroid bombardment. But now it seems the origin of Earth’s water is more complex and more interesting that we’ve thought.

Last month an article in Science showed that much of Earth’s water existed before the formation of the solar system. The authors demonstrated this by looking a levels of deuterium in terrestrial water. Deuterium is an isotope of hydrogen that has a proton and neutron in its nucleus, rather than just a proton. As a result, it’s almost twice as heavy as regular hydrogen, and this means the way it chemically reacts is slightly different from regular hydrogen.

Deuterium isn’t very common compared to hydrogen, and exists at about 26 parts per million. When the team measured levels of deuterium in the water of Earth and other solar system bodies, they found the water contained deuterium at about 150 parts per million. This is interesting, because deuterium water is more likely to form in interstellar space. Water formed in the heat of a young solar system isn’t likely to produce much deuterium water. Given measured deuterium levels, the authors calculate that about half of Earth’s water was produced in the depths of space, before the solar system was formed.

This month another paper in Science found that water arrived on Earth earlier than expected. In this paper the team compared chondrite minerals on Earth with chondrite asteroids, specifically ones that likely originated from Vesta. Chondrite asteroids have a high quantity of water chemically bound to them, and one idea is that they could have been the source of Earth’s water. When they looked at the chemical makeup of terrestrial chondrites, they found them to be remarkably similar. This likely means terrestrial chondrites were themselves the source of Earth’s water. If that’s the case, then Earth was likely a water world a hundred million years earlier than the bombardment model predicts.

So it seems that Earth’s seas are more ancient both in origin and composition than we once thought.

Paper: Cleeves, L. I., et al. The ancient heritage of water ice in the solar system. Science, 345 (6204), p. 1590 – 1593 (2014)

Paper: Sarafian et al. Early accretion of water in the inner solar system from a carbonaceous chondrite–like source. Science, 346 (6209) p. 623-626 (2014)

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On June 30, 1908 a bolide streaked across the sky in the region near the Podkamennaya Tunguska River in Russia. When it exploded, the airburst leveled more than 2,000 square kilometers of trees. It is now known as the Tunguska event. This particular region of Russia is extremely remote so the damage was limited to trees and other local flora and fauna. But the impact is estimated to have been on the order of 10 -15 megatons. If it had occurred over a major city, the bolide would have destroyed it as surely as dropping a modern nuclear weapon. A similar sized event occurred in Arizona 40,000 years ago, and produced a crater a kilometer wide.

Meteor impacts are known to occur from time to time on Earth, such as the Chelyabinsk meteor 0f 2013, but impacts the size of Tunguska are fortunately quite rare. It’s estimated that events of that scale only occur about once every 300 years. So imagine if such an event occurred every 5 years or so. Every few years, and some part of the planet gets a crater, or an airburst. Most would be in remote areas, but some wouldn’t. Of course we know that meteors follow a power law distribution in size distribution. So for every Tunguska event, there would be thousands of Chelyabinsks. So perhaps 200 of them every year. Then there are the larger ones. Big impacts creating 10km-wide craters every century or so. And the ever larger ones every millennia.

Now imagine Earth gets pummeled at this rate for 200 million years.

Such a thing likely occurred on Earth about 4 billion years ago. It is a period known as the Late Heavy Bombardment (LHB). It is also known as the lunar cataclysm, because the first evidence of the LHB was found on the Moon.  When the Apollo astronauts brought back lunar rock samples, it was found that they all showed evidence of impact melts around this period. This would imply that the Moon must have been heavily bombarded during that time. If the LHB model is correct, then a similar bombardment should have occurred on Earth and other inner planets. But at the time there was no evidence for it. Then in 2002, isotopic measurements of rocks in Greenland and Canada showed signs of a similar impact period.

There have been several models proposed to account for this bombardment period, but one is that of planetary migration in the early solar system. One model, known as the Nice (pronounced neese) model, posits that Jupiter was roughly at its current distance when it entered a 1:2 resonance with Saturn. The resulting resonance drove Neptune (initially closer than Uranus) to the outer edge of the solar system, pushed Uranus and Saturn outward, and scattered much of the remaining protoplanetary material. Some of it was flung inward, creating the LHB period, while some was flung to the farthest reaches of the solar system, forming the Oort cloud.

While the LHB is the most popular explanation for the lunar rocks, there are some who disagree and have proposed alternate models. We can’t be certain yet that such a heavy bombardment period actually happened. But it certainly seems that Earth’s early period was quite tumultuous.

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For the past week I’ve been in a remote part of northern Minnesota, vacationing at my Grandmother’s house.  In that part of world you can pretty much throw a stick and hit a lake, which is part of the reason it is known as the Land of 10,000 lakes.  Being in a region with so much water tends to give the impression that Earth is water-rich world.

Water on Europa and Earth compared.
Credit: Kevin Hand, Jack Cook, Howard Perlman.

That’s certainly true when compared to other inner solar system planets like Venus and Mars, which are dry in comparison, but Earth is not the most water-rich body in our solar system. In the outer solar system most bodies are rich with water. Jupiter’s moon Europa, for example has only 1.5% of Earth’s volume, but has 2 – 3 times more water.  What makes Earth unusual is not that it has lots of water, but rather that it is a rocky world with surface water.  Other water-rich bodies in our solar system tend to be icy.

Just how Earth came to have so much surface water is still a bit of a mystery. One idea is that a combination of Earth’s strong magnetic field and distance from the Sun allowed Earth to retain much of the water emitted from rocks as the planet cooled. Another is that water came to Earth through cometary or asteroid bombardment. Surprisingly, isotopic measurements of Earth’s water actually match asteroids better than comets, so our water could have come from water-rich meteors.

Whatever the reason, it is the water lakes that makes our world exceptional.  Other worlds have water oceans covered in ice, or methane lakes as on the surface of Titan, but water lakes are a thing only observed on Earth.

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The magnetic field of the Earth is often portrayed as a large magnet that runs through the center of the Earth, with the magnetic poles located basically at the north and south poles of the Earth, but this is only a rough approximation. Earth’s magnetic field is generated in its core. The core of the Earth has a solid central region surrounded by a fluid outer region. This outer region undergoes convection, and its motion generates the magnetic field through what is known as a dynamo effect.

This is similar to the way the Sun’s magnetic field is generated. For this reason, the Earth’s magnetic field (like the Sun’s) is not static. Over the past several centuries, for example, we have seen the magnetic field gradually change, and can even observe how the location of the north and south poles shifts over time.

Credit: U.S. Geological Survey (USGS)

We know that the Earth’s magnetic field has undergone reversals through geological evidence. For example, the mid-atlantic ridge is a boundary between tectonic plates that are gradually pulling about at a rate of a few centimeters per year. As they pull apart, magma flows through the fissure to create new ocean floor. When the magma solidifies, its magnetic alignment with the Earth’s magnetic field is lock in stone. By measuring the magnetic field of the ocean floor near the ridge, we can observe how the orientation of the Earth’s magnetic field has changed over time.

What we find is that over the past 20 million years Earth’s magnetic field has reversed every 200,000 – 300,000 years or so. Because we can observe the changes over time, we also know that these reversals typically happen gradually, over thousands of years. We also know that the Earth’s magnetic field doesn’t completely disappear during a reversal. Instead it becomes chaotic during the transition period before settling down again.

Because the Earth’s magnetic field provides protection against solar flares and the like, it has been suggested that a magnetic reversal could lead to catastrophic changes in the Earth’s ecosystem. But studies comparing magnetic reversals with things like extinction events have found no evidence of the reversals affecting life on Earth in any significant way. There are species that depend upon Earth’s magnetic field, such as migrations of birds, and these could be seriously impacted by a rapid change in our magnetic field, but these effects would be localized, and not catastrophic.

So there’s no need to fear a magnetic reversal. Earth’s magnetic field has undergone a reversal fairly frequently on a geological scale, but such reversals don’t pose a threat to life on Earth. Long term it just means we’d need to switch the labels on our compasses.

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The Earth and Moon are slowly moving apart. The moon’s distance from the Earth is not constant, because the Moon’s orbit is not perfectly circular. Over the course of a month the moon comes as close as 363,000 km and as far as 405,000 km due to the eccentricity of its orbit. However its average distance is slowly increasing.

We have very precise measurements of the Moon’s distance made by laser ranging. Retroreflectors were placed on the Moon during the Apollo missions, around the time when the picture above was taken. The retroreflectors let us bounce laser beams off the Moon, which lets us measure the distance of the Moon to within millimeters. What we’ve found after decades of observation is that the average distance of the Moon is increasing at a rate of about 3.8 centimeters per year. Not much compared to the range of its distance, but it builds up over time.

The reason for this increasing distance has to do with a property known as angular momentum. Angular momentum is a measure of the rotation of a system. In the case of the Earth and Moon, it includes the rotation of the Earth about its axis and revolution of the Moon around the Earth. The angular momentum of a system is constant, which means if the Earth loses angular momentum, the Moon must gain angular momentum.

It turns out the Earth does lose angular momentum. The days on Earth are getting longer by about 17 milliseconds per century. This slowdown is due to the tides of the Earth. As the oceans slosh back and forth, they slow down the Earth. But the tides are largely driven by the gravitational pull of the Moon, so as the Earth’s rotation slows, the Moon gains angular momentum. It moves just a bit faster in its orbit, and that causes its orbit to get just a bit larger.

This increasing distance to the Moon is often used by “young Earth” creationists, who argue that since the Moon was closer in the past, it can’t possibly be billions of years old. If you use the current rate of increase and the current average distance, you get a maximum age of the Moon of 10 billion years, which is much longer than the actual 4.5 billion year age of the Moon. But the argument is that if the Moon was closer, the tides would be stronger, therefore the rate of distance increase would have been greater in the past.

But we actually know the rate of increase was less in the past. We have geological records of ancient estuaries, where the tides were particularly high. With each tide a layer of silt was deposited, which creates tidal rhythmites. Since the length of the Earth’s year is the same in the past as it is now, you can look at seasonal variations in the rhythmites to determine how many days there were in an ancient year. For example, we find that 620 million years ago a day was about 22 hours long instead of 24. From this we can determine that over geologic scales the Moon’s orbit has been expanding by about 2.2 centimeters per year, which is less than the current level.

This difference is due to the fact that the Earth is changing shape. The Earth is not a perfect sphere. It is thicker at the equator than at the poles. Part of this is due to the Earth’s rotation, but part of it is due to weight of ice at the polar regions. At the end of the last ice age about 10,000 years ago the ice of the polar regions melted, and the Earth’s crust continues to spring back to a more spherical shape. That means even more angular momentum is transferred from the Earth to the Moon, making its distance increase faster than it would by tides alone.

So there’s no contradiction between the age of the Moon and the fact that it is slowly moving away from us.

Tides go in, tides go out. For billions of years.

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