Venus: Death of a Planet

From the fires of a sun’s birth, twin planets emerged. Venus and Earth. Two roads diverged in our young solar system. Nature draped one world in the greens and blues of life. While enveloping the other in acid clouds, high heat, and volcanic flows. Why did Venus take such a disastrous turn? And what light can Earth’s sister planet shed on the search for other worlds like our own? For as long as we have gazed upon the stars, they have offered few signs that somewhere out there are worlds as rich and diverse as our own. Recently, though, astronomers have found ways to see into the bright lights of nearby stars. They’ve been discovering planets at a rapid clip, using orbiting observatories like NASA’s Kepler space telescope, and an array of ground-based instruments. The count is almost a thousand and rising. These alien worlds run the gamut, from great gas giants many times the size of our Jupiter, to rocky, charred remnants that burned when their parent star exploded. Some have wild elliptical orbits, swinging far out into space, then diving into scorching stellar winds.

Still others orbit so close to their parent stars that their surfaces are likely bathed in molten rock. Amid these hostile realms, a few bear tantalizing hints of water or ice, ingredients needed to nurture life as we know it. The race to find other Earths has raised anew the ancient question, whether, out in the folds of our galaxy, planets like our own are abundant, and life commonplace? Or whether Earth is a rare Garden of Eden in a barren universe? With so little direct evidence of these other worlds to go on, we have only the stories of planets within our own solar system to gauge the chances of finding another Earth. Consider, for example, a world that has long had the look and feel of a life-bearing planet. Except for the moon, there’s no brighter light in our night skies than the planet Venus, known as both the morning and the evening star. The ancient Romans named it for their goddess of beauty and love. In time, the master painters transformed this classical symbol into an erotic figure, then a courtesan.

It was a scientist, Galileo Galilei, who demystified planet Venus, charting its phases as it moved around the sun, drawing it into the ranks of the other planets. With a similar size and weight, Venus became known as Earth’s sister planet. But how Earth-like is it? The Russian scientist Mikkhail Lomonosov caught a tantalizing hint in 1761. As Venus passed in front of the Sun, he witnessed a hair thin luminescence on its edge. Venus, he found, has an atmosphere. Later observations revealed a thick layer of clouds. Astronomers imagined they were made of water vapor, like those on Earth. Did they obscure stormy, wet conditions below? And did anyone, or anything, live there? The answer came aboard an unlikely messenger, an asteroid that crashed into Earth.

That is, according to the classic sci-fi adventure, “The First Spaceship on Venus.“ A mysterious computer disk is found among the rubble. With anticipation rising on Earth, an international crew sets off to find out who sent it, and why. What they find is a treacherous, toxic world. No wonder the Venusians want to switch planets. It was now time to get serious about exploring our sister planet. NASA sent Mariner 2 to Venus in 1962, in the first-ever close planetary encounter. Its instruments showed that Venus is nothing at all like Earth. Rather, it’s extremely hot, with an atmosphere made up mostly of carbon dioxide. The data showed that Venus rotates very slowly, only once every 243 Earth days, and it goes in the opposite direction. American and Soviet scientists found out just how strange Venus is when they sent a series of landers down to take direct readings. Surface temperatures are almost 900 degrees Fahrenheit, hot enough to melt lead, with the air pressure 90 times higher than at sea level on Earth.

The air is so thick that it’s not a gas, but a “supercritical fluid.” Liquid CO2. On our planet, the only naturally occurring source is in the high-temperature, high-pressure environments of undersea volcanoes. The Soviet Venera landers sent back pictures showing that Venus is a vast garden of rock, with no water in sight. In fact, if you were to smooth out the surface of Venus, all the water in the atmosphere would be just 3 centimeters deep. Compare that to Earth, where the oceans would form a layer 3 kilometers deep. If you could land on Venus, you’d be treated to tranquil vistas and sunset skies, painted in orange hues. The winds are light, only a few miles per hour, but the air is so thick that a breeze would knock you over. Look up and you’d see fast-moving clouds, streaking around the planet at 300 kilometers per hour. These clouds form a dense high-altitude layer, from 45 to 66 kilometers above the surface. The clouds are so dense and reflective that Venus absorbs much less solar energy than Earth, even though it’s 30% closer to the Sun. These clouds curve around into a pair of immense planetary hurricanes as the air spirals down into the cooler polar regions.

Along the equator, they rise in powerful storms, unleashing bolts of lightning. Just like earth, these storms produce rain, only it’s acid rain that evaporates before it hits the ground. At higher elevations, a fine mist forms, not of water but of the rare metal tellurium, and iron pyrites, known as fool’s gold. It can form a metallic frost, like snowflakes in hell. Scientists have identified around 1700 major volcanic centers on Venus ranging from lava domes, and strange features called arachnoids or coronae, to giant volcanic summits. The planet is peppered with volcanoes, perhaps in the millions, distributed randomly on its surface. Venus is run through with huge cuts thousands of kilometers long that may well be lava channels. Our sister planet is a volcanic paradise, in a solar system shaped by volcanism. The largest mountain on Earth, Hawaii’s Mauna Kea volcano, measures 32,000 feet from sea floor to summit.

Rising almost three times higher is the mother of all volcanoes: Olympus Mons on Mars. Jupiter’s moon Io, is bleeding lava. It’s produced deep underground by the friction of rock on rock, caused by the gravitational pull of its mother planet. Then there’s Neptune’s moon Triton, with crystals of nitrogen ice shooting some 10 kilometers above the surface. Saturn’s moon Titan, with frozen liquid methane and ammonia oozing into lakes and swamps. On our planet, volcanoes commonly form at the margins of continents and oceans. Here, the vast slabs of rock that underlie the oceans push beneath those that bear the continents. Deep underground, magma mixes with water, and the rising pressure forces it up in explosive eruptions. On Venus, the scene is very different. In the high-density atmosphere, volcanoes are more likely to ooze and splatter, sending rivers of lava flowing down onto the lowlands. They resemble volcanoes that form at hot spots like the Hawaiian islands.

There, plumes of magma rise up from deep within the earth, releasing the pressure in a stream of eruptions. To see a typical large volcano on Venus, go to Sappas Mons, at 400 kilometers across and 1.5 kilometers high. The mountain was likely built through eruptions at its summit. But as magma reached up from below, it began to drain out through subsurface tubes or cracks that formed a web of channels leading onto the surrounding terrain. Is Venus, like Earth, still volcanically active? Finding the answer is a major goal of the Venus Express mission, launched in 2005 by the European Space Agency. Armed with a new generation of high-tech sensors, it peered through the clouds. Recording the infrared light given off by several large mountains, it found that the summits are brighter than the surrounding basins. That’s probably because they had not been subject to as much weathering in this corrosive environment.

This means that they would have erupted sometime within the last few hundred thousand years. If these volcanoes are active now, it’s because they are part of a deeper process that shapes our planet as well. On Earth, the release of heat from radioactive decay deep in its mantle is what drives the motion of oceanic and continental plates. It’s dependent on erosion and other processes associated with water. With no water on Venus, the planet’s internal heat builds to extreme levels, then escapes in outbreaks of volcanism that may be global in scope. This may explain why fewer than a thousand impact craters have been found on Venus. Anything older than about 500 million years has literally been paved over. So why did Venus diverge so radically from Earth when it was born in same solar system and under similar circumstances? There is growing evidence, still circumstantial, that Venus may in fact have had a wetter, more Earth-like past.

One of the most startling findings of the early Venus missions was the presence of deuterium, a form of hydrogen, in Venus’ upper atmosphere. It forms when ultraviolet sunlight breaks apart water molecules. Additional evidence recently came to light. Venus Express trained its infrared sensors on the planet’s night side, to look at how the terrain emits the energy captured in the heat of the day. This picture is a composite of over a thousand individual images of Venus’ southern hemisphere. Higher elevation areas, shown in blue, emit less heat than the surrounding basins. That supports a hypothesis that these areas are made not of lava, but of granite. On Earth, granite forms in volcanoes when magma mixes with water. If there’s granite on Venus, then there may well have been water. If Earth and Venus emerged together as twin blue marbles, then at some point, the two worlds parted company.

Earth developed ways to moderate its climate, in part by removing carbon dioxide, a greenhouse gas, from its atmos phere. Plants, for one, absorb CO2 and release oxygen in photosynthesis. One square kilometer of tropical jungle, for example, can take in several hundred tons of co2 in just a year. That’s nothing compared to the oceans. In a year’s time, according to one recent study, just one square kilometer of ocean can absorb 41 million tons of CO2. Earth takes in its own share of CO2. When rainfall interacts with rocks, a chemical reaction known as “weathering” converts atmospheric CO2 to carbonate compounds. Runoff from the land washes it into rivers and the seas, where they settle into ocean sediments. With little water and no oceans, Venus has no good way to remove CO2 from its atmosphere. Instead, with volcanic eruptions adding more and more CO2 to the atmosphere, it has trapped more and more of the sun’s heat in a runaway greenhouse effect. Venus is so hot that liquid water simply cannot survive on the surface. Nor, it seems, can it last in the upper atmosphere.

The culprit is the Sun. The outer reaches of its atmosphere, the corona, is made up of plasma heated to over a million degrees Celsius. From this region, the sun sends a steady stream of charged particles racing out into the solar system. The solar wind reaches its peak in the wake of great looping eruptions on the surface of the Sun, called coronal mass ejections. The blast wave sweeps by Venus, then heads out toward Earth. Our planet is fortified against the solar blast. Plumes of hot magma rise and fall in Earth’s core as it spins, generating a magnetic field that extends far out into space. It acts as a shield, deflecting the solar wind and causing it to flow past. It’s this protective bubble that Venus lacks. Venus Express found that these solar winds are steadily stripping off lighter molecules of hydrogen and oxygen.

They escape the planet on the night side, then ride solar breezes on out into space. All this may be due to Venus’ size, 80% that of Earth. This prevents the formation of a solid iron core, and with it the rising and falling plumes that generate a strong magnetic field. There may be another reason too, according to a theory about the planet’s early years. A young planet Venus encountered one or more planet-sized objects, in violent collisions. The force of these impacts slowed its rotation to a crawl, and reversed it, reducing the chances that a magnetic field could take hold. This theory may have a surprising bearing on Earth’s own history. Scientists believe the sun was not always as hot as it is. In fact, going back several billion years, it was cool enough that Earth should have been frozen over. Because it was not, this is known as the faint young sun paradox. Earth’s salvation may well be linked to Venus’ fate.

The idea is that the Earth occupied an orbit closer to the Sun, allowing it to capture more heat. The gravity of two smaller planets with unstable orbits would have gradually pushed it out to its present orbit. The pair would eventually come together, merging to form the Venus we know. As dead as Venus is today, it has brought surprising dividends in the search for life. On its recent crossing between Earth and the Sun, astronomers were out in force. In remote locations where the viewing was optimal, such as the Svalbard islands north of Norway. The data gathered here would be added to that collected by solar telescopes on the ground and in space. To object for most was to experience a spectacle that will not occur again till the year 2117. It was also to capture sunlight passing through Venus’ atmosphere.

Today, the Kepler Space Telescope is searching for planets around distant stars by detecting dips in their light as a planet passes in front. Telescopes in the future may be able to analyze the light of the planet itself. If elements such as carbon or oxygen are detected, then these worlds may well be “Earth-like.” Venus provides a benchmark, and some valuable perspective. So what can we glean from the evolution of planet Venus? As we continue to scan the cosmic horizons, the story of Venus will stand as a stark reminder. It takes more than just the right size, composition, and distance from the parent star, for a planet to become truly Earth-like. No matter how promising a planet may be, there are myriad forces out there that can radically alter its course. For here was a world, Venus, poised perhaps on the brink of a glorious future.

But bad luck passed its way. Now, we can only imagine what might have become of Earth’s sister planet? 8.

Top 10 Recently Discovered Earth Like Planets

Welcome to Top10Archive! The longer we stay on Earth, the more apparent it becomes that maybe we should have a backup plan should we live long enough to completely dry ‘er up. On our quest to find the perfect place to call Second Home, we’ve come across these incredible exoplanets. Factoring in the Earth Similarity Index or ESI, we’ve compiled the Top 10 Earth-like planets discovered over the past decade. 10. Kapteyn B In June of 2014, the High Accuracy Radial Velocity Planet Searcher discovered the potentially habitable exoplanet Kapteyn B. Found to reside in a system estimated at over 11 billion years old, about 7 billion years older than our own solar system, Kapteyn B orbits the red subdwarf star Kapteyn and is 12.8 light-years away from Earth. Kapteyn B has an ESI of .67 and, while found within a habitable zone capable of liquid water, is believed to have a temperature of approximately -91° F or roughly -68° C and, therefore, too cold to sustain water in a liquid form, but with enough C02 in its atmosphere, this may not even be a factor.

Working against the argument of habitability is the fact that some researchers, such as Paul Robertson at Penn State University, think Kapteyn B may not even exist and may just be a starspot mimicking a planetary signal. 9. Gliese 667 Cc Orbiting around the red dwarf star Gliese 667 C some 23 light years away, the exoplanet Gliese 667 Cc is within the habitable zone and has an ESI of .84. In November of 2011, astronomers noticed the super-Earth and started to find similarities to our own planet. The habitability of Gliese 667 Cc depends on where you’re aiming to terraform as the two hemispheres display complete opposite properties. One side is completely shrouded in permanent darkness while the other is constantly facing towards the red dwarf. It’s believed that, between these hemispheres, there is a sliver of space that may experience temperatures suitable for human life. There is, however, a possibility of extreme tidal heating upwards of 300 times that of Earth, calling into question whether, at times, if Gliese 667 Cc may be a little too hot for habitation.

8. Kepler 442b Launched in 2009, NASA’s Kepler space observatory has succeeded on numerous occasions in its mission to find Earth-sized planets. Announced in January of 2015, alongside the discovery of Kepler-438b, 442b has an ESI of .83 and a radius of 1.34 radians, quite a bit larger than Earth’s radius of .009 radians. While located within the habitable zone and deemed one of the most Earth-like planets in regards to temperature and size, life would be quite a bit different on 442b. For instance, a year would only be 112.3 days long and we’d experience only 70% of the sunlight that we’re used to receiving on Earth. Since the axial tilt is believed to be fairly small, we also shouldn’t expect to enjoy the quarterly change in seasons that we’re accustomed to. 7. Proxima B With an ESI of .87, Proxima b may be one of the most Earth-like exoplanets to date, but that doesn’t mean it’s the greatest candidate for habitability.

Though it shares many characteristics with Earth and touts a higher ESI, if you haven’t noticed yet, that’s not a guaranteed proponent of habitability. In fact, Proxima b, which is only 4.2 light-years away, is likely uninhabitable due to incredibly high stellar wind pressures. Compared to Earth, Proxima b is thought to be subjected to pressures of more than 2,000 times what we experience. Coupled with the radiation from its host star, it’s possible that the exoplanet would have no atmosphere to sustain life. In October of 2016, researchers at the National Center for Scientific Research in France hypothesized a chance for surface oceans and a thin atmospheric layer, though proof has yet to be discovered. 6. Kepler 438b In January of 2015, the newly found Kepler 438b, located 470 light years away, was deemed one of the most “Earth-like” planets ever discovered, making it an incredible candidate for the potential of life. Though it has a potential ESI of .88 and still carries similarities to our home world, research later that year determined that, while still “Earth-like,” 438b may be missing qualities needed for habitation – such as an atmosphere.

The planet’s nearby star emits flares 10 times more powerful than the Sun, leading to the possibility of a stripped atmosphere. There’s still hope that Kepler-438b, which is 12% larger and receives 40% more light than Earth, may be usable if it has a magnetic field like our own. 5. Wolf 1061 c At an ESI of .76, Wolf 1061 c is a potentially rocky super-Earth exoplanet discovered in December of 2015, some 14 light-years away from Earth. Orbiting Wolf 1061 at .084 AU, the exoplanet is closer to the inner edge of the habitable zone and is believed to be tidally locked. With one side permanently fixated on its star, the possibility of an extreme difference in temperatures on either side of the planet is incredibly likely. On the warmer side, liquid water may be sustainable, though it’s hypothesized to have an icy equilibrium temperature of -58° F or about -50° C, that could be offset by a thick atmosphere that allows for a transfer of heat away from the side of the planet facing Wolf 1061. 4. Kepler 62 e A Super-Earth found within the habitable zone of the Kepler 62 star, this exoplanet, which was discovered in 2013, has an ESI of .

83 and has some of the imperative qualities of potentially livable planets. On top of being rocky, the planet is also believed to be covered in an extensive amount of water. One factor working against 62 e as a habitable zone is the 20% increase in stellar flux from what we experience on Earth, which can trigger temperatures as high as 170° F or about 77 ° C, and start a detrimental greenhouse effect. In relation to Earth, 62 e is 60% larger and orbits the Kepler 62 star 243 days quicker and receives 20% more sunlight than Earth does. 3. Kepler 62f Kepler 62 f may only have an ESI of .67, but this super-Earth, discovered at the same time as 62e at about 1,200 light-years away from Earth, poses one of the best scenarios for habitability.

Where the exoplanet may fall short in its ability to sustain life is its possible lack of an atmosphere, which would lead to any surface water to be ice. At 1.4 times larger than Earth and with an orbital period of 267 days, life on 62f would be fairly similar to life on Earth – that is, of course, if its atmosphere were similar to that of our own. As of now, much remains unknown about the theoretically habitable planet, including whether or not it’s mostly terrestrial or predominantly covered in water. 2. Kepler-186f Kepler 186f of the Kepler 186 system may only have an ESI of .61, but the 2014 discovery is the first Earth-like exoplanet to have a radius similar to Earth’s – measuring in at about 10% larger. Found 500 light-years from Earth in the Cygnus constellation, 186f has an orbital period of 130 days and only receives 1/3 the energy from its star that Earth receives from the Sun. In terms of livability, 186f is within the habitable zone, but unknown atmospheric factors make how habitable it may be impossible to determine.

Like Kepler 442b, 186f has a low obliquity that keeps it from experiencing seasons like Earth. Of the four other planets in the Kepler system, 186f is believed to not be tidally locked like its neighbors and may be the only one far enough away from the Kepler star to sustain water. 1. Kepler 452b Also known as Earth 2.0, the discovery of Kepler 452b by the Kepler space telescope was announced in July of 2015. Found 1,400 light-years away from Earth, the super-Earth, which has an ESI of .83, was located in the habitable zone of a G-type star that shares a very similar mass and surface temperature of our Sun. While 452b’s smaller radius indicates it may have a rocky, terrestrial surface, the habitability of the exoplanet remains widely unknown, though it is believed to be subjected to a runaway greenhouse effect. The exoplanet is approximately 60% larger than Earth and has a year that’s only 5% longer than our own, earning it the title of Earth’s Cousin.

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The Earth: Crash Course Astronomy

The Earth is a planet. That’s a profound statement, and one that’s not really all that obvious. For thousands of years, planets were just bright lights in the sky, one-dimensional points that wandered among the fixed stars. How could the Earth be one of them? With the invention of the telescope those dots became worlds, and with spacecraft they became places. The Earth went from being our unique home in the Universe to one of many such…well, planets. The Earth is the largest of the terrestrial planets, the four smaller, denser, rocky worlds orbiting close in to the Sun. It’s about 13,000 kilometers across, and has a single, large Moon which we’ll learn a lot more about next week. Unlike the other three terrestrial planets, Earth has something very important: Water. Or, more specifically, liquid water on its surface, where it can flow around, evaporate, become clouds, rain down, and then mix up chemicals so they can do interesting, complex things—like support life.

Earth’s ability to sustain life depends on that water. It also depends on Earth’s atmosphere, of course—breathing has its advantages—and both, weirdly enough, depend on Earth’s magnetic field to exist. And that, in turn, depends on what’s going on deep inside our planet. So, let’s take a look. Like the Sun, the Earth is a many-layered thing. At its very center is the core, which actually has two layers, the inner core and the outer core. The inner core is solid, and made mostly of iron and nickel. These are heavy elements, and sank to the center of the planet when it was forming, leaving lighter elements like oxygen, silicon, and nitrogen to rise to the surface. The solid inner core is about 1200 kilometers in radius, or about 10% the radius of the Earth. The outer core is also mostly iron and nickel, but it’s liquid. The material in it can flow. It’s about 2200 kilometers thick. The temperature in the Earth’s core is tremendously high, reaching 5500° C. The pressure is huge as well, as you might expect with the weight of an entire planet sitting on top of it. You might think at such a high temperature, iron would be a liquid, but iron can stay solid if the pressure is high enough.

In the inner core, the pressure is extremely high, and even though it’s hot, iron is solid. In the outer core, where it’s still hot, but the pressure is a little bit lower, iron is a liquid. Above the core is the mantle.It’s about 2900 kilometers thick. The consistency of the mantle is weird; most people think it’s like lava, but really it’s like very thick hot plastic. It behaves more or less like a solid, but over long periods of time, geologic periods of time, it can flow. We’ll get back to that in a sec. On top of the mantle is the crust, a solid layer of rock. The overall density of the rock in the crust is less than in the mantle, so in a sense it floats on the mantle. There are two types of crust on Earth: Oceanic crust, which is about 5 kilometers thick, and continental crust, which is a much beefier 30-50 kilometers thick. Still, the crust is very thin compared to the other layers. The crust isn’t a solid piece, though; it’s broken up into huge plates, and these can move.

What drives the movement of these plates is the flow of the rock in the mantle, and that, in turn, is powered by heat. The core of the Earth heats the bottom of the mantle. This causes convection; the warmer material rises. It’s not exactly a speed demon, though: The rate of flow is only a couple of centimeters per year, so it takes about 50 or 60 thousand years for a blob to move a single kilometer. The hot material rises toward the surface, but it’s blocked by the crust. The magmatic rock pushes on the plates, causing them to slide around very slowly. Your fingernails grow at about the same rate the continents move. Over millions of years, though, this adds up, changing the surface geography of the Earth—where you see continents now is not at all where they were millions of years ago. In some places, generally where the plates come together, the crust is weaker.

Magma can push its way through, erupting onto the surface, forming volcanoes. Other volcanoes, like Hawaii or the Canary Islands, are thought to be from a plume of hotter material punching its way right through the middle of a continental plate. As the plate moves, the hot spot forms a linear chain of volcanoes over millions of years. Volcanoes create new land as material wells out, but they also pump gas out of the Earth too. A large part of Earth’s atmosphere was supplied from volcanoes! The interior of the Earth is hot; in the core, it’s about as hot as the surface of the Sun! Where did that heat come from? Most of it is leftover from the Earth’s formation, more than 4.5 billion years ago. As rock and other junk accumulated to form the proto-Earth, their collisions heated them up. As the Earth grew that heat built up, and it’s still toasty inside even today. Also, as the Earth formed and gained mass it began to contract under its own gravity, and this squeezing added heat to the material. Another source is elements like uranium deep inside the Earth, which add heat as the atoms radioactively decay.

And a fourth source of heat is from dense material like iron and nickel sinking to the center of the Earth, which warms things up due to friction. All of these things add up to a lot of heat, which is why, after all these billions of years, the Earth still has a fiery heart. The outer core of the Earth is liquid metal, which conducts electricity. The liquid convects, and this motion generates magnetic fields, similar to the way plasma in the Sun generates magnetic fields. The Earth’s rotation helps organize this motion into huge cylindrical rolls that align with the Earth’s axis. The overall effect generates a magnetic field similar to a bar magnet, with a magnetic north pole and south pole, which lie close to the physical spin axis poles of the Earth. The loops of magnetism surround the Earth, and play a very important role: They deflect most of the charged particles from the solar wind, and they trap some, too. Without the geomagnetic field, that solar wind would hit the Earth’s atmosphere directly.

Over billions of years, that would erode the Earth’s air away, like a sand blaster stripping paint off a wall. Mars, for example, doesn’t have a strong magnetic field, and we think that’s why its atmosphere is mostly gone today. But we do have an atmosphere, and it’s more than just air blowing around. Earth’s atmosphere is the layer of gas above the crust. Because it’s not solid, it doesn’t just stop, it just sort of fades away with height. By accepted definition—and by that I mean it’s not really science, it’s more of a “Eh, let’s just do it this way” kind of thing—the line between Earth’s atmosphere and space is set at 100 kilometers up. This is what’s called the Kármán line, and if you get above it, congratulations! You’re an astronaut. The atmosphere is, by volume, about 78% nitrogen, 21% oxygen, 1% argon of all things, and then an assortment of trace gases. There’s water vapor, too, almost all of it below a height of about 8-15 kilometers. This part of the atmosphere is warmest at the bottom, which means we get convection in the air, creating currents of rising air, which carry water with them, forming clouds, which in turn is why we have weather. At a height of about 25 kilometers on average is a layer of ozone, a molecule of oxygen that’s good at absorbing solar ultraviolet light.

That kind of light can break apart biological molecules, so the ozone layer is critical for our protection. Incidentally, the Earth’s magnetic field does more than trap solar wind particles; it also channels some of them down into the atmosphere, where they slam into air molecules about 150 kilometers up. This energizes the molecules, which respond by emitting light in different colors: Nitrogen glows red and blue, oxygen red and green. We call this glow the aurora, and it happens near the geomagnetic poles—far north and south. The lights can form amazing ribbons and sheets, depending on the shape of the magnetic field. I’ve never seen an aurora. Some day. You may not be aware of the atmosphere unless the wind is blowing, but it’s there. It exerts a pressure on the surface of the Earth of about a kilogram per square centimeter, or nearly ten tons per cubic meter! There’s roughly ton of air pushing down on you right now! You don’t feel it because it’s actually pushing in all directions—down, to the sides, even up—and our bodies have an internal pressure that balances that out. The Earth also has liquid water on its surface, unique among the planets.

The continental crust is higher than oceanic crust, so water flows down to fill those huge basins. The Earth’s surface is about 70% covered in water. Most likely, some of this water formed when the Earth itself formed, and some may have come from comet and asteroid impacts billions of years ago. The exact proportion of locally sourced versus extraterrestrial water is still a topic of argument among scientists. Earlier, I mentioned trace molecules of gas in the atmosphere. One of these is carbon dioxide, which only constitutes about 0.04% of the lower atmosphere. But it’s critical. Sunlight heats the ground, which emits infrared light. If this infrared light were allowed to radiate into space, the Earth would cool. But carbon dioxide traps that kind of light, and the Earth doesn’t cool as efficiently.

This so-called greenhouse effect warms the Earth. Without it, the average temperature on Earth would be below the freezing point of water! We’d be an iceball. This is why climate scientists are concerned about carbon dioxide. A little is a good thing, but too much can be very dangerous. Since the Industrial Revolution, we’ve added a lot of the gas to our atmosphere, trapping more heat. By every measure available, the heat content of the Earth is increasing, upsetting the balance. It’s melting glaciers in Antarctica and Greenland, as well as sea ice at the north pole. Sea levels are going up, and some of the extra CO2 in the air is absorbed by the oceans, acidifiying them. There’s an old concept in science fiction called terraforming: Going to uninhabitable alien planet and engineering it to be more Earthlike. I don’t know what the opposite process would be called, but it’s what we’re doing to Earth right now.

The Earth is the only habitable planet in the solar system. And you know what? We should keep it that way. Today you learned that the Earth is a planet, with a hot core, a thick layer of molten rock called the mantle, and a thin crust. The outer core generates a strong magnetic field, which protects the Earth’s atmosphere from the onslaught of the solar wind. Motion in the mantle creates volcanoes, and the surface is mostly covered in water. The Earth’s atmosphere is mostly nitrogen, and it’s getting warmer due to human influence.