3. The invisible buffer zone with space - atmospheres, magnetospheres and the solar wind

Atmospheres of the terrestrial planets

The breath of life

Our atmosphere forms an indispensable interface with nearby space, but it is often invisible. After all, you look right through the air in your room. Our atmosphere usually goes unseen on a warm, dry windless day. Yet, the slow drift of floating clouds or the sight of birds and airplanes supported by their motion proves that there is something substantial surrounding us. We can sense the touch of the wind on a stormy day, and on cold days we feel the air against our skin.

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When astronauts look down at the Earth at sunrise or sunset, they detect the thin atmosphere that warms and protects us, and permits us to breathe. It is only 10 thousand meters from the ground to the top of the sky, or no further than you might run in an hour. Everything beyond that thin layer of air is the black void of space. And everything below it is what it takes to sustain life.

The major constituents of dry air on Earth are nitrogen molecules (77 percent), oxygen molecules (21 percent) that we breathe, and argon atoms (0.93 percent). Carbon dioxide is a miniscule 0.035 percent. There is no hydrogen in our air, and most of the hydrogen on Earth is found in water. The water vapor in wet air is variable in amount, usually no more than 1 percent.

Earth's weather

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The temperate zones are dominated by high-altitude jet streams blowing eastward at speeds up to 40 meters per second, in a sinuous path which resembles the meandering of a river. Between the trade winds and the jet stream, the pattern is primarily a series of high pressure cells rotating clockwise in the north and counter-clockwise in the south. In the polar regions, the weather pattern is primarily a series of low pressure cells rotating counter-clockwise in the north and clockwise in the south.

Atmospheres of Venus and Mars

Russian Spacecraft Venera 7 directly measured the atmosphere of Venus, measuring the temperature and pressure all the way down to the bottom of the atmosphere, where the temperature reaches a sizzling 735 degrees kelvin. Down there the thick, heavy atmosphere produces a pressure of 92 bars that is, 90 times the sea-level pressure on Earth. It consists of 96 percent carbon dioxide. The atmosphere of Venus contains about ten thousand times as much carbon dioxide as is present in our air. This massive carbon dioxide atmosphere is responsible for the high surface temperature of Venus through the greenhouse effect.

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Strong winds are blowing the highest clouds around Venus at speeds of up to 100 meters per second, racing around the planet's equator once every four Earth days. Curiously enough, Venus's surface rotates in the same westward direction but with a much longer period of 243 Earth days. So the winds blow the entire outer atmosphere around the planet much more rapidly than the planet spins. Although terrestrial jet streams move at up to half the speed of the high-flying clouds on Venus, they are limited to narrow zones high in the Earth's atmosphere.

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The exact chemical composition of the atmosphere on Mars was determined by direct measurements in 1976 when the Viking 1 and Viking 2 landers arrived at the surface. Like Venus, the atmosphere on Mars is mostly carbon dioxide (95 percent) with just a whiff of nitrogen (2.7 percent). The landers confirmed that the local surface pressure is at or below 0.01 bars. They also showed that the surface temperature is almost always below the freezing point of water.

The planet breathes its atmosphere in and out as its southern polar cap grows and shrinks, producing a seasonal change in atmospheric pressure by about 30 percent. It is as if the planet was a giant lung that slowly breathes in and exhales the same gas, carbon dioxide. When the surface temperature drops during the southern winter, the atmospheric carbon dioxide condenses and freezes to enlarge the polar cap, resulting in a drop in atmospheric pressure. In the southern summer, the ice sublimates (goes directly from solid to vapor, without becoming liquid) back into the atmosphere, increasing the atmospheric pressure. The polar cap waxes and wanes due to this seasonal component of carbon dioxide ice.

Atmospheres of the giant planets

Jupiter has very nearly the same composition as the Sun, made up mainly of the light gases hydrogen and helium. Saturn has about the same composition, with a bit more helium, but Uranus and Neptune are depleted of these two gases relative to the heavier hydrogen compounds like methane, ammonia and water.

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Definite spectroscopic proof that molecular hydrogen is the most abundant element in Jupiter and Saturn’s upper atmosphere did not occur until the 1960s and late 1970s, when high-dispersion infrared spectroscopy, from both the ground and space, showed several weak absorption features due to molecular hydrogen. Precise values for the ingredients of Jupiter's atmosphere were obtained in 1999 from the Galileo probe.

The helium abundance for Uranus and Neptune is consistent with that expected from a solar composition, but helium has been significantly depleted from Saturn's upper atmosphere and somewhat reduced in Jupiter's. Theoretical calculations indicate that helium rain has been falling toward the center of Saturn for the past two billion years, generating heat and producing lower amounts of helium in its outer atmosphere. Helium rain must be similarly settling toward Jupiter's core, but in lesser amounts.

Raging winds on the giant planets

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Astronomers have been using telescopes, both small and large, to scrutinize weather patterns on Jupiter and Saturn for more than a century. They observe clouds of various ices, such as ammonia, ammonium hydrogen sulfide and water, that are formed in the cold outer atmospheres. These clouds have been pulled into counter-flowing winds by rapid planetary rotation, moving in opposite eastward or westward directions at constant speed and remaining confined to specific latitudes. These windswept clouds move in alternating light-colored bands, called zones, and dark ones, known as belts. Since Jupiter is all atmosphere, with no solid surface to rub against or continents to disturb the flow, its winds are free to rage unabated in response to the planet's spin, with large-scale configurations that have remained unchanged for as long as they have been observed.

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The weather patterns on Uranus and Neptune, where clouds of methane ice are observed, resemble those on Earth, which has low-latitude trade winds that blow westward and a meandering eastward current, the jet stream, in each hemisphere. Nevertheless, the Earth has the weakest winds in the solar system; its fastest jet streams move at speeds of about 40 meters per second. In contrast, Jupiter's winds move at constant speeds of up to 180 meters per second, and Uranus' fastest winds are just a little faster. The high clouds on Venus and Mars also move at faster speeds than those on Earth, both with speeds of up to 100 meters per second. The winds on Neptune and Saturn move at speeds of up to 400 and 450 meters per second, respectively, ten times the fastest winds on Earth. The speed of the Earth's winds are measured with respect to the rapidly rotating surface beneath them; for the giant planets that have no solid surface the internal rotation rate is inferred from their magnetic fields that are generated within their cores.

Titan - a satellite with a substantial atmosphere

Saturn's largest moon, Titan, is the only satellite with a substantial atmosphere. Detailed investigations with instruments aboard Voyager 1 in 1980 showed that the dominant gas surrounding Titan is molecular nitrogen, N2, at 82 to 99 percent, similar to Earth (71 percent). In fact, the satellite is enveloped by about 10 times more nitrogen then we are, yielding a surface pressure 1.5 times greater than the sea-level pressure of Earth's atmosphere. The surface temperature on Titan is 94 degrees kelvin, as expected for a body so far from the Sun.

The spectrometers on Voyager 1 showed that the next-most abundant gas enveloping Titan is methane, CH4, with an abundance between 1 and 6 percent. Methane molecules rise up to high levels in Titan's atmosphere, where they are broken apart by ultraviolet sunlight and electrons coming from Saturn's magnetic environment. These molecular fragments recombine to form heavier hydrocarbon molecules such as ethane, C2H6, and familiar gases like acetylene, C2H2, propane, C2H8, and hydrogen cyanide, HCN.

It doesn't rain water on Titan, but it could rain fuel, in large drops that fall like snow. Given the known atmospheric composition and the temperatures, scientists speculate that thin clouds of methane ice crystals may form in the lower atmosphere. Ethane and propane can rain all the way down to the surface, forming seas, lakes and ponds. The patchy reservoirs of liquid hydrocarbons could be driving the weather cycle on Titan, with towering clouds of methane and a rainy drizzle of ethane.

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We are not completely certain that there are any liquid seas on Titan, for we cannot see through its smog. The Voyager 1 cameras showed that an opaque haze completely enshrouds the satellite. The smog is unimaginably worse than a bad day in Los Angeles or Mexico City. Compared with any urban smog on Earth, there are relatively few smog particles per unit volume of Titan's atmosphere, but the haze extends to an altitude of about 200 thousand meters. This makes the smog thick enough to completely hide Titan's surface from view.

When the Cassini/Huygens spacecraft arrives at Saturn in mid-2004, it is expected to tell us what lies beneath Titan's obscuring veil of orange smog.

The planets are inside the expanding Sun

The Sun's two winds

The space just outside the Earth is not empty. It is filled with pieces of the Sun. Our star is expanding out in all directions, filling interplanetary space with electrically-charged particles that are forever blowing from the Sun. This solar wind moves past the planets and engulfs them, carrying the Sun's rarefied atmosphere out to the space between the stars. So, we are actually living in the outer part of the Sun.

Unlike any wind on Earth, the solar wind is a tenuous mixture of charged particles and magnetic fields streaming radially outward in all directions from the Sun at supersonic speeds of hundreds of thousands of meters per second. It is mainly composed of electrons and protons, set free from the Sun's abundant hydrogen atoms, but it also contains lesser amounts of heavier ions. The seemingly eternal wind carries a magnetic field with it, with one end anchored in the Sun. This interplanetary magnetic field has a spiral shape due to the combined effects of the radial solar wind flow and the Sun's rotation.

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The existence of the solar wind was suggested from observations of comet tails about half a century ago. When a comet is tossed into the inner solar system, the dirty ice on its surface is vaporized, forming two tails that always point away from the Sun. One is a curved dust tail, pushed away from the Sun by the pressure of sunlight. The other is a straight ion tail that is affected by the solar wind.

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The radial, supersonic outflow creates a huge bubble of electrons, protons and magnetic fields, with the Sun at the center and the planets inside, called the heliosphere, from Helios the Greek word for the "Sun". Within the heliosphere, conditions are regulated by the Sun. Its domain extends out to about 150 AU, or about 150 times the mean distance between the Earth and Sun, marking the outer boundary or edge of the solar system. Out there, the solar wind has become so weakened by expansion that it can no longer repel interstellar forces.

Magnetospheres

Earth's magnetic dipole

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The Earth's magnetic field is described by magnetic field lines that emerge out of the south magnetic pole, loop through nearby space and re-enter at the north magnetic pole. The lines are close together near the magnetic poles where the magnetic force is strong, and spread out near the magnetic equator where the magnetism is weaker than at the poles. You cannot see the invisible magnetism, but instruments can be used to measure it.

The magnetic field strength at the Earth's magnetic equator is 0.0000305 tesla, or 0.305 x 10-4 T. Maps of the surface magnetic fields of Earth show stronger fields near the poles where the magnetic field lines congregate, at roughly twice the strength of the field at the equator.

Earth's protective magnetosphere

Fortunately for life on Earth, the terrestrial magnetic field deflects the Sun’s wind away from the Earth, like a rock in a stream or a windshield that deflects air around a car. It hollows out a protective cavity in the solar wind called the magnetosphere. The magnetosphere of the Earth, or any other planet, is that region surrounding the planet in which its magnetic field dominates the behavior of electrically-charged particles such as electrons, protons and other ions. It diverts most of the solar wind around our planet at a distance far above the atmosphere, thereby protecting humans on the ground from possibly lethal solar particles.

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The dipolar (two poles) magnetic configuration applies near the surface of the Earth, but further out the magnetic field is distorted by the Sun’s perpetual wind. Although it is exceedingly tenuous, far less substantial than a terrestrial breeze or even a whisper, the solar wind is powerful enough to mold the outer edges of the Earth’s magnetosphere into a changing asymmetric shape, like a tear drop falling toward the Sun.

After passing through the bow shock, the solar wind encounters and flows around the magnetopause, the boundary between the solar wind and the magnetosphere. The magnetic field carried in the solar wind merges with that of the planet, and stretches it out into a long magnetotail on the night side of Earth. The magnetic field points roughly toward the Earth in the northern half of the tail and away in the southern. The field strength drops to nearly zero at the center of the tail where the opposite magnetic orientations lie next to each other and currents can flow.

Trapped particles

The Earth's protective magnetic cocoon is not perfect. Energetic charged particles flowing from the Sun can penetrate the magnetic defense and become trapped within the magnetosphere. This was realized in 1958 when James A. Van Allen (1914 - ) and his students used instruments aboard the Explorer 1 and 3 satellites to unexpectedly discover a large flux of high-energy electrons and protons that girdle the Earth far above the atmosphere, moving within two belts that encircle the Earth’s magnetic equator but do not touch it. They resemble a gigantic, invisible, torus-shaped doughnut. This was the first major discovery of the Space Age.

These regions are sometimes called the inner and outer Van Allen radiation belts. Van Allen used the term “radiation belt” because the charged particles were then known as corpuscular radiation; the nomenclature does not imply either electromagnetic radiation or radioactivity. The radiation belts lie within the inner magnetosphere at distances of 1.5 and 4.5 Earth radii from the center of the Earth, creating a veritable shooting gallery of high-speed electrons and protons in nearby space.

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Thus, the electrons and protons bounce back and forth between the north and south magnetic poles. It takes about one minute for an energetic electron to make one trip between the two polar mirror points. The spiraling electrons also drift eastward, completing one trip around the Earth in about half an hour. There is a similar drift for protons, but in the westward direction. The bouncing can continue indefinitely for particles trapped in the Earth’s radiation belts, until the particles collide with each other or some external force distorts the magnetic fields.

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The magnetic fields of the solar wind and the Earth's magnetotail are sometimes pointing in opposite directions, and when this happens the two fields become linked, just as the opposite poles of two toy magnets stick together. When the solar and terrestrial magnetic fields touch each other, the magnetotail can be punctured, providing a back door entry that funnels energy and particles into the magnetosphere. Once inside the magnetic trap, the charged particles can be additionally accelerated to higher energies.

Planets with magnetospheres

Magnetic fields are ubiquitous in the solar system. Earth, Mercury and all the giant planets have strong magnetic fields generated within the planet, and Jupiter and Saturn have extensive magnetospheres. A magnetic field has been found on Mars, but the field's patchy nature suggests that it is not the result of an active internal dynamo. Instead the magnetic field on Mars is probably a remnant of former times, frozen into expanses of solidifying lava. Venus is the only major planet to have no detectable magnetic field. A magnetic field has even been found on at least one satellite, Jupiter's Ganymede.

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The general form of the Jovian magnetosphere resembles that of the Earth. but its dimensions are at least 1,200 times greater. It is larger than the Sun in size, with a bow shock at more than 3 billion meters or at least 42 times the planet's radius. Pioneer 10 and Voyager 1 first encountered the Jovian bow shock at 95 and 86 planetary radii, but the shock moved in and out due to the variable solar wind. Jupiter's largest satellites are all embedded within its magnetosphere and interact with it.

Jupiter's magnetic field was first recognized in 1954-55 by Earth-based observations of the planet's intense radio emission, and then directly measured by visiting spacecraft. The radio signals are generated by high-speed electrons trapped within the planet's magnetic field. The synchrotron radio emission is beamed in a direction nearly parallel to the magnetic equator, and it is therefore swept past an Earth-based observer as the planet rotates and brings the magnetic equator in and out of alignment with the line of sight. Periodic variations in the strength of the radio emission indicate that the magnetic field rotates with a period of precisely 9 hours 55 minutes 41 seconds. Since the magnetic fields are generated deep within the planet, this is assumed to be Jupiter's rotation period; it differs from the rotation speed inferred from visible clouds that are blown in different directions and at various speeds by powerful winds.

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The magnetism near most of the planets can be described by a central magnetic dipole, the equivalent of placing a small but powerful bar magnet near the center of the planet, and with a magnetic axis that is nearly aligned with the planet's rotational axis. But on Uranus and Neptune the dipolar magnet has to be displaced toward the surface, by about one-third the radius of Uranus and more than half Neptune's radius, resulting in a variable magnetic field strength. At the cloud tops of Uranus and Neptune, the magnetic fields are about ten times stronger in the hemisphere that is closest to the buried, offset magnet, and their magnetic equators weave across the clouds.

The large offsets of Uranus's and Neptune's magnetic dipoles suggest that the fields are generated at an intermediate depth rather than within a deep central core like the other planets that have dipolar magnetic fields. Convection and rotation apparently produce currents of electrically-conductive, ionized water within a spherical shell far removed from the center of these two planets. The magnetic axes of Uranus and Neptune are titled at a large angle with respect to their rotational axis, by 58.6 degrees for Uranus and 46.8 degrees for Neptune. They have fully developed magnetospheres that are well-represented by the offset, tilted dipoles. The rotation periods of the magnetic fields of Uranus and Neptune, inferred from their periodic radio emission, are 17.24 and 16.11 hours, respectively.

Auroras

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Curtains of green and red light dance and shimmer across the night sky in the Earth's polar regions, far above the highest clouds. This light is called the aurora after the Roman goddess of the rosy-fingered dawn, a designation that has been traced back to Galileo Galilei (1564-1642). The auroras seen near the north and south poles have been given the Latin names aurora borealis, for northern lights, and aurora australis, for southern lights.

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Nowadays we can use spacecraft to view both the northern and southern lights from space. The Space Shuttle has even flown right through the northern lights. While inside the display, astronauts could close their eyes and see flashes of light caused by the charged aurora particles speeding through their eyeballs.

When viewed from above, the auroras form a luminous oval centered at each magnetic pole, resembling a fiery halo. The aurora oval is constantly in motion, expanding toward the equator or contacting toward the pole, and constantly changing in brightness. Such ever-changing aurora ovals are created simultaneously in both hemispheres and can be viewed at the same time from the Moon.

The auroras are themselves caused by energetic electrons bombarding the upper atmosphere. The reason that auroras are usually located near the polar regions is that the Earth's magnetic fields guide the energetic electrons there. Electrical currents as great as a million amperes can be produced along the aurora oval, and the electric power generated during the discharge is truly awesome - about ten times the annual consumption of electricity in the United States.

When the electrons slam into the upper atmosphere, at speeds of about 50 thousand meters per second, they collide with the oxygen and nitrogen atoms there and excite them to energy states unattainable in the denser air below. The pumped-up atoms quickly give up the energy they acquired from the electrons, emitting a burst of color in a process called fluorescence. It is something like electricity making the gas in a neon light shine or a fluorescent lamp glow. The process also resembles the beam of electrons that strikes the screen of your color television set, making it glow in different colors depending on the type of chemicals, or phosphors, that coat the screen.

Even though changing conditions on the Sun may trigger the northern and southern lights, we now know that the electrons that cause the auroras arrive indirectly at the polar regions, from the Earth's magnetic tail, and that these electrons can be energized locally within the magnetosphere. Changing solar wind conditions can temporarily pinch off the Earth’s magnetotail, opening a valve that lets the solar-wind energy cross into the magnetosphere and additionally shoot energy stored in the magnetic tail back toward the aurora zones near the poles. During this magnetic reconnection process, the magnetic fields heading in opposite direction - having opposite north and south polarities – break and reconnect at 140 to 160 million meters downwind of Earth on its night side. Electrons are pushed up and down the tail, and can be accelerated within the magnetosphere as they travel back toward the Earth and then down into the upper atmosphere at the poles.

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The brightest auroras in the solar system are those of Jupiter; they are one thousand times more powerful than Earth's, at about 1014 Watts. Like their terrestrial counterparts, the curtains of light on Jupiter are found in two oval-shaped regions circling the magnetic poles of the planet, just above the clouds. The aurora glows are produced in these high-latitude regions because that is were the magnetic field directs electrically-charged particles, electrons, protons and other ions. When these particles hit the planet's upper atmosphere, they collide with atoms and molecules there, leaving them in an excited state. As on Earth, the atoms and molecules release the extra energy in the form of light, and return to their normal state.

Unlike Earth's colored light show, Jupiter's aurora ovals were first observed from space at ultraviolet wavelengths because that is where most of the atoms and molecules radiate the most intense light. More recently, the Galileo spacecraft has obtained visible light images of thin, patchy aurora arcs originating at about 500 thousand meters above the cloud tops. Both internal and Sun-driven processes probably account for the brilliant curtains of light detected in Jupiter's upper atmosphere, just above the clouds

Scientists speculate that one reason that Jupiter's aurora is so powerful is that they are driven by both internal processes and the solar wind. Electron and ions spewed out by volcanoes on Jupiter's satellite Io are captured by the intense, rapidly rotating magnetic field and spiral inward at high energies toward the planet's polar regions. As the rotating magnetic field sweeps past Io, an invisible current of charged particles, equal to about 1 million amperes, is generated (Section 9.4). The current flows along Jupiter's magnetic field lines into the polar regions, bolting in and out of the planet's upper atmosphere, and producing bright trails in the ultraviolet images.

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Saturn's ultraviolet auroras are most likely caused when the gusty solar wind sweeps over the planet, perhaps like the Earth's aurora. But unlike the Earth, Saturn's aurora oval has only been seen from spacecraft in ultraviolet light, at least so far. It could not be detected from beneath the Earth's atmosphere that absorbs the ultraviolet. We now turn our attention to our home planet, Earth, third rock from the Sun.