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

    • An atmosphere is a gaseous layer of molecules, with smaller amounts of atoms and ions, which surrounds a planet or natural satellite, held near them by their gravity.

    • The present-day atmospheres of the terrestrial planets are thought to be secondary, having originated after planetary formation about 4.6 billion years ago. Their atmospheres may have been released from volcanoes or acquired during collisions of comets and asteroids.

    • The giant planets retain their primeval atmospheres, created when these planets formed, capturing significant amounts of hydrogen and helium gas.

    • An atmosphere is characterized by the pressure and temperature of the molecules in it.

    • Pressure increases with the temperature and density of the gas.

    • An atmosphere allows the warmth of sunlight in but prevents the escape of infrared heat radiation from the planetís surface; this global warming by heat-trapping gases in an atmosphere is now known as the greenhouse effect.

    • The ability of a planet or satellite to retain an atmosphere depends on both the temperature of that atmosphere and the gravitational pull of the planet or satellite. If the gas is hot, the molecules move about with a greater velocity and are more likely to escape the gravitational pull of the planet. A planet with a larger mass is more likely to retain an atmosphere.

    • Only the massive giant planets, like Jupiter and Saturn, have a high enough escape velocity that they can retain all atoms and molecules, including the lightest element, hydrogen.

    • Not all molecules move at the same average speed, or at the thermal velocity. Some of them move faster and others move slower, with velocities described by the Maxwellian distribution.

    • The lighter, high-velocity molecules can slowly leak out or evaporate from the top of an atmosphere where collisions no longer dominate the velocity distribution; this process is called Jeans escape or thermal evaporation.

    • A thin membrane of air protects, ventilates and incubates us.

    • Winds move air from hot to cold regions, in an attempt to equalize the temperature differences.

    • The surface of Venus now lies under a hot and heavy atmosphere of carbon dioxide; its greenhouse effect has raised the surface temperature on Venus to a torrid 735-kelvin, hot enough to melt lead and zinc.

    • High-velocity winds on Venus whip its highest clouds around the planet 60 times faster than the planet rotates.

    • Mars now has an exceedingly thin, dry and cold atmosphere of carbon dioxide, with less than one-hundredth the surface pressure of the Earthís atmosphere, but global winds can stir up enough dust to completely cover Mars.

    • The red planet breathes about one-third of its atmosphere in and out, as its southern polar cap grows and shrinks with the Martian seasons.

    • Mercury and the Earthís Moon are surrounded by a tenuous, varying mist of atoms that is continuously escaping into the surrounding space and being replenished from below. It is essentially a layer of exit, or escape, known as an exosphere.

    • The atmospheres of Earth, Venus and Mars have evolved to their present states, as the result of their varying distances from the Sun, a runaway greenhouse effect on Venus, and the development of life on Earth.

    • The Sun generated so little heat more than 2 billion years ago that the Earth's oceans should have been frozen solid. This faint-young-Sun paradox could be resolved if a thick carbon-dioxide atmosphere warmed the planet back then or if the young Sun was more magnetically active or more massive than it is now.

    • Like the Sun, the most abundant element in the giant planets is the lightest element, hydrogen, and the next-most-abundant element is helium; but Uranus and Neptune have lesser amounts of these two gases and relatively greater amounts of the heavier hydrogen compounds like methane, ammonia and water.

    • Jupiter is all atmosphere, with no solid surface to rub against or continents to disturb the flow. Its Great Red Spot has existed for more than 300 years; the location and speed of powerful winds and some violent storms on the giant planet have remained unchanged for at least one century.

    • Helium rain has been falling toward the center of Saturn for the past two billion years, significantly depleting the amount of helium in the planetís upper atmosphere.

    • Saturn's largest moon, Titan, has a substantial Earth-like atmosphere, which is mainly composed of nitrogen and has a surface pressure comparable to that of the Earthís atmosphere. An opaque smog-like haze hides the surface of Titan from view.

    • A thin film of oxygen envelops Jupiterís large moon Europa, while a tenuous mist of sulfur dioxide surrounds Jupiterís moon Io.

    • An energy-laden, electrically charged solar wind blows out from the Sun in all directions and never stops, carrying with it a magnetic field rooted in the star.

    • The radial, supersonic outflow of the solar wind creates a huge bubble of electrons, protons and magnetic fields, with the Sun at the center and the planets inside, called the heliosphere.

    • The Earth has a dipolar magnetic field, amplified and sustained by dynamo action in its liquid core.

    • Ancient magnetic rocks indicate that Earthís magnetic poles keep switching places every 10 thousand to 10 million years, and that our planetís magnetic field may now be heading for a flip.

    • The terrestrial magnetic field deflects the solar wind, hollowing out a cavity called the magnetosphere. The magnetosphere of any planet is the volume of space from which the main thrust of the solar wind is excluded.

    • Energetic electrons and protons have penetrated the Earth's magnetic defense. Some of them are confined within two doughnut-shaped radiation belts that encircle the Earth's equator but do not touch it.

    • The magnetosphere contains particles from the Sun that arrive via the solar wind and penetrate the Earthís magnetic defense through a temporary opening in it, which is produced by magnetic reconnection when the solar magnetic field and the Earthís magnetic field point in opposite directions at the place where they touch.

    • A plasmasphere is found in the inner part of the Earthís magnetosphere and located just outside the upper terrestrial atmosphere, called the ionosphere. The plasmasphere contains oxygen ions, protons and electrons derived from the ionosphere.

    • Cosmic rays produce neutrons in the Earthís atmosphere, and a small fraction of these neutrons move out into the Earthís inner radiation belt before they disintegrate, producing electrons and protons in places they could not otherwise have reached.

    • Of the eight major planets, six are known to now generate detectable, global magnetic fields; only Mars and Venus do not now have such a dipolar magnetic field.

    • Jupiter's magnetosphere is the largest enduring structure in the solar system, more than ten times larger than the Sun. It was discovered when Earth-based radio telescopes unexpectedly detected the synchrotron radio emission of high-speed electrons trapped in the giant planet's immense magnetic field.

    • The magnetic fields of Uranus and Neptune are offset by large amounts from their centers, and tilted by enormous angles from their rotation axes.

    • The Earth's aurora is a spectacular multi-colored light show that shines like a cosmic neon sign.

    • When viewed from space, the aurora forms an oval centered on the magnetic poles of the Earth; similar aurora ovals have been detected in ultraviolet light at both the north and south magnetic poles of Jupiter and Saturn.

Copyright 2010, Professor Kenneth R. Lang, Tufts University