l>Planetary Science

Atmospheres

A planet"s atmosphere helps shield a planet"s surface ar from harsh radiation indigenous theSun and also it moderates the amount of energy lost to space from the planet"s interior. An atmosphere additionally makes it feasible for liquid to exist on a planet"s surface by offering the pressure needed to save the fluid from boiling away to space---life on the surface ar of a earth or moon requires an atmosphere.All that the planets began out with settings of hydrogen and helium. The innerfour planets (Mercury, Venus, Earth, and also Mars) lost their original atmospheres. The atmospheres they have actually now are from gases released from their interiors, but Mercuryand Mars have even lost most of their an additional atmospheres. The outer 4 planets(Jupiter, Saturn, Uranus, and also Neptune) to be able to keep their initial atmospheres.They have an extremely thick environments with proportionally tiny solid cores when thethe inner four planets have thin environments with proportionally huge solid parts.The nature of every planet"s atmosphere are summarized in thePlanet Atmospheres table (will show up in a new window). Two crucial determinants inhow thick a planet"s atmosphere will be are the planet"s escape velocity and thetemperature the the atmosphere.

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Escape of an Atmosphere

The thickness of a planet"s setting depends top top the planet"s gravity and also thetemperature the the atmosphere. A world with weaker heaviness does not have actually as solid a host on the molecules that make up its atmosphereas a earth with more powerful gravity. The gas molecules will be an ext likely come escape theplanet"s gravity. If the setting is cool enough, climate the gas molecules will not bemoving fast enough to escape the planet"s gravity. Yet how strong is ``strong enough""and how cool is ``cool enough"" to host onto an atmosphere? to answer the you require toconsider a planet"s escape velocity and how the molecule speeds count on thetemperature.Escape VelocityIf you litter a absent up, it will increase up and also then fall ago down because of gravity. If you litter it up with a faster speed, it will certainly rise greater before heaviness brings it ago down. If you litter it increase fast sufficient it just escapes the heaviness of the planet---the absent initially had actually a velocity equal to the escape velocity. The escape velocity is the early velocity required to escape a massive body"s gravitational influence. In the Newton"s regulation of gravity chapter the to escape velocity is discovered to = Sqrt<(2G × (planet or moon mass))/distance)>. The street is measured indigenous the earth or moon"s center.

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Since the massis in the peak of the fraction, the escape velocity rises as the mass increases. A moremassive planet will have stronger gravity and, therefore, a greater escape velocity.Also, due to the fact that the distance is in the bottom the the fraction, the to escape velocitydecreases as the distance increases. The to escape velocity is reduced at greaterheights above the planet"s surface. The planet"s gravity has a weaker hold on themolecules in ~ the top of the setting than those close to the surface, so those highup molecules will certainly be the an initial to ``evaporate away.""Do not confuse the distance from the planet"s center with the planet"sdistance from the Sun. The escape velocity does NOT depend on how much the planet isfrom the Sun. Girlfriend would use the Sun"s distance only if you wanted to calculation theescape velocity from the Sun. In the very same way, a moon"s escape velocitydoes NOT depend on how much it is from the world it orbits.TemperatureThe temperature that a material is a measure of the median kinetic (motion) energyof the molecule in the material. As the temperature increases, a hard turns right into agas as soon as the particles are moving fast enough to break complimentary of the chemistry bonds thatheld them together.

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The particles in a hotter gas space moving faster thanthose in a cooler gas that the same type. Making use of Newton"s regulations of motion, the relationbetween the speeds of the molecules and their temperature is discovered to it is in temperature = (gas molecule mass)×(average gas molecule speed)2 / (3k),where k is a universal continuous of nature dubbed the ``Boltzmann constant"".Gas molecules of the same type and at the very same temperature will have a spread of speeds---some moving quickly, some relocating slower---so usethe average speed.If you switch the temperature and also velocity, you can derive the median gas moleculevelocity = Sqrt<(3k × temperature/(molecule mass))>. Rememberthat the mass right here is the small mass of the gas particle, no the planet"s mass. Sincethe fixed is in the bottom of the fraction, the more massive gas molecules will moveslower on typical than the lighter gas molecules. For example, carbon dioxidemolecules relocate slower on median than hydrogen molecules at the exact same temperature.Because huge gas molecules relocate slower, planets with weaker gravity (e.g., theterrestrial planets) will tend to have atmospheres made of just massive molecules.The lighter molecules choose hydrogen and helium will have actually escaped.
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Whereas the process described above leads to evaporation molecule by molecule, another form of atmospheric lose from heating happens once the setting absorbs ultraviolet light, warms up and expands upward leading to a planetary wind flowing outside to space. Planets through a many hydrogen in their environments are particularly subject to this sort of atmospheric loss indigenous heating. The very light hydrogen deserve to bump more heavier molecules and also atoms exterior in the planetary wind.

Does Gravity win or Temperature?

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If a world does not have actually a magnetic field (for reasons described later), the solar wind have the right to strip an environment through a process called sputtering. there is no a magnetic field, the solar wind is able to hit the planet"s environment directly. The high-energy solar wind ions deserve to accelerate atmosphere particles in ~ high altitudes to an excellent enough speed to escape. An additional way of atmosphere escape dubbed photodissociation occurs when high-energy sunshine (e.g., ultraviolet or x-rays) hits high-altitude molecule in the planet"s atmosphere and also breaks them apart right into individual atom or smaller molecules. These smaller sized particles have the very same temperature together the larger molecules and, therefore, as defined above, will relocate at quicker speeds, probably fast enough to escape.

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The processes explained so much in this section job-related particle come particle and also work over long time periods as the atmosphere leaks away fragment by particle. In comparison impacts by comets or asteroids can inject a large amount of energy an extremely quickly as soon as the projectile vaporizes upon impact. The broadening plume of warm gas drives off the air over the impact site, with the larger the impact energy, the broader is the cone the air that is removed above the influence site. The affect removal process was probably specifically effective for Mars (being so close come the asteroid belt) and the huge moons that Jupiter (so close come Jupiter"s strong gravity that attracts numerous comets and asteroids).

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Go ago to previous section -- next go to following sectionGo come Astronomy notes homelast updated: June 5, 2019Is this web page a copy that Strobel"s Astronomy Notes?Author of original content: Nick Strobel