l>Planetary Science

Atmospheres

A planet"s environment helps shield a planet"s surface from harsh radiation native theSun and it moderates the lot of power lost to an are from the planet"s interior. One atmosphere likewise makes it feasible for liquid to exist ~ above a planet"s surface ar by supplying the pressure essential to save the fluid from boiling away to space---life ~ above the surface of a world or moon needs an atmosphere.All of the planets started out with environments of hydrogen and helium. The innerfour planets (Mercury, Venus, Earth, and Mars) lost their original atmospheres. The settings they have now space from gases released from your interiors, however Mercuryand Mars have also lost most of their an additional atmospheres. The outer 4 planets(Jupiter, Saturn, Uranus, and also Neptune) to be able to save their initial atmospheres.They have an extremely thick settings with proportionally tiny solid cores when thethe inner 4 planets have thin atmospheres with proportionally big solid parts.The properties of every planet"s setting are summary in thePlanet Atmospheres table (will appear in a brand-new window). Two an essential determinants inhow special a planet"s setting will be space the planet"s escape velocity and thetemperature the the atmosphere.

You are watching: Explain why the outer planets did not lose the lighter gases in their atmospheres

Escape of an Atmosphere

The thickness of a planet"s setting depends on the planet"s gravity and also thetemperature that the atmosphere. A earth with weaker gravity does not have actually as strong a hold on the molecules that make up its atmosphereas a world with more powerful gravity. The gas molecules will be much more likely to escape theplanet"s gravity. If the environment is cool enough, climate the gas molecules will not bemoving fast enough to to escape the planet"s gravity. Yet how strong is ``strong enough""and how cool is ``cool enough"" to organize onto one atmosphere? come answer that you need toconsider a planet"s escape velocity and also how the molecule speeds count on thetemperature.Escape VelocityIf you litter a rock up, the will climb up and also then fall ago down because of gravity. If you throw it up v a quicker speed, it will certainly rise greater before heaviness brings it earlier down. If you litter it increase fast enough it simply escapes the gravity of the planet---the rock initially had actually a velocity same to the escape velocity. The escape velocity is the early stage velocity required to to escape a enormous body"s gravitational influence. In the Newton"s regulation of heaviness chapter the to escape velocity is discovered to = Sqrt<(2G × (planet or moon mass))/distance)>. The distance 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 boosts as the fixed increases. A moremassive planet will have actually stronger gravity and, therefore, a greater escape velocity.Also, due to the fact that the distance is in the bottom that the fraction, the escape velocitydecreases as the distance increases. The escape velocity is reduced at greaterheights over the planet"s surface. The planet"s gravity has a weaker hold on themolecules at the optimal of the setting than those close come the surface, therefore those highup molecules will certainly be the an initial to ``evaporate away.""Do not confuse the distance from the planet"s facility with the planet"sdistance indigenous the Sun. The to escape velocity go NOT count on how far the earth isfrom the Sun. Girlfriend would use the Sun"s distance only if you want to calculation theescape velocity from the Sun. In the same way, a moon"s escape velocitydoes NOT depend on how far it is from the world it orbits.TemperatureThe temperature that a material is a measure of the typical kinetic (motion) energyof the molecules in the material. Together the temperature increases, a hard turns right into agas as soon as the corpuscle are relocating fast sufficient to break cost-free of the chemical bonds thatheld castle together.

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The particles in a hot gas room moving much faster thanthose in a cooler gas the the same type. Utilizing Newton"s regulations of motion, the relationbetween the speed of the molecules and also their temperature is uncovered to be temperature = (gas molecule mass)×(average gas molecule speed)2 / (3k),where k is a universal consistent of nature referred to as the ``Boltzmann constant"".Gas molecules of the same form and in ~ the same temperature will have a spread of speeds---some relocating quickly, some relocating slower---so usethe average speed.If you move the temperature and velocity, you deserve to derive the mean gas moleculevelocity = Sqrt<(3k × temperature/(molecule mass))>. Rememberthat the mass right here is the tiny mass that the gas particle, no the planet"s mass. Sincethe massive is in the bottom that the fraction, the more massive gas molecules will moveslower on typical than the lighter gas molecules. For example, carbon dioxidemolecules move slower on typical than hydrogen molecules at the same temperature.Because enormous gas molecules relocate slower, planets with weaker gravity (e.g., theterrestrial planets) will have tendency to have atmospheres made that just massive molecules.The lighter molecules choose hydrogen and helium will have escaped.
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Whereas the process described over leads come evaporation molecule through molecule, another form of atmospheric lose from heater happens when the setting absorbs ultraviolet light, warms up and also expands upward resulting in a planetary wind flowing external to space. Planets v a lot of hydrogen in their settings are specifically subject to this kind of atmospheric loss indigenous heating. The really light hydrogen can bump more heavier molecules and also atoms external in the planetary wind.

Does Gravity win or Temperature?

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If a earth does not have actually a magnetic field (for reasons defined later), the solar wind can strip an atmosphere through a process called sputtering. without a magnetic field, the solar wind is able to hit the planet"s atmosphere directly. The high-energy solar wind ions can accelerate environment particles at high altitudes to an excellent enough speeds to escape. An additional means of atmosphere escape dubbed photodissociation occurs when high-energy sunshine (e.g., ultraviolet or x-rays) hits high-altitude molecules in the planet"s atmosphere and breaks castle apart right into individual atom or smaller sized molecules. These smaller particles have actually the same temperature together the larger molecules and, therefore, as defined above, will relocate at faster speeds, maybe fast enough to escape.

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The processes explained so far in this section work-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 vast amount the energy an extremely quickly once the projectile vaporizes top top impact. The broadening plume of hot gas drives off the air over the impact site, with the larger the influence energy, the broader is the cone that air the is removed over the impact site. The impact removal procedure was probably specifically effective because that Mars (being so close to the asteroid belt) and the big moons that Jupiter (so close come Jupiter"s strong gravity the attracts many comets and asteroids).

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Go back to previous ar -- next walk to next sectionGo to Astronomy notes homelast updated: June 5, 2019Is this web page a copy the Strobel"s Astronomy Notes?Author of original content: Nick Strobel