l>Planetary Science


A planet"s atmosphere helps shield a planet"s surface ar from harsh radiation native theSun and also it moderates the quantity of energy lost to an are from the planet"s interior. One atmosphere likewise makes it possible for fluid to exist top top a planet"s surface ar by supplying the pressure required to save the fluid from boiling far to space---life on the surface ar of a earth or moon needs an atmosphere.All of the planets began out with environments of hydrogen and helium. The innerfour planets (Mercury, Venus, Earth, and Mars) lost their original atmospheres. The settings they have actually now room from gases exit from your interiors, but Mercuryand Mars have also lost most of their secondary atmospheres. The outer four planets(Jupiter, Saturn, Uranus, and also Neptune) to be able to store their original atmospheres.They have very thick atmospheres with proportionally tiny solid cores if thethe inner 4 planets have actually thin atmospheres with proportionally huge solid parts.The properties of each planet"s environment are summarized in thePlanet Atmospheres table (will appear in a brand-new window). Two an essential determinants inhow special a planet"s setting will be room the planet"s escape velocity and also thetemperature the the atmosphere.

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

Escape of one Atmosphere

The thickness of a planet"s environment depends ~ above the planet"s gravity and thetemperature that the atmosphere. A world with weaker gravity does not have actually as strong a hold on the molecules that make up its atmosphereas a planet with more powerful gravity. The gas molecules will certainly be much more likely come escape theplanet"s gravity. If the atmosphere is cool enough, climate the gas molecules will certainly not bemoving fast enough to escape the planet"s gravity. But how strong is ``strong enough""and exactly 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 rely on thetemperature.Escape VelocityIf you throw a rock up, that will climb up and then fall back down due to the fact that of gravity. If you litter it up with a quicker speed, it will certainly rise higher before heaviness brings it back down. If you litter it up fast enough it simply escapes the heaviness of the planet---the absent initially had actually a velocity equal to the escape velocity. The escape velocity is the early stage velocity required to escape a massive body"s gravitational influence. In the Newton"s regulation of gravity chapter the to escape velocity is found to = Sqrt<(2G × (planet or moon mass))/distance)>. The street is measured from the earth or moon"s center.


Since the massis in the top of the fraction, the escape velocity increases as the fixed increases. A moremassive world will have stronger heaviness and, therefore, a greater escape velocity.Also, because the distance is in the bottom that the fraction, the escape velocitydecreases as the street increases. The to escape velocity is lower 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, for this reason those highup molecules will be the an initial to ``evaporate away.""Do not confuse the street from the planet"s facility with the planet"sdistance from the Sun. The escape velocity walk NOT rely on how much the planet isfrom the Sun. You would use the Sun"s distance only if you want to calculate theescape velocity from the Sun. In the very same way, a moon"s escape velocitydoes NOT rely on how much it is indigenous the earth it orbits.TemperatureThe temperature of a material is a measure up of the median kinetic (motion) energyof the molecule in the material. Together the temperature increases, a heavy turns into agas as soon as the particles are moving fast enough to break totally free of the chemical bonds thatheld lock together.

The particles in a name is gas are moving much faster thanthose in a cooler gas that the same type. Making use of Newton"s legislations of motion, the relationbetween the speeds of the molecules and their temperature is uncovered to it is in temperature = (gas molecule mass)×(average gas molecule speed)2 / (3k),where k is a universal continuous of nature called the ``Boltzmann constant"".Gas molecule of the same form and at the very same temperature will have a spread of speeds---some relocating quickly, some moving slower---so usethe average speed.If you switch the temperature and velocity, you can derive the mean gas moleculevelocity = Sqrt<(3k × temperature/(molecule mass))>. Rememberthat the mass here is the small mass of the gas particle, no the planet"s mass. Sincethe massive is in the bottom the the fraction, the much more massive gas molecules will certainly moveslower on median than the lighter gas molecules. Because that example, carbon dioxidemolecules relocate slower on mean than hydrogen molecule at the exact same temperature.Because huge gas molecules relocate slower, planets v weaker gravity (e.g., theterrestrial planets) will have tendency to have environments made of just enormous molecules.The lighter molecules like hydrogen and helium will have actually escaped.

Whereas the procedure described over leads come evaporation molecule through molecule, another form of atmospheric loss from heating happens once the setting absorbs ultraviolet light, warms up and expands upward bring about a planetary wind flowing outward to space. Planets with a many hydrogen in their atmospheres are especially subject come this sort of atmospheric loss from heating. The really light hydrogen deserve to bump more heavier molecules and also atoms external in the planetary wind.

Does Gravity win or Temperature?


If a world does not have actually a magnetic ar (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 atmosphere particles in ~ high altitudes to good enough speed to escape. An additional means of atmosphere escape referred to as photodissociation occurs as soon as high-energy sunlight (e.g., ultraviolet or x-rays) hits high-altitude molecule in the planet"s atmosphere and also breaks lock apart right into individual atom or smaller sized molecules. These smaller sized particles have actually the same temperature as the bigger molecules and, therefore, as defined above, will relocate at much faster speeds, probably fast sufficient to escape.

See more: How To Find The Charge Of A Transition Metal Ions, For Example: In The Problem: Fe+2 + Mno4

The processes explained so much in this section job-related particle to particle and work over lengthy time periods as the environment leaks away particle by particle. In contrast impacts by comets or asteroids can inject a huge amount of energy an extremely quickly once the projectile vaporizes top top impact. The expanding plume of warm gas drives turn off the air above the influence site, through the bigger the affect energy, the more comprehensive is the cone of air that is removed over the impact site. The impact removal process was probably an especially effective because that Mars (being therefore close to the asteroid belt) and the large moons the Jupiter (so close come Jupiter"s solid gravity the attracts plenty of comets and also asteroids).

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