l>Planetary Science

Atmospheres

A planet"s setting helps shield a planet"s surface from harsh radiation from theSun and also it moderates the amount of power shed to area from the planet"s interior. An atmosphere additionally renders it possible for liquid to exist on a planet"s surchallenge by giving the press needed to save the liquid from boiling amethod to space---life on the surface of a planet or moon needs an environment.All of the planets began out via environments of hydrogen and helium. The innerfour planets (Mercury, Venus, Earth, and also Mars) shed their original atmospheres. The atmospheres they have now are from gases released from their interiors, however Mercuryand Mars have actually also lost the majority of of their second environments. The external 4 planets(Jupiter, Saturn, Uranus, and also Neptune) were able to keep their original atmospheres.They have very thick atmospheres via proportionally little solid cores while thethe inner 4 planets have thin settings with proportionally large solid parts.The properties of each planet"s atmosphere are summarized in thePlanet Atmospheres table (will appear in a brand-new window). Two essential factors inhow thick a planet"s environment will certainly be are the planet"s escape velocity and also thetemperature of the setting.

You are watching: How is the size of a planet related to the thickness of its atmosphere

Escape of an Atmosphere

The thickness of a planet"s setting relies on the planet"s gravity and also thetemperature of the setting. A world with weaker gravity does not have actually as solid a host on the molecules that make up its atmosphereas a earth through stronger gravity. The gas molecules will certainly be even more most likely to escape theplanet"s gravity. If the atmosphere is cool sufficient, then the gas molecules will certainly not bemoving quick enough to escape the planet"s gravity. But just how strong is ``strong enough""and just how cool is ``cool enough"" to organize onto an atmosphere? To answer that you require tothink about a planet"s escape velocity and how the molecule speeds depend on thetemperature.Escape VelocityIf you throw a rock up, it will certainly climb up and then autumn ago dvery own because of gravity. If you throw it up via a much faster speed, it will climb higher before gravity brings it ago down. If you throw it up fast enough it simply escapes the gravity of the planet---the rock initially had a velocity equal to the escape velocity. The escape velocity is the initial velocity essential to escape a enormous body"s gravitational influence. In the Newton"s Law of Gravity chapter the escape velocity is uncovered to = Sqrt<(2G × (earth or moon mass))/distance)>. The distance is measured from the world or moon"s facility.

*

Due to the fact that the massis in the height of the fractivity, the escape velocity boosts as the mass boosts. A moresubstantial earth will certainly have actually stronger gravity and, therefore, a greater escape velocity.Also, because the distance is in the bottom of the fraction, the escape velocitydecreases as the distance boosts. The escape velocity is lower at greaterheights over the planet"s surchallenge. The planet"s gravity has a weaker organize on themolecules at the top of the atmosphere than those close to the surconfront, so those highup molecules will be the first to ``evapoprice ameans.""Do not confusage the distance from the planet"s center via the planet"sdistance from the Sun. The escape velocity does NOT depend on how far the planet isfrom the Sun. You would certainly use the Sun"s distance just if you wanted to calculate theescape velocity from the Sun. In the same means, a moon"s escape velocitydoes NOT depend on just how much it is from the planet it orbits.TemperatureThe temperature of a material is a measure of the average kinetic (motion) energyof the molecules in the product. As the temperature increases, a solid transforms right into agas once the particles are moving fast enough to break free of the chemical bonds thatorganized them together.

*
The pshort articles in a hotter gas are relocating much faster thanthose in a cooler gas of the exact same form. Using Newton"s regulations of movement, the relationbetween the speeds of the molecules and also their temperature is discovered to be temperature = (gas molecule mass)×(average gas molecule speed)2 / (3k),wbelow k is a universal continuous of nature called the ``Boltzmann constant"".Gas molecules of the very same form and also at the very same temperature will certainly have actually a spread of speeds---some relocating conveniently, some moving slower---so usethe average speed.If you switch the temperature and velocity, you can derive the average gas moleculevelocity = Sqrt<(3k × temperature/(molecule mass))>. Rememberthat the mass here is the tiny mass of the gas pshort article, not the planet"s mass. Sincethe mass is in the bottom of the fraction, the more huge gas molecules will movesreduced on average than the lighter gas molecules. For example, carbon dioxidemolecules move sreduced on average than hydrogen molecules at the same temperature.Since huge gas molecules move slower, planets with weaker gravity (e.g., theterrestrial planets) will tfinish to have actually settings made of simply enormous molecules.The lighter molecules choose hydrogen and helium will certainly have actually escaped.
*

Whereas the procedure explained above leads to evaporation molecule by molecule, another type of atmospheric loss from heating happens as soon as the atmosphere absorbs ultraviolet light, warms up and also increases upward leading to a planetary wind flowing outward to room. Planets with a lot of hydrogen in their settings are specifically subject to this sort of atmospheric loss from heating. The exceptionally light hydrogen deserve to bump heavier molecules and atoms outward in the planetary wind.

Does Gravity Victory or Temperature?

*

If a earth does not have a magnetic area (for reasons defined later), the solar wind have the right to strip an environment with a process dubbed sputtering. Without a magnetic area, the solar wind is able to hit the planet"s atmosphere directly. The high-energy solar wind ions deserve to acceleprice environment pposts at high altitudes to excellent sufficient speeds to escape. An extra way of environment escape called photodissociation occurs as soon as high-power sunlight (e.g., ultraviolet or x-rays) hits high-altitude molecules in the planet"s atmosphere and also breaks them apart right into individual atoms or smaller molecules. These smaller sized pshort articles have actually the very same temperature as the larger molecules and also, therefore, as described above, will relocate at much faster speeds, perhaps fast sufficient to escape.

See more: Why Don T Animals Have Belly Buttons ? Do Dogs Have Belly Buttons

The procedures defined so far in this area job-related pshort article to ppost and also job-related over lengthy time periods as the atmosphere leaks amethod ppost by pwrite-up. In comparison effects by comets or asteroids have the right to inject a huge amount of power exceptionally quickly when the projectile vaporizes upon impact. The widening plume of warm gas drives off the air over the impact website, through the bigger the impact energy, the wider is the camong air that is rerelocated above the affect website. The influence removal process was more than likely especially reliable for Mars (being so close to the asteroid belt) and the huge moons of Jupiter (so close to Jupiter"s strong gravity that attracts plenty of comets and also asteroids).

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