helio y velocidad de escape

7/29/2019 Helio y Velocidad de Escape

1/5

hapter index in this window Chapter index in separate window

This material (including images) is copyrighted!. See my copyright notice for fair use practices.

A planet's atmosphere helps shield a planet's surface from harsh radiation from the Sun and it moderates the amount ofnergy lost to space from the planet's interior. An atmosphere also makes it possible for liquid to exist on a planet's

urface by supplying the pressure needed to keep the liquid from boiling away to space---life on the surface of a planet

r moon requires an atmosphere. All of the planets started out with atmospheres of hydrogen and helium. The inner fourlanets (Mercury, Venus, Earth, and Mars) lost their original atmospheres. The atmospheres they have now are from

ases released from their interiors, but Mercury and Mars have even lost most of their secondary atmospheres. Theuter four planets (Jupiter, Saturn, Uranus, and Neptune) were able to keep their original atmospheres. They have veryhick atmospheres with proportionally small solid cores while the the inner four planets have thin atmospheres withroportionally large solid parts.

he properties of each planet's atmosphere are summarized in the Planet Atmospheres table (will appear in a newwindow). Two key determinants in how thick a planet's atmosphere will be are the planet's escape velocity and theemperature of the atmosphere.

Escape of an Atmosphere

he thickness of a planet's atmosphere depends on the planet's gravity and the temperature of the atmosphere. A planetwith weaker gravity does not have as strong a hold on the molecules that make up its atmosphere as a planet with

ronger gravity. The gas molecules will be more likely to escape the planet's gravity. If the atmosphere is cool enough,hen the gas molecules will not be moving fast enough to escape the planet's gravity. But how strong is ``strong enough''nd how cool is ``cool enough'' to hold onto an atmosphere? To answer that you need to consider a planet's escapeelocity and how the molecule speeds depend on the temperature.

scape Velocity

f you throw a rock up, it will rise up and then fall back down because of gravity. If you throw it up with a faster speed,will rise higher before gravity brings it back down. If you throw it up fast enough it just escapes the gravity of the

lanet---the rock initially had a velocity equal to the escape velocity. The escape velocity is the initial velocity neededo escape a massive body's gravitational influence. In theNewton's Law of Gravity chapter the escape velocity is foundo = Sqrt[(2G (planet or moon mass))/distance)]. The distance is measured from the planet or moon's center.


2/5

ince the mass is in the top of the fraction, the escape velocity increases as the mass increases. A more massive planetwill have stronger gravity and, therefore, a higher escape velocity. Also, because the distance is in the bottom of the

action, the escape velocity decreases as the distance increases. The escape velocity is lower at greater heights abovehe planet's surface. The planet's gravity has a weaker hold on the molecules at the top of the atmosphere than thoseose to the surface, so those high up molecules will be the first to ``evaporate away.''

o not confuse the distance from the planet's center with the planet's distance from the Sun. The escape velocity does

NOT depend on how far the planet is from the Sun. You would use the Sun's distance only if you wanted to calculate thescape velocityfrom the Sun. In the same way, a moon's escape velocity does NOT depend on how far it is from thelanet it orbits.

emperature

he temperature of a material is a measure of the average kinetic (motion) energy of the molecules in the material. Ashe temperature increases, a solid turns into a gas when the particles are moving fast enough to break free of thehemical bonds that held them together.

he particles in a hotter gas are moving quicker than those in a cooler gas of the same type. Using Newton's laws of

motion, the relation between the speeds of the molecules and their temperature is found to be temperature = (gasmolecule mass)(average gas molecule speed)2 / (3k), where kis a universal constant of nature called the ``Boltzmannonstant''. Gas molecules of the same type and at the same temperature will have a spread of speeds---some movinguickly, some moving slower---so use the average speed.

f you switch the temperature and velocity, you can derive the average gas molecule velocity = Sqrt[(3k

mperature/(molecule mass))]. Remember that the mass here is the tiny mass of the gas particle, not the planet's mass.


3/5

ince the mass is in the bottom of the fraction, the more massive gas molecules will move slower on average than theghter gas molecules. For example, carbon dioxide molecules move slower on average than hydrogen molecules at theame temperature. Because massive gas molecules move slower, planets with weaker gravity (eg., the terrestrial planets)

will tend to have atmospheres made of just massive molecules. The lighter molecules like hydrogen and helium will havescaped.

he dependence of the average speed of the gas molecules on their mass also explains the compositional structure

bserved in planet atmospheres. Since the distance a gas molecule can move away from the surface of a planet dependsnly on how fast it is moving and the planet's gravity, the lighter gas molecules can be found both close to the surfacend far above it where the gravity is weaker. The gas molecules high up in the atmosphere are most likely to escape.he massive gas molecules will stay close to the planet surface. For example, the Earth's atmosphere is made ofitrogen, oxygen, and water molecules and argon atoms near the surface but at the upper-most heights, hydrogen andelium predominate.

Whereas the process described above leads to evaporation molecule by molecule, another type of atmospheric loss fromeating happens when the atmosphere absorbs ultraviolet light, warms up and expands upward leading to aplanetaryindflowing outward to space. Planets with a lot of hydrogen in their atmospheres are especially subject to this sort oftmospheric loss from heating. The very light hydrogen can bump heavier molecules and atoms outward in the planetary

wind.


4/5

oes Gravity Win or Temperature?

he effects of gravity and temperature work opposite to each other. A higher temperature tries to dissipate antmosphere while higher gravity tries to retain an atmosphere. If the particle's average speed is close to the escape

elocity, then those type of gas particles will not remain for billions of years. The general rule is: if the average gasmolecule speed for a type of gas is less than than 0.2(the escape velocity), then more than 1/2 of that type of gas will

e left after one billion years. If the average speed is greater than that critical value, then more than 1/2 of that type ofas will be gone after one billion years. A flowchart of this is given on the escaping atmosphere page.

ecause the jovian planets are massive and cold, they have THICK atmospheres of hydrogen and helium. The terrestriallanets are small in mass and warm, so they have thin atmospheres made of heavier molecules like carbon dioxide oritrogen.

est and improve your understanding of these concepts with the UNL Astronomy Education program's AtmosphericRetention module (link will appear in a new window). Note that it does use some simplifications but it provides a nice

way to show the roles of temperature and escape velocity in determining how thick a planet's atmosphere will be.

Atmosphere Escape via Non-Thermal Processes

he processes described above occur from the heating of the atoms and molecules in an atmosphere to the point wherehey can escape the planet's gravity. They are calledthermal processes. Other ways involve the presence or lack of a

magnetic field and asteroid or comet impacts. Ions are atoms that have an extra charge (usually by losing an electron).ons will spiral around magnetic field lines so aplanet's magnetic field(discussed more in a later section) will have a lotf ions trapped in it. When a fast-moving hydrogen ion (a proton) bumps into a neutral atom it can steal an electron toecome a neutral atom that is not trapped by the magnetic field and it escapes the planet's gravity. This is calledcharge-

xchange. Some of the magnetic field lines are so wide that they get stretched out by the high-speed stream of ions fromhe Sun calledsolar wind. The stretched out lines do not loop back and just open out into interplanetary space. Ionspiraling around these open magnetic field lines can escape along those lines in what is called a polar wind.


5/5

f a planet does not have a magnetic field (for reasons described later), the solar wind can strip an atmosphere through a

rocess calledsputtering. Without a magnetic field, the solar wind is able to hit the planet's atmosphere directly. Theigh-energy solar wind ions can accelerate atmosphere particles at high altitudes to great enough speeds to escape. An

dditional way of atmosphere escape calledphotodissociation occurs when high-energy sunlight (e.g., ultraviolet or-rays) hits high-altitude molecules in the planet's atmosphere and breaks them apart into individual atoms or smaller

molecules. These smaller particles have the same temperature as the larger molecules and, therefore, as describedbove, will move at faster speeds, possibly fast enough to escape.

he processes described so far in this section work particle to particle and work over long time periods as thetmosphere leaks away particle by particle. In contrast impacts by comets or asteroids can inject a huge amount ofnergy very quickly when the projectile vaporizes upon impact. The expanding plume of hot gas drives off the air abovehe impact site, with the larger the impact energy, the wider is the cone of air that is removed above the impact site. Thempact removal process was probably particularly effective for Mars (being so close to the asteroid belt) and the large

moons of Jupiter(so close to Jupiter's strong gravity that attracts numerous comets and asteroids).

Go back to previous section -- Go to next section

Go to Astronomy Notes home

ast updated: June 12, 2010

Is this page a copy of Strobel's Astronomy Notes?

uthor oforiginal content:Nick Strobel

helio y velocidad de escape

Documents