How Is The Size Of A Planet Related To The Thickness Of Its Atmosphere

Atmospheres

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A planet's atmosphere helps shield a planet'south surface from harsh radiation from the Dominicus and it moderates the amount of free energy lost to space from the planet'due south interior. An atmosphere also makes it possible for liquid to be on a planet'southward surface by supplying the pressure needed to keep the liquid from boiling away to space---life on the surface of a planet or moon requires an atmosphere. All of the planets started out with atmospheres of hydrogen and helium. The inner 4 planets (Mercury, Venus, Globe, and Mars) lost their original atmospheres. The atmospheres they have now are from gases released from their interiors, simply Mercury and Mars take fifty-fifty lost most of their secondary atmospheres. The outer four planets (Jupiter, Saturn, Uranus, and Neptune) were able to continue their original atmospheres. They have very thick atmospheres with proportionally minor solid cores while the the inner four planets have sparse atmospheres with proportionally large solid parts.

The properties of each planet's atmosphere are summarized in the Planet Atmospheres table (volition appear in a new window). Two key determinants in how thick a planet'south temper will exist are the planet's escape velocity and the temperature of the atmosphere.

Escape of an Temper

The thickness of a planet'due south temper depends on the planet'due south gravity and the temperature of the atmosphere. A planet with weaker gravity does not accept every bit strong a hold on the molecules that make upwards its atmosphere as a planet with stronger gravity. The gas molecules volition exist more likely to escape the planet's gravity. If the atmosphere is cool enough, and so the gas molecules will non be moving fast plenty to escape the planet's gravity. But how stiff is ``strong plenty'' and how cool is ``cool enough'' to hold onto an atmosphere? To answer that you demand to consider a planet'due south escape velocity and how the molecule speeds depend on the temperature.

Escape Velocity

If you throw a rock up, it volition rise upwards and so autumn back downwardly because of gravity. If you throw it up with a faster speed, it will rise higher before gravity brings it back down. If you lot throw it up fast enough it just escapes the gravity of the planet---the rock initially had a velocity equal to the escape velocity. The escape velocity is the initial velocity needed to escape a massive body's gravitational influence. In the Newton's Police of Gravity chapter the escape velocity is found to = Sqrt[(two1000 × (planet or moon mass))/altitude)]. The distance is measured from the planet or moon's middle.

escape velocity

Since the mass is in the top of the fraction, the escape velocity increases as the mass increases. A more than massive planet volition take stronger gravity and, therefore, a higher escape velocity. Also, because the distance is in the bottom of the fraction, the escape velocity decreases as the distance increases. The escape velocity is lower at greater heights in a higher place the planet'south surface. The planet'south gravity has a weaker agree on the molecules at the summit of the atmosphere than those shut to the surface, so those loftier up molecules will be the first to ``evaporate away.''

Practise not misfile the distance from the planet's centre with the planet's distance from the Sun. The escape velocity does Non depend on how far the planet is from the Sunday. You would use the Sunday'south distance only if you wanted to calculate the escape velocity from the Dominicus. In the same way, a moon's escape velocity does Not depend on how far it is from the planet it orbits.

Temperature

The temperature of a material is a measure of the average kinetic (motion) energy of the molecules in the cloth. As the temperature increases, a solid turns into a gas when the particles are moving fast plenty to break gratuitous of the chemical bonds that held them together. raising the temperature changes the phase of material

The particles in a hotter gas are moving quicker than those in a cooler gas of the aforementioned blazon. Using Newton's laws of movement, the relation between the speeds of the molecules and their temperature is institute to be temperature = (gas molecule mass)×(boilerplate gas molecule speed)^two / (3k), where yard is a universal constant of nature called the ``Boltzmann abiding''. Gas molecules of the same type and at the aforementioned temperature will have a spread of speeds---some moving apace, some moving slower---and so use the average speed.

If you switch the temperature and velocity, you can derive the average gas molecule velocity = Sqrt[(3k × temperature/(molecule mass))]. Remember that the mass here is the tiny mass of the gas particle, not the planet's mass. Since the mass is in the bottom of the fraction, the more than massive gas molecules will move slower on boilerplate than the lighter gas molecules. For case, carbon dioxide molecules movement slower on average than hydrogen molecules at the same temperature. Because massive gas molecules move slower, planets with weaker gravity (e.g., the terrestrial planets) will tend to have atmospheres made of simply massive molecules. The lighter molecules similar hydrogen and helium will accept escaped.

speed of particles depends on temperature and mass

The dependence of the average speed of the gas molecules on their mass also explains the compositional structure observed in planet atmospheres. Since the distance a gas molecule can motion away from the surface of a planet depends only on how fast it is moving and the planet's gravity, the lighter gas molecules can be institute both close to the surface and far in a higher place it where the gravity is weaker. The gas molecules high up in the atmosphere are most likely to escape. The massive gas molecules will stay close to the planet surface. For example, the Earth's temper is made of nitrogen, oxygen, and h2o molecules and argon atoms near the surface but at the upper-most heights, hydrogen and helium predominate.

the evaporation of a planet's atmosphere

Whereas the process described to a higher place leads to evaporation molecule by molecule, another type of atmospheric loss from heating happens when the temper absorbs ultraviolet calorie-free, warms up and expands upward leading to a planetary current of air flowing outward to space. Planets with a lot of hydrogen in their atmospheres are peculiarly subject to this sort of atmospheric loss from heating. The very lite hydrogen can bump heavier molecules and atoms outward in the planetary wind.

Does Gravity Win or Temperature?

strong gravity keeps an atmosphere but high temperature dissipates an atmosphere

The effects of gravity and temperature work opposite to each other. A higher temperature tries to misemploy an temper while higher gravity tries to retain an atmosphere. If the particle's average speed is close to the escape velocity, then those blazon of gas particles will non remain for billions of years. The general dominion is: if the average gas molecule speed for a type of gas is less than than 0.2×(the escape velocity), so more than 1/ii of that type of gas volition be left subsequently one billion years. If the average speed is greater than that critical value, and then more than 1/2 of that blazon of gas will be gone after i billion years. A flowchart of this is given on the escaping temper page.

Because the jovian planets are massive and cold, they accept THICK atmospheres of hydrogen and helium. The terrestrial planets are small-scale in mass and warm, and so they have thin atmospheres fabricated of heavier molecules like carbon dioxide or nitrogen.

Test and improve your agreement of these concepts with the UNL Astronomy Education program'due south Atmospheric Retention module (link volition appear in a new window). Note that it does use some simplifications but it provides a overnice mode to show the roles of temperature and escape velocity in determining how thick a planet'due south temper will be.

Atmosphere Escape via Not-Thermal Processes

The processes described higher up occur from the heating of the atoms and molecules in an temper to the point where they tin escape the planet's gravity. They are chosen thermal processes. Other ways involve the presence or lack of a magnetic field and asteroid or comet impacts. Ions are atoms that take an actress charge (usually past losing an electron). Ions will spiral around magnetic field lines so a planet's magnetic field (discussed more in a later section) will take a lot of ions trapped in information technology. When a fast-moving hydrogen ion (a proton) bumps into a neutral atom it tin can steal an electron to go a neutral atom that is not trapped by the magnetic field and information technology escapes the planet's gravity. This is called accuse-exchange. Some of the magnetic field lines are so wide that they go stretched out past the loftier-speed stream of ions from the Sun chosen solar wind. The stretched out lines do non loop back and just open out into interplanetary space. Ions spiraling around these open magnetic field lines can escape along those lines in what is called a polar wind.

non-thermal escape of atmosphere

If a planet does not have a magnetic field (for reasons described afterward), the solar wind tin strip an temper through a process called sputtering. Without a magnetic field, the solar current of air is able to hit the planet'south temper directly. The high-free energy solar air current ions can advance temper particles at loftier altitudes to groovy plenty speeds to escape. An additional way of temper escape chosen photodissociation occurs when high-energy sunlight (eastward.k., ultraviolet or x-rays) hits high-altitude molecules in the planet's temper and breaks them autonomously into private atoms or smaller molecules. These smaller particles accept the same temperature every bit the larger molecules and, therefore, as described above, will move at faster speeds, maybe fast enough to escape.

The processes described and so far in this section piece of work particle to particle and work over long time periods as the atmosphere leaks away particle by particle. In dissimilarity impacts by comets or asteroids can inject a huge amount of energy very quickly when the projectile vaporizes upon touch. The expanding plume of hot gas drives off the air above the bear on site, with the larger the impact free energy, the wider is the cone of air that is removed higher up the touch on site. The bear on removal process was probably peculiarly effective for Mars (being so close to the asteroid belt) and the big moons of Jupiter (so close to Jupiter's potent gravity that attracts numerous comets and asteroids).

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