Gravity and the Atmosphere
Name: Steven T.
Date: Thursday, November 28, 2002
I was wondering why the force or pull of gravity does
not collapse our atmosphere? Why can elements lighter than air like
hydrogen or nitrogen float and be resistant to gravity when gravity is
supposed to pull everything down at the same rate?
There are a couple of ways to answer your question. First, because the
atmosphere is a gas and because gases can be compressed, the atmosphere is
denser at lower altitudes In a way the atmosphere is collapsed by gravity.
But more then that, because of the kinetic energy of the molecules of the
atmosphere and because of their random notion, air molecules move in all
directions including in opposition to gravity. When we observe this - and
we can by watching larger particles in suspended in the air - we see what is
called Brownian motion. You might find reading about this motion of
interest in dealing with your question.
Nothing is "resistant" to gravity. Gravity pulls on all matter.
Hydrogen floats because there are other molecules in our atmosphere that are
more dense (weigh more / volume of space occupied) than hydrogen. So those
higher density molecules simply push the lighter H2 (hydrogen) out of the
way. It does this much like water (density 1 g / cc) pushes oil ( ~ 0.90 g
/ cc ) out of the way and makes the oil "float". Other higher density
molecules will displace those lighter molecules. You might get a better
education in this matter if you go to the Internet and do a search on
I do no understand what you mean by collapsing our atmosphere. But it is the
gravity that the Earth generates that holds our atmosphere close to the
surface of the planet.
As a very rough rule of thumb, and irrespective of compound type (N2 or O2
or H2...etc..., I do believe that the "amount" of molecules or to put it
another way the pressure of the atmosphere vs. altitude is an exponential
P (atmosphere) = function ( altitude ) = Po * exp ( ALTITUDE / C )
Po = 1_atm = 14.69 psia (at sea level)
ALTITUDE IS SIMPLY HEIGHT ABOVE SEA LEVEL
C is a mathematical constant that makes the equation true for our given
atmosphere ... ie ... 21% oxygen and 79% nitrogen.
Gravity DOES pull the atmosphere down. That is why it stays around. The
moon and celestial objects like the planet mercury, that are not massive
enough have lost most of their atmosphere. Our atmosphere has a certain
pressure because of the balance of the gravitational attractive force and
the resistance of a gas to compression, as stated fairly accurately by the
ideal gas law: PV= nRT where P is the pressure in atmospheres, V is the
volume in liters, n is the number of moles, T is the absolute temperature in
kelvins, and R is the gas constant, R=0.0825
liter-atmospheres/moles-kelvins. The force of gravity decreases with
altitude, but there are other atmospheric factors that come into play.
Search the term "standard atmosphere 1976" and you will find numerous
calculators that give the pressure and other properties as a function of
altitude. One such is: http://www.digitaldutch.com/atmoscalc/,
The lighter gases tend to escape because of the smaller force of gravity on
them F = G mM/r^2 the smaller the value of m ( the mass of the
atom/molecule) the smaller the attractive force of gravity.
Hydrogen and any other gaseous molecules indeed have mass and are being
pulled to earth by earth's gravity. But temperature (kinetic energy) is
keeping them from collapsing --like colliding ping-pong balls in a
wind-chamber. Under temperature conditions at the equator/tropics some
molecules of hydrogen can be raised to the upper atmosphere and a smaller
subset achieve escape velocity and indeed leak-off into space. Its a small
% - most that get to the upper atmosphere lose energy and are held by
earth's gravity. Larger molecules like nitrogen and oxygen cannot escape at
all. If earth was a larger planet, nothing would escape as is the case of
Jupiter. Earth's early atmosphere 4 billion years ago is believed to be
much like Jupiter is today allowed lots of hydrogen to drift into
space. But earth changed because of its size (weaker gravitational forc)
and higher average temp (closer distance from the sun) and its evolutionof
photosynthetic organisms. Its not surprising that Jupiter's atmosphere s
much thicker and hydrogen-rich due to lower average temp and much highergravity.
At very low temperatures the atmosphere would collapse around 300 F below
zero. And that would need to be the warmest temp on a planet if similar to
The moon and asteroids have no atmosphere because their gravity is just too
small to hold any gases. With no counter pressure, liquids boil away at
very low temperature. As a gas they are lost to space. But could there be
a balance where "body" size and temperature could be acheived - a body
larger than our moon with less average temperature?
Gravity is not the only force acting on the molecules of our atmosphere.
Gravity is not the strongest force, either.
The molecules in the atmosphere are moving very fast. This is because of
the temperature. Until the temperature gets down around -250 degrees
Celsius, all molecules move around very quickly. When moving around, they
bump into each other. All this bouncing is what supports the molecules at
higher altitudes. The common word for this constant bouncing around is
pressure. The closer molecules get to each other, the more often they bang
into each other. Molecules that are closer together experience more
pressure. When they get too close together, the pressure pushes them apart.
When they get too far apart, gravity pulls them back together.
Hydrogen and nitrogen float in the air because oxygen is heavier. Imagine a
large jar. In it are many little balls. Some are lighter, some are
heavier. You mix all the balls up. If you then jiggle the jar for several
hours, the heavier balls will work their way to the bottom. The light balls
will end up near the top. All the rapid bouncing does the same thing with
our atmosphere. The heavier molecules are pulled harder by gravity.
Gravity only pulls objects down at the same rate if NOTHING else is pushing
or pulling on the objects. For heavy objects such as a steel bar, the force
from the air molecules is nothing compared to the immense weight of the
steel. A steel bar falls with an acceleration of 9.8 m/s^2. For light
objects such as a ping-pong ball, the force from the air molecules is
important. A ping-pong ball falls with a much smaller acceleration.
Exactly what it is depends on how fast the ball is moving. This is exactly
what air resistance is. For objects as light as individual molecules, the
force from the other molecules is large enough to completely counter the
effects of gravity.
Dr. Ken Mellendorf
Illinois Central College
Gravity is a force and the strength of the force due to gravity
depends on the mass of the object it acts upon. A bubble of air is
pulled down with less force than an equal volume of water above it so
the water displaces the air and the bubble rises. This same buoyancy
occurs on a molecular scale. A molecule of hydrogen is pulled down with
less force than the nitrogen molecules around them (because the hydrogen
molecules have less mass) and the hydrogen rises as a result.
It does not all collapse to the surface of the earth because the
molecules have kinetic energy and are in constant (rapid) motion in
three dimensions. The gravitational force on individual molecules is
very small and they have sufficient energy to collide and rebound to
keep an average distance between molecules that fills the volume of our
If enough molecules stick together, for example when water condenses
into clouds, the weight of the individual water droplet becomes too
heavy for the energy of the molecules around them to keep them
suspended. The result is that the heavy droplets fall to the sky as
rain. Fortunately for us, oxygen and nitrogen molecules don't do this
under the conditions that exist in our atmosphere.
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