Gravity and Atmospheres

Key idea: Atmospheres are held in place by the force (pull) of gravity. A gravitational force exists between all objects which have mass. In the case of atmospheres, the gas molecules and other particles in the atmosphere are bound by gravity to the host planet or satellite.

Neptune with Spots Astronomical tidbit: Nearly all of the planets in the Solar System have atmospheres, though the atmospheres are quite different from each other. A number of the larger satellites in the Solar System have atmospheres as well. The photograph on the left shows the planet Neptune with bright clouds and huge dark storms in its atmosphere. The large dark feature at the upper left is called the Great Dark Spot. The large feature at the lower right is called the Small Dark Spot. The two Spots and the other features in Neptune's atmosphere travel around the planet at different rates. The photograph was taken by the Voyager 2 spacecraft during its August, 1989 encounter with Neptune, the outermost of the gas giant planets.

Surface Gravity

When a force acts on a body, that body is accelerated. The acceleration due to gravity on the surface of the Earth is 9.8 meters per second2 (note: second2 is seconds to the 2nd power). This means that if an object is dropped near the surface of the Earth, its velocity due to gravity will increase by 9.8 meters per second every second. The surface gravity on Mars is only 38% that of Earth. If an object is released above the surface of Mars, it will accelerate only 38% as quickly as it will on Earth. The "surface gravity" on Jupiter (at the 1-bar pressure level) is 22.9 times that of Earth. To convert from meters to feet, multiply by 3.28 because 1 meter = 3.28 feet.

Weight is also determined by gravity. Weight is the force which you exert on your bathroom scale due to the gravitational attraction of the Earth!!! If a person weighs 100 pounds on Earth, then he or she will weigh only 38 pounds on Mars because the surface gravity there is 38% of what it is on Earth.

Questions: How much will that same person weigh on Jupiter (at the 1-bar level)? How much will each of you weigh on Mars and Jupiter???

Surface gravity (g) depends on the mass (M) of the planet and on the planet's radius (r). The formula which relates these parameters is:

g = G * M / r2

where G is called the gravitational constant. If you know the mass of a planet (or satellite) and its radius, then you can use this formula to calculate its surface gravity. By looking at the formula, you can see that if the mass of the body doubles while its radius remains the same, then its surface gravity also doubles. On the other hand, if the mass remains the same but the radius doubles, then the gravity on the surface will decrease by a factor of four!!!

Activity: For mathematically advanced students, calculate the surface gravity for various planets and satellites in the Solar System and for the Sun itself!

Escape Velocity

Gas molecules (or any massive objects for that matter) which have a velocity greater than the escape velocity for a given planet will not be bound to the planet by the force of gravity. Such molecules will have sufficient energy to escape to distant space! The escape velocity at the Earth's equator is 11.2 kilometers per second (note that 1 kilometer is equal to 1000 meters). The escape velocity at the surface of Mars is 5.0 kilometers per second and at Jupiter (1-bar level again) is 59.6 kilometers per second.

Questions: What is the escape velocity of the Earth in units of miles per hour? How fast would a person have to throw an object from the surface of the Earth so that it would not return to the surface but rather escape to space? Can a person throw a ball that fast? How do you suppose that gas molecules might gain sufficient energy to travel that fast???

Like surface gravity, the escape velocity is determined by the mass of the planet and by its radius. The more massive the planet, the greater is its escape velocity and the more gas it is likely to have in its atmosphere.

Activity: For mathematically advanced students again, learn to calculate the escape velocity from the mass and radius of a body, and calculate the escape velocity for various planets and satellites in the Solar System.

M87 Nucleus and Jet Astronomical tidbit: A black hole is an astrophysical object whose mass is compressed into such a small volume that the escape velocity for the black hole exceeds the speed of light!!! Because light cannot escape the gravitational attraction of such an object, the object is said to be black. It is not possible for any object to exceed the speed of light, so mass cannot escape from a black hole either. Black holes are believed to form from the collapsed cores of large stars which have burned all their inner fuel. Supermassive black holes are thought to exist in the centers of many galaxies where tremendous amounts (millions of times the mass of the Sun!) of gas are gravitationally compacted.

The center of the giant elliptical galaxy M87 is shown in the photo at the right. Astronomers believe that a supermassive black hole exists in the nucleus of M87. The hole itself is black, but fast moving gas is heated as it is spirals toward and then into the black hole, and the hot gas shines brightly in the photograph. A jet of high-speed electrons is also shown as it exits from the vicinity of the black hole and travels at nearly light speed toward the upper right of the picture. The photograph of M87 was taken by the Hubble Space Telescope in 1994. The giant galaxy M87 is located about 50 million light years away from Earth!!! This means that it takes light 50 million years to reach Earth from M87. By contrast, light from the Sun takes only 8 minutes to reach the Earth.

Thermal Motion

Gas molecules move faster when they are hot than when they are cold, and faster moving molecules are more likely to exceed a planet's escape velocity than slower moving molecules. Mercury is the closest planet to the Sun (and therefore very hot) and it has no atmosphere, whereas Pluto is the furthest planet from the Sun (and therefore very cold) and it does have an atmosphere. This is so even though the escape velocity of Mercury is four times more than that of Pluto, and it would be expected to have more of an atmosphere than Pluto if the atmospheres were at the same temperature.

Activity/demonstration: Add colored dye to water at different temperatures and observe how quickly is disperses.

At the same temperature, light molecules move faster than heavy ones. That is why atmospheres consist largely of heavy gases like oxygen and carbon dioxide instead of light gases like hydrogen and helium. The lighter molecules move too fast to be easily bound to planets or satellites by gravity.

Last updated: November 17, 1999
Joe Twicken /
Rob Wigand