Helium escape velocity

The main reason that Mercury does not have a distinctive atmosphere is that it is small and because it is close to the Sun. Mercury s small radius indicates that it has a low escape velocity, just 2.5 mi (4.2 km)/sec. Mariner 10 did detect a very thin atmosphere of hydrogen and helium on Mercury. It is believed, however, that Mercury s wispy atmosphere is composed of atoms that have been temporarily captured from the solar wind. Ground-based observations have found that a sodium and potassium atmosphere exists on the daylight side of Mercury. These atoms are probably released through the interaction of ultraviolet radiation with surface rocks. 

The escape velocity necessary for objects to leave the gravitational field of the Earth is 11.2 km Calculate the ratio of the escape velocity to the root-mean-square speed of helium, argon, and xenon atoms at 2000 K. Does your result help explain the low abundance of the light gas helium in the atmosphere Explain. 

Gravity Every molecule on Earth is pulled toward the planet by gravity, even the very lightest gas molecules. To overcome a planet s gravitational pull, an object, be it a space ship or a helium atom, must reach at least the escape velocity. Almost all of the molecules in the atmosphere are below this velocity because of the temperature of the planet Earth (higher temperature = faster molecules). 

Water was surely abundant in early atmospheres and throughout Earth s history with a cooled lithosphere. If there were water. Earth had an oxidizing atmosphere, while hydrogen liberated from water escaped from Earth s atmosphere just as it continues to do today. Both hydrogen gas and helium, when released into Earth s atmosphere, escape from Earth never to return, because the thermal velocity of these gases is above the escape velocity of a mass of their size. They are two of a very few substances that are not automatically recycled. We have no other choice than to recycle unless garbage is to be disposed of in outer space, a highly unlikely event. 

Helium derives its name from the Greek word helios, sun. The first evidence for helium was discovered in the solar spectrum taken during a solar eclipse in 1868. Helium is the second most abundant element in the universe after hydrogen. Helium atoms are light and travel at velocities sufficient to escape from the earth s atmosphere. 

FIGURE 228. The distribution of velocities of gas molecules at 25°C. Note how much greater the velocities are for ultralight hydrogen molecules and helium atoms. These substances thus escape the earth s atmosphere, unlike the heavier gases. (Adapted from Brown et ah, Chemistry—the Central Science). 

Because helium atoms are very light, once released they can achieve velocities great enough to escape from Earth s gravity. Ihe decay ot radioactive elements resupplies helium, creating a roughly constant amount in the atmosphere. 

Fig. 7.14. (a) Typical velocity map ion images of CF3 fragments escaping from helium... 

Helium in a large tank escapes vertically through a hole in the tank s top at a velocity of 38.7 m/sec. The helium in the tank is above air in the same tank. What is the height of the helium in the tank ... 

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