Question

Helium (He, atomic weight 4 g mol-1) and argon (Ar, atomic weight 40 g mol-1) are both produced in the Earth’s interior and exhaled to the atmosphere. Helium is produced by radioactive decay of uranium and thorium; argon is produced by

radioactive decay of potassium-40 (40K). Both helium and argon, being noble gases, are chemically and biologically inert and are negligibly soluble in the ocean. Present-day atmospheric mixing ratios of helium and argon are 5.2 ppmv and 9340 ppmv, respectively.

1. Atmospheric argon has no sink and has therefore gradually accumulated since

Earth’s formation 4.5x109 years ago. In contrast, atmospheric helium has a sink. What is it?

2. Show that the average source of argon to the atmosphere over Earth’s history is PAr = 1.5x107 kg yr-1

Answer 1

Answer for 1:

All of the atmospheric gases are held by the gravitational force. However some lighter gases such as hydrogen and helium, escape into space because of their low molecular weights (less than 10). The gravitational field with which it holds the gases is measured in terms of escape velocity which is directly proportional to (T/M)^1/2 (where T = temperature in kelvins and M = molecular weight). Thus helium is continuously lost into space as it is produced from the radioactive decay of elements in the earth’s crust. So the space acts as the sink for lighter gases such as hydrogen and helium.