Helium is leaking off the planet, a little every second
Across the universe, helium is everywhere. After hydrogen, it is the most abundant element, about a quarter of the mass of all the elements. Most of it formed in the Big Bang, and stars keep making more by fusing hydrogen.
It was even found in the Sun before anyone found it on Earth. In August 1868, during a total solar eclipse in India, the astronomer Jules Janssen saw a bright yellow line in the Sun's light that no known element explained. Norman Lockyer saw it too, concluded it came from a new element, and named it after the Greek word for the Sun, helios.
On Earth, though, helium is rare. It makes up only a few parts per million of the air we breathe, and that share stays low even though new helium is made all the time.
Too light to keep
Gravity holds an atmosphere in place, but not every gas equally. At the same temperature, light atoms move faster than heavy molecules, and high in the thinnest air a few move fast enough to leave for good. Helium is the second-lightest element, and as a noble gas it forms almost no compounds, so nothing ties it down.
Earth loses about 50 grams of helium every second, mostly through what is called the polar wind: near the poles, open magnetic field lines give charged particles in the upper atmosphere a way out into space. Fifty grams a second sounds small, but it comes to roughly 1,600 tonnes a year.
New helium comes from below. Uranium and thorium in rocks decay slowly and send out alpha particles, which are helium nuclei. When they come to rest in the rock, they become helium atoms. Throughout the Earth's crust and upper mantle, this makes an estimated 3,000 tonnes a year.
Helium that seeps up into the air is effectively lost. The helium we can use is the part that never got there: it collected underground under the same conditions that trap natural gas. In most gas fields it is only a trace, but in a few it reaches more than 7%. That is where commercial helium comes from, separated by cooling the gas until its other parts turn liquid.
So on any human timescale, helium is a resource that doesn't come back. A balloon that floats away or a leak that escapes sends its helium up and, eventually, off the planet. Its largest single use is cooling: liquid helium keeps the superconducting magnets of MRI scanners cold. Some scientists argue that prices have been too low to discourage waste.
Where does the helium we use come from?
Helium is too light for Earth to keep: what reaches the air drifts off into space, so the helium we use comes from gas trapped underground.
