Tuesday, March 15, 2011

"Where Did Helium Come From?" or "Jupiter & Saturn, Helium Hoarders"

All matter in the Universe is made up of three-quarters hydrogen and one-quarter helium (by weight), with relatively insignificant amounts of all the other elements (not including dark matter, whatever that is).  This crazy abundance of hydrogen and helium has been around in nearly the same ratio from minutes after the big bang over 13 billion years ago.  Wowza!


Helium.  (see story of Larry Walters)

Partial evidence for the incredible abundance of hydrogen and helium in the universe can be seen right here in our own solar system, in the form of the "gas giant" planets Jupiter and Saturn, which are both composed primarily of hydrogen and helium, with small but significant amounts of carbon, oxygen, nitrogen, and other elements.  The composition of these planets is thought to be similar to that of the "primordial solar nebula" from which the solar system was formed.  Woah!!

Jupiter, as imaged by Voyager 1 in 1979.  Mesmerizing!!!!  You can see a few of Jupiter's 63 moons too! 

Saturn, as imaged by the Hubble Space Telescope.  Saturn has aurora too, just like earth!

Earth on the other hand, is composed primarily of iron, oxygen, and silicon, with only minuscule amounts of helium.  As you can see in this awesome chart that compares the compositions of the planets in the solar system, the composition of Earth is waaaaay different than that of Jupiter and Saturn (click to enlarge)(thanks to researchers at the Australian National University):

Densities and Elemental Compositions of the Planets.  Earth is divided into two sections, with Earth on the left and our moon on the right.

Why the huge difference between the gas giants and Earth?  Where did all our helium go?

Well, as helium loves to do, it floated away!  Even though scientists believe that Earth began with a pretty decent amount of helium in its atmosphere, Earth is not massive enough to gravitationally hold on to this extremely light element, so the helium just rose to the top of our atmosphere and never stopped rising.  Jupiter and Saturn however are 300- and 100-times (respectively) more massive than the earth, and they have plenty of gravity to hold on to their helium.  So sweet.

BUT:  If helium in our atmosphere just floats away, and you are still able to buy helium for birthday balloons or to float you and your lawn chair, where did that helium come from?  Well, if it can't be up above in the atmosphere, that leaves us with only one option, down below in the earth.  It's true!  We get all our helium from underground!  It gets trapped in pockets along with natural gas, and is piped out of the ground by energy companies, who then separate the helium from the other natural gas chemicals (i.e. methane, ethane, nitrogen, etc).

Here is the craziest part:  That helium was not around when Earth formed.  So where did it come from???  Well, it is constantly being produced by the radioactive decay (like in a nuclear reactor) of the elements thorium and uranium (images courtesy Theodore Gray).

The radioactive decay of these elements ejects alpha particles, which are pretty much the same thing as helium atoms.  This produces around 3000 metric tons of helium per year in the earth's crust!  That is where almost all our helium comes from!


These ejected helium atoms travel at thousands of miles/hour, and are slowed down by bouncing off other atoms, producing heat in the process.  That is one of the primary reasons the core of the earth is still molten, even after 5 billion years!  This "geothermal" heat eventually radiates from the earth and can be captured in places where the heat flow is high, like Iceland, which gets one quarter of their energy from geothermal sources.

Global Heat Flow (source).  Notice Iceland is in a yellow zone.
So crazy!!!