Which almost all argon has.
But there's also an isotope that has a weight of 38 and
one that has a weight of 36, which are much more rare.
You might think, well, why would you measure the ratio of 36 to 38 instead of
something that's rarer to something that's very common?
The answer is because these two have nearly the same abundance.
They're only different by about a factor of five,
whereas these are different by factors of 1,000, 10,000.
And so it's difficult to make a measurement at the same time
between something that has a factor of 10,000 times more than something else.
So measuring these two things at the same time, the abundance of these two things at
the same time in the same thing is a much more easier observational task.
How do you do this observationally?
You do it through some sort of mass spectrometer, which, for example,
ionizes the argon, sends it through a magnet.
And that magnet causes the trajectory to bend.
And it bends a little bit more if it's lighter and
a little bit less if it's less light.
So where that argon hits on a detector tells you its mass, and
you can then measure the mass ratio.
This technique has been done on the terrestrial atmosphere, it's right here.
And interestingly, it's been done on the Jovian atmosphere, and
even on the solar atmosphere.
How do you do the Jovian atmosphere?
Well, this was done from the Galileo probe that actually went
into the Jovian atmosphere, measured this ratio.
How do you do it for the Sun?
Well, we didn't send things into the Sun to measure it.
But we did have a spacecraft, Genesis,
it was called, that sat out in space, collected solar wind particles.
Brought them back down to the Earth, brought them back down to the Earth,
forgot to deploy the parachute, and crashed into the Earth.
But materials was still recovered, and things like
the 38 to 36 argon ratio were measured for the Sun, for Jupiter, for the Earth.
Sun and Jupiter are taken to be the primordial abundance
of argon 36 to argon 38 in the initial Solar System.
And it's nice, they're about the same value.
How the about the Earth?
Well, the Earth is down by a little bit, but really not very much.
There's no evidence that the Earth has lost very much atmosphere.
Okay, I'll let you see the first measurement.
The first measurement came from the Viking lander that got to the surface of Mars,
measured the atmospheric composition, measured the isotopic ratios.
And sure enough, well, it's either higher than the Earth and
Jupiter and the Sun, or somewhere in between.